METHOD FOR THE TIMELY SYNCHRONIZATION OF TWO RECEIVERS FOR QUANTUM COMMUNICATION

Abstract

The invention relates to a method for the temporal synchronization of detection events of two receivers for quantum communication, preferably for quantum key exchange (QKD), comprising the generation of entangled photon pairs in a source, transmission of one individual photon of each entangled photon pair to each receiver, detection of the individual photons at each receiver and storage of the detection events, and the temporal synchronization of the detection events of the two receivers, preferably for quantum key exchange (QKD). Essentially, during the generation of the entangled photon pairs for the temporal synchronization of the detection events of the two receivers, the emission rate of the individual photons of the entangled photon pairs in the source is changed over a time interval Δtmod, and the change in the emission rate of the individual photons is detected at both receivers for the purpose of synchronizing the detection events.

Description

The present invention relates to a method for the temporal synchronization of detection events of two receivers for quantum communication, preferably for quantum key exchange (QKD). In the era of digital communication, the security of data transmissions is becoming increasingly important. Traditional encryption methods are reaching their limits, especially given the rapid advances in computer technology and the development of quantum computers. Against this backdrop, quantum communication, and in particular quantum key exchange (QKD), stands out as a solution that can ensure secure data transmission. Quantum key exchange (QKD) is based on the principles of quantum mechanics, which allow two communication partners to securely create a shared key while detecting eavesdropping attempts. For quantum key exchange (QKD), entangled photon pairs can be used, in which one photon of each entangled photon pair is measured at each receiver, or the prepare-and-measure method, in which one receiver prepares a quantum state and the second receiver measures it. A key element in these methods for generating a shared key is the precise synchronization of the temporal detection events, in which the individual photons are detected by the receivers. This synchronization is essential for establishing a quantum key exchange protocol that can detect manipulation attempts and thus ensure the integrity of the transmitted information. To utilize the quantum correlations of entangled photon pairs, the detection times must be determined with high precision. This is particularly challenging when the two receivers, and consequently their detection systems, are located in different places. The goal of synchronization in quantum communication is to define the narrowest possible time window for both receivers, within which two detection events are considered "simultaneous" or "correlated." This time window should be small because its length is directly proportional to the amount of noise. Fundamentally, this window is limited by the temporal precision of the detectors used and by the ability to keep the receivers' clocks synchronized with each other. A major difficulty in time synchronization lies in finding the initial delay between detection events. This initial delay arises, on the one hand, from the different initialization times of the clocks at the respective receivers (different switch-on times) and, on the other hand, from the fact that the photons travel different distances from the source to the two receivers, as well as from systematic electronic delays caused by the detectors and electronics used. For example, in continuously pumped entangled photon sources, the emission probability of the entangled photon pairs is distributed approximately uniformly over time. To achieve synchronization, known methods detect the entangled photons at both receivers over a specific time period and then compare the detection event times, from which the initial delay can be calculated. However, this calculation is very time-consuming and resource-intensive. Furthermore, a disadvantage arises that high photon losses, i.e., large distances between receivers or poor detectors, result in poor statistical analysis, further increasing the computation time. In such cases, synchronization may not be achieved. A time synchronization method is known from Lohrmann et al., “Classical clock synchronization for quantum communications using the quantum channel”, Applied Optics, Vol. 62, No. 32, 2023. This method uses a separate, low-power, low-frequency synchronization signal that shares the quantum channel and is detected by the single-photon detectors already present in the quantum communication system. From DE 10 2023 106 662 A1, a device for recovering a clock signal from a transmitter is known using transmitted quantum particles. The timestamps of the received particles are used to determine a phase-space position. The clock signal is then recovered based on the time drift of this phase-space position. In known systems, GPS clocks are therefore used at both receivers for synchronization purposes to determine the initial delay, as described, for example, in S. Neumann, A. Buchner, L. Bulla, M. Bohmann, R. Ursin, "Continuous entanglement distribution over a transnational 248 km fiber link," Nature Communications, Volume 13, 6134 (2022). The disadvantages of this approach are the additional costs and increased system complexity due to the extra components required. Based on this problem, the invention aims to provide an improved, more cost-effective and simpler method for synchronizing detection events of two receivers for quantum communication, as well as for the temporal synchronization of generation and detection events of a source and a receiver for quantum communication, preferably for quantum key exchange (QKD). The problem underlying the invention is solved by a method for the temporal synchronization of detection events of two receivers for quantum communication, preferably for quantum key exchange (QKD), according to independent claim 1, wherein advantageous further developments of the method according to the invention are specified in the dependent claims. Accordingly, the invention relates to a method for the temporal synchronization of detection events of two receivers for quantum communication, preferably