QKD communication systems and methods

Abstract

Quantum key distribution (QKD) communication techniques where time-bin phase (T-P) encoded photons generated by a QKD transmitter system are converted into hybrid time-bin and polarization encoded photons, transmitted over a communication channel and received by a QKD receiver system, are converted back into T-P encoded photons using an interferometer structure.

Description

QKD COMMUNICATION SYSTEMS AND METHODSTECHNOLOGICAL FIELD

[0001] The present disclosure is generally in the field of quantum key distribution (QKD), and more specifically relates to QKD communication receivers.BACKGROUND ART

[0002] References considered to be relevant as background to the presently disclosed subject matter are listed below:

[0003] Davide S. et al "Cross-encoded quantum key distribution exploiting time-bin and polarization states with qubit-based synchronization" , arXiv:2111.13383 (2021).

[0004] Costantino A. et al "All-fiber self-compensating polarization encoder for quantum key distribution" , Optics Letters Vol. 44, No. 10 (2019).

[0005] IS. Hodges et al " Polar ization / Time-bin basis conversion of entangled photons", CLEO Technical Digest OSA (2012).

[0006] Kupchak C. et al "Time-bin-to-polarization conversion of ultrafast photonic qubits" PHYSICAL REVIEW A 96, 053812 (2017).BACKGROUND

[0007] This section intends to provide background information concerning the present application, which is not necessarily prior art.

[0008] Quantum key distribution (QKD) systems can be implemented using various photonic degrees of freedom (DOFs). Two common DOFs are time-bin (and the phase between the timebins) and polarization. Time-bin encoding represents a good candidate for fiber-optic channels, as birefringence does not perturb this kind of states. This encoding exploits the time-of-arrival of photons and the relative phase between time bins. Polarization encoding is usually preferred for free-space QKD implementations, because atmospheric transmission does not change the polarization state of the transmitted qubits, thus allowing parties to share a stable polarization reference frame.

[0009] In contrast, polarization encoding has some drawbacks when propagating through a fiber channel, mainly due to random changes of the fiber birefringence introduced by ambient conditions and mechanical stress. This causes a random rotation of polarization, increasing the quantum bit error rate (QBER) and potentially lowering the secret key rate. Consequently, apolarization compensation system may be required for fiber links using polarization encoding. While time-bin encoding is robust to fiber birefringence, it has the disadvantage of requiring phase stabilization of the interferometers used for encoding and decoding.

[0010] In free space optics (FSO), polarization is generally considered a more robust DOF compared to phase encoding, as phases are less robust to atmospheric effects and wavefront distortions can lead to poor (destructive) interference at the receiver, degraded signal and potential loss of information. However, for mobile platforms like satellites, the movement relative to the transmitter can cause the polarization of photons to change, requiring polarization compensating components. Polarization optics, while suitable for free space, may present challenges in terms of bandwidth for information encoding, larger footprint, and complexity in manufacturing and alignment, compared to other techniques.

[0011] The development of quantum networks that include both fiber-optic and free-space links is envisioned. In such hybrid networks, the choice of the optimal encoding scheme is not trivial and depends on the characteristics of the quantum channel. This has led to interest in techniques that allow conversion between different photonic encoding formats.

[0012] The prior art includes implementations of QKD systems that convert between encoding schemes to better suit the transmission channel or measurement requirements. Some solutions known from patent literature are briefly described hereinbelow.

[0013] JP2008160368 discloses conversion of polarization-coded quantum information to phase-coded quantum information to achieve stable fiber transmission. Two polarization beam splitters are 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.

[0014] CN113810191 discloses a quantum key distribution system, encoder, decoder and method based on circular polarization. The system comprises a single photon source, a PBS1, a first end phase modulator, a PBS2, two 1 / 4 slides, a PBS3, a second end phase modulator, a PBS5 and two single photon detectors. At the Alice end, a single photon is divided into two orthogonal phase equal polarization components through PBS1, phase modulation is carried out on the vertical polarization components, then beam combination is carried out throughPBS2, and then the polarization components are converted into a left-hand circular polarization component and a right-hand circular polarization component through the 1 / 4 slide; 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 PBS4. Polarization control needed by the system after space alignment is completed is reduced, and the working efficiency of the system is improved.

[0015] RU2771775 discloses a method and apparatus for quantum key distribution (QKD). The claimed technical result is achieved by supplementing the QKD apparatus with additional modules ensuring conversion of polarisation encoding into phase-time encoding in the transmitter and reverse conversion from phase-time encoding to polarisation encoding in the receiver.

[0016] US8509446 discloses a network system for quantum key distribution (QKD) for free space and fiber networks. The system generates a couple of photons which have different wavelength and inputs each of the photons into the asymmetric Mach-Zehnder interferometer to obtain time-bin entangled state. It provides polarization information with one part of the photons. Then it can obtain hybrid quantum entanglement. The system may be used hybrid quantum key distribution system applied for both free space and fibers.

[0017] WO2021078723 discloses a polarization modulation method of photonic pulses, in particular for generating quantum cryptographic keys, ensures an optimum stability of the outgoing polarization states, and comprises the steps of: generating a plurality of photonic pulses with an unspecified polarization state obtained by the overlapping of both the horizontal and vertical polarization modes thereof, and routing them in a first polarization-maintaining fiber; splitting said horizontal and vertical polarization modes and routing them in respective terminals of a second polarization-maintaining fiber forming a ring, whereby they travel such ring clockwise and counter-clockwise respectively, or vice versa; inducing a respective phase modulation of both said polarization modes, in one point of said ring which is spaced from said terminals by optical paths having different length along said ring, thereby a polarizationmaintaining fiber delay line is determined; and recombining said polarization modes in one single photonic pulse beam, and routing the resulting beam through said first polarizationmaintaining fiber, by obtaining, at the output thereof, a polarization state of the pulses depending upon the difference between said phase modulations.

[0018] WO18214888 discloses a 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. Through adoption of the quantum key distribution system according to the present invention, the requirement of quantum key distribution by hybrid transmission over free space and an optical fiber channel can be satisfied.GENERAL DESCRIPTION

[0019] Methods, apparatus and system, are disclosed for converting encoded photons from one encoding basis to another, for improved communication between QKD transmitter and receiver systems with enhanced security. The embodiments disclosed herein find particular applications in quantum communication over free space links by converting phase-time encoded photons into a hybrid polarization and time-bin encoded photons, and vice versa.

[0020] While the prior art tends to rely on polarization-to-time conversion using beam splitting and polarization rotation, or on polarization modulation methods requiring complex phase modulators and Sagnac interferometers, embodiments hereof employ a unique hybrid encoding scheme that fundamentally enhances security. In a broad aspect, QKD systems disclosed herein comprises a transmitter (Alice) system configured with a time-bin phase state photon encoding (T-P) to hybrid time-bin and polarization state photon encoding converter, and a receiver (Bob) system configured with a corresponding hybrid to time-bin phase (H2T) converter.

