Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/70—Photonic quantum communication
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
  • H04B10/112—Line-of-sight transmission over an extended range
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/25—Arrangements specific to fibre transmission
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/532—Polarisation modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/54—Intensity modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/548—Phase or frequency modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
  • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
  • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
  • H04L9/0852—Quantum cryptography

Definitions

• the present invention is generally in the field of quantum key distribution (QKD) particularly over free space medium.
• QKD quantum key distribution
• QKD is gaining considerable recognition for its ability to securely generate secret cryptographic keys between distant parties, guaranteeing that the cryptographic keys thereby generated cannot be intercepted or tampered with, based on principles of quantum mechanics.
• the cryptographic keys generated by QKD systems can be used for symmetric key cryptography in applications demanding high levels of privacy and long-term secrecy.
• a secret cryptography key is securely generated between two distant parties, usually referred to as "Alice” (the QKD transmitter) and "Bob" (the QKD receiver), by encoding information as quantum states of single photons, also known as qubits.
• QKD implementations exploit certain properties of these quantum states to ensure its security, guaranteeing that any attempt to eavesdrop/tap the quantum communication channel will introduce detectable errors into the key generation process indicative of the eavesdropping attempt.
• QKD systems can be implemented using various photonic degrees of freedom (DOF), such as polarization, time-bin, and orbital angular momentum, over optical fiber, free-space and satellite links.
• DOF photonic degrees of freedom
• Two degrees of freedom commonly used in QKD systems are time -bin and phase (between the time-bins) photon state encoding, and polarization photon state encoding.
• Time-bin phase state photon encoding is commonly used in optical fibers based QKD implementations
• the polarization state photon encoding is commonly used in free space optics (FSO) QKD implementations. This is mainly because information is easily encoded in time-bin phase state of photons, and well preserved when propagating along optical fibers, whereas polarization states encoding tends to get mixed along the optical fibers.
• polarization states encoding is the more robust DOF, since phase states encodings are less robust to atmospheric effects, whereas wave fronts of time bin phase state photon encoding waves are easily distorted, which results in information corruption..
• Polarization optics e.g., Pockels cells
• Both active and passive polarization optics have a larger footprint, and are harder to manufacture and align.
• time bin phase state photon encoding techniques a single photon (or less on average) is typically divided into two separated pulses (referred to herein as time-bins Po and P T ) with quantum information encoded as relative phase and/or intensity difference therebetween.
• time-bins Po and P T quantum information is encoded utilizing two basic orthogonal degrees of polarization states of a single photon e.g., horizontal polarization
• Russian Patent Publication No. RU2771775 discloses a method and apparatus for quantum key distribution (QKD) supplementing the QKD apparatus with additional modules ensuring conversion of polarization encoding into phase-time encoding in the transmitter and reverse conversion from phase-time encoding to polarization encoding in the receiver.
• QKD quantum key distribution
• Japanese Patent Publication No. JP2008160368 discloses polarization beam splitters used to divide an optical pulse signal resulting from coding quantum information to polarization degrees of freedom of single photons, into two optical pulse signals having a time difference in accordance with polarization, and polarization of one optical pulse signal is rotated by a polarization rotator controlled by a modulation controller, whereby quantum information coded to polarization degrees of freedom is transferred to time degrees of freedom of pulses. Thereafter, the quantum information is transmitted as two divided optical pulse signals by fibers and is restored into information coded to polarization degrees of freedom in a transmission destination in accordance with reverse procedures.
• WO2018214888 discloses polarization and phase entangled coding method and apparatus, and a quantum key distribution system.
• the method comprises converting a first photon in a polarization entangled photon pair generated by a polarization entangled light source from using polarization coding to using phase coding, and forming the first photon converted to use phase coding and a second photon in the polarization entangled photon pair into a polarization and phase entangled photon pair.
• the quantum key distribution system based on the polarization and phase entangled coding apparatus can make full use of transmission advantages of different coding schemes over different channels to enable conversion of a photon from using polarization coding to using phase coding when transmissions are performed over different channels.
• Chinese Patent Publication No. CN113810191 discloses a quantum key distribution system wherein at the Alice end a single photon is divided into two orthogonal phase equal polarization components through a 1 st polarization beam splitter, phase modulation is carried out on the vertical polarization components, then beam combination is carried out through a 2 nd polarization beam splitter, and then the polarization components are converted into a lefthand circular polarization component and a right-hand circular polarization component through the 1/4 slide, and at the Bob end the polarization component is changed into a vertical polarization component and a horizontal polarization component again through the 1/4 slide, the recovered horizontal polarization component is subjected to phase modulation, and then phase information is converted into polarization information after beam combination through a 4 th polarization beam splitter.
• US Patent Publication No. US8509446 discloses QKD free space and fiber network systems that generate a couple of photons which have different wavelength, and input each of the photons into an asymmetric Mach-Zehnder interferometer to obtain time-bin entangled states to provides polarization information with one part of the photons.
• EP3742664 discloses a QKD system comprising an emitter adapted to generate a QKD free-space signal, a transmitter station adapted to receive the free-space signal from the emitter, and a remote QKD receiving station supporting a QKD receiver located at a different location than the transmitter station, wherein the transmitter station is adapted to receive said free space signal from the emitter and to forward said signal through a fiber link to the QKD receiver in said remote QKD receiving station.
• Embodiments hereof are configured to use time-bin phase state qubit (T-P) encoding in QKD systems with additional optical elements configured to convert the time-bin phase state encoded photons into hybrid time-bin and polarization state encoded photons, or pure polarization states, with a simple, compact and manufacturable design.
• additional optical elements can be used to covert the hybrid time-bin and polarization state encoded photons or pure polarization states encoded photons, into T-P encoding e.g., for extracting the information encoded therein.
• the disclosed embodiments can be advantageously exploited for QKD network implementations.
• Embodiments hereof can be exploited for separating between secure and insecure components of FSO based QKD systems, for allowing placement of qubit signaling equipment in a remote insecure site (e.g., telecommunication tower, building roofs, or suchlike), and placement of sensitive components (also referred to herein as secured units e.g., cryptographic key generation and communication units) of the system in well secured communication centers (e.g., safeguarded/underground facilities, building basement, or suchlike).
• the secure cryptographic key generation data/signals, and/or other secured communication can be communicated with the qubit signaling equipment mounted at the remote insecure site over optical fiber(s).
• the qubit signaling equipment is configured for FSO communication with other QKD system(s) over free space medium e.g., as terrestrial communication between different cities, different country /city regions, between different countries, or over uplink satellite, downlink satellite, or intersatellite communication links.
• the qubit e.g., FSO
• the qubit e.g., FSO
• the qubit e.g., FSO
• the communication between the secured units of the system and its FSO communication system is preferably carried out over the optical fiber(s) utilizing time bin phase state (T-P) photon encoding, and the communication between the FSO communication systems of the different QKD systems is carried out over the free space medium utilizing the hybrid time-bin and polarization state photon encoding, such as disclosed herein.
• the FSO communication system can be configured to convert the T-P encoded photons thereby received into time -bin and polarization state photons suitable for quantum communication over the free space medium.
• the FSO communication system can be configured to convert the hybrid time-bin and polarization state encoded photons thereby received into T-P encoded photons suitable for communication with its secured units over optical fiber(s).
• the secured units of the QKD transmitter and/or of the QKD receiver are located in a well secured location, and their FSO communication systems can be installed at a separate relatively insecure location.
• FSO communication systems can be installed at a separate relatively insecure location.
• the fiber-based transmitter designs suggested below provide a significant advantage, as it allows mechanical design flexibility due to the ease of routing optical fibers, low power consumption since telecom components are typically readily optimized in power consumption, and a robust design since less optomechanical and electro-mechanical modules are required.
• a T-P to hybrid time-bin and polarization state photon encoding (T2H) converter is used in some embodiments in the FSO communication system of the QKD transmitter.
• the T2H converter can be configured to utilize an imbalanced (e.g., Mach Zehnder or Michelson interferometer with Faraday mirrors having short and long arms) interferometer for producing at least three observable (e.g., time separated) signal patterns for the polarization state photon encoding thereby produced.
• a beam combiner can be used in the imbalanced interferometer to combine the optical signals propagating along its short and long arms.
• the beam combiner can be configured to split (e.g. , 50:50, or other suitable ratio) the combined optical signals emerging from the imbalanced interferometer into two separate optical fiber polarizing arms configured to apply the polarization sate photon encoding.
• ⁇ - ⁇ -)) polarizer can be used in one of the optical fiber polarizing arms for transmitting therefrom horizontally polarized optical signals
• J)) polarizer can be used in the other optical fiber polarizing arm for transmitting therefrom vertically polarized optical signals.
• at least one X/2 phase retarder is used in at least one of the optical fiber polarizing arms to affect the desired polarization states to the optical signals emerging from the imbalanced interferometer.
