Methods and system for fast synchronization in long range quantum key distribution

Abstract

A set of frames of a predefined bit length and comprising a plurality of subframes are generated, where a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern. The set of frames are transmitted to receiving device. A histogram equal to the predefined bit length is generated. The first pattern is correlated with the histogram to detect a first subframe and the local time reference of the receiving device is adjusted such that first pattern comes to start of histogram. A new histogram is repeatedly generated and second pattern is correlated therewith to detect all other subframes therein, where on each detection, local time reference of the receiving device is adjusted such that second pattern comes to start of the corresponding detected subframe. A synchronization complete message is transmitted to transmitter device upon detection of all the subframes in histogram.

Description

[0001] METHODS AND SYSTEM FOR FAST SYNCHRONIZATION IN LONG RANGE QUANTUM KEY DISTRIBUTION

[0002] FIELD OF THE INVENTION

[0003] The present description relates to the field of quantum communication systems and, more particularly, relates to systems and methods for synchronizing communication devices in quantum communication systems.

[0004] BACKGROUND OF THE INVENTION

[0005] Owing to the well-known advantages associated with Quantum technologies, its adaptation in communication systems has seen a multi-fold increase in recent times. For instance, encryption key distribution systems that are based on quantum cryptography techniques are deployed by enterprises and service providers nowadays, in view of the security aspects brought about by quantum technology. Such encryption key distribution systems are also referred to as Quantum Key Distribution (QKD) systems.

[0006] The setup of a QKD system involves synchronizing a transmitter device, say, Alice, and a receiver device, say, Bob, so as to ensure correct bit identification takes place during the communication between Alice and Bob. The synchronization of Alice and Bob includes two-fold steps of clock synchronization and phase synchronization. The clock synchronization is performed over a separate clock synchronization channel or using clock data recovery in the classical channel, and involves syncing the clocks of Alice and Bob. The phase synchronization involves aligning the phase or the start of a transmitted pulse between Alice and Bob.

[0007] Thus, to ensure accuracy of the QKD system, it is imperative that the aforementioned synchronization steps are performed prior to the distribution of keys or any other data as intended. While the security aspects of such QKD systems are known, setting up of such QKD systems may face a technical challenge of increased setup time, for instance, in case of long range QKD systems.

[0008] Reference may be made to EP3787220A1 which enclosed quantum communication synchronization and alignment procedure. Reference may be made to JP2008058704 which enclosed the weak communication frame synchronization method and system.

[0009] OBJECTIVE OF THE INVENTION

[0010] The main objective of the present invention is to provide a method for fast synchronization in long range quantum key distribution.

[0011] Another objective of the present invention is to provide a system for fast synchronization in long range quantum key distribution.

[0012] SUMMARY OF THE INVENTION

[0013] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0014] In an embodiment, a method of synchronizing a transmitter device and a receiver device in a communication system is disclosed. The method comprises, generating, by the transmitter device, a set of frames where each frame is of a predefined bit length and comprises a plurality of subframes, and where a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern. The method further comprises, repeatedly transmitting the set of frames to the receiving device over a communication channel. The method further comprises, generating, by the receiver device, a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the transmitter device. The method further comprises, correlating the first pattern with the histogram to detect a first subframe in the histogram and adjusting the local time reference of the receiving device such that the first pattern comes to the start of histogram. The method further comprises, repeatedly generating a new histogram and correlating the second pattern with the new histogram to detect all other subframes in the histogram, where on each detection, the local time reference of the receiving device is adjusted such that the second pattern comes to the start of the corresponding detected subframe and the new histogram is generated after the local time reference of the receiving device is adjusted. The method further comprises, transmitting a synchronization complete message to the transmitter device upon detection of all the subframes in the histogram and adjustment of the local time reference of the receiving device corresponding to the detection of all the subframes.

[0015] In an embodiment, a communication system is disclosed. The communication system comprises, a transmitter device configured to generate a set of frames where each frame is of a predefined bit length and comprises a plurality of subframes, and where a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern. The transmitter device is further configured to transmit, repeatedly, the set of frames to the receiving device over a communication channel. The communication system further comprises a receiver device configured to generate a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the transmitter device. The receiver device is further configured to correlate the first pattern with the histogram to detect a first subframe in the histogram and adjust a local time reference of the receiving device such that the first pattern comes to the start of histogram. The receiver device is further configured to repeatedly generate a new histogram and correlate the second pattern with the new histogram to detect all other subframes in the histogram, where on each detection, the local time reference of the receiving device is adjusted such that the second pattern comes to the start of the corresponding detected subframe and the new histogram is generated after the local time reference of the receiving device is adjusted. The receiver device is further configured to transmit a synchronization complete message to the transmitter device upon detection of all the subframes in the histogram and adjustment of the local time reference of the receiving device corresponding to the detection of all the subframes.