[0021] The hybrid encoding scheme used in embodiments hereof provides probability distributions that can mask the original quantum states, making it very difficult, or entirely impossible, for eavesdroppers to determine the photon encoding basis or state information. This is an important security enhancement not achieved by simple polarization-to-time conversions. Unlike the complex multi-component systems of the prior art, utilizing phase modulators and delay lines, embodiments hereof use a more elegant interferometer approach that inherently provides better stability.

[0022] Feedback mechanisms are used in possible embodiments for polarization correction and phase stabilization to compensate inter alia for environmental effects during free space transmission, while maintaining a simpler architecture. Additionally, unlike fiber-centric prior systems that struggle with free space deployment, embodiments hereof specifically enable the quantum transmitter (Alice) and receiver (Bob) to be located in secure facilities / environments while the optical transmi ssion / recepti on equipment can be remotely positioned, providing a unique architectural advantage for practical deployment.

[0023] In one aspect there is provided a quantum key distribution (QKD) communication method comprising generating time-bin phase (T-P) encoded photons by a QKD transmitter system, converting the T-P encoded photons into hybrid time-bin and polarization encoded photons, transmitting the hybrid encoded photons over a communication channel, receiving the hybrid encoded photons by a QKD receiver system, converting the received hybrid encoded photons back into T-P encoded photons using an (e.g., balanced and / or unbalanced) interferometer structure, and detecting the converted T-P encoded photons to establish a quantum cryptographic key.

[0024] The converting of the T-P encoded photons into hybrid encoded photons at the transmitter system can comprise splitting the linearly polarized T-P encoded photons through a non-polarizing beamsplitter into first and second optical paths, applying polarization rotation and time delay to photons in the first optical path, and recombining photons from the first and second optical paths using a polarization beam combiner.

[0025] The converting the hybrid encoded photons back into the T-P encoded photons can comprise splitting the hybrid encoded photons by polarization using a polarizing beamsplitter, processing the split photons through parallel arms of a balanced interferometer, wherein one arm includes a polarization rotating element, recombining the processed photons using a nonpolarizing beamsplitter, and detecting the recombined photons using dual single-photon detectors. The method may comprise monitoring polarization alignment of the received hybrid encoded photons and applying feedback correction to maintain proper polarization orientation before the polarizing beamsplitter.

[0026] The hybrid encoded photons comprise in some embodiments quantum states distributed across multiple time bins with orthogonal polarization components, such that measurement of any single parameter provides no information about the encoding basis.

[0027] The method can comprise: (i) generating the time-bin phase (T-P) encoded photons at a secured location of the QKD transmitter system, and converting the T-P encoded photons into the hybrid time-bin and polarization encoded photons and transmitting them over thecommunication channel at a less secured location of the QKD transmitter system; and / or (ii) receiving the hybrid encoded photons and converting them back into the T-P encoded photons at a less secured location of the QKD receiver system, and detecting the converted T-P encoded photons at a secured location of said QKD receiver system.

[0028] In another aspect there is provide a QKD communication system comprising: a transmitter system including a secured subsystem configured to generate T-P encoded photons, a photon state encoding converter configured to convert the T-P encoded photons into hybrid time-bin and polarization encoded photons, and an optical transmitter configured to transmit the hybrid encoded photons over a communication channel; a receiver system including an optical receiver configured to receive the hybrid encoded photons over said communication channel, a photon states decoding converter configured to convert the hybrid encoded photons back into T-P encoded photons, and a secured subsystem configured to detect the T-P encoded photons and generate cryptographic keys.

[0029] The photon encoding converter comprises in possible embodiments an asymmetric interferometric structure including a non -polarizing beamsplitter configured to split input T-P photons into first and second arms of the asymmetric interferometric structure. The first arm can include a polarization rotating element and a time delay element. A polarization beam combiner can be used to recombine photons from both arms. The secured subsystems can be physically separated from their respective optical transmission components and connected via optical waveguides. The system of may comprise communication transceivers configured to exchange basis reconciliation and error correction information between the transmitter and receiver systems.

[0030] The photon states decoding converter can comprise: a polarizing beamsplitter configured to separate hybrid time-bin and polarization encoded photons into orthogonal polarization components; a balanced interferometric structure having first and second arms optically coupled to the polarizing beamsplitter, wherein the first arm includes a half-wave retarder; a non-polarizing beamsplitter configured to recombine photons from the first and second arms of the balanced interferometric structure.

[0031] The system comprises in possible embodiments a polarization correction feedback loop located before the polarizing beamsplitter and configured to compensate for channel -induced polarization rotation, and / or a phase correction feedback loop located in one of the arms of the balanced interferometric structure and configured to maintain interferometer stability.

[0032] The photon states decoding converter can comprise a free space interferometric structure and a multimode fiber configured to collect output photons for detection. The systemcomprises in possible embodiments a polarization controller located before the photon states decoding converter and configured to actively correct polarization distortions in the received hybrid encoded photons.

[0033] The photon states decoding converter can be a dual-stage photon decoding converter comprising a conversion stage including an asymmetric interferometric structure configured to convert hybrid encoded photons to intermediate encoded photons, and an analyzer stage including an asymmetric interferometric structure configured to facilitate detection of the encoded photon states, and an optical waveguide connecting the first and second stages.

[0034] The transmitter system can be configured to introduce a first time delay to pulses of the hybrid time-bin and polarization encoded photons thereby generated and the receiver system can be configured to introduce a second time delay 12 to pulses of the T-P encoded photons thereby generated. The first time delay can be greater than the second time delay 12 so as to facilitate and to optimize state discrimination between properly aligned and misaligned polarization states.

[0035] In yet another aspect there is provide a photon decoding converter for quantum key distribution comprising a polarizing beamsplitter configured to separate hybrid time-bin and polarization encoded photons into orthogonal polarization components and a balanced interferometric structure having first and second arms. One of these arms can include a polarization rotation element. A non-polarizing beamsplitter can be sued to recombine photons from the first and second arms. The converter can comprise first and second single-photon detectors configured to detect the recombined photons.

[0036] The converter can comprise a feedback loop configured to correct polarization misalignment of input hybrid encoded photons, and / or a feedback loop configured to correct phase variations between the first and second arms.

[0037] The converter comprises in some embodiments a free space interferometric structure replacing and a multimode fiber configured to collect output photons for detection.

[0038] A polarization controller can be positioned before the polarizing beamsplitter and to actively correct polarization distortions in the received hybrid encoded photons.

[0039] In yet another aspect there is provide a dual-stage photon decoding converter comprising a conversion stage including an unbalanced interferometric structure configured to convert hybrid encoded photons to intermediate T-P encoded photons, an analyzer stage including an unbalanced interferometric structure configured to facilitate detection of the encoded photon states, and an optical waveguide connecting said conversion and analyzer stages. The converter can comprise a detection apparatus comprising first and second single-photon detectors configured to measure pulses of the intermediate T-P encoded photons at specific time intervals.