• a beam combiner can be used to combine the optical signals emerging from the optical fiber polarization arms and thereby produce hybrid time -bin and polarization state encoded photons.
• the beam combiner is configured to split the hybrid time-bin and polarization state encoded photons thereby produced into at least two light output components e.g., output optical fibers. At least one of the light output components can be used as an output of the T2H converter e.g., optically coupled to the FSO transmitter. At least another one of the light output components from the beam combiner can be used for monitoring (e.g.
• a state characterization system which can includes polarization optics and photodiodes
• the hybrid time-bin and polarization state encoded photons produced by the T2H converter are used (e.g., in the T2H converter or at its output) to attenuate the transmitted signal to a single photon level, as required in QKD applications.
• the monitoring of the light output components from the beam combiner preferably operates over higher intensities, and thus does not require single photon detectors.
• At least one X/2 phase retarder is used in the long (and/or short) arm of the imbalanced interferometer to affect the photon polarization states.
• the beam combiner of the imbalanced interferometer can be configured to split the combined optical signals of the imbalanced interferometer into the at least two (e.g., 50:50, or other suitable ratio) light output components.
• At least one of the light output components from the beam combiner of the imbalanced interferometer can be used as an output of the T2H converter e.g., optically coupled to the FSO transmitter.
• At least another one of the light output components from the beam combiner of the imbalanced interferometer can be used for monitoring (e.g., by a state characterization system, which can includes polarization optics and photodiodes) the hybrid time -bin and polarization state photons produced by the T2H converter.
• a state characterization system which can includes polarization optics and photodiodes
• an attenuator is used (e.g., in the T2H converter or at its output) to attenuate the transmitted signal to a single photon level, , as required in QKD applications.
• the monitoring of the light output components from the beam combiner preferably operates over higher intensities, and thus does not require single photon detectors.
• the imbalanced interferometer of the T2H converter is configured to covert each T-P state encoded photon thereby received into three distinguishable optical pulse signals, wherein the first optical pulse signal is generated responsive to passage of the first time-bin (Po) of the T-P state encoded photon through the short arm of the imbalanced interferometer, the second optical pulse signal is generated responsive to an interference pattern of the first and second time-bins (Po,P T ) of the T-P state encoded photon after they respectively pass through the long and short arms of the imbalanced interferometer, and the third optical pulse signal is generated responsive to passage the second time-bin (P T ) of the T-P state encoded photon through the long arm of the imbalanced interferometer.
• the first optical pulse signal is generated responsive to passage of the first time-bin (Po) of the T-P state encoded photon through the short arm of the imbalanced interferometer
• the second optical pulse signal is generated responsive to an interference pattern of the first and second time-bins (Po,P T
• a hybrid time-bin and polarization to T-P state photon encoding (H2T) converter is used in some embodiments in the FSO communication system of the QKD receiver.
• the H2T converter can be configured according to implementations disclosed in the above-mentioned patent publications and/or reference [1].
• the interferometer of some embodiments disclosed herein are implemented such that the optical signals propagating through their arms (when in orthogonal polarization orientations) are combined without interference.
• a photon state polarization converter comprising: an imbalanced interferometer configured to generate the following three optical signal pulses responsive to time -bin and phase states encoded photons thereby received: (i) a first optical pulse signal responsive to a first time-bin of the time-bin and phase states encoded photon emerging through a short arm of the imbalanced interferometer; (ii) a second optical pulse signal responsive to an interference pattern between said first time-bin emerging through a long arm of the imbalanced interferometer and a second time-bin of the time -bin and phase states encoded photon emerging through the short arm; and (iii) a third optical pulse signal responsive to the second time -bin emerging through the long arm of the imbalanced interferometer; and at least one polarization orientation manipulating device configured to affect orthogonal polarization orientations to light components from the imbalanced interferometer and thereby generate hybrid time-bin and polarization states encoded photons.
• the photon state polarization converter comprising in some embodiments two polarization arms optically coupled to the imbalanced interferometer and configured for generating the two orthogonally polarized optical signals.
• Each one of the polarization arms can comprise a respective polarization orientation manipulating device.
• the photon state polarization converter can comprise a half wavelength phase retarder in one of the polarization arms.
• the time -bin and phase states encoded photons can have a defined linear polarization orientation.
• the photon state polarization converter can comprise a half wavelength phase retarder in one of the arms of the imbalanced interferometer.
• the time -bin and phase states encoded photons can have a defined diagonal polarization orientation.
• the photon state polarization converter can comprise a half wavelength phase retarder in one of the arms of the imbalanced interferometer and a polarizing beam splitter for transmitting the time-bin and phase states encoded photons into the long and short arms of the imbalanced interferometer.
• the time -bin and phase states encoded photons can have a defined linear polarization orientation.
• the photon state polarization converter can comprise a half wavelength phase retarder in one of the arms of the imbalanced interferometer and a polarizing beam combiner for combining optical signals emerging from the long and short arms of the imbalanced interferometer.
• the photon state polarization converter comprises in some embodiments an optical switch device configured to selectively transmit the first time -bin of the time-bin and phase states encoded photon into the long arm of the imbalanced interferometer and the second timebin of the time -bin and phase states encoded photon into the short arm of the imbalanced interferometer.
• the photon state polarization converter comprises in some embodiments a first intensity modulator configured to selectively block passage of the first time-bin of the time-bin and phase states encoded photon through the short arm of the imbalanced interferometer, and a second intensity modulator configured to selectively block passage of the second time-bin of the time-bin and phase states encoded photon through the long arm of the imbalanced interferometer.
• the photon state polarization converter can comprise a first driver unit configured to operate the first intensity modulator to periodically block passage of the first time-bin of the time-bin and phase states encoded photon through the short arm, and a second driver unit configured to operate the second intensity modulator to periodically block passage of the second time-bin of the time -bin and phase states encoded photon through the long arm.
• the driver units can be configured to set timing, frequency and/or bias to periodically block the respective time-bins.
• the photon state polarization converter can comprise an intensity modulator configured to selectively block passage of the first and third optical pulse signals through an output of the imbalanced interferometer.
• a quantum communication system comprising transmitter and receiver systems configured to communicate over a free space medium, the transmitter system comprising a quantum transmitter configured to generate time -bin and phase states modulated photons, and the photon state polarization converter of any of the embodiments disclosed herein optically coupled to the quantum transmitter for converting time-bin and phase states encoded photons thereby generated into hybrid time -bin and polarization states encoded photons for transmission to the receiver system over the free space medium.
• the quantum transmitter can be mounted in a secure location relatively remote to the photon state polarization converter and optically coupled thereto by one or more optical fibers.
• the receiver system can comprise a quantum receiver optically coupled to a photon state polarization converter configured to convert hybrid time -bin and polarization states photons received over the free space medium into time-bin and phase states encoded photons.
• the quantum receiver is optionally mounted in a secure location relatively remote to the photon state polarization converter and optically coupled thereto by one or more optical fibers.
• a polarization photon states encoder comprising: a coherent light source of a defined linear polarization orientation; an interferometer optically coupled to the coherent light source and configured with a polarizing beam combiner for combining optical signals emerging from arms thereof; first and second modulators configured to respectively modulate amplitude and phase of optical signals passing through the arms; and a half wavelength phase retarder configured to shift polarization orientation of optical signals passing through one of the arms.
• the polarization photon states encoder can comprise separate amplitude and phase modulators in each of the arms.
• the polarization photon states encoder comprises a single intensity modulator in each of the arms configured to modulate both amplitude and phase of the optical signals.
• the polarization photon states encoder comprises a single intensity modulator in each of the arms configured to modulate both amplitude and phase of the optical signals.
• a quantum communication system comprising transmitter and receiver systems configured to communicate over free space medium or optical fibers, wherein the transmitter system comprising a quantum transmitter comprising the polarization photon states encoder according to any of the embodiments disclosed herein.
• a method of converting photon state polarization comprising passing time-bin and phase states encoded photons through an imbalanced interferometer configured to generate a first optical pulse signal responsive to a first time-bin of the time-bin and phase states encoded photon emerging through a short arm of the imbalanced interferometer, a second optical pulse signal responsive to an interference pattern between the first time-bin emerging through a long arm of the imbalanced interferometer and a second time -bin of the time-bin and phase states encoded photon emerging through said short arm, and a third optical pulse signal responsive to the second time-bin emerging through the long arm of the imbalanced interferometer, and setting orthogonal polarization orientations to light components from said imbalanced interferometer and thereby
• the method can comprise setting one light components from the imbalanced interferometer into a horizontal orientation and another light component therefrom into a vertical polarization orientation.
• the method can comprise applying a half wavelength phase shift to one the light components from the imbalanced interferometer.
• the time -bin and phase states encoded photons can have a defined linear polarization orientation, and in this case the method can comprise applying a half wavelength phase shift to optical signals passing through one of the arms of the imbalanced interferometer.