for quantum key exchange (QKD), comprising: Generation of entangled photon pairs in a source, transmission of one single photon of each entangled photon pair to each receiver, detection of the single photons at each receiver and storage of the detection events; Temporal synchronization of the detection events of the two receivers, preferably for quantum key exchange (QKD); The essential point is that, during the generation of the entangled photon pairs for the temporal synchronization of the detection events of the two receivers, the emission rate of the individual photons of the entangled photon pairs in the source is changed in a time interval Δtmod, and that the change in the emission rate of the individual photons is detected at both receivers for the synchronization of the detection events. The problem underlying the invention is also solved by a method for the temporal synchronization of generation and detection events of a source and a receiver for quantum communication, preferably for quantum key exchange (QKD), according to independent claim 2, wherein advantageous further developments of the method according to the invention are specified in the dependent claims. Accordingly, the invention relates to a method for the temporal synchronization of generation and detection events of a source and a receiver for quantum communication, preferably for quantum key exchange (QKD), comprising: Generation of single photons in a source and storage of the generation events, transmission of the single photons to the receiver, detection of the single photons at the receiver and storage of the detection events; Temporal synchronization of the generation and detection events of the single photons of the source and the receiver, preferably for quantum key exchange (QKD); The essential point is that, during the generation of individual photons for the temporal synchronization of the generation and detection events of the source and the receiver, the emission rate of the individual photons in the source is changed in a time interval Δtmod, and that the change in the emission rate of the individual photons at the receiver is detected for the synchronization of the generation and detection events. It is advantageous for both methods according to the invention that single photons, or the detection and generation events of single photons and photon pairs, are used for temporal synchronization. This means that single-photon detectors can be used in existing, functional quantum key exchange (QKD) systems to perform synchronization using both methods according to the invention, and no further additional components, such as a GPS system or additional transmitting and receiving systems, are necessary. It is advantageous that both methods according to the invention further result in the synchronization being carried out with the components for quantum key exchange (QKD), which already takes into account a delay caused by different components, such as the detectors and the electronics. In this context, emission rate refers to the generation of single photons or photon pairs per second and their transmission to one or two receivers. The number of detection events at the two receivers, preferably averaged over a time interval, is proportional to the emission rate of the source. Additional losses can occur during the transmission and detection of single photons or entangled photon pairs, which may prevent the exact emission rate of the source from being determined at the receivers, but allows for the detection of changes in the emission rate at the receivers. In a preferred further development, it may be provided that the single photons or entangled photon pairs are used not only for synchronization but also for key generation in a quantum key exchange (QKD) procedure. This means that the source generates entangled photon pairs or single photons, and a portion of the entangled photon pairs or single photons is used for synchronization, while another or the same portion is used for key generation. A detection event is understood to be the detection, preferably the point in time of detection, of a single photon or a single photon of an entangled photon pair at the respective receiver. A generation event is understood to be the generation, preferably the point in time of generation, of a single photon at the source. Temporal synchronization of the detection events preferably means that two detection events, preferably the times of the two detection events that originated from two corresponding single photons of a photon pair, are assigned to each other at the two receivers. Preferably temporal synchronization of the generation event and the detection event means that the generation event and the detection event, preferably the times of the generation event and the detection event that concern the same single photon, are assigned to each other at the source and the receiver. The term "individual photons of an entangled photon pair" refers to the fact that each entangled photon pair consists of two individual photons. These individual photons are generally referred to as the signal photon and idler photon of an entangled photon pair. In a preferred embodiment, the time interval Δtmod may have a length of at least 1 ns, preferably at least 1 ms, preferably at least 1 s, and preferably at least the length of the dead time or jitter of the detectors used. The upper limit of the time interval Δtmod may be arbitrarily large, preferably a maximum of 1 h. In a preferred further development, it can be provided that, during the generation of single photons in the source and storage of the generation events, a trigger event for the possible generation of a single photon is stored. Single photons can be generated, for example, by pulsed pumping of a nonlinear element with a pulsed laser beam, where in this case the pulse of the laser beam can represent a trigger event. It is essential that in such a method the generation of a single photon is subject to a certain probability, and thus a single photon is not necessarily generated with every pulse of the laser beam. In a preferred embodiment, it can be provided that single-photon events are detected as detection events at one or the two receivers, preferably in one or more single-photon detectors, preferably in one or more superconducting nanowire single-photon detectors (SNSPDs), and / or one or more avalanche photodiodes (APDs). Advantageously, this allows the synchronization method to be performed with the same detectors used for quantum communication or quantum key exchange (QKD). In a preferred further development, it can be provided that the change in the emission rate occurs through a change in the generation of the entangled photon pairs or the single photons at the source, and / or that the change in the emission rate occurs through a change in the transmission of the entangled photon pairs or the single photons from the source to the one or two receivers. Changing the emission rate through a change in transmission means here that the change in the emission rate for the entangled photon pairs or the single photons occurs through a change in transmission within the source's sphere of influence, i.e., for both receivers in the same way at the source. The advantage of this is that the change in the emission rate at the source or within the source's sphere of influence causes an identical change at both receivers, which can be detected independently at each receiver. In a preferred embodiment, the change in generation at the source can be achieved by modifying a laser beam used to generate single photons or entangled photon pairs, preferably by changing the intensity, and / or activating or deactivating the laser beam, and / or blocking the laser beam or the single photons or entangled photon pairs, preferably by an electronic or optical switch, and / or changing the pulse frequency of a pump laser beam, and / or misaligning one or more components of the laser beam or of generating the single photons or entangled photon pairs, preferably a mirror, preferably a tip-tilt mirror. A key advantage of modifying the laser beam used to generate the single photons or entangled photon pairs is the simple, precise, and rapid nature of changing the emission rate.An advantage when changing the intensity, and / or activating or deactivating the laser beam, and / or blocking the laser beam or individual photons or entangled photon pairs, preferably by an electronic or optical switch, and / or changing the pulse frequency of a pump laser beam, is that the source does not need to be readjusted after synchronization, i.e., it is immediately functional again. In a preferred embodiment, alternatively or simultaneously with the change in the source, the change in transmission from the source can be achieved by altering or misaligning one or more optical components for transmitting the individual photons, preferably an aperture, and / or a mirror, preferably a tip-tilt mirror, and / or an electronic and / or optical switch. It is essential that the change in the emission rate during transmission is carried out in the same way for both receivers. In a preferred embodiment, it can be provided that when the emission rate is changed, at least one decrease and / or at least one increase of the emission rate occurs; preferably, the emission rate decreases to 0 or increases to a value relevant to quantum key exchange, or above or below a value relevant to quantum key exchange. Preferably, the emission rate can be increased by a factor of at least 2, preferably at least 5, and most preferably at least 10 above a value relevant to quantum key exchange for synchronization purposes. Preferably, it can be provided that the emission rate is first increased before a decrease, or that the emission rate is first decreased before an increase. In both cases, the emission rate can be set to a value relevant to quantum key exchange before and / or after this. In a preferred further development, it may be provided that one or both receivers detect the waste or the increase in the emission rate, preferably detect a falling or rising edge of the emission rate, and / or that one or both receivers detect a reduced or increased emission rate after the waste or the increase in the emission rate. In a preferred further development, the detection of changes in the emission rate at each receiver can be achieved through statistical analysis of the detection events, preferably by analyzing the time intervals between two or more detection events, and / or by analyzing the number of detection events in a first time window. Analyzing the number of detection events in the first time window means determining whether there are more, fewer, or no detection events in this first time window, compared to another time window or to the emission rate for quantum key exchange (QKD). The statistical analysis advantageously allows synchronization to occur even with transmission losses to the receivers, since synchronization is not based on a specific single photon. In a preferred further development, it can be provided that, to detect the change in the emission rate, preferably by statistical evaluation of the detection events, a comparison of the number of detection events in several successive time windows Δtdet,ier is performed, where preferably Δtdet,i < Δtmod, and preferably an average of the detection events of the several time windows Δtdet,i is compared with each other. This advantageously improves the synchronization to one or more of the time windows Δtdet,iver. Here, i, with i = 1, 2, represents the first receiver or the second receiver. In a preferred embodiment, the time window Δtdet,i can be selected independently for each receiver, preferably according to a minimum number of detection events N at the respective receiver i with i = 1, 2. A necessary minimum number of detection events may depend on losses during transmission to the individual receivers and their detectors, making it advantageous to select a separate time window for each receiver. In a preferred embodiment, the detection of a change in the emission rate may be deemed to have occurred if at least one, two, three, or more consecutive time windows Δtdet,ie exhibit a changed emission rate, preferably if the number of detection events has changed by at least one, two, three, or more standard deviations of the number of detection events in the one, two, three, or more windows Δtdet,i. Preferably, the change in the emission rate may be to a value previously used for quantum key exchange (QKD). In a preferred further development, it may be provided that, for temporal synchronization, the first of the successive time windows Δtdet,imit changed emission rate