[0040] In yet another aspect there is provided a method for detecting in a QKD receiver system polarization misalignments for encoded photons received from a QKD transmitter system. The method comprising measuring photon detection events at predetermined time intervals corresponding to expected quantum state arrivals, comparing measured detection probabilities with reference probability distributions for aligned polarization states, identifying additional detection peaks at unexpected time intervals indicating polarization misalignment, and generating feedback signals to correct the polarization misalignment based on the identified peaks.

[0041] In yet another aspect there is provided a QKD transmitter system configured to introduce a first time delay to pulses of said encoded photons and the QKD receiver system is configured to introduce a second time delay 12 to the pulses of the encoded photons. The first time delay can be greater than the second time delay 12. The reference probability distributions may correspond to quantum states |0), |1), |+), and |-) with specific detection probabilities at time intervals 0, 12, TI, T1+T2, 2i2, and 2TI.

[0042] In yet another aspect there is provided a QKD receiver system configured to implement the polarization alignment detection method of any of the embodiments disclosed herein, wherein detection of photons at time intervals where no photons are expected for aligned polarization indicates need for polarization correction.

[0043] In possible embodiments, the light source of the transmitter (Alice) can be a type of entangled photon source (e.g., parametric down conversion). The converter embodiments disclosed herein can be used to convert polarization based entanglement photon states into time-bin and phase based entanglement photon states, or hybrid entanglement of polarization and time bin and phase photon states. It is noted that any of the converter embodiments disclosed herein can be used separately for other / different applications z.e., in applications not necessarily related to QKD systems (e.g., quantum communication networks). In addition, the transmitter (Alice) and receiver (Bob) system embodiments disclosed herein can be independently and separately used in any suitable application z.e., using Alice's transmitter system, or Bob's receiver to system, to communicate with other type of optical communication systems.

[0044] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Although methods and materials similar or equivalent to those described herein canbe used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings. Features shown in the drawings are meant to be illustrative of only some embodiments of the invention, unless otherwise implicitly indicated. In the drawings same reference signs are used to indicate members (configural elements) having identical or corresponding functions and / or structures, and in which:

[0046] Fig. 1 schematically illustrates transmitter (Alice) and receiver (Bob) embodiments in a QKD system according to possible embodiments;

[0047] Fig. 2 schematically illustrates a transmitter-side photon encoding converter according to possible embodiments using asymmetric interferometer structure;

[0048] Fig. 3 schematically illustrates receiver-side photon encoding converter according to possible embodiments using balanced interferometer structure;

[0049] Figs. 4A to 4C schematically illustrates alternative receiver-side photon encoding converter configurations according to possible embodiments, wherein Figs. 4A and 4B demonstrate using free space interferometers with multimode fiber (MMF) output to address fiber coupling challenges and Fig. 4C shows a configuration utilizing an unbalanced interferometer structure utilizing single mode fibers (SMFs);

[0050] Fig. 5 schematically illustrates a receiver-side photon encoding converter according to possible embodiments with active polarization control system to correct polarization distortions before interferometer processing; and

[0051] Fig. 6 schematically illustrates a receiver-side photon encoding converter according to possible embodiments with dual-stage conversion and analysis processes.DETAILED DESCRIPTION OF EMBODIMENTS

[0052] One or more specific and / or alternative embodiments of the present disclosure will be described below with reference to the drawings, which are to be considered in all aspects as illustrative only and not restrictive in any manner. It shall be apparent to one skilled in the art that these embodiments may be practiced without such specific details. In an effort to providea concise description of these embodiments, not all features or details of an actual implementation are described at length in the specification. Elements illustrated in the drawings are not necessarily to scale, or in correct proportional relationships, which are not critical. Emphasis instead being placed upon clearly illustrating the principles of the invention such that persons skilled in the art will be able to make and use the embodiments hereof, once they understand the principles of the subject matter disclosed herein. This invention may be provided in other specific forms and embodiments without departing from the essential characteristics described herein.

[0053] The following disclosure addresses the challenge of transmitting quantum -encoded photons through free space while maintaining security and signal integrity. Various approaches are disclosed for converting between different photon encoding schemes to optimize transmission through different communication media. Particularly, in possible embodiments temporal phase relationships in time-phase (T-P) encoding are transformed into spatial- temporal-polarization relationships in hybrid encoding. This conversion maintains quantum information content while distributing it across multiple degrees of freedom, making the encoding more robust for free space transmission and less vulnerable to single-parameter eavesdropping attacks.

[0054] An unbalanced (e.g., Mach-Zehnder) interferometer structure is used in some embodiments at the transmitter to split by a non-polarizing beamsplitter T-P encoded photons into two optical paths with different characteristics. In one non-limiting embodiment, in a short arm of the unbalanced interferometer structure the photons maintain original polarization and timing, and in its long arm they undergo polarization rotation and time delay. A polarization beam combiner recombines the optical signals from the short and long arms of the unbalanced interferometer structure, thereby producing hybrid time-bin polarization encoded photon states, in which the original T-P photon encoding states are converted to hybrid states wherein information is encoded in both arrival time (to, ti, t?) and polarization (e.g., horizontal - H, vertical - V), with probabilities distributed across multiple time bins.

[0055] Optical monitoring can be used through a secondary beamsplitter that diverts a portion of the converted photons to a monitoring device while directing the remainder for transmission. This enables real-time verification of conversion quality and detection of potential system compromises without affecting the transmitted quantum states. Optionally, variable optical attenuators are used in one or both arms of the unbalanced interferometer structure to compensate for optical loss imbalances between paths, component-induced attenuation variations, and / or environmental fluctuations affecting path transmission. Automated feedbackis used in some embodiments to adjusts attenuation levels to maintain equal photon probabilities in both arms and ensuring proper hybrid state generation. Polarizationmaintaining fibers can be used in the arms of the interferometer structure to preserve polarization states during conversion, for maintaining quantum state fidelity.

[0056] The receiver (Bob) system utilizes in some embodiments a symmetric / balanced interferometer structure (z.e., of equal arm lengths) having a polarization rotating element in one of its arms to convert the hybrid encoded photons received from transmitter (Alice) system back to time-phase encoding. The hybrid photons are split by a polarizing beamsplitter into orthogonal polarization components of the hybrid photons directed into the arms of the symmetric interferometer structure, which thereafter recombined for detection (e.g., by dual SPADs). Feedback loops are used in some embodiments for slow polarization correction to compensate for channel-induced polarization rotation, and / or fast phase correction to maintain interferometer stability. A balanced design is thereby achieved that can provide superior thermal stability compared to asymmetric configurations.

[0057] Optionally, the interferometer is implemented utilizing free space optics followed by multimode fiber collection, which is particularly usable for satellite system designs and mobile platform deployments where precise single-mode fiber coupling is challenging. Such free space optics configurations eliminate stringent fiber alignment requirements, while maintaining conversion fidelity.