• the time-bin and phase states encoded photons can have a defined diagonal polarization orientation, and in this case the method can comprise splitting the time-bin and phase states encoded photons into orthogonally polarized light components into the long and short arms of the imbalanced interferometer and applying a half wavelength phase shift to optical signals passing through one of the arms of the imbalanced interferometer.
• the time -bin and phase states encoded photons can have a defined linear polarization orientation, and the can method comprise in this case applying a half wavelength phase shift to optical signals passing through one of the arms of the imbalanced interferometer and combining orthogonally polarized optical signals emerging from the long and short arms of the imbalanced interferometer.
• the method can comprise selectively transmitting the first time-bin of the time-bin and phase states encoded photon into the long arm of the imbalanced interferometer and selectively transmitting the second time -bin of the time-bin and phase states encoded photon into the short arm of the imbalanced interferometer.
• the method can comprise selectively blocking passage of the first time-bin of the timebin and phase states encoded photon through the short arm of the imbalanced interferometer, and selectively blocking passage of the second time-bin of the time-bin and phase states encoded photon through the long arm of the imbalanced interferometer.
• the method of comprises in some embodiments selectively blocking passage of the first and third optical pulse signals through an output of the imbalanced interferometer.
• FIG. 1A to 1C schematically illustrates FSO quantum communication system according to possible embodiments
• FIGs. 2A to 2C schematically illustrate FSO based receiver and transmitter configurations of possible embodiments, wherein Fig. 2A shows components of a possible receiver system, Fig. 2B shows components of a possible transmitter system, and Fig. 2C shows a possible polarization plane aligner implementation;
• Figs. 3A to 3E schematically illustrate T2H converter configurations of possible embodiments for converting time-bin and phase state encoded photons into hybrid time -bin and polarization encoded photons, wherein Fig. 3A demonstrates use of polarizers in a polarization stage of the converter, Fig. 3B demonstrates use of a half wavelength phase retarder in the polarization stage of the converter, and Figs. 3C to 3E demonstrates use of a half wavelength phase retarder in the long arm of an imbalanced interferometer of converter implementations configured to receive polarized P-T states encoded photons;
• Figs. 4A to 4C schematically illustrate T2H converter configurations of possible embodiments configured to transmit one component responsive to an interference pattern of time-bins of linearly polarized P-T states encoded photons, wherein the converter of Fig. 4A utilizes optical switching at the input of the imbalanced interferometer, the converter of Fig. 4B utilizes intensity modulators at the arms of the imbalanced interferometer, and the converter of Fig. 4C utilizes an intensity modulator at the output of its imbalanced interferometer;
• Fig. 5 schematically illustrate a setup for generation of polarization states encoded photons according to possible embodiments;
• Fig. 6 schematically illustrates a setup for processing polarization state encoded photons according to possible embodiments
• Fig. 7 schematically illustrates a closed loop control scheme according to possible embodiments for phase and/or polarization drifts compensation
• Figs. 8A and 8B schematically illustrate point-to-multipoint passive network configuration according to possible embodiments.
• Time-bin and polarization states photon encoding configuration are disclosed, suitable for quantum communication over free space medium.
• a specially designed T2H converter utilizing an imbalanced interferometer is used in some embodiments to convert time-bin and phase states encoded photons into hybrid time-bin and polarization states encoded photons suitable for the quantum communication over the free-space medium.
• a combiner at the output of the imbalanced interferometer may be configured to combine optical signals from the long and short arms of the imbalanced interferometer and split the combined signals into at least two separate optical signal components/transmission arms.
• the T2H converter can utilize a polarization stage having two polarizing and/or phase retardation elements in one or both of the optical signal transmission arms, and configured to apply polarization states to the converted photons.
• a different polarizer can be used in each one of the optical signal transmission arms for orthogonally polarizing the optical signal components passing through the optical signal transmission arms one with respect to the other.
• a half wavelength phase retarder can be used in one of the signal transmission arms to affect the orthogonal polarization between the optical signal components passing through the optical signal transmission arms.
• the T2H converter is configured to convert time-bin and phase encoded photons received therein with a certain polarization, by utilizing a half wavelength phase retarder in one of the arms on the imbalanced interferometer e.g., instead of the polarization stage.
• one of the signal transmission arms from combiner of the imbalanced interferometer may be used for outputting the hybrid time -bin and polarization states encoded photons, and the other signal transmission arms may be used for monitoring this photons or omitted.
• a 1 :2 optical switch element is used to selectively introduce the first time -bin (Po) of the time-bin and phase states encoded photons only into the long arm of the imbalanced interferometer having the half wavelength phase retarder, and to selectively introduce the second time -bin (P T ) of the time -bin and phase states encoded photons only into the short arm of the imbalanced interferometer.
• the combined optical signals generated by the beam combiner of the imbalanced interferometer are only responsive to interference patterns of the first and second time-bins (Po,P T ) of the time-bin and phase states encoded photons obtained therein.
• an intensity modulator is used in the long arm of the imbalanced interferometer having the half wavelength phase retarder, and another intensity modulator is used in the short arm of the imbalanced interferometer.
• the intensity modulators are configured to selectively block passage of the first time-bin (Po) of the time-bin and phase states encoded photons through the short arm of the imbalanced interferometer and to selectively block passage of the second time-bin (P T ) of the time-bin and phase states encoded photons through the long arm of the imbalanced interferometer.
• Fig. 1A schematically illustrates a communication system 10 configured in some embodiments for quantum communication over free space medium lOq.
• the communication system 10 comprises a transmitter system 12 and a receiver system 11 configured for quantum communication over a free space communication link lOq, and for conventional data (e.g., packets based) communication over a standard communication link (e.g., over optical fiber(s) and/or optical and/or radiofrequency based over free space) 10s.
• conventional data e.g., packets based
• a standard communication link e.g., over optical fiber(s) and/or optical and/or radiofrequency based over free space
• the transmitter 12 comprises secured sub-system 12u comprising sensitive components, a T2H converter 12t optically coupled to the secured sub-system 12u over optical fiber(s) 12o, and a free space optical (e.g., using telescope and/or collimating optics) system 12e optically coupled to the T2H converter 12t.
• secured sub-system 12u comprising sensitive components
• T2H converter 12t optically coupled to the secured sub-system 12u over optical fiber(s) 12o
• the secured sub-system 12u comprises a quantum communication transmitter (qTx) 12p, and router and cryptography key components 12y for carrying out cryptographic key generation and communication.
• the router and cryptography key components 12y can be coupled to transceiver unit 12n configured to carry the communication over the standard communication link (e.g., over optical fiber(s) and/or optical and/or radiofrequency based over free space) 10s.
• the router and cryptography key components 12y can be configured to communicate with the quantum communication transmitter 12p over (e.g.
• the secured sub-system 12u is located in a secured location which can be placed relatively remote from the insecure components of the system, such as the T2H converter, the FSO system 12e, and/or the transceiver unit 12n.
• the receiver system 11 comprises in this non-limiting example a FSO system (e.g., using telescope and/or collimating optics) lie in line-of-sight (EOS) with the FSO system of the transmitter system 12, a hybrid time-bin and polarization states to time-bin and phase states photon encoding (H2T) converter lit, a quantum receiver (qRx) and router and cryptography key generation unit Ilf, and a transceiver unit lln electrically coupled to the router and cryptography key generation unit Ilf and configured to carry the communication over the standard communication link 10s with the transceiver unit 12n of the transmitter system 12.
• the secure and insecure components of the receiver system 11 are located in the same location, which may be secured and/or not suitable for the physical separation e.g., if mounted on a satellite.
• This configuration allows generation of T-P states encoded photons in the secured location in which the quantum transmitter 12p is located, and securely transmitting them to the T2H converter 12t over optical fiber(s) 12o for conversion into the hybrid time -bin and polarization states photon encoding and transmission over the free space medium lOq to the receiver system 11 by the FSO system 12e.
• Fig. IB schematically illustrates a communication system 10' similar to the communication system 10 of Fig. 1A, but in which the secure and insecure components of the transmitter system 12 i.e., quantum transmitter (qTx), router and cryptography key generation unit 12f, the FSO system 12e, the T2H converter 12t, and the transceiver 12n, are located in the same secure place, which may not be suitable for the physical separation e.g., if mounted on a satellite.
• the secure and insecure components of the transmitter system 12 i.e., quantum transmitter (qTx), router and cryptography key generation unit 12f, the FSO system 12e, the T2H converter 12t, and the transceiver 12n
• the receiver system 11' in this example is separated into a secured sub-system llu comprising the quantum receiver (qRx) lip and router and cryptography key generation unit lly, and an insecure sub-system comprising the a hybrid time-bin and polarization states to time-bin and phase photon encoding (H2T) converter lit, and the FSO system lie.
• a secured sub-system llu comprising the quantum receiver (qRx) lip and router and cryptography key generation unit lly
• an insecure sub-system comprising the a hybrid time-bin and polarization states to time-bin and phase photon encoding (H2T) converter lit, and the FSO system lie.