[0016] BRIEF DESCRIPTION OF DRAWINGS

[0017] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0018] FIG. 1 illustrates an example network environment, in accordance with one or more embodiments of the present subject matter;

[0019] FIG. 2 illustrates a schematic block diagram of a transmitter device and a receiver device, in accordance with one or more embodiments of the present subject matter; FIG. 3 illustrates a schematic block diagram of components of the transmitter device and the receiver device, in accordance with one or more embodiments of the present subject matter;

[0020] FIG. 4 illustrates generation of set of frames by a transmitter device, in accordance with one or more embodiments of the present subject matter; and

[0021] FIG. 5 illustrates a flowchart of a method synchronizing a transmitter device and a receiver device in a communication system, in accordance with one or more embodiments of the present subject matter.

[0022] Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present invention. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] The detailed description set forth below is intended as a description of various configurations of the subject technology and is not intended to represent the only configurations in which the subject technology can be practiced. The appended drawings are incorporated herein and constitute a part of the detailed description. The detailed description includes specific details for the purpose of providing a thorough understanding of the subject technology. However, the subject technology is not limited to the specific details set forth herein and can be practiced using one or more other implementations. In one or more implementations, structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology.

[0025] Proper operation of QKD (Quantum Key Distribution) system relies on accurate synchronization of Alice and Bob. It ensures the identification of the transmitted pulse index from the detected photon’s arrival time. This pulse index, usually referred as timestamp, is sent to Alice so that subsequent steps of sifting and key distillation can proceed. Any inaccuracy in this synchronization will led to high QB ER (Quantum Bit Error Rate) and thus failure of the QKD system key generation.

[0026] In the conventional system, the synchronization is achieved using two parts - first one is the clock synchronization which is done using separate clock synchronization channel or using clock data recovery in the classical channel, and the second step the alignment of the phase or the start of the transmitted pulse. The phase alignment is done by transmitting a known repeating sequence initially from Alice and forming a histogram of the detector click event. The clock synchronization runs continuously however the alignment of the frame is one-time process. Additionally, a tracking system based on over-sampling of the quantum link allows maintaining the phase between Alice and Bob thus allowing continuous operation after calibration, without any need for resynchronization.

[0027] The repetition period of the transmitting sequence for the phase alignment between Alice and Bob must be more than the latency in the quantum channel. For long range QKD system (> 100KM) the repetition period should be more than 33ms. As the probability of the photon detection event is very low, it takes a lot of time (in the order of minutes) to form an inferable histogram with a long repetition period in the Bob side.

[0028] The present subject matter discloses method(s) and system(s) pertaining to synchronization of communication devices in Quantum Key Distribution (QKD) systems.

[0029] According to an embodiment of the present subject matter, a method of synchronizing a transmitter device and a receiver device in a communication system, such as a QKD system, is provided. In said embodiment, the transmitter device may transmit a set of frames, each of a predefined bit length, to a receiver device. In an example, the predefined bit length may be determined based on a latency of the communication channel, as would be explained in greater detail with reference to the description of accompanying drawings.

[0030] Each of the transmitted set of frames includes a plurality of subframes, where a first subframe is based on a first pattern and other subframes are based on a second pattern. As would be appreciated, the first pattern and the second pattern are patterns of bits. Furthermore, in an example, the first pattern and second pattern are selected in a manner such that both of them have low autocorrelation. Yet further, the first pattern and the second pattern may be determined beforehand and may be communicated amongst the transmitter device and the receiver device. In an example, the first subframe being based on the first pattern may be understood as all the subframe being formed of bits as per the first pattern. An example of this would be the other first subframe included in the each of the set of frames transmitted by the transmitter device.