[0058] A polarization control unit is used in some embodiments before the symmetric interferometer structure to actively correct polarization distortions introduced during transmission of the hybrid encoded photons received from the transmitter. Multimode fiber collection can be used with real-time polarization adjustment based on feedback from the detection system. This configuration enables operation in environments with significant polarization drifts, such as mobile platforms or long-distance free space links.

[0059] In some embodiments a dual-stage conversion and analyzer architecture is utilized at the receiver system to convert the hybrid photons back into the T-P encoding and to detect their encoding states at the detection system. In this configuration a first unbalanced interferometer structure is used for initial conversion from hybrid to intermediate T-P encoding, and thereafter a second unbalanced interferometer structure analyzer is used to facilitate detection of the encoded photon states.

[0060] In some embodiments the photon encoding conversion units of the transmitter and of the receiver systems are configured to introduce different delay times and 12, respectively to the hybrid encoded photons generated by the transmitter system and to their T-P convertedstated generated by the receiver system, (where 12 < TI) to optimize state discrimination. Singlemode fiber connection can be between the conversion and analyzer stages, and / or in the arms of the second unbalanced interferometer structure of the analyzer stage, to maintain security. Enhanced state readout methodology is thereby obtained using specific time segments for measurement. These embodiments can be optimized for long-distance transmission and high- security requirements, where maximum conversion fidelity and eavesdropping detection sensitivity may be critical.

[0061] Embodiments hereof provide superior security through the use of hybrid encoding, that inherently masks quantum information beyond what polarization modulation alone can achieve, with simplified architecture (compared to complex Sagnac interferometer systems) and improved thermal stability through balanced interferometer designs. Embodiments hereof also provide deployment flexibility by allowing separating security-critical quantum components from optical equipment, and optimized performance for both satellite-based quantum communication and hybrid fiber / free-space networks, addressing limitations of prior art systems designed primarily for single medium transmission.

[0062] The term 'interferometer structure' as used herein refers to optical structures configured to manipulate optical signals similar to interferometers, but which not necessarily require metering / detector equipment. The terms 'asymmetric interferometer' and 'unbalanced interferometer' are interchangeably used herein to refer to interferometer structures having arms of different lengths. The arms of such asymmetric / unbalanced interferometric structures are also referred to herein as a 'short arm' and a 'long arm' .

[0063] For an overview of several example features, process stages, and principles of the invention, the examples of conversion of hybrid time-bin and polarization state photon encoding into time-bin phase state photon encoding illustrated schematically and diagrammatically in the figures are intended for a QKD implementations. These conversion techniques are shown as one example implementation that demonstrates a number of features, processes, and principles used for quantum communication over free space, but they are also useful for other applications and can be made in different variations. Therefore, this description will proceed with reference to the shown examples, but with the understanding that the invention recited in the claims below can also be implemented in myriad other ways, once the principles are understood from the descriptions, explanations, and drawings herein. All such variations, as well as any other modifications apparent to one of ordinary skill in the art and useful in quantum communication applications may be suitably employed and are intended to fall within the scope of this disclosure.

[0064] The present application provides solutions for converting time-bin and phase encoded photons into polarization-based coded photons, and for encoding the polarization-based coded photons back into time-bin and phase encoded photons, with a simple, compact, and manufacturable design, that has further advantages for QKD networks. A schematic diagram for a Free Space Optical (FSO) implementation according to possible embodiments is depicted in Fig. 1.

[0065] Fig. 1 shows 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 the 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) / waveguide(s) and / or over free space by radiofrequency- RF / optical based link) 10s. The transmitter 12 comprises a secured sub-system 12u comprising security-sensitive components, a time-bin phase state photon encoding (T-P) to hybrid time-bin and polarization state photon encoding (T2H) converter 12t optically coupled to the secured sub-system 12u over optical fiber(s) / waveguide(s) 12o, and an optical transmitter (e.g., a free space optical transmitter utilizing telescope and / or collimating optics) system 12e optically coupled to the T2H converter 12t.

[0066] In some embodiments 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 out the communication over the standard communication link (e.g., over optical fiber(s) / waveguide(s) and / or over free space by radiofrequency - RF / optical based link) 10s. The router and cryptography key components 12y can be configured to communicate with the quantum communication transmitter 12p over (e.g., key channel and / or service channel) internal communication channel 12i utilizing a standard parallel or serial data communication bus (e.g., USB, Ethernet, SCSI, or suchlike), or wirelessly (e.g., WiFi, Bluetooth, Zigbee, or suchlike) if all security-sensitive components are located in a well secured facility / environment). Optionally, but in some embodiments preferably, the secured sub-system 12u is located in a secured location which can be placed relatively remote from the components of the system that are less security-sensitive e.g., the T2H converter 12t, the optical transmitter system 12e, and / or the transceiver unit 12n.

[0067] The receiver system 11 comprises in this non-limiting example an optical receiver system (e.g., FSO receiver utilizing telescope and / or collimating optics) lie in line-of-sight (LOS) with the optical transmitter system 12e 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 lly, and a transceiver unit lln electrically coupled to the router and cryptography key generation unit lly and configured to carry out the communication over the standard communication link 10s with the transceiver unit 12n of the transmitter system 12.

[0068] This configuration of system 10 allows generation of T-P states encoded photons (PO,PT) in the secured location (12u) in which the quantum transmitter 12p is located, and securely transmitting them to the T2H converter 12t over optical fiber(s) / waveguide(s) 12o for conversion into the hybrid time-bin and polarization states photons encoding (Sto,Sti,St2) for transmission over the free space medium lOq to the receiver system 11 by the optical transmitter system 12e. Optionally, but in some embodiments preferably, components of the receiver system 11 are accordingly separated into a secured sub-system llu comprising the quantum receiver (qRx) lip and router and cryptography key generation unit lly, and a less security-sensitive sub-system comprising the hybrid time-bin and polarization states to timebin and phase photon encoding (H2T) converter lit, and the optical receiver system lie.

[0069] The quantum receiver lip can be configured to securely communicate with the H2T converter lit over optical fiber(s) / waveguide(s) llo, and the router and cryptography key components lly can be configured to communicate with the quantum communication receiverllp over an internal communication channel Hi (e.g., key channel and / or service channel) utilizing a standard parallel or serial data communication bus (e.g., USB, Ethernet, SCSI, or suchlike), or wirelessly (e.g., WiFi, Bluetooth, Zigbee, or suchlike) if all sensitive components are located in a well secured facility / environment).

[0070] This configuration allows converting by the H2T converter lit the hybrid time-bin and polarization states photon (Sto,Sti,St2) encoding received over the free space medium lOq by the optical receiver system He back into T-P states encoded photons (Po,Pt), and securely transmitting the T-P states encoded photons (Po,Pt) to the secured location (Hu) in which the quantum receiver lip is located, over the optical fiber(s) / waveguide(s) Ho. At the receiver 11, the H2T converter lit converts the hybrid time-bin and polarization states photon encoding (Sto,Sti,St2) received over the free space medium lOq by the optical receiver system lie into T-P states encoded photons (Po,Pt), and securely transmits the T-P states encoded photons(Po,Pt) over the optical fiber(s) / waveguide(s) llo to the secured location (llu) in which the quantum receiver lip is located.