• the quantum receiver lip can be configured to securely communicate with the H2T converter lit over optical fiber(s) llo, and the router and cryptography key components lly can be configured to communicate with the quantum communication transmitter lip over (e.g., key channel and/or service channel) a standard parallel or serial data communication bus (e.g., USB, Ethernet, SCSI, or suchlike), or wirelessly (e.g., WiFi, Bluetooth, Zigbee, or suchlike - assuming all sensitive components are located in a well secures facility).
• a standard parallel or serial data communication bus e.g., USB, Ethernet, SCSI, or suchlike
• wirelessly e.g., WiFi, Bluetooth, Zigbee, or suchlike - assuming all sensitive components are located in a well secures facility.
• FIG. 1C schematically illustrates a communication system 10" similar to the communication systems 10 and 10' of Figs. 1A and IB respectively, but in which the secure and insecure components of the transmitter system 12 and of the receiver system 11' are separated, as explained hereinabove in details. Accordingly, in the communication system 10" of Fig.
• T-P states encoded photons are generated in the secured location in which the quantum transmitter 12p is located, and securely transmitted to the T2H converter 12t over optical fiber(s) 12o for converting them into the hybrid time -bin and polarization states photon encoding and transmission over the free space medium lOq to the receiver system 11 by the FSO system 12e.
• the H2T converter lit converts the hybrid time-bin and polarization states photon encoding received over the free space medium lOq by the FSO system lie into T-P states encoded photons, and securely transmits the T-P states encoded photons over the optical fiber(s) llo to the secured location in which the quantum receiver lip is located.
• the quantum transmitter (qTx) 12p and/or receiver (qRx) lip of embodiments hereof can be implemented by any of the embodiments disclosed in International Patent Application Nos. PCT/IL2021/050322, PCT/IL2021/050822, PCT/IL2022/051129, PCT/IL2023/050018, of the same Applicant hereof, the disclosures of which are incorporated herein by reference.
• T2H converters of embodiments hereof rely on basic properties of T-P state photon encoding schemes, as exemplified hereinbelow.
• T-P state photon encoding is typically based on transmission of 4 (four) orthogonal phase and/or amplitude/intensity states, each composed of two consecutive phase/intensity encoded optical signal pulses, designated as:
• Table 1 summarizes the intensity (normalized to 1) and phase on each time- bin/pulse of a T-P state encoded photon according to possible embodiments.
• Table 1 transmitted states with time-bin and phase (T-P) photon coding
• T is the time difference between the Po and P T time-bins of the T-P photon encoding.
• the measurement apparatus receiving these T-P state encoded photon typically utilizes an un-balanced interferometer to differentiate between the ⁇ p),
• the optical signal outputted from the un-balanced interferometer for each T-P state encoded photon is characterized by at least the following 3 (three) distinguishable optical pulse signals:
• St 0 responsive to the first optical time-bin pulse signal P o of the T-P state encoded photon emerging through the short arm of the imbalanced interferometer.
• optical pulse signals is thus also T.
• the optical signals outputted by the imbalanced interferometer are simply the result of splitting and combining the optical signals (or probabilities) from both the short and long arms of the imbalanced interferometer.
• the optical signals St 1 outputted by the imbalanced interferometer is responsive to interference patterns occurring due to the interference between the optical signals P 0 ,P T emerging through the long and short arms of the imbalanced interferometer.
• one output arm split from the receiver's imbalanced interferometer (e.g., interferometer 62 in Fig. 6 having the output arms 63) will receive optical signals which are responsive to constructive interference of the
• Table 2 summarizes the normalized probability of a photon to exit the unbalanced interferometer from each arm at a certain time-bin t 0 , t and t 2 , wherein Anno is the short arm and Armi is the long arm of the imbalanced interferometer. Table 2 - photon probability for each interferometer output arm (63) per each input state
• these photon detection probabilities can be verified if each arm of the imbalanced interferometer (62) is terminated with a single photon (e.g., Geiger-mode avalanche photodiode or a superconducting nanowire detector) detector configured to measure the optical signals emerging therethrough.
• a single photon e.g., Geiger-mode avalanche photodiode or a superconducting nanowire detector
• the measuring single photon detector e.g., 21a and 21b in Fig. 2A
• the measuring single photon detector e.g., 21a and 21b in Fig. 2A
• the measurement time-bin t 0 , tq or t 2
• the T2H converter 12t is configured to produce the four encoded photonic states presented in Table 3, which are derived from three distinguishable time -bin and polarization encoded optical signal pulses (also referred to herein as hybrid photon encoding states) St 0 , St ⁇ St 2 , for each T-P encoded photon ⁇ PQ,P T > thereby received.
• the P o is the first transmitted optical time-bin pulse signal of the T-P encoded photon
• P T is the second transmitted optical time-bin pulse signal of the T-P encoded photon.
• a first possible hybrid T-P and polarization photon state encoding scheme/basis is described hereinbelow.
• This hybrid encoding scheme/basis can be easily generated by polarization of the optical signals emerging from the imbalanced interferometer, it is simple to measure, and robust to atmospheric effects.
• Table 4 presents the transmitted photon encoding states/basis of such possible hybrid time-bin and polarization photon state encoding in possible embodiments.
• H horizontal polarization ⁇ H
• V vertical polarization
• K) superposition states can differ in different embodiments, and can also randomly change over time between the different linear, circular and elliptical polarizations.
• the measurement is performed in some embodiments at the QKD receiver after passing the transmitted signal through a polarizing beam splitter, PBS 21 in Fig. 2A, configured to split the optical signals thereby received into two or more light component arms.
• One light component arm from the PBS 21 can be configured for passage of only the horizontally ( ⁇ H )) polarized light components received in the PBS 21 (e.g., to detector 21a), and another light component arm thereof can be configured for passage of only the vertically (
• Table 5 summarizes the probabilities for photons to pass through each of the light component arms of the PBS (Arm H and Arm v ) at the receiver.
• each of the PBS's light component arms at the receiver is terminated with a single photon detector - DetO for Arm H and Detl for Arm v .
• the interpretation of the states is the same as shown in Table 3.
• a second hybrid time-bin and polarization photon encoding scheme/basis according to other possible embodiments hereof is disclosed hereinbelow.
• This hybrid photon state encoding scheme utilizes superpositions of the ⁇ H ) and
• the measurements are performed at the receiver 11/11' in some embodiments after passing the optical signal through a PBS 21 having at least two light component arms (of detectors 21a, 21b) into which the split optical signals from the PBS 21 are directed.
• one light component arm of the receiver's PBS passes only the transmitted ⁇ H ) +
• half wavelength phase (X 2) retarder lOr may be used either in the receiver system 11/11', anterior or posterior to its FSO system lie, in the transmitter system 12/12', or anywhere in between them, to align the PBS 21 and the transmission setup one with respect to the other.
• Table 7 summarizes the probabilities for photons to pass through the receiver's PBS 21 to each of its light component arm (Arm 0+ and Arm 1 _) i.e., when each of the light component arms from the receiver's PBS 21 is terminated with a single photon detector, Detector-a 21a for Arm 0+ and Detector-b 21b for Arm l _.
• Detector-a 21a for Arm 0+
• Detector-b 21b for Arm l _.
• Table 7 the optical signal probability /power at each output light component arm from the receiver's PBS
• the measurement basis used at the receiver (e.g., randomly) changes for every time -bin, (e.g., by measuring Sto in the H/V basis, .svi in the diagonal p/m basis, and St2 in the H/V basis) better SNR can be achieved.
• P) polarization; and t 2T are always in the
• Table 8 the optical signal probability /power for each light component arm detector of the receiver's PBS
• Yet another hybrid time-bin and polarization scheme/basis (4 th option) of embodiments hereof utilizes the
• This configuration increases the level of security compared to embodiments that include the
• a T-P encoder 12c of the quantum transmitter 12p generates the PO.PT time-bin and phase encoded quantum states e.g., according to Table 1 hereinabove.
• the generated PO.PT time-bin and phase quantum state encoded photons are then converted by the T2H unit 12d (e.g., using any of the T2H converters disclosed herein) into hybrid time-bin and polarization state encoded photons Sto,Sti,St2.
• the output of the T2H converter 12d can be then passed through a polarization plane aligner 20, and thereafter transmitted over the free space medium lOq by the FSO system 12e.
• the polarization plane aligner 20 can be located either at the transmitter system 12/12', at the receiver system 11/11', or anywhere between the transmitter and receiver systems.
• the polarization plane aligner 20 is configured to compensate for rotations along the Bloch-sphere in the polarization plane between the output of the transmitter 12/12' system and the PBS 21 of the receiver system 11/11'. Such rotations can be the result of the relative rotation between the output optical fiber of the transmitter system 12/12' and the polarizer e.g., PBS 21 of the receiver system 11/11', for example.