[0031] In an example, the other subframes being based on the second pattern may be understood as all the other subframes being formed of bits as per the second pattern. An example of this would be the other subframes included in the first frame of the set of frames transmitted by the transmitter device. In another example, the other subframes being based on the second pattern may be understood as one or more of the other subframes being formed of bits as per the second pattern and at least one other subframe being formed by rotating one or more bits of the second pattern. An example of this would be the other subframes includes in frames other than the first frame.

[0032] As mentioned above, the transmitter device transmits the set of frames to the receiver device and in an example, the transmitter device may repeatedly transmit the set of frames to the receiving device over the communication channel. In an example embodiment, the transmitter device may rotate at least one subframe in each of the other frames as per a predefined rotation scheme which would be described in greater detail with reference to the description of accompanying drawings.

[0033] Continuing with the above embodiment, on the receiving end, the receiver device may generate a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the transmitter device. The local time reference may be understood as a time reference based on a clock of the receiver device. The clock received from the transmitter device may be understood as data indicative of clock operation of a clock of the transmitter device.

[0034] Once the histogram is generated, the receiving device may correlate the first pattern with the histogram to detect a first subframe in the histogram. On detecting the first subframe in the histogram, the receiving device may adjust the local time reference of the receiving device such that the first pattern comes to the start of histogram.

[0035] Subsequently, the receiving device may repeatedly generate a new histogram and correlate the second pattern with the histogram to detect all other subframes in the histogram. Accordingly, on each detection of the other subframe in the histogram, the receiving device may adjust the local time reference of the receiving device such that the second pattern comes to the start of the corresponding detected subframe and generate the new histogram. In other words, the receiving device operates the histogram in a rotating histogram mode whereby the other subframes of the first frame are detected and accordingly, the local time reference of the receiving device is adjusted.

[0036] Once the adjustments of the local time reference for all the subframes is complete, the receiving device may transmit a synchronization complete message to the transmitter device. Accordingly, on receiving the synchronization complete message, the transmitter device may proceed with subsequent communication.

[0037] The present subject matter automatically achieves synchronization between Alice and Bob for long range QKD and measurement of quantum channel latency using correlation of the received photon histogram and transmitted pattern.

[0038] Unlike the conventional method where the pattern repetition period of the transmitting sequence for the phase must be more than the latency in the quantum channel, here the transmitting sequence is dynamic with frame length is much smaller comparing to latency in the quantum channel.

[0039] In conventional method, the received photon histogram length is equals to pattern repetition period which makes the synchronization step longer. In this proposed method, total histogram length equals to a frame length which is much less and all the photon detection event contributes to the generation of histogram resulting the synchronization step shorter.

[0040] According to aspects of the present subject matter, synchronization can be achieved for both direct photon detection (Viz. COW data SPAD) and constructive / destructive arm photon detection after an interferometer (Viz. COW monitoring SPAD and DPS SPADs).

[0041] FIG. 1 illustrates a network environment 100 implementing a transmitter device 102, hereinafter interchangeably referred to as ‘device 102’, and a receiver device 104, hereinafter interchangeably referred to as ‘device 104’, according to an embodiment of the present subject matter. In an example, the devices 102 and the device 104 may be computing devices that are capable of generating random information encoded weak coherent optical pulses or qubits. Furthermore, the device 104 may be a device that is configured to measure the state of a single photon. As would be appreciated, both the device 102 and the device 104 may be termed as QKD apparatus, in an example. Accordingly, the device 102 and the device 104 may be configured to do post processing of keys, such as like sifing, error estimation, error correction and privacy amplification. Furthermore, the device 102 and the device 104 may be configured to exchange information through the authenticated classical channel. In an example, the device 102 and the device 104 may form a communication system, such as a quantum key distribution (QKD) system.

[0042] In an example, as a part of synchronization of the device 102 with the device 104, the phase of a transmitted signal between the device 102 and the device 104 may be aligned. Once the phase is aligned, the subsequent communication between the device 102 and the device 104 may then take place.

[0043] To achieve the above objective, in an example embodiment, the device 102 may be configured to generate and transmit a set of frames to the device 104 until the synchronization between the device 102 and the device 104 is completed. In an example, each of the set of frames include quantum particles, such as photons, which may be encoded with information, such as bit values. Furthermore, each frame may include a plurality of subframes, where a first subframe includes quantum particles as per a first pattern of bits and all the other subframes include quantum particles based on a second pattern of bits.

[0044] As mentioned above, the device 102 and the device 104 may determine and fix the first pattern and the second pattern beforehand. Further, in an example, the first pattern and the second pattern are such that they have very low autocorrelation.