is used as the synchronization time for both receivers. In a preferred embodiment, the temporal synchronization at each receiver can be precisely calibrated to the time window Δtdet,i, preferably having a length between 10 ms and 1 ns. More preferably, the time window Δtdet,i corresponds to a multiple of the mean time between two detection events, preferably at least 10 times, preferably at least 100 times. Preferably, the lower limit of the time window Δtdet,i is the dead time of the detectors. Such an embodiment can improve the accuracy of the synchronization to the time window Δtdet,i. In a preferred advanced training, it may be provided that, for synchronization purposes, the times of the detection events at the first and second receivers are aligned based on the detection of changes in the emission rate. This can be achieved, for example, by determining a time difference at one receiver resulting from the detection of changes in the emission rate of the two receivers, and adding or subtracting this time difference from the time of its own detection events. In a preferred further development, the detection of individual photons of entangled photon pairs for temporal synchronization can be performed either dependently or independently of an entanglement property of the entangled photon pairs or a preparation property of the individual photons. The advantage of synchronization independent of the entanglement or preparation property is that no measurement of the entanglement or preparation property is required for synchronization, which generally results in a higher detection rate. The advantage of synchronization dependent on the entanglement or preparation property is that the quantum key exchange (QKD) system does not need to be reconfigured, and only specific detection events can be used for synchronization.For example, the entanglement property of the entangled photon pairs and / or the preparation property of the individual photons can be polarization, time, energy, and / or a torque. Accordingly, for example, in synchronization that depends on the entanglement property, such as with polarization-entangled photon pairs, only detection events of photons measured with a specific polarization can be used. In a preferred further development, it may be provided that after the temporal synchronization, preferably to the time window Δtdet,igenau, a further temporal synchronization takes place, preferably based on an evaluation of the temporal intervals of the detection events. The invention is described in more detail below with reference to the accompanying drawings: Fig. 1 schematic representation of a temporal sequence of detection events at a first receiver; Fig. 2 schematic representation of a temporal sequence of detection events at the first and a second receiver; Fig. 1 shows a schematic representation of a temporal progression of detection events of single photons or single photons of entangled photon pairs at a receiver, where time is plotted along the x-axis and the individual detection events are represented as vertical lines across the temporal progression. In Fig. 1, the dashed line represents the emission rate of the source corresponding to the detection events, which changes over a time interval Δtmodin of the source. The change in the emission rate at the source is reflected by the changed number of detection events at the receiver, as shown in Fig. 1. Due to losses in the transmission of single photons or entangled photon pairs, the number of detection events at the two receivers is not identical to the emission rate, but proportional to the emission rate of the source. Crucially, in the methods according to the invention, the emission rate value itself is not used for synchronization, but only a change in the emission rate is detected. To synchronize the detection events of two receivers, entangled photon pairs are generated in the source, and one individual photon from each entangled photon pair is transmitted to a first receiver and a second receiver. The individual photons of the entangled photon pairs are detected at both receivers. This means that the time of detection of an individual photon at both receivers is stored as a detection event. For synchronization, the emission rate of the individual photons of the entangled photon pairs is varied over the time interval Δtmodin of the source, as shown in Fig. 1. Furthermore, for synchronization at the receivers, this change in the emission rate is detected by the detection events at both receivers, as shown in Figs. 1 and 2. To synchronize the detection events of two receivers, in one embodiment using entangled photon pairs, the individual photons are detected at both receivers. Fig. 2 (top) shows the temporal progression of the detection events for the first receiver and (bottom) for the second receiver. Due to differing clocks and the distance of the two receivers from the source, the detection events of the individual photons of the entangled photon pairs are temporally shifted relative to each other at the two receivers. To synchronize the detection events of the two receivers i with i = 1, 2, in the embodiment shown in Fig. 2, the emission rate of the source is determined for each receiver i in several successive time windows Δtdet,i, for example, by comparing the number of detection events, preferably by statistical evaluation of the detection events. This is done to determine in which time windows Δtdet,ides of the respective receiver i the emission rate of the source changed. By determining the time interval Δt of the first time window Δtdet,i with the changed emission rate, the detection events of both receivers can be synchronized. Here, i with i = 1, 2 represents the first receiver or the second receiver. As shown in Fig. 2, the time windows Δtdet,1 of the first receiver can have a different length than the time windows Δtdet,2 of the second receiver. For synchronization, for example, the times of the detection events at the first receiver and the second receiver can be aligned based on the detection of the change in the emission rate by determining a time difference Δt at the second receiver, which results from the time offset of the detection of the change in the emission rate of the two receivers, and adding this time difference to the time of the second receiver's own detection events for synchronization.