• the polarization plane aligner 20 is configured in some embodiments to align the hybrid time-bin and polarization states encoded photons generated by the T2H unit 12d, such that at the optical signals from the transmitter system 12/12' e.g., the transmitted H (V) polarization, is aligned with the polarization of the receiver system 11/11' e.g., the H (V) polarization of the receiver's PBS 21.
• the polarization plane aligner 20 is exemplified in Fig. 2B in the transmitter system 12', but it can be similarly installed in the receiver system 11/11' e.g., after the FSO system lie, or anywhere between the transmitter 12/12' and receiver 11/11' systems.
• the polarization alignment is performed e.g., at the receiver system 11/11', the states of the hybrid tine-bin and polarization state encoded photons are analyzed and measured by the receiver's PBS 21 and the single -photon detectors 21a, 21b coupled to its light component arms.
• Fig. 2C demonstrates a polarization plane aligner 20 according to possible embodiments.
• the polarization plane aligner 20 is generally configured to convert between linear, circular, and elliptical polarization states. It is used in possible embodiments to compensate for polarization scrambling that occurs in optical fibers, or while passing through other physical mediums, and to align the plane of reference in case of free space optical links, such as a satellite rotating in space relative to a ground station.
• a free space implementation of the polarization plane aligner 20 can utilize a cascade combination of half (X 2) and quarter (X/4) phase retarder wave-plates rl, r2, r3,....
• a fiberbased implementation can use stress-induced birefringence Polarization Controllers, such as the motorized fiber polarization controller MPC320 manufactured by Thorlabs.
• Such polarization aligning instruments are usually opto-mechanical devices that are tuneable by electrical control signals.
• such a polarization plane alignment instrument 20 generally requires a simultaneous closed-loop control of 3 DOF, as demonstrated in Fig. 2C.
• the polarization plane alignment 20 comprises a sequence of a X/4 phase retarder (rl), a X/2 phase retarder (r2), and X/4 phase retarder (r3), each coupled to a respective rotation angle control actuator Actl, Act2 and Ac3, and optional gear system gl, g2 and g3.
• optical coupling 22 is used for monitoring the polarization of the optical signals inputting and/or outputting the polarization plane alignment 20 by a detector 27.
• a control unit 28 can be used to process the measurement signals from the detector 27 and generate based thereon control signals for activating at least one of the actuators Actl, Act2 and/or Ac3, in order to convert the polarization orientation of the optical signal between the linear, circular, and elliptical polarization states.
• the purpose of generating polarization encoded photons in some embodiments is to perform QKD over a free-space medium (not over optical fiber). It is thus assumed that the state of polarization is maintained from the output of the transmitter (12/12') to the input of the receiver (11/11').
• the optical coupling 22 of the photons in and out of the optical fiber 20f can be carried out by collimators, lenses, telescopes etc. While progressing through a single-mode optical fiber, the level of polarization slowly degrades. While progressing through a polarization maintaining fiber (PM) the
• PM polarization maintaining fiber
• the optics of the transmitter and receiver can be designed to have a well defined plane of reference with a well-defined
• the alignment can be done independently of the correction of the phase between the
• V) polarization orientations is accumulated in the optical fibers and interferometers used in the system, and eventually sums to a total phase (/) to tai that can be generally expressed as follows: pTxConverter T (pTxFiber T (pmedium T (pRxInterferometer T P RxConverter T (pRxFiber ptotal
• )totai 0.
• the total phase ⁇ p total is se t to 0 (zero) by changing the transmission wavelength. Changing the wavelength results in changing the phase accumulated in each of the system segments, and especially in the interferometers where each polarization accumulates the phase differently.
• closed control-loop based on the measured values at the end of the fiber is used in some embodiments to change/adjust the wavelength (with a dithering algorithm for example) to compensate in real time for all of the phases accumulated in the system and distinguish between the different superposition states with a high resolution (visibility > 99% for example).
• Fig. 3A shows a T2H converter 30 according to possible embodiments.
• the T2H converter 30 comprises an imbalanced interferometer 23r having a long arm Armi and a short arm Arms, a polarization stage 24p having two polarization arms Armh,Armv optically coupled to the imbalanced interferometer 23r, a signal output arm 25t coupled to the polarization stage 24p, and an optional signal monitoring arm 25m coupled to the polarization stage 24p.
• the imbalanced interferometer 23r comprises a beam splitter (BS e.g., having a 50:50 splitting ratio) 23 configured to receive the T-P state encoded photons from the T-P unit 12c over the optical fiber 12o and spilt the same into the optical fibers of its long and short arms Armi, Arms.
• BS beam splitter
• a non-polarizing beam combiner 24 can be used to combine the optical signals emerging from the long and short arms Armi, Arms of the imbalanced interferometer 23r.
• the non-polarizing beam combiner 24 can be further configured to split (e.g., 50:50 splitting ratio) the combined optical signals of the long and short arms Armi, Arms into optical fibers of the polarization arms Armh,Arm v of the polarization stage 24p.
• a non-polarizing beam splitter 25 can be used to combine the optical signals emerging from the polarization arms Armh,Arm v of the polarization stage 24p, and to split (e.g., 50:50 splitting ratio) the combined optical signals of the polarization arms Armh,Arm v into optical fibers of the output arm 25t and of the optional signal monitoring arm 25m. It is noted that in embodiments hereof the nonpolarizing beam splitter 25 requires PM connectivity.
• the T-P input 12c of the T2H converter 30 is a non-polarized time -bin and phase encoded states photons. These T-P photons pass through an unbalanced interferometer 23r over the non-PM optical fibers.
• the imbalanced interferometer 23r can be similarly implemented as a Michelson interferometer with Faraday mirrors, for example.
• the imbalanced interferometer 23r is configured such that the lag/time- difference between its long and short arms Armi, Arms is T, such that the two optical time-bin signal pulses Po,Pt of the T-P photons can timely interfere with each other at the non-polarizing combiner 24 after passage through the arms Armi, Arms of the unbalanced interferometer 23r.
• the phase shift of the imbalanced interferometer 23r and the (e.g., laser) wavelength of the optical signal time -bin pulses Po,Pt thereby received, can be set such that the output optical signals of the imbalanced interferometer 23r emerging from the beam combiner 24 is substantially as presented hereinabove in Table 2.
• the output optical signals on each light component polarization arm Armh,Arm v coupled to the non-polarizing beam combiner 24 then passes through the respective H-polarizer 24h and V-polarizer 24v, and therefrom into a PM optical fiber(s). After passing through the polarizers 24h,24v the states of the optical signals in the polarization arms Armh and Arm v are substantially as presented in Table 10:
• Table 10 the optical signal states in the polarization arms of the polarization stage 24p after passage through the H and V polarizers 24h,24v
• the polarization arms Armh,Arm v are configured to exhibit the same time delay (e.g., with lengths difference configured in accordance with the different propagation velocity of each PM fiber axis).
• the polarization arms Armh, Armv are configured to combine the optical signals propagating therethrough in the beam combiner 25 with no interference, since the polarizations of the hybrid states encoded photons of the polarizing arms Armh,Arm v are orthogonal.
• the polarization of the output optical signals 25t in some of the times and/or states is a superposition of both H and V polarized optical signals emerging through the polarizing arms Armh,Arm v .
• the phase shift of the combiner 25 is not necessarily stabilized, the state of the combined optical signals from the combiner 25 may change over time between the different linear, circular and elliptical polarization orientations.
• the output optical signals from the arms of the combiner 25 can be used for monitoring the hybrid states encoded photons produced by the T2H converter 30 (or dumped), and for outputting the transmitted optical signal states, which can be transmitted to the receiver (e.g., over FSO) e.g., after passing through the polarization plane aligner 20.
• the timing and polarization of the different transmitted optical signal states are substantially as presented in Table 4 hereinabove.
• Fig. 3B illustrates a possible T2H converter 31 implementation structured similar to the T2H converter 30 of Fig. 3A, but utilizing one or more phase retarders 24w in the PM arms Armh', Arm v ' of the polarization stage 24p', instead of the H and/or V polarizers (24h,24v).
• the T-P input optical signals 12c' are polarized time-bin and phase encoded photons. Accordingly, the optical fibers and combiner/splitter (23', 24', 25') of the T2H converter 31 employ PM optical elements.
• the polarization orientation of the input signal 12c' is linear and aligned with either the slow or fast axis of the PM fiber.
• the linearly polarized photons 12c' are passed through the PM imbalanced interferometer (e.g., Mach Zehnder or a Michelson interferometer with Faraday mirrors for example) 23r'.
• the PM imbalanced interferometer e.g., Mach Zehnder or a Michelson interferometer with Faraday mirrors for example
• the lag/time-difference between the long and short arms Arm , Arms' of the imbalanced PM interferometer 23r' is similarly set to T seconds, such that the two optical signal time-bin pulses Po',Pt' of the T-P and polarized states encoded photons can overlap after passage through the arms Armi', Arms' and interfere with each other at the PM beam combiner 24'.