[0045] In an implementation, the device 102 may generate the set of frames in a manner such that except the first frame, one or more subframes of the other frames are rotated as per a predefined rotation scheme. Accordingly, the device 102 generates the first frame such that a first subframe thereof includes quantum particles as per the first pattern of bits. Further, the other subframes of the first frame includes quantum particles as per the second pattern of bits.

[0046] When generating the other frames, the device 102 generates the first subframe thereof as is done for the first frame, i.e., as per the first pattern of bits. Thus, all the frames of the set of frames have an identical first frame which includes quantum particles as per the first pattern of bits. Once the first frame of other frames is generated, the device 102 may be configured to generate the other subframes of the other frames such that at least one subframe in each of the other frames is rotated as per the predefined rotation scheme. The predefined rotation scheme is explained in detail in FIG. 4 below.

[0047] Furthermore, in an example embodiment, the device 102 generates the set of frames in a manner such that each frame of the set of frames is of a predetermined bit length. In an example, the predefined bit length may be equal to nl+n2*m, where nl is the number of bits in the first pattern, n2 is the number of bits in the second pattern, and m is the number of subframes in each frame of the set of frames. The value of the m is based on a maximum latency of the communication channel.

[0048] Thus, the device 102 generates the set of frames as described above. In an example, as and when the frames are generated, the device 102 simultaneously and repeatedly transmits the set of frames to the device 104. In an example, the device 102 may be configured to transmit the set of frames to the device 102 until the synchronization is complete.

[0049] Continuing with the above embodiment, the device 104 may be configured to generate a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the device 102. The local time reference may be understood as a time reference based on a clock of the device 104. The clock received from the device 102 may be understood as data indicative of clock operation of a clock of the device 102.

[0050] For generating the histogram, in an example, the device 104 may be configured to detect the photons transmitted by the device 102. Herein, each photon detection event at the device 104 may amount to detection of one bit. Thus, when the device 104 determines that the number of photon detection events equals the number of bits in the predefined bit length, the device 104 completes the generation of the histogram. As would be understood, the histogram may correspond to bits of multiple subframes.

[0051] After generating the histogram, in an example embodiment, the device 104 may be configured to correlate the first pattern with the histogram to detect a first subframe in the histogram. As mentioned above, the first subframe includes bits as per the first pattern of bits. Thus, the device 104 may correlate the bits of the first pattern with the histogram to identify the first subframe in the histogram. Accordingly, based on the correlation of the first pattern with histogram, the device 104 may determine the latency and subsequently adjust the local time reference of the device 104 such that the first pattern may come to the start of histogram. As a result of the adjustment, the last subframe for which the time reference is adjusted may be properly aligned with the corresponding pattern. Thus, in the present case, the first subframe may be properly aligned with the corresponding pattern.

[0052] After the first frame is aligned, in an example embodiment, the device 104 may be configured to generate a new histogram, correlate the second pattern with the newly generated histogram to detect another subframe, and adjust the local time reference of the device 104 such that the second pattern comes to the start of the corresponding detected subframe. After the adjustment, the device 104 may again generate a new histogram and again correlate the second pattern with this newly generated histogram to detect further subframes.

[0053] Thus, as may be understood, the device 104 may be configured to repeatedly generating a new histogram and correlating the second pattern with the histogram to detect all other subframes in the histogram. On each detection, the device 104 adjusts the local time reference of the device 104 such that the second pattern comes to the start of the corresponding detected subframe. Further, the device 104 generates a new histogram after the local time reference of the receiving device is adjusted.

[0054] In an example embodiment, once the adjustments of the local time reference for all the subframes is complete, the device 104 may transmit a synchronization complete message to the device 102.

[0055] FIG. 2 illustrates a schematic block diagram 200 illustrating components of the transmitter device 102 and the receiver device 104, according to an embodiment of the present subject matter. As shown in the FIG. 2, the transmitter device 102 may include a laser / pulse generator 202, a modulator 204, and a Field Programmable Gate Arrays (FPGA) 206. Examples of the modulator 204 may include, but are not limited to, an Electro-optic phase modulator or intensity modulator.

