Quantum key distribution system, transmitter, receiver and method

Abstract

An auto-compensated quantum key distribution (QKD) system 400 includes a QKD transmitter 300 and a QKD receiver 100. The QKD transmitter generates an initial light pulse and sends it to the QKD transmitter. At the QKD transmitter, the initial optical pulse train is split (310) to form a reference optical pulse train and a second optical pulse train. The second optical pulse train is phase modulated (314) and time delayed (308) relative to the reference optical pulse train to form a signal optical pulse train. The reference light pulse train and the signal light pulse train are transmitted from the QKD transmitter to the QKD receiver. At the QKD receiver, a portion of the power of the reference light pulse train is tapped (106) to form a trigger light pulse. The remainder of the reference light pulse train is blocked by the VOA (110). The signal light pulse train is forwarded by the VOA for detection at a light detector (114, 116). In response to detecting the trigger light pulse, the QKD receiver is enabled to phase modulate a portion of the train of signal light pulses (112) and detect a final light pulse resulting from the phase modulation and forwarding of the signal pulse.

Description

Technical Field

[0001] The present invention relates to an automatically compensating QKD system, a QKD transmitter for an automatically compensating QKD system, a QKD receiver for an automatically compensating QKD system, and a method of operating an automatically compensating QKD system. Background Art

[0002] Quantum communication systems exploit the possibility of transmitting information encoded in quantum states, prepared in such a way that an eavesdropper between the two communicating parties cannot avoid introducing detectable interference. In optical communications, quantum information is encoded in the physical properties of photons, such as their polarization state or phase.

[0003] Quantum key distribution (QKD) offers a solution to the key distribution problem in symmetric cryptographic systems. In theory, quantum encryption should be applied to the entire message being transmitted, encrypted using a one-time pad. However, this would unacceptably compromise the capacity and latency of the communication channel, as feasible QKD systems operate at speeds of only a few megabits per second and require processing time for the sender and receiver to reach error-free agreement on the final key. In practice, QKD is used only to generate and distribute keys, not to transmit message data. The keys are then used in conjunction with classical cryptographic algorithms to encrypt and decrypt messages transmitted over classical high-capacity communication channels.

[0004] In QKD based on the BB84 protocol, as described, for example, by A. Ruiz-Alba et al., “Practical Quantum Key Distribution based on the BB84 protocol,” Waves, 2011, pp. 4-14, the sender Alice generates a random bit, i.e., a “0” or a “1,” and encodes it in one of two different basis vectors through appropriately chosen physical properties of photons. One of the basis vectors is then used to encode the “0” bit, and the other basis vector is used to encode the “1” bit.

[0005] Since the receiver, Bob, does not know Alice's choice of basis, he measures the basis of the incoming photon by randomly selecting one of the two possible basis vectors. If he uses the same basis vector used by Alice, then he will deterministically measure the correct bit value. In contrast, if he chooses the wrong basis vector, the result of his measurement will be a random projection of the possible values ​​of the encoded basis vector, which has a 50% probability of giving the correct result. After having exchanged long sequences of photons, Alice and Bob compare the basis vectors they used for encoding and measurement, respectively, communicating via a "classical" channel. They only retain the random bits generated and detected with the matching basis vectors, and these bits are said to constitute the "screened key". In an ideal system without noise, imperfections and interference, the screened keys are identical and can be used as private keys.

[0006] In the so-called "plug and play" self-compensating QKD system reported by D Stucki et al., "Quantum key distribution over 67 km with a plug & play system", New J. Phys., Vol. 4, No. 41.1-41.8, 2002, the phase difference between the two pulses traveling from Bob to Alice and back is used to encode the key. In this system, as in the prior art Figure 1 As shown in , the optical pulse generated at Bob is split by a 50 / 50 beam splitter BS and travels through the short and long arms of an interferometer including a phase modulator PM and a 50ns delay line DL to a polarization beam splitter PBS. All optical fibers and optical components at Bob are polarization-maintaining, and the linear polarization is rotated 90° in the short arm, so that both pulses exit from the same output end of the PBS. Therefore, paired pulses are output from Bob and transmitted to Alice. At Alice, the pulses are reflected by the Faraday mirror (experiencing a 90° polarization rotation), attenuated at the variable attenuator ATT, and transmitted back to Bob with orthogonal polarization. To implement the BB84 protocol in this system, the phase modulator PM at Alice applies a phase shift selected from 0 and π or π / 2 or 3π / 2 to the second pulse, and Bob selects the measurement basis by using his PM to apply a phase shift of 0 or π / 2 to the first pulse. Since the pulses transmitted from Alice to Bob now have orthogonal polarizations, each pulse takes a different path through the interferometer at Bob and arrives at the BS at the same time, where they interfere; since each pulse takes the same path from Bob to Alice and back, the system automatically compensates. Depending on the measurement basis selected by Bob, the resulting pulse output from the BS is detected at single-photon detectors SPAD1 and SPAD2.