• the phase shift of the imbalanced PM interferometer 23r' and the (e.g., laser) wavelength of the optical signals time -bin pulses Po',Pt' thereby received, can be set such that the output optical signals of the imbalanced PM interferometer 23r' emerging from the PM beam combiner 24' is substantially as presented hereinabove in Table 2.
• the optical signals emerging through the arms Armi', Arms' from the PM beam combiner 24' are polarized in the same orientation e.g., along the fast axis (denoted as H hereinabove).
• the optical signals passing through at least one of the arms (e.g., Armh') of the polarization stage 24p' are passed through a half-wavelength waveplate phase retarder (X/2) 24w.
• X/2 half-wavelength waveplate phase retarder
• the polarization of the optical signal outputted from the PM beam combiner 25' in some of the times and/or states is a superposition of the H and V polarizations in these arms.
• the state of the optical signal outputted from the PM beam combiner 25' can change over time between linear, circular and elliptical polarization orientations.
• one output of the PM beam combiner 25' is used for monitoring the hybrid time-bin and polarization encoded photon states produced by the T2H converter 30' (or dumped).
• the other output of the PM beam combiner 25' can be used (e.g., attenuated if required to an intensity level required by the QKD protocol) to transmit the hybrid time-bin and polarization states encoded photons (e.g., over free space optics) to the QKD receiver e.g., after passing through the polarization plane aligner (20).
• the timing and polarization of the different transmitted states of the hybrid states encoded photons are presented in Table 4 hereinabove.
• the polarization encoding is carried out after the passing the optical signals through the imbalanced interferometer 23r.
• the beam splitters/combiners (23/23', 24/24', 25/25') of the T2H converters 30, 31 and 32 preferably non-polarizing beam splitters/combiners.
• the polarization encoding is performed within the imbalanced interferometer 23r/23r'.
• Fig. 3C illustrates a possible T2H converter 32 implementation structured similar to the T2H converters 30,31 of Figs. 3A and 3B, but utilizing instead of the polarization stage 24p/24p' one or more phase retarders 24w in the arms Armi,Arm s of the imbalanced interferometer 23r.
• Optical components e.g., fibers, combiners, splitters,...
• the T2H converter 32 of Fig. 3C are based on non-PM optical elements.
• the input optical signals from the T-P unit 12c' are polarized time -bin and phase encoded photons PO',PT'.
• the polarization orientation of the input optical signals 12c' is linear e.g., H-polarized.
• the polarized photons Po',Pt' from the T-P unit 12c' pass through the unbalanced (e.g., fiber-based) interferometer (e.g., Mach Zehnder or Michelson interferometer) 23r.
• the time- difference/lag between the short and long arms Arm s ,Armi of the imbalanced interferometer 23r is set to T seconds, such that the two transmitted time -bins Po',Pt' of each transmitted photon can overlap.
• a half waveplate phase retarder (X/2) 23w is incorporated in one of the arms (e.g., the long arm Armi) of the imbalanced interferometer 23r for converting the timebins Po ,PT of the transmitted photons from one polarization orientation to another/orthogonal polarization orientation e.g., from H-polarization into V-polarization.
• polarization controller e.g. , stress-induced birefringence
• the phase shift of the imbalanced interferometer 23r and/or the (e.g., laser) wavelength of the input optical signal 12c' is set such that the output optical signals of the imbalanced interferometer 23r (emerging from the combiner 24) has stable polarization states, substantially as presented in Table 6.
• the combiner 24 can be configured to split (e.g., with a 50:50 split ratio) the combined optical signals from the imbalanced interferometer 23r into two of more optical output arms.
• one optical output arm of the imbalanced interferometer 23r is used for monitoring 25m the hybrid states encoded photons generated by the T2H converter 32 e.g., to optimize the polarization control and wavelength control (or dumped).
• Another optical output arm 25t of the imbalanced interferometer 23r can be used to transmit hybrid states encoded photons (e.g., over FSO), to the QKD receiver e.g., after passing through the polarization plane aligner (20).
• T2H converters 30, 31 and 32, of Figs. 3A, 3B and 3C there may be a need to maintain a high visibility interference, even when the ambient temperature is undergoing changes.
• FIG. 3D Another possible T2H converter 33 embodiment is shown in Fig. 3D, which is similar in many aspects to the configuration of the T2H converter 32 shown in Fig. 3C.
• a main difference between these T2H converter embodiments is that the input and output stages of the T2H converter 33 of Fig. 3D are constructed from non-PM optical (e.g., fibers, combiners, splitters, etc.) elements, and a imbalanced PM-based interferometer stage 23r'.
• non-PM optical e.g., fibers, combiners, splitters, etc.
• the input optical signals 12c' are polarized time-bin and phase encoded photons Po',Pt'.
• the input of the imbalanced PM-based interferometer 23r' is a polarizing beam splitter (PBS) 23", such that the one polarization (e.g., H- polarization) component of the photons Po',Pt' propagates into one optical arm (e.g., the long arm Armi), and the orthogonal polarization (e.g., V-polarization) component thereof propagates into the other arm (e.g., the short arm Arms), of the imbalanced PB-based interferometer 23r'.
• PBS polarizing beam splitter
• the input optical signals 12c' are diagonally (+/-) polarized e.g., H+V polarized. Accordingly, the polarized time-bin and phase encoded photons Po',Pt' from the T-P device 12c' are split into two optical signal components having perpendicular polarization orientations, which pass through the respective arms Arms.Armi of the unbalanced (e.g., fiber-based) PM-based interferometer (e.g., Mach Zehnder or Michelson interferometer) 23r'. The time-difference/lag between the interferometer arms is T, such that the two transmitted time -bins Po',Pt' can overlap at the combiner 24.
• the unbalanced e.g., fiber-based
• PM-based interferometer e.g., Mach Zehnder or Michelson interferometer
• the phase-difference of the interferometer and/or the (e.g., laser) wavelength of the input optical signal 12c' can be set such that the output optical signals emerging from the combiner 24 of the imbalanced PM-based interferometer 23r' has stable polarization states, substantially as presented in Table 6, that propagates into the output stage (e.g., made of non- PM single mode fibers).
• the hybrid state encoded output photons 25t emerging from the combiner 24 of the imbalanced PM-based interferometer 23r' can be transmitted (e.g., over FSO) to the QKD receiver e.g., after passing through the polarization plane aligner (20).
• the output photons emerging from the combiner 24 are split for monitoring 25m e.g., in order to optimize the polarization control and wavelength control (or dumped).
• Fig. 3E Another possible embodiment is shown in Fig. 3E, which is similar in many aspects to the configuration of the T2H converter 32 shown in Fig. 3C.
• the output stages of the T2H converter 34 of Fig. 3E is constructed from non-PM optical (e.g., fibers, combiners, splitters, etc.) elements, while its input (12c', 12o) and interferometric stages (23r) are PM-based (i.e., utilizing PM-based optical elements) imbalanced PM-based interferometer stages 23r'.
• non-PM optical e.g., fibers, combiners, splitters, etc.
• PM-based i.e., utilizing PM-based optical elements
• the beam splitter at the input of the imbalanced PM-based interferometer 23r' is a non-polarizing (e.g., 50:50 ratio) splitter, configured such that the input optical signals 12c' of the T2H converter 34 propagates into both the short and the arms of the imbalanced PM-based interferometer 23r' regardless of its polarization (e.g., H-polarization).
• a least one the arms Armi,Arm s of the imbalanced PM-based interferometer 23r' (e.g., the long arm Arun) comprises a phase retarder (e.g., half waveplate) element 23w inline, to convert the polarization (e.g., H polarization) of the optical signals propagating therethrough into the orthogonal polarization direction (e.g., V-polarization).
• a phase retarder e.g., half waveplate
• the input optical signals 12c' of the T2H converter 34 is polarized time -bin and phase encoded photons Po',Pt'.
• the input optical signals 12c' are linearly polarized (e.g., H-polarization).
• These polarized photons Po',Pt' pass through an the unbalanced (e.g., fiber-based) PM-based interferometer (e.g., Mach Zehnder or Michelson interferometer) 23r'.
• the time-difference/lag between the short and long arms Armi,Arm s of the imbalanced PM-based interferometer 23r' is set to T seconds, such that the two transmitted time -bins Po',Pt' can overlap at the polarization beam combiner 24" at the output of the imbalanced PM-based interferometer 23r'.
• the phase-difference of the imbalanced PM-based interferometer 23r' and/or the (e.g., laser) wavelength of the input optical signalsl2c' are configured such that the output optical signals of the imbalanced PM-based interferometer 23r' has a stable polarization state, substantially as presented in Table 6, that propagates into the output stage (e.g., made of non- PM single mode fibers).
• the hybrid state encoded output photons 25t emerging from the polarization beam combiner 24" of the imbalanced PM-based interferometer 23r' can be transmitted (e.g., over FSO) to the QKD receiver.