[0056] As is further shown, the receiver device 104 may include a delay line interferometer 208, a single photon detector 210, and an FPGA 212. Examples of the delay line interferometer 208 may include, but are not limited to, a 1 -bit delay interferometer which can be of Mach-Zehnder or Michelson type. Examples of the single photon detector 210 may include, but are not limited to, Single Photon Avalanche Detector (SPAD) or a Superconducting Nanowire Single-Photon Detector (SNSPD). In an example, the device 102 and the device 104 may be configured to operate as per a predefined protocol, such as the COW protocol and the DSP protocol for forming a QKD system, such as that described in FIG. 1. In such a system, the device 102 and the device 104 may operate as per the aspects of the present subject matter. That is, the device 102 may generate and transmit a set of frames, as per the predefined rotation scheme defined in detail in FIG. 4. More particularly, in an example, the pulse generator 202 may be configured to generate a plurality of optical pulses. Furthermore, the FPGA 206 may be configured to generate RF signals based on the first pattern and the second pattern as per the predetermined scheme described in the description of FIG. 4. Accordingly, in an example, the RF signals thus generated are used by the modulator 204 to modulate the optical pulses generated by the pulse generator 202, as per a predefined QKD protocol, such as COW or DPS protocol.

[0057] Further, the device 104 may be configured to operate as per a rotating histogram mode and accordingly perform the synchronization as described in detail herein at least with respect to FIG. 1 and FIG. 5. Herein, the single photon detector 210 detects the photon detection events. The FPGA 212 performs the histogramming based on the detected photon events. In an example, the delay interferometer 208 may be tuned prior to the synchronization.

[0058] FIG. 3 illustrates a schematic block diagram of components of the transmitter device and the receiver device, in accordance with one or more embodiments of the present subject matter. As shown in the FIG. 3, the FPGA 206 of the device 102 may include a frame generator 302, a control unit 304, and a communication unit 306. Further, the FPGA 212 of the device 104 may include a time to digital module 308, a histogram generator 310, the control unit 304, and the communication unit 306.

[0059] In an example embodiment, in the FPGA 206 of the device 102, the control unit 304 may be configured to manage the operation of other modules, such as when to start the synchronization process and when to stop. The frame generator 302 may be configured to generate RF signals based on the first pattern and the second pattern as per the predetermined scheme described in the description of FIG. 4. In an example, the RF signals thus generated are used by the modulator 204 to modulate the optical pulses generated by the pulse generator 202, as per a predefined QKD protocol, such as COW or DPS protocol.

[0060] In an example embodiment, in the FPGA 212 of the device 104, the photon detection event is detected by the single photon detector 210. In an example, the photon detection event signal is timestamped by the Time to digital module 308 using a clock recovered from the communication unit 306. In an example, the histogram generator 310 may be configured to obtain / accu- mulate a predetermined number of photon arrival timestamps from the time to digital module 308 and form histogram as described in detail in description of FIG. 1 and FIG. 5 herein. In an example, the control unit 304 may be configured to maintain and update the time reference after each iteration for each subframe. The time reference, in an example, may be used by the histogram generator 310 for repeated generation of the histogram. Furthermore, the control unit 304 of the device 104 may be further configured to control beginning and ending of each histogram iteration, determine the correlation, find latency for each subframe, adjust the time references, and enable the rotating histogramming mode. Further, the control unit 304 may be configured to inform the device 102 about the completion status as well.

[0061] In an example, the communication unit 306 may be used to communicate or exchange messages between device 102 and the device 104 through a classical channel. Further, the communication unit 306 may be configured for performing the clock recovery in the device 104.

[0062] FIG. 4 illustrates an operation flow 400 of a predefined rotating scheme for rotation of at least one subframe of other frames of the set of frames, according to an embodiment of the present subject matter.

[0063] In an example, as soon as communication system of FIG. 1 is ready for transmission, in a first step, frame 402 is sent on the quantum channel as a first frame using device 102’s time reference. The first nl bit of frame 402 is Pl, next n2 bit is P2 and P2 pattern is repeated for ‘m’ times. The length of the frame 402 is (nl +m*n2) bits.

[0064] The second frame 404 is similar to the first frame 402 with a change that is in the Subframe- 1. In Subframe 1 of frame 404, Pattern P2 is rotated left (or right) by one bit, say P2’, and is sent as Subframe- 1. Similarly, next frames are generated by rotating P2’ to P2”, P2” to P2’” and this goes up to n2 frames. For the sake of brevity, only frame 406 depicting rotation of P2’ to P2” is shown herein.