[0071] It should be understood that the receiver system 11 and / or the transmitter system 12 depicted in Fig. 1 may include additional components and that some of the components described herein may be removed and / or modified without departing from a scope of the embodiments disclosed herein. The transmitter 12 and / or receiver 11 systems of embodiments hereof, and / or their components, 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, PCT / IL2023 / 051022, PCT / IL2023 / 051248, of the same Applicant hereof, the disclosures of which are incorporated herein by reference.

[0072] Table 1 shows the hybrid time-bin phase and polarization photon states transmitted by the transmitter (Alice) system 12 according to possible embodiments, after basis conversion module (BCM) applied by the T2H converter 12t to change their photon encoding.TABLE 1

[0073] Fig. 2 schematically illustrates a photon encoding converter (T2H) 12t of the transmitter (Alice) system 12 according to possible embodiments, utilizing an asymmetric interferometer structure 23r (e.g., Mach-Zehnder). In this non-limiting example, linearly polarized (e.g., H-polarized) and T-P encoded photons (Po,Pt) pass through a (e.g., 50%-50%) non-polarizing beamsplitter (NPBS) 23 into two polarization maintaining (PM) output optical fiber / waveguide arms, Armi (long arm) and Arms(short arm) of the asymmetric interferometer structure 23r. In one of the output optical fiber / waveguide arms (Armi) a polarization rotation element (e-g-, . / 2 rotation / phase retarder, or other means to flip between the H and V polarizations) 23w, and a time shift / delay (e.g., about T = 500 ps) 23d, are applied. Variable optical attenuators (VOA) are provided in some embodiments in one, or both, of the PM optical fiber / waveguide arms Armi,Armsfor testing / calibration and / or compensating attenuation imbalances in the arms Armi,Armsof the asymmetric interferometer structure 23r.

[0074] The light signal portions passing through the optical fiber / waveguide arms Armi,Armsreach the polarization beam combiner (PBC) 24 with orthogonal polarities, and they arecombined by the PBC 24 for transmission to the receiver 11. The combined light signals from the PBC 24 are hybrid time-bin and polarization photons (Sto,Sti,St2 - to, ti, or t2 in Table 1) to be transmitted (e.g., by the optical transmitter system 12e) to the receiver (Bob) system 11.

[0075] Optionally, but in some embodiments preferably, before the transmission to the receiver (Bob) system 11, the combined light signals from the PBC 24 are passed through a non-PM optical fiber / wav eguide 25t to a (e.g., 90%-10%) NPBS 25 configured to direct a portion e.g., 10%) of the photons to the optical transmitter 12e for transmission to the receiver (Bob) system 11, and directing the remaining e.g., 90%) portion of the combined light signals to a monitor device 25m. The monitor device 25m can be used to monitor the hybrid time-bin and polarization photons (Sto,Sti,St2) e.g., for initial calibration.

[0076] This way, a potential eavesdropper (Eve) cannot acquire any information about the photon encoding bases by measuring the hybrid time-bin and polarization photon (Sto,Sti,St2) signal. The main reason for that, is that the |1) T-P state is split equally between \H) and | V) polarizations, and the probability of measuring a signal sent in the photon encoding base |0, 1) is equal of the probability of measuring a signal sent in the photon encoding base |+, — ) i.e., probabilities of z^A+V^). Thus, for the same reason, an eavesdropper (Eve) cannot get any information about the photon encoding states from measuring the hybrid time-bin and polarization photon (Sto,Sti,St2) signal transmitted over the free space medium lOq.

[0077] Fig. 3 schematically illustrates a H2T converter lit used in the receiver system 11 according to possible embodiments, utilizing a balanced interferometer structure 33r (i.e., without delay line - having equal arm lengths e.g., Mach-Zehnder) to convert the hybrid timebin and polarization photon (Sto,Sti,St2) signal from the optical receiver system lie into T-P states encoded photons (Po,Pt). The H2T converter lit comprises a polarization rotation element e.g., ^ / 2 rotation / phase retarder, or other means to flip between the H and V polarizations) 33w in one of the arms Armi,Arm2 of the balanced interferometer structure 33r, and an optional feedback loop FL1 before the balanced interferometer structure 33r, and / or an optional feedback loop FL2 in one of its arms Armi or Arirn, respectively.

[0078] The hybrid time-bin and polarization photons (Sto,Sti,St2) from the optical receiver system lie are split by a polarizing beamsplitter (PBS) 33 into two orthogonally (e.g., horizontally and vertically) polarized light signal portions directed into respective arms Armi,Arm2 of the balanced interferometer structure 33r. The polarization rotation element 33w provided in one of the arms (Armi) flips the polarization of the portion of the hybrid timebin and polarization (Sto,Sti,St2) signal portion passing therethrough into an orthogonalpolarization direction (e.g., from horizontal polarization into vertical polarization). Thus, the two light signal portions from the arms Armi,Arm2 of the balanced interferometer structure 33r reach the NPBS 34 at substantially the same polarization orientation, causing them to interfere one with the other therein.

[0079] The (e.g., a slow) feedback loop FL1 can be used in some embodiments to correct the polarization (e.g., correct changes in the photon polarization induced after passing the PBC 24 of the transmitter's T2H converter 12t, such as photon transition caused by a single-mode fiber) of the received hybrid time-bin and polarization (Sto,Sti,St2) signal, before it enters the polarizing beamsplitter (PBS) 33 of the balanced interferometer structure 33r. The polarization correction can be applied by the feedback loop FL1 by adjusting angular orientation of one or more polarizer elements (not shown e.g., using in FL1 a MPC320 motorized polarization controller of THORLABS). For example, the correction of the polarization orientation of the received hybrid time-bin and polarization (Sto,Sti,St2) signal by the feedback loop FL1 can be carried out in accordance with the encoding state shown in Tables 4A-4C and 5A-5B.

[0080] The (e.g., fast) feedback loop FL2 can be used in some embodiments to correct any phase changes between the orthogonal (e.g., \H) and |7)) polarization caused by the photon transition in the interferometer structures (e.g., 23r and 33r) of the transmitter (12) and of receiver (11). For example, the feedback loop FL2 can be configured to measure the intensity of the hybrid time-bin and polarization (Sto,Sti,St2) signal portion passing through one of the arms (i.e., the feedback loop FL2 can be provided in Armi or Arn ) and transmit corresponding control data / signals to the transmitter system 12 over a side channel (e.g., classical / conventional communication channel 10s) to control / adjust the light signals generated by the (e.g., laser) light source (12r in Fig. 1) of the quantum communication transmitter (qTx) 12p of the transmitter system 12 e.g., by controlling the temperature of the laser source (12r). Other control methods for pulse overlap and / or optical phase control can be used, such as mechanical delay lines, fiber stretching or thermal control of one or more of the interferometer structures at the transmitter and / or receiver systems.