• the output optical signals emerging from the polarization beam combiner 24" can be split for monitoring e.g., to optimize the polarization control and wavelength control (or dumped).
• Figs. 4A and 4B schematically demonstrate T2H converter embodiments 35,36 configured to generate pure polarization encoding states from the time-bin and phase encoded T-P photon encoding states.
• active optical components are used in the T2H converters 35,36 to generate the pure polarization encoding states.
• the advantage of these embodiments is that the encoded optical signals (e.g., in the QKD transmitter/ Alice system) can be transmitted over long distances (e.g., tens of kilometers) of optical fiber and only then converted to the pure polarization coding.
• the T2H converter embodiments 35,36 are similar in many aspects to the configuration of the T2H converter 34 shown in Fig. 3E, but utilizes in possible embodiments an intensity modulator (IM) having one input and at least two outputs, or any other fast 1:2 optical switch 23s, to selectively direct the input T-P encoded input photons 12c' into one of the arms Armi,Arm s of the imbalanced PM-based interferometer 23r*.
• the optical switch (or intensity modulator) 23s replaces the passive splitter (23') at the input of the unbalanced PM-based interferometer 23r' of the T2H converter 34 shown in Fig. 3E.
• pure polarization states can be generated by switching the Po' pulse of the T-P encoded photons 12c' only to the long arm Armi of the unbalanced PM-based interferometer 23r*, and directing the P T ' pulse of the T-P encoded photons 12c' only to the short arm Arms of the imbalanced PM-based interferometer 23r*.
• the first and third optical pulse signals (Sto and St2) of the hybrid encoded photons are eliminated, and only the middle optical pulse signal (Sti) of the hybrid encoded photons are transmitted from the imbalanced PM-based interferometer 23r*.
• At least one intensity modulator IM1,IM2 is used in each one of the arms Arm s ,Armi of the unbalanced PM-based interferometer 23r'.
• at least one intensity modulator IM1 can be used in the long arm Armi to block the PT pulse of the T-P encoded photons 12c'
• at least another one intensity modulator IM1 can be used in the short arm Arms the unbalanced PM-based interferometer 23r' to block the Po' pulse of the T-P encoded photons 12c'.
• a modulator which is not polarization sensitive can be used at the output of the T2H photon states converters disclosed herein, for blocking the photons at tO and t2, and transmitting only the photons at tl, thereby obtaining pure polarization encoded data.
• These configurations have the advantage over standard pure -polarization state photon encoders that the time -bin and phase encoded data can be transmitted over substantially long distances of the optical fibers 12o', until it is required to convert them into the pure or hybrid polarization state encoded photons e.g., for free space transmission.
• These configurations utilizing active components also eliminate the need to secure the location in which the T2H converter is mounted, since information is not encoded by the active components.
• the T2H converter embodiments hereof have active elements, the information regarding the encoded states that are transmitted by the system is not required for their operation and thus remain concealed . Therefore, no information is revealed to an adversary during the T2H photon states conversion, and the T2H conversion unit doesn’t need to be secured like the quantum encoding components (12u).
• the modulation applied in Fig. 4A by the 1:2 optical switch or intensity modulator 23s, and in Fig. 4B by intensity modulators IM1,IM2 can be carried out periodically in accordance with the photon transmission frame-rate, and regardless of the actual data being transmitted. Different synchronization techniques can be applied to get the transmitter frequency, and some timing calibration may be required to precisely set the switching times.
• Fig. 4C shows a T2H converter 37 utilizing an intensity modulator IM to selectively block the Sto and St2 optical signals at the output of the imbalanced interferometer 23r', and permit passage of the Sti optical signal through the output stage 25t.
• the driver unit D5 can be accordingly configured to periodically block the Sto,St2 optical signals, such that only the pure polarization encoded data Sti emerges out of the T2H converter 37.
• Fig. 5 illustrates a pure-polarization fiber-based QKD encoder 50 according to possible embodiments.
• the following polarization states generation technique requires only to rotate the frame of reference (the plane formed in 3D space by the H and V polarization vectors) for aligning the QKD transmitter and receiver systems to the H and V basis.
• the phase difference between the optical signals in both of the arms Armi,Arm2 that forms the superposition states can be compensated between the H and V axes without moving parts, as illustrated in Fig. 7.
• the frame of reference can be aligned either by physically rotating the output PM fiber, or by rotating the receiver's aperture, or by using a half wave-plate phase retarder. It is noted that in this embodiment the optical signals propagating through the arms Armi,Arm2 of the balanced interferometer 58 are combined without interference due to their orthogonal polarization orientation.
• the QKD encoder 50 shown in Fig. 5 comprises PM input (12o') and intermediate interferometric (23r') stages, and an output stage that can be configured as either PM or non- PM.
• the QKD encoder 50 comprises a balanced interferometer 58 coupled to a light (e.g., continuous wave/CW or pulsed laser) source 12s configured to generate coherent linearly polarized optical signals.
• the electrical delays DL1,D12 of the respective driver units D1,D2 are configured to optimize the overlap (in time) of the optical signals propagating in the arms Armi,Arm2 and combined at the output of the balanced interferometer 58.
• the linearly (e.g., horizontally) polarized optical signals received from the light source 12s via the PM -based input stage is split (e.g., 50:50 ratio) by the non-polarizing PM beam splitter into (e.g., 50:50 ratio) the arms Armi,Arm2 of the PM PM interferometer 23r'.
• the intensities of the components of the polarized optical signals propagating along the arms Armi,Arm2 are modulated by their respective intensity modulators IM1,IM2 according to the driving signals generated by their respective driver units D1,D2.
• the phase of the intensity modulated polarized optical signal component propagating along at least one of the arms is further shifted by the shift (e.g., /2 waveplate) retarder 23w, such that the polarity orientations of the optical signal components in the arms Armi,Arm2 are perpendicular one with respect to the other.
• the intensity modulated and orthogonally polarized optical signal components in the arms Armi,Arm2 are combined at the polarization PM combiner 24" into the output stage for transmission of polarized states encoded photons 25t e.g., over FSO. Accordingly, in this configuration there is only one time bin, which is the pure polarization state that emerges from the combiner 24" at the output of the interferometer 58.
• the DL1,DL2 functionality of the driver unit D1,D2 are configured to accurately overlap the H and V polarized optical signals from the arms of the balanced interferometer 58.
• the QKD encoder 50 can be configured to generate all quantum states required for decoy-state BB84 implementations, which typically include different amplitudes of the ⁇ H),
• one or both of the intensity modulators IM1,IM2 is a Mach-Zehnder intensity modulator.
• Table 11 presents voltage levels that can be generated by the D1,D2 driver units of the intensity modulators IM1,IM2 for generating polarized encoded photons according to possible embodiments. Other embodiments can include both amplitude and phase modulators in each arm.
• Table 11 voltage levels generated by the drivers of the intensity modulators for generating polarized encoded photons
• a (e.g., tuneable) delay line mechanism can be applied to one (or both) of the driver units D1,D2.
• Fig. 6 illustrates a photon processing setup 60 configured according to possible embodiments to convert pure-polarization state encoded photons into time-bin state encoded photons.
• the photon processing setup 60 generally comprises an input stage 12i for receiving polarization state encoded photons, and a (e.g., passive) converter stage 61 optically coupled to the input stage 12i and configured to convert polarization state encoded photons into T-P state encoded photons.
• a (e.g., passive) converter stage 61 optically coupled to the input stage 12i and configured to convert polarization state encoded photons into T-P state encoded photons.
• the polarization state encoded photons 12i received via the input stage are split by the polarization PM beam splitter 61p of the converter stage 61 into two orthogonally (e.g., vertically and horizontally) polarized components, directed into respective long and short arms 611,61s of the imbalanced interferometer.
• the input of the PBS 61p is thus aligned with the H/V plane of the polarization state encoded photons 12i.
• the (e.g., vertically) polarized component in the long arm 611 undergoes a 180° phase shift as it passes through the phase retarder 61w, thereby converting its polarization into the orthogonal (e.g., horizontal) polarization orientation of the polarized component in the short arm 61s.
• the optical signals from the long and short arms 611,61s are thus received at the beam combiner 61c in the same polarization orientation, but in a time-difference/lag of r seconds, wherein r is the time difference between time-bins of the optical signals emerging from the long and short arms 611,61s of the imbalanced interferometer 61.
• the ⁇ H) polarized state photons are passed through the short arm 61s and thereby converted into
• 7) polarized state photons are passed through the long arm 611 and converted into
• H + V) polarized state photons are converted into ⁇ p) T-P encoded state photons, and the diagonal
• the photon processing setup 60 is further configured to detect the encoded states of the photons thereby processed.
• the photon processing setup 60 further comprises a photon encoding state analyzer stage 62 optically coupled to the converter stage 61 and configured generate optical signals indicative of the photon state encoding, and an output stage 63 optically couped the photon encoding state analyzer stage 62 and configured to detect the encoding state of the processed photons.