[0065] Referring now to frame 408, after n2 frames, Subframe- 1 again becomes P2. This time Sub- frame-2 is rotated once to P2’. And keeping Subframe-2 to P2’, Subframe- 1 is rotated n2 times to generate next n2 frames. Then Subframe-2 rotated to P2’ ’ and next n2 frames are generated. This goes up to n2*n2 frames. As would be understood, after n2*n2 frames, Subframe-3 is rotated once and n2*n2 frames are generated using earlier steps. Similarly, Subframe-4 rotates after n2*n2*n2 frame and pattern - m rotates after n2A(m-l) frames. Thus, as per operation flow 400, actual frame repetition is n2Am frames. This will enable to measure the quantum channel latency up to (nl +m*n2)* n2Am pulse period. In an example, according to the quantum channel range, the value of ‘m’ may be fixed.

[0066] FIG. 5 illustrates a flowchart of a method 500 of synchronizing a transmitter device and a receiver device in a communication system, according to an embodiment of the present subject matter. In an example, the method 500 may be implemented in computing devices, such as the device 102 and the device 104. Furthermore, details of the steps of the method 500 which are already described herein in the above figures are not included in detail herein below.

[0067] In an example, before synchronization is initiated, the transmitter device and the receiver device may respectively initialize their internal optical components. Thereafter, the transmitter device and the receiver device may establish an authenticated connection through the classical channel for information exchange. In an example, in case there is any delay interferometer in the receiver device, the interferometer tuning is also performed.

[0068] Once the authenticated connection is formed, the transmitter device then informs the receiver device to enter into the synchronization state. Once the receiver device enters into synchronization state, it first acknowledges the same to the transmitter device, resets its own time reference, and starts with histogram formation. After receiving the acknowledgement from the receiver device, the transmitter device resets it's time reference and starts transmission of frames. The method 500 below elucidates the steps for synchronizing the transmitter device and the receiver device.

[0069] At step 502, a set of frames is generated by a transmitting device, where each frame is of a predefined bit length and comprises a plurality of subframes, and where a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern.

[0070] In an example, the set of frames may be understood as a set of optical pulses with information encoded thereon. More particularly, the transmitter device generates quantum particles, such as a single photon, which can be encoded with information, such as bit values. A collection of such single photons effectively constitutes a frame, and can be further classified into subframes. In an example, predefined bit length may be equal to nl+n2*m. Herein, nl is the number of bits in the first pattern and n2 is the number of bits in the second pattern. Furthermore, m is the number of subframes in each frame of the set of frames. In an example, the value of the m may be determined based on a maximum latency of the communication channel. More particularly, according to the aspects of the present subject matter, quantum channel latency may be adjusted up to (nl + m*n2) n2Am pulse period. In an example, the quantum channel latency may include the latency of the quantum channel optical fiber and latency of all the components in the quantum channel (viz. modulator, single photon detector, timestamp unit in FPGA etc.). Herein, the value of nl and n2 i.e. the length of pattern Pl and P2 may be pre-decided. Thus, as an example, say if a system is expected to support 100KM and pulse period is Ins, the quantum channel latency will be approximately ((100*10A3) / (2* 10A8)) seconds, i.e., 0.5ms. For such a system, m may be taken large enough so that the value of (nl + m*n2)*n2Am should be greater than ((0.5*10A-3) / (10A-9)) i.e., 5*10A5.

[0071] In an example, a first frame of each of the set of frames may be based on a first pattern and other subframes may be based on a second pattern, as has been explained in detail in reference to the above figures.

[0072] At step 504, the set of frames may be repeatedly transmitted to a receiving device over a communication channel by the transmitting device. In an example, the transmission of the set of frames may continue till the completion of the synchronization.

[0073] At step 506, a histogram of bit length equal to the predefined bit length is generated by the receiver device, based on a local time reference and clock received from the transmitting device. In an example, each photon detection event at the receiver device end may amount to detection of one bit. Thus, when the number of photon detection events equals the number of bits in the predefined bit length, the histogram is generated. Accordingly, the histogram, thus generated, corresponds to bits of multiple subframes.