[0081] The NPBS 34 combines the two light signal portions from the arms Armi,Arm2 to restore the T-P encoding states of the signals, and directs portions (e.g., 50%-50%) of the combined light signal into two respective detectors (e.g., single-photon avalanche diodes - SPAD1,SPAD2) 34a and 34b, configured to respectively detect their various T-P states e.g., as exemplified in Table 2. This configuration can be used in a scenario where the link (llo) between Bob's converter (lit) and its quantum receiver (lip) is trusted. The feedback loop(s) FL1 and / or FL2 are configured to adjust the polarization and phase of the hybrid time-bin andpolarization (Sto,Sti,St2) signals transmitted by the transmitter system 12, to ensure that the T- P signals (Po,Pt) obtain therefrom reach the detectors 34a, 34b distributed in accordance with the polarization axis of the transmitter system 12 (before any arbitrary polarization rotations occur). Such feedback loop FL1 and / or FL2 is optionally also used in the H2T converter embodiments (lit', lit" and 50) shown in Figs. 4, 5 and 6.

[0082] Regular T-P signal detection (without T2H conversion) can be detected by the detectors 34a, 34b as demonstrated in Table 2.TABLE 2

[0083] Advantages of the configuration of Fig. 3 includes inter alia', (i) enhanced security, since the photon encoding basis used by Alice (transmitter system 12) is impossible for determination by eavesdroppers (Eve); (ii) single-mode optical fibers (SMF) can be used to allow mounting the optical receiver system (e.g., FSO / telescope) lie remotely away from the analyzer unit (qRx) lip; (iii) since there is no interference in different time-bin coded photons there is no need to correct the wavefront; and (iv) a balanced interferometer (33r) is more stable (thermally) than an unbalanced interferometer structure.

[0084] Optionally, MMFs are used instead of SMFs, and high quality interference can be achieved using mode filtering and wavefront correction, utilizing passive optical elements or adaptive optics techniques.

[0085] In possible embodiments, however, the implementation of this system for satellite communication could be problematic since the signal should get inside a single-mode fiber on the optical receiver system (e.g., FSO / telescope) lie. This issue can be addressed in possible embodiments as follows: (a) using a free space interferometer 33r' and a multimode fiber (MMF) before or after the NPBS 34, as exemplified in Fig. 4A; and / or (b) collecting the signal to a multi-mode fiber before or after the PBS 33, as exemplified in Fig. 4B.

[0086] Accordingly, in the embodiment shown in Figs. 4A and 4B the H2T converter llt / llt' is implemented with a FSO interferometer 33r' utilizing e.g., utilizing FSO elements, such as reflecting elements / mirrors.

[0087] In some embodiments, as shown in Fig. 4C, the conversion stage utilizes an asymmetric interferometer 44 configured to introduce a 2T time delay (e.g., by the time delay unit 33d). After the optical signal received from Alice is converted by the asymmetric interferometer 44 into T-P encoded photons (Po,Pt), it is directly split by NPBS 34 and measured by the detectors 34a, 34b. In this embodiment it is easier to distinguish between the states, but it may be more difficult for use if the distance between the optical receiver system (lie) and Bob's qRx (lip) is greater than few meters.

[0088] In possible embodiments, the polarization of the hybrid time-bin and polarization (Sto,Sti,St2) signal from the transmitter 12 can be actively corrected by a polarization controller 37 e.g., configured for multi-mode fibers (or any other polarization controller e.g., MPC320 motorized polarization controller of THORLABS), as exemplified in Fig. 5. The polarization correction performed by polarization controller 37 is applied to optical signals passing through a MMF channel between the optical receiver system lie and the PBS 33 of the balanced interferometer structure 33r.

[0089] In embodiments hereof, a long (e.g., 100 meters) balanced interferometer structure (e.g., 33r") can be used to locate components requiring relatively lower security (e.g., H2T) of the receiver system 11 in a location relatively remote to the components that are placed in secured locations / environments (e.g., the detectors 34a, 34b). In this configuration the polarization control task is substantially simplified, because SMF until the PBS 33 is not required. The length of the interferometer structure 33r" can be the separation distance between the optical receiver system lie and the secured location of the BOB's quantum receiver (qRx) Up, which in this case, may the only part that needs to be secured because eavesdropping on this fiber cannot reveal meaningful data.

[0090] However, keeping the balanced interferometer structure 33r" stable may be hard with long arms. Thus, in possible embodiments, the two polarizations arms Armi,Arm2 are implemented using a PM fiber, on which the optical signals are sent such that one polarization is embedded in the fast axis and the other polarization is embedded in the slow axis, of the transmitted signal, so as to compensate for the “walk-off’ (z.e., time difference evolving between the fast and slow axes) of both polarizations e.g, by switching the H and V polarizations from the fast and slow axes in the middle of the fiber i.e., the polarization rotation element 33w should be placed at the center of the arm Armi (or Arn )

[0091] In another possible embodiment the two polarizations arms Armi and Arirn are implemented using a single dual-core fiber. In this embodiment, both cores of the dual-core fiber experience the same conditions, which improves the stability of the interferometerstructure. In embodiments using a dual-core fiber to implement the arms Armi and Armz of the balanced interferometer structure 33r", an MMF-to Dual-core adapter llq can be used to optically couple between MMF fiber llo and the Dual core fiber of the interferometer structure 33r"

[0092] In yet other possible embodiments, a duplex single-mode fiber is used to implement the two polarizations arms Armi,Arm2, allowing a shorter (e.g., few meters) implementation of the interferometer structure 33r".

[0093] In the H2T converter embodiment 50 shown in Fig. 6, a two stage photon processing scheme is used. In the first / conversion stage 6a, the hybrid encoded photons (Sto,Sti,St2) from the optical receiver system lie are passed through a first unbalanced interferometer structure 61 configured to convert them back into T-P encoded photons utilizing a polarization rotation element (e.g.,phase retarder) 33w in one of its arms. In the second / analyzer stage 6b, the T-P encoded photons from the first / conversion stage 6a are passed through a second unbalanced interferometer structure 62 configured to facilitate detection of the encoded photon states by the detectors 34a, 34b.

[0094] For example, linearly (e.g., H) polarized photons passing through Armi of the first unbalanced interferometer structure 61 are delayed by time shift / delay (e.g., T in a range of 100 to 1000 ps, optionally about 500 ps) unit 33d and then their polarization is changed by the polarization rotation element 33w into an opposite linear (e.g., V) polarization orientation. Optionally, the time shift / delay unit 33d, and / or the polarization rotation element 33w, is mounted in the short arm Arms. The signals passing through the two arms Armi,Armsof the unbalanced interferometer structure 61 are then merged in the NPBS 34 and directed via the optical fiber / waveguide (e.g., SMF) 66, that connects the conversion stage 6a to the T-P analyzer (time domain) stage 6b for measurement by the detectors 34a, 34b.