• the coupling between the converter stage 61 and the analyzer stage 62 is obtained using a significantly long optical fiber 61e (e.g., from few meters and up to few tens of kilometers, optionally hundred of meters, depending on the total attenuation the system can tolerate). This achieved in some embodiment using a single-mode optical fiber (SMF) for the coupling 61e.
• SMF single-mode optical fiber
• the H/V plane of the incoming polarization state encoded photons 12i can be (e.g., controllably or mechanically) aligned with the H/V plane of the polarization splitter 61p at the input of the imbalanced (e.g., Mach Zehnder or Michelson) interferometer 61.
• the imbalanced e.g., Mach Zehnder or Michelson
• the ⁇ H + 7) and the ⁇ H — V) polarization state photons obtained at the beam combiner 61c are converted to the different phase superposition states, ⁇ p) and
• the optical signals received at the combiner 61c from the long and short arms of the imbalanced interferometer 61 are in the same polarization orientation, for generating the needed time-bin and phase photon encoding photon states transmitted for the analysis at the analyzer stage 62.
• the non-polarizing (e.g., 50:50 ratio) beam splitter 62p splits the T-P states encoded photons from the converter stage 61 into the long and short arms 621,62s of the of the imbalanced (e.g., Mach Zehnder or Michelson) interferometer 62.
• the time- difference/lag between the long and short arms 621,62s is configured for r seconds for properly combining the time -bins at the optical signals pulses of the T-P states encoded photons at the beam combiner (also non-polarizing splitter) 62c.
• the non-polarizing beam splitter 62c is configured to (e.g., 50:50 ratio) split the output optical signals from the converter stage 61 into two detection arms, each optically coupled to a respective (e.g. , single-photon avalanche diode) detector SPAD1,SPAD2.
• a respective detector SPAD1,SPAD2 At the output stage 63 of the photon processing setup 60, the accumulated phase of the ⁇ p) and
• the converter stage 61 generating the time-bin and phase encoded photons may be located substantially remote from the analyser and output stages 62,63.
• This property of the photon processing setup 60 is advantageous as it permits locating the sensitive QKD system far away from the passive optics (e.g., FSO) and T2H converter.
• the phase at the analyser interferometer ( 3 ) can be set to compensate for all of the accumulated phases e.g., of the interferometers in the QKD transmitter ( -Q and in the Converter ( 2 )- This can be done by laser wavelength fine-tune or by interferometer thermal control, in a closed loop with the SPAD measured values (by QBER minimization for example).
• Embodiments hereof may utilize different half-wavelength waveplate implementations.
• fiber-based half wavelength waveplate there are a few ways to implement fiber-based half wavelength waveplate.
• there are devices that apply the desired birefringence property of waveplates by stress and/or twisting of the optical fiber there are free-space optical elements (e.g., quartz or mica optical windows) with intrinsic birefringence, packed inline with the optical fiber.
• free-space optical elements e.g., quartz or mica optical windows
• a half wavelength waveplate may be required to rotate the optical signal to the slow axis of the PM fiber.
• this is implemented by fusing two PM-based optical fibers with 90° rotation, to directly transfer optical signals aligned with the fast-axis light to the slow-axis of the optical fiber.
• This method achieves the effect of the waveplate without the requirement of any additional optical elements on the optical path.
• Other fusing angles can be used to affect other polarization orientations. By busing fast to slow axis, the different propagation speed can be compensated if required.
• any combination of half and quarter waveplates can be used, which may be configured as either fiber-based or free space optics waveplates.
• the T2H converters 32, 33 and 34, of Figs. 3C, 3D and 3E, may require stabilization of the phase of the optical signals to generate the diagonal state photon polarizations. This is because if the phase is not controlled, other elliptical polarizations are likely to be obtained. However, the orthogonality of the encoded states will still be maintained.
• a polarization plane aligner 20 e.g., located within the QKD transmitter, receiver, or anywhere along the optical path therebetween, can compensate both for the polarization rotations within the transmitter and on the optical path, as long as the rate of polarization changes is slow e.g., smaller than n radians/second, thereby simplifying the transmitter operation.
• Fig. 7 demonstrates a control scheme 77 configured to compensate phase and/or polarization drifts between the transmitter 12 and receiver 11 systems.
• the control unit llu of the receiver system 11 is configured to receive from the detectors (e.g., SPAD1 and SPAD2 in Fig. 6) 63 data/signal of the encoded data of the optical signal Sti, and of the orthogonal (H/V) polarization states of the optical signal Sto and St2.
• a polarization control module can be used in the control unit llu to process and analyze the orthogonal (H/V) polarization states of the optical signal Sto and St2, and determine based thereon polarization orientation corrections, if so need.
• the control unit llu can then generate control signals 20c for either the polarization plane aligner 20, or other angular position rotary adjustment means at the transmitter or receiver, in order to correct any polarization orientation misalignments between the transmitter and receiver systems 12,11.
• the control unit llu can further include a phase control module configured to process the optical signal Sto, Sti, and/or Sts and determine based thereon phase drifts of the received photons.
• the control unit llu can be configured to generate control signals/date 20g to correct phase drifts by adjusting the wavelength of light source 12s at the transmitter system 12.
• the control signals/date 20g can be transmitted to the transmitter system 12 over the standard communication link 10s, that is further adapter in possible embodiment for system control.
• the control unit 12u of the transmitter comprises in some embodiments a wavelength control module configured to receive the control signals/date 20g from the receiver 11 accordingly control a wavelength tuning unit 12w of the light source 12s.
• the polarization plane aligner 20 is configured to receive the information required to align the polarization state between the QKD transmitter and the QKD receiver. For example, as follows:
• the QKD transmitter can be configured to transmit (in addition to the quantum communication beam) an alignment laser beam with high enough power to be measured on the QKD receiver side. If the medium (e.g., the atmosphere) between the QKD transmitter and QKD receiver is non-birefringent, any wavelength supported by the optical components can be used for the alignment laser beam. If the medium is birefringent, as with realistic optical fibers, the wavelength of the alignment beam is similar to that of the data-carrying optical signal/laser.
• the medium e.g., the atmosphere
• the polarization state is mapped, and the feedback for correction of the polarization is transferred to the polarization plane aligner 20.
• the QKD receiver is configured to transmit the alignment laser beam to the QKD transmitter.
• the QKD transmitter can have a polarization state mapping system (e.g., polarimeter), configured to send feedback to the polarization plane aligner 20 to get the required polarization state at the QKD receiver side.
• a polarization state mapping system e.g., polarimeter
• the relative orientation of the transmitter and receiver can be pre-calibrated or measured (by an accelerometer for example).
• the calculated relative polarization shift is then corrected by the polarization plane aligner 20.
• the polarization can be controlled to minimize the bit error rate e.g., using feedback data/signals from the detection system at the receiver during the QKD parameter estimation stage.
• the passive T2H converters disclosed herein are employed for point-to-Multipoint (P2MP) QKD network communication, as explained hereinbelow.
• Figs. 8A and 8B illustrate two P2MP configurations, with a 1:64 passive optical network (PON) network, for example.
• Fig. 8A shows a P2MP PON network 70 having a single QKD receiver 70r and multiple QKD transmitters 70t
• Fig. 8B shows a P2MP PON network 71 having a single QKD transmitter 70t and multiple QKD receivers 70r.
• the unbalanced interferometer e.g., 62 in Fig. 6
• Bob's (/'. ⁇ ?., QKD receiver 70r) side determines the time difference T between the two transmitted time-bins (Po,P T ) of the T-P encoded photons, and therefore the clock frequency of the QKD transmitter 70t.
• TDM time- division-multiplexing
• the stabilization of the single QKD transmitter 70t (laser/light source) to multiple interferometers QKD receivers 70r becomes a challenge.
• the unbalanced interferometer is typically located at the QKD transmitter 70t side. Therefore the single QKD transmitter 70t can determine the clock frequency for the multiple QKD receivers 70r, enabling the operation of such P2MP network. All QKD receivers 70r can measure simultaneously, without the TDM requirement, making the network deployment and management much simpler: the QKD receivers 70r can be connected and disconnected without affecting the rest of the connected QKD receivers 70r.
• the software can be stored in a computer program product and loaded into the computer system using the removable storage drive, the memory chips or the communications interface.
• the control logic when executed by a control processor, causes the control processor to perform certain functions of the invention as described herein.
• features of the invention are implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs) or field -programmable gated arrays (FPGAs).
• ASICs application specific integrated circuits
• FPGAs field -programmable gated arrays
• Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s).
• features of the invention can be implemented using a combination of both hardware and software.
• the present invention provides QKD system configurations useful for FSO communication, and related methods. While particular embodiments of the invention have been described, it will be understood, however, that the invention is not limited thereto, since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the claims.

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