[0074] At step 508, the first pattern is correlated with the histogram to detect the first subframe in the histogram and the local time reference of the receiving device is adjusted such that the first pattern comes to the start of histogram. As mentioned above, the first subframe is based on the first pattern. Thus, the bits of the first pattern are correlated with the histogram and accordingly the first subframe may be detected in the histogram. Based on the correlation of the first pattern with histogram, latency may be determined and accordingly the local time reference of the receiving device may be suitably adjusted so that the first pattern comes to the start of histogram.

[0075] At step 510, a new histogram is repeatedly generated and the second pattern is correlated with the new histogram to detect all other subframes in the histogram, where on each detection, the local time reference of the receiving device is adjusted such that the second pattern comes to the start of the corresponding detected subframe and the new histogram is generated after the local time reference of the receiving device is adjusted. In other words, in an example, a rotating histogram mode is enabled where a position of a click event with respect to the subframes of the histograms is detected. Accordingly, a click time-bin index corresponding to the click event is rotated based on time interval elapsed since an initial local time reference. As an example, in this mode, the click event which lies in Subframe-0 directly constitute into histogram. If the click event comes into Subframe- 1, the click time -bin index is rotated by number of frames, i.e., nl +m*n2 bit interval, elapsed since the beginning, i.e., initial local reference. Similarly, if click event comes into Subframe-2, the time -bin index is rotated by number of n2-frames, i.e., n2*(nl +m*n2) bit interval elapsed since the initial local reference. Similarly for the click events in other sub-frames are also rotated according to its time-bin index and the time interval elapsed since the initial local reference.

[0076] In an example, after the first subframe is detected and the local time reference of the receiving device is adjusted, the new histogram is formed. Subsequently, the second pattern is now identified in the new histogram to detect a subframe other than the first subframe. Once identified, the local time reference is adjusted, and the histogram generation and correlation is again repeated until all the other subframes are identified and corresponding local time reference adjustments are done.

[0077] At step 512, a synchronization complete message is transmitted to the transmitter device upon detection of all the subframes in the histogram and adjustment of the local time reference of the receiving device corresponding to the detection of all the subframes.

[0078] Furthermore, in an example embodiment, the above steps may be implemented by the SPADs which are directly detecting the photons in the quantum channel. However, in some QKD protocols, SPADs (Viz. monitoring SPAD in COW or both SPADs in DPS) may be connected in constructive or destructive arm of a delay interferometer. In such cases, each detected photon may correspond to the state of two consecutive transmit pulses, i.e., two consecutive bits of transmitted frame. This will led to interference of last bit of a subframe with first bit of next subframe and as the subframes are dynamic (rotating), this will make all the bits of the receiving histogram incorrect.

[0079] Accordingly, further in the method 500, one extra bit may be padded at the end of each subframe. The extra padded bit will be the MSB of the same subframe, i.e., after transmitting the last bit of a subframe, the first bit of the same subframe is sent again and then the subsequent subframe is sent. Thus, the received histogram will have an extra bit for each subframes. Accordingly, the first bit of the histogram of the each subframes may be discarded and the rest part may be correlated with the expected pattern for the subframe to measure the latency.

[0080] The foregoing descriptions of exemplary embodiments of the present disclosure have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiment was chosen and described in order to best explain the principles of the disclosure and its practical application, to thereby enable others skilled in the art to best utilize the disclosure and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions, substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but is intended to cover the application or implementation without departing from the spirit or scope of the claims of the present disclosure.

[0081] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.

[0082] While specific language has been used to describe the disclosure, any limitations arising on account of the same are not intended. As would be apparent to a person in the art, various working modifications may be made to the apparatus in order to implement the inventive concept as taught herein.

Claims

AMENDED CLAIMS received by the International Bureau on 04 September 2023 (04.09.2023)WE CLAIM:

1. A method (500) of synchronizing a transmitter device and a receiver device in a communication system comprising steps of: generating (502), by the transmitter device, a set of frames, wherein each frame is of a predefined bit length and comprises a plurality of subframes, and wherein a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern; repeatedly transmitting (504) the set of frames to the receiver device over a communication channel; generating (506), by the receiver device, a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the transmitter device; correlating (508) the first pattern with the histogram to detect a first subframe in the histogram and adjusting the local time reference of the receiving device such that the first pattern comes to the start of histogram; repeatedly generating (510) a new histogram and correlating the second pattern with the new histogram to detect all other subframes in the histogram, wherein on each detection, (a) the local time reference of the receiving device is adjusted such that the second pattern comes to the start of the corresponding detected subframe and (b) the new histogram is generated after the local time reference of the receiving device is adjusted; and transmitting (512) a synchronization complete message to the transmitter device upon detection of all the subframes in the histogram and adjustment of the local time reference of the receiver device corresponding to the detection of all the subframes.