[0095] The T-P analyzer stage 6b comprises a NPBS 64 configured to (e.g., 50%-50% split the T-P encoded photons (Po,Pt) thereby received over the optical fiber / waveguide 66 into long and short arms Armi,Armsof the asymmetric interferometer structure 62 e.g., having time shift / delay (e.g., T in a range of 100 to 1000 ps, optionally about 500 ps) unit 62w in its long arm Armi (or in the short arm Arms), and a NPBS 65 configured to combine the optical signals from the long and short arms Armi,Armsof the asymmetric interferometer structure 62 and split (e.g., 50%-50%) the combined optical signal into measurement arms of the detectors 34a, 34b

[0096] Table 3 shows the photon encoded states produced by the first / conversion stage(time domain) 6a and transmitted to the analyzer stage 6b, and their probabilities, in a specific embodiment wherein the time delay 33d is set to T and the polarization rotator 33w is configured to orthogonally rotate the optical signal (e.g., rotate the H polarization signal from Table 1 into V polarization). This configuration allows using a relatively long (e.g., upto 100 Km) SMF fiber between the conversion stage 6a and the analyzer stage 6b.TABLE 3

[0097] Using the fact that all states transmitted by Alice (12) at ti are distinguishable, in possible embodiments the transmitter system (12) is configured to transmit only the ti states shown in Table 1 above e.g., using an intensity modulator (as exemplified in international publication No. PCT / IL2023 / 051248) in each of the arms (Armi,Arms) of Alice's interferometer (23r). In this setup the receiver system (11) is configured to measure the only the state at ti transmitted by Alice, when arriving at Bob’s receiver system (11).

[0098] In some embodiments the (T2H) converter 12t (e.g., at least the asymmetric interferometer structure 23r of the transmitter / Alice system 12) is configured to introduce a rl time delay, and the H2T converter 50 (e.g., at least the unbalanced interferometer structure 61 and the unbalanced interferometer structure 62 of the receiver / Bob system 11) is configured to introduce a T2 (< T1) time delay (e.g., T1=800 ps and T2 = 500 ps). Configuring the transmitter and receiver systems 12,11 to introduce such T1 and T2 delay times, where T2<T1 alleviates in distinguishing between a properly aligned polarization at the receiver / Bob system 11, as demonstrated in Tables 4C and 5B.

[0099] Table 4C demonstrates photon signal states received by the detectors 34a, 34b when polarization alignment conditions are achieved e.g., using the feedback loop ELI. Table 5B demonstrates photon signal states measured by the detectors 34a, 34b when the polarization between the transmitter system 12 and the receiver / Bob system 11 is misaligned. When not aligned, the detection of photons can occur at time intervals where no photons are expected when the polarization is aligned. The additional peaks, (differences are shown slanted in the table), enable the implementation of a closed-loop mechanism for polarization correction.

[0100] The feedback loop FL1 can be used as a correction loop configured to monitor the signals measured by the detectors 34a, 34b and based thereon locally correct the polarization of the optical signals received over from the optical receiver system lie. The feedback loop FL2 can be configured to similarly monitor the signals measured by the detectors 34a, 34b and locally correct polarization on one of the arms Armi,Armsif it determines that misaligned "+" and / or photon states of Table 5C are being received.

[0101] TABLE 4A: encoded photons before and after the T2H converter 12t of the transmitter system 12.

[0102] TABLE 4B: encoded photons after the H2T converter lit of the receiver system 11, when polarization of the photon reaching the detectors 34a, 34b is aligned with the polarization axis of the transmitter system 12.

[0103] TABLE 4C: encoded photons reaching the detectors 34a (SPAD1) and 34b (SPAD2) of the receiver system 11, when polarization of the photons reaching the detectors is aligned with the polarization axis of the transmitter system 12.

[0104] TABLE 5A: encoded photons after the H2T converter lit of the receiver system 11, when polarization of the photon reaching the detectors are rotated.

[0105] TABLE 5B: encoded photons reaching the detectors 34a (SPAD1) and 34b (SPAD2) of the receiver system 11, when polarization of the photons reaching the detectors are rotated.

[0106] With this configuration, the state readout at the receive (Bob) system (11) can solely rely on the time segments TI, 2 T2, TI+ T2, 2 TI, and 2 T2+TI (the double-line bordered cells in Tables 4C and 5B above). This permits using the detection scheme of Table 2, except for states|0> and |1> that are “switching places” i.e., state |0> which is sent on time 0 is discovered on time 2 T2, and state |1> which is sent with a time difference of TI is discovered on time “0” (shown in grayed cells in Tables 5C and 6C).

[0107] It is noted that though the feedback loop FL2 is shown in Fig- 6 in the second unbalanced interferometer structure 62, FL2 can be alternatively placed in the first unbalanced interferometer structure 61.

[0108] Feasible ranges of values for both T1 and T2, are the consequence of various laser parameters e.g., coherence length, repetition rate, duty cycle, and pulse location after parameter oscillation. In addition, when operating in laser gain-switch mode to ensure phase randomization between laser cycles there is a limitation on the duty cycle to ensure the return to spontaneous emission operation. Another approach of discrete phase randomization may allow even longer stable operation time for increased rl and T2, or higher repetition rate operation - following the stable laser profile which is not completely closed during laser cycles.

[0109] It is noted that interferometer structures used in embodiments hereof can be implemented in various different forms i.e., not only necessarily Mach Zhender interferometer structures e.g., using a Michelson interferometer structure. It should be understood that throughout this disclosure, where a process or method is shown or described, the steps / acts of the method may be performed in any order and / or simultaneously, and / or with other steps / acts not-illustrated / described herein, unless it is clear from the context that one step depends on another being performed first. In possible embodiments not all of the illustrated / described steps / acts are required to carry out the method.

[0110] As will be appreciated by one of skill in the art, the present invention may be embodied as a method, system, computer program product, or a combination of the foregoing. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may generally be referred to herein as a "system". Furthermore, the present invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium.[OHl] As described hereinabove and shown in the figures, the present invention provides QKD communication technique particularly 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 by the embodiments as shown in the drawings and / or as described in the specification, since these are given by way of example onlyand not by way of limitation, and since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.

Claims

1. CLAIMS:

1. A quantum key distribution (QKD) communication method comprising: generating time-bin phase (T-P) encoded photons by a QKD transmitter system; converting the T-P encoded photons into hybrid time-bin and polarization encoded photons; transmitting the hybrid encoded photons over a communication channel; receiving the hybrid encoded photons by a QKD receiver system; converting the received hybrid encoded photons back into T-P encoded photons using an interferometer structure; and detecting the converted T-P encoded photons to establish a quantum cryptographic key.

2. The method of claim 1 wherein converting the T-P encoded photons into the hybrid time-bin and polarization encoded photons is carried out using an asymmetric interferometric structure.