Claims

A method for the temporal synchronization of detection events of two receivers for quantum communication, comprising: - generation of entangled photon pairs in a source, transmission of one individual photon of each entangled photon pair to each receiver, detection of the individual photons at each receiver, and storage of the detection events; - temporal synchronization of the detection events of the two receivers; characterized in that, during the generation of the entangled photon pairs for the temporal synchronization of the detection events of the two receivers, an emission rate of the individual photons of the entangled photon pairs in the source is changed in a time interval Δtmod, and that the change in the emission rate of the individual photons is detected at both receivers for the synchronization of the detection events. A method for the temporal synchronization of generation and detection events of a source and a receiver for quantum communication, comprising: - generation of single photons in the source and storage of the generation events, transmission of the single photons to the receiver, detection of the single photons at the receiver and storage of the detection events; - temporal synchronization of the generation and detection events of the single photons of the source and the receiver; characterized in that, during the generation of the single photons for the temporal synchronization of the generation and detection events of the source and the receiver, an emission rate of the single photons in the source is changed in a time interval Δtmod, and that the change in the emission rate of the single photons at the receiver is detected for the synchronization of the generation and detection events. Method for temporal synchronization according to claim 1 or 2, characterized in that single photon events are detected as detection events at one or the two receivers. Method for temporal synchronization according to one of claims 1 to 3, characterized in that the change in emission rate is effected by a change in the generation of the entangled photon pairs or the single photons in the source, and / or that the change in emission rate is effected by a change in the transmission of the entangled photon pairs or the single photons from the source to the one or the two receivers. Method for temporal synchronization according to one of claims 1 to 4, characterized in that the change in generation in the source is effected by changing a laser beam for generating the single photons or the entangled photon pairs, and / or that the change in transmission from the source is effected by changing or misaligning one or more optical components for transmitting the single photons. Method for temporal synchronization according to one of claims 1 to 5, characterized in that when the emission rate is changed, at least a decrease or at least an increase in the emission rate occurs. Method for temporal synchronization according to claim 6, characterized in that one or both receivers detect the decrease or increase in the emission rate, and / or that one or both receivers detect a decreased or increased emission rate after the decrease or increase in the emission rate. Method for temporal synchronization according to one of claims 1 to 7, characterized in that the detection of the change in the emission rate at each receiver is carried out by statistical evaluation of the detection events, and / or by evaluation of the number of detection events in a first time window. Method for temporal synchronization according to one of claims 1 to 8, characterized in that, for the detection of the change in the emission rate, a comparison of the number of detection events in several successive time windows Δtdet,ier is performed. Method for temporal synchronization according to claim 9, characterized in that the time windows Δtdet,i are selected independently of each other for each receiver, with i = 1, 2 for the respective receiver i. Method for temporal synchronization according to claim 9 or 10, characterized in that the detection of the change in the emission rate is deemed to have occurred if at least one, two, three or more successive time windows Δtdet,ie exhibit a changed emission rate. Method for temporal synchronization according to claim 11, characterized in that for temporal synchronization the first of the successive time windows Δtdet, with changed emission rate is used as the synchronization time for both receivers. Method for temporal synchronization according to one of claims 9 to 12, characterized in that the temporal synchronization at the respective receiver is carried out with high accuracy to the time window Δtdet. Method for temporal synchronization according to one of claims 1 to 13, characterized in that the detection of the individual photons of the entangled photon pairs for temporal synchronization is dependent on or independent of an entanglement property of the entangled photon pairs or preparation property of the individual photons. Method for temporal synchronization according to one of claims 1 to 14, characterized in that a further temporal synchronization takes place after the temporal synchronization.

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