2. The method (500) as claimed in claim 1, wherein each of the other subframes of the first frame comprises a plurality of quantum bits forming the second pattern.

3. The method (500) as claimed in claim 2, wherein at least one subframe in each of the other frames is rotated as per a predefined rotation scheme.

4. The method (500) as claimed in claim 3, wherein each of the other frames in the set of frames comprise a first subframe based on the first pattern and other subframes based on the second pattern, wherein at least one subframe of each of the other subframes is unique.

5. The method (500) as claimed in claim 1, wherein the method (500) further comprises rotating, by the receiving device, the histogram as per a predefined scheme, wherein rotating the histogram comprising of; determining a position of a click event with respect to the subframes of the histograms; and rotating a click time -bin index corresponding to the click event based on time interval elapsed since an initial local time reference.

6. The method (500) as claimed in claim 1, wherein the predefined bit length is equal to nl+n2*m, wherein nl is the number of bits in the first pattern, n2 is the number of bits in the second pattern, and m is the number of subframes in each frame of the set of frames, wherein the value of the m is based on a maximum latency of the communication channel.

7. The method (500) as claimed in claim 1, wherein the method (500) comprises including, by the transmitter device, one bit at a last position of each subframe of the set of frames.

8. A communication system (100, 200), comprising a transmitter device (102) and a receiver device (104), wherein the transmitter device (102) is configured to: generate a set of frames, wherein each frame is of a predefined bit length and comprises a plurality of subframes, and wherein a first frame comprises a first subframe based on a first pattern and other subframes based on a second pattern; and transmit, repeatedly, the set of frames to a receiver device (104) over a communication channel; and the receiver device (104) is configured to:generate a histogram of bit length equal to the predefined bit length, based on the received set of frames, a local time reference, and clock received from the transmitter device (102); correlate the first pattern with the histogram to detect a first subframe in the histogram and adjust a local time reference of the receiving device such that the first pattern comes to the start of histogram; repeatedly generate a new histogram and correlate the second pattern with the new histogram to detect all other subframes in the histogram, wherein on each detection, the local time reference of the receiver device (104) is adjusted such that the second pattern comes to the start of the corresponding detected subframe and the new histogram is generated after the local time reference of the receiver device (104) is adjusted; and transmit a synchronization complete message to the transmitter device (102) upon detection of all the subframes in the histogram and adjustment of the local time reference of the receiving device corresponding to the detection of all the subframes.

9. The communication system (100, 200) as claimed in claim 8, wherein each of the other subframes of the first frame comprises a plurality of quantum bits forming the second pattern.

10. The communication system (100, 200) as claimed in claim 9, wherein at least one subframe in each of the other frames is rotated as per a predefined rotation scheme.

11. The communication system (100, 200) as claimed in claim 10, wherein each of the other frames in the set of frames comprise a first subframe based on the first pattern and other subframes based on the second pattern, wherein at least one subframe of each of the other subframes is unique.

12. The communication system (100, 200) as claimed in claim 8, wherein the receiver device (104) is further configured to rotate the histogram as per a predefined scheme, wherein as per the predefined scheme, the receiver device (104) is configured to; determine a position of a click event with respect to the subframes of the histograms; androtate a click time -bin index corresponding to the click event based on time interval elapsed since the initial local time reference.

13. The communication system (100, 200) as claimed in claim 8, wherein the predefined bit length is equal to nl+n2*m, wherein nl is the number of bits in the first pattern, n2 is the number of bits in the second pattern, and m is the number of subframes in each frame of the set of frames, wherein the value of the m is based on a maximum latency of the communication channel.

14. The communication system (100, 200) as claimed in claim 8, wherein the transmitter device (102) is further configured to include one bit at a last position of each subframe of the set of frames.STATEMENT UNDER ARTICLE 19With reference to the search report ad written opinion of ISA / IN, the applicant has amended claims.

1. The applicant would like to submit that the claims 1-14 have been amended by incorporating the reference numbers, (refer to as filed specification)The applicant undertakes that no new subject matter has been added in claims and the amended claims do not go beyond disclosure of international application as-filed.

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