3. The method of claim 1 or 2 wherein converting the T-P encoded photons into hybrid encoded photons at the transmitter system comprises splitting the linearly polarized T-P encoded photons through a non-polarizing beamsplitter into first and second optical paths; applying polarization rotation and time delay to photons in the first optical path; and recombining photons from the first and second optical paths using a polarization beam combiner.

4. The method of any one of the preceding claims wherein converting the hybrid encoded photons back into the T-P encoded photons comprises splitting the hybrid encoded photons by polarization using a polarizing beamsplitter; processing the split photons through parallel arms of a balanced interferometer, wherein one arm includes a polarization rotating element; recombining the processed photons using a non-polarizing beamsplitter; and detecting the recombined photons using dual single-photon detectors.

5. The method of claim 4 comprising monitoring polarization alignment of the received hybrid encoded photons; and applying feedback correction to maintain proper polarization orientation before the polarizing beamsplitter.

6. The method of any one of the preceding claims wherein the hybrid encoded photons comprise quantum states distributed across multiple time bins with orthogonal polarization components, such that measurement of any single parameter provides no information about the encoding basis.

7. The method of any one of the preceding claims comprising: (i) generating the time-bin phase (T-P) encoded photons by at a secured location of the QKD transmitter system, and converting the T-P encoded photons into the hybrid time-bin and polarization encoded photonsand transmitting them over the communication channel at a less secured location of said QKD transmitter system; and / or (ii) receiving the hybrid encoded photons and converting them back into the T-P encoded photons at a less secured location of the QKD receiver system, and detecting the converted T-P encoded photons at a secured location of said QKD receiver system.

8. A QKD communication system comprising: a transmitter system including a secured subsystem configured to generate T-P encoded photons, a photon state encoding converter configured to convert the T-P encoded photons into hybrid time-bin and polarization encoded photons, and an optical transmitter configured to transmit the hybrid encoded photons over a communication channel; a receiver system including an optical receiver configured to receive the hybrid encoded photons over said communication channel, a photon states decoding converter configured to convert the hybrid encoded photons back into T-P encoded photons, and a secured subsystem configured to detect the T-P encoded photons and generate cryptographic keys.

9. The system of claim 8 wherein the photon encoding converter comprises an asymmetric interferometric structure including a non-polarizing beamsplitter configured to split input T-P photons into first and second arms of said asymmetric interferometric structure, wherein the first arm includes a polarization rotating element and a time delay element, and a polarization beam combiner configured to recombine photons from both arms.

10. The system of claim 8 or 9 wherein the secured subsystems are physically separated from their respective optical transmission components and connected via optical waveguides.

11. The system of any one of claims 8 to 10 comprising communication transceivers configured to exchange basis reconciliation and error correction information between the transmitter and receiver systems.

12. The system of any one of claims 8 to 11 wherein the photon states decoding converter comprise: a polarizing beamsplitter configured to separate hybrid time-bin and polarization encoded photons into orthogonal polarization components; a balanced interferometric structure having first and second arms optically coupled to said polarizing beamsplitter, wherein the first arm includes a half-wave retarder; a non-polarizing beamsplitter configured to recombine photons from said first and second arms of the balanced interferometric structure.

13. The system of 12 comprising a polarization correction feedback loop located before the polarizing beamsplitter and configured to compensate for channel -induced polarization rotation, and / or a phase correction feedback loop located in one of the arms of the balanced interferometric structure and configured to maintain interferometer stability.

14. The system of any one of claims 8 to 11 wherein the photon states decoding converter comprises a free space interferometric structure and a multimode fiber configured to collect output photons for detection.

15. The system of any one of claims 8 to 11 comprising a polarization controller located before the photon states decoding converter and configured to actively correct polarization distortions in the received hybrid encoded photons.

16. The system of any one of claims 8 to 11 wherein the photon states decoding converter is a dual-stage photon decoding converter comprising a conversion stage including an asymmetric interferometric structure configured to convert hybrid encoded photons to intermediate encoded photons, and an analyzer stage including an asymmetric interferometric structure configured to facilitate detection of the encoded photon states, and an optical waveguide connecting the first and second stages.

17. The system of any one of claims 8 to 16 wherein the transmitter system is configured to introduce a first time delay TI to pulses of the hybrid time-bin and polarization encoded photons thereby generated and the receiver system is configured to introduce a second time delay 12 to pulses of the T-P encoded photons thereby generated, and wherein said first time delay TI is greater than said second time delay 12 so as to facilitate and to optimize state discrimination between properly aligned and misaligned polarization states.

18. A photon decoding converter for quantum key distribution comprising a polarizing beamsplitter configured to separate hybrid time-bin and polarization encoded photons into orthogonal polarization components; a balanced interferometric structure having first and second arms, wherein the first arm includes a polarization rotation element; a non-polarizing beamsplitter configured to recombine photons from the first and second arms.

19. The converter of claim 18 comprising first and second single-photon detectors configured to detect the recombined photons.

20. The converter of claim 18 or 19 comprising a feedback loop configured to correct polarization misalignment of input hybrid encoded photons and / or a feedback loop configured to correct phase variations between the first and second arms.

21. The converter of any one of claims 18 to 20 comprising a free space interferometric structure replacing and a multimode fiber configured to collect output photons for detection.

22. The converter of any one of claims 18 to 21 a polarization controller positioned before the polarizing beamsplitter and configured to actively correct polarization distortions in the received hybrid encoded photons.

23. A dual-stage photon decoding converter comprising a conversion stage including an unbalanced interferometric structure configured to convert hybrid encoded photons to intermediate T-P encoded photons, an analyzer stage including an unbalanced interferometric structure configured to facilitate detection of the encoded photon states, and an optical waveguide connecting said conversion and analyzer stages.

24. The converter of claim 23 comprising a detection apparatus comprising first and second single-photon detectors configured to measure pulses of the intermediate T-P encoded photons at specific time intervals.

25. A method for detecting in a QKD receiver system polarization misalignments for encoded photons received from a QKD transmitter system, the method comprising: measuring photon detection events at predetermined time intervals corresponding to expected quantum state arrivals, comparing measured detection probabilities with reference probability distributions for aligned polarization states, identifying additional detection peaks at unexpected time intervals indicating polarization misalignment, and generating feedback signals to correct the polarization misalignment based on the identified peaks.

26. The method of claim 25 wherein the QKD transmitter system is configured to introduced a first time delay to pulses of said encoded photons and the QKD receiver system is configured to introduced a second time delay 12 to said pulses of said encoded photons, and wherein said first time delay is greater than said second time delay 12, and wherein the reference probability distributions correspond to quantum states |0), |1), |+), and |-) with specific detection probabilities at time intervals 0, 12, TI, T1+T2, 2i2, and 2TI.

27. A QKD receiver system configured to implement the polarization alignment detection method of any one of claims 25 to 26 wherein detection of photons at time intervals where no photons are expected for aligned polarization indicates need for polarization correction.

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