[0007] At Alice's location, the PM must phase modulate the second pulse of the pair. The time interval between the first and second pulses of the pair is on the order of tens of nanoseconds, so the timing of the drive signal to the PM must be very accurate. At Bob's location, the PM must phase modulate the return pulse from Alice. The time interval between the pulse pairs is approximately 200 ns, so the round-trip time of the pulses must be known to better than this accuracy. The SPADs at Bob's location are gated so that they are set to active when the return pulse arrives, which also requires precise knowledge of the round-trip time of the pulses.

[0008] WO 2022 / 135704 A1 discloses an auto-compensating QKD system in which the statistics of photon detections during an initial phase enable the determination of a time grid of pulse arrivals at a QKD receiver. During a subsequent key distribution phase, detections are expected to separate quantum key signal detections at a single-photon avalanche detector (SPAD) from noise-induced detections at the SPAD based on their time differences from the time grid estimated during the initial phase. Summary of the Invention

[0009] An object is to provide an improved automatically compensating QKD system. Another object is to provide an improved QKD transmitter for an automatically compensating QKD system. Another object is to provide an improved QKD receiver for an automatically compensating QKD system. Another object is to provide an improved method of operating an automatically compensating QKD system.

[0010] In one aspect, a QKD receiver for an automatically compensated quantum key distribution (QKD) system is provided. The QKD receiver includes an optical source configured to generate an initial optical pulse train, an optical input / output port, an optical tap, a first optical detector, a variable optical attenuator (VOA), an optical phase modulator, a second optical detector, a third optical detector, and processing circuitry. The optical input / output port is configured to output the initial optical pulse train and to receive a reference optical pulse train followed by a signal optical pulse train. The optical tap is configured to tap a portion of the power of the reference optical pulse train to form trigger optical pulses. The first optical detector is configured to output a trigger signal in response to detecting at least one of the trigger optical pulses. The VOA is disposed after the optical tap. The VOA is configurable to block or divert the optical pulse train. The VOA has a first state and a second state. In the first state, the VOA is configured to block the reference optical pulse train, and in the second state, the VOA is configured to divert optical pulses from the signal optical pulse train. The optical phase modulator is configured to phase modulate a portion of the signal optical pulse train. The second and third optical detectors are configured to detect a final optical pulse generated by the signal optical pulse train after being forwarded by the VOA and phase modulated by the optical phase modulator. The processing circuit is configured to receive the trigger signal and generate at least one control signal in response. The at least one control signal is configured to cause the VOA to be in an open state. The at least one control signal is further configured to enable the second and third optical detectors to detect the final optical pulse.

[0011] The QKD receiver enables continuous time alignment of the operation of the QKD receiver and the QKD transmitter during quantum key distribution within an auto-compensated QKD system by separately processing the reference optical pulse train and the signal optical pulse train. Thus, the QKD receiver mitigates detection timing uncertainty caused by thermomechanical fluctuations in the transmission line between the QKD transmitter and the QKD receiver. This advantageously enables adaptive operation of the QKD receiver without requiring the key transmission to be stopped while time alignment is reestablished, resulting in high availability of the QKD system.

[0012] In an embodiment, the light source includes a laser and an optical interferometer. The laser is configured to generate a seed light pulse train of seed light pulses. The optical interferometer includes a beam splitter and a polarization beam splitter connected by a short arm and a long arm. The long arm includes a delay line. The optical phase modulator is arranged in one of the long arm or the short arm. The beam splitter is configured to split the power of the seed pulse into the short arm and the long arm. The VOA is arranged between the polarization beam splitter and the optical tap, and in the second state, the VOA is configured to forward the signal light pulse train to the polarization beam splitter. The optical phase modulator is configured to phase modulate a portion of the signal light pulse train in the one arm. The short arm is configured to impart a first polarization to the seed pulse and the long arm is configured to impart a second orthogonal polarization to the seed pulse, so as to form the initial light pulse train comprising paired orthogonally polarized light pulses. The VOA advantageously blocks the reference optical pulse train and forwards the signal optical pulse train to the optical interferometer for forming the final optical pulse for detection by the second optical detector and the third optical detector.

[0013] In an embodiment, the light source includes a laser and an optical interferometer. The laser is configured to generate a seed light pulse train of seed light pulses. The optical interferometer includes a beam splitter and a polarization beam splitter connected by a short arm and a long arm. The long arm includes a delay line. The optical phase modulator is disposed in one of the long arm or the short arm. The beam splitter is configured to split the power of the seed pulse into the short arm and the long arm. The optical phase modulator is configured to phase modulate a portion of the signal light pulse train in the one arm. The VOA is disposed in front of the second optical detector. In a second state, the VOA is configured to forward the final light pulse to the second optical detector. The QKD receiver further includes a second VOA. The second VOA is disposed in front of the third optical detector. The second VOA is configurable to block or divert the light pulse train. The second VOA has a first state and a second state. In the first state, the second VOA is configured to block the reference light pulse train. In the second state, the second VOA is configured to forward the final light pulse to the third optical detector. The VOA and the second VOA advantageously block the reference optical pulse train and forward a final optical pulse formed as a result of the signal optical pulse train transmitting through the optical interferometer to the second optical detector and the third optical detector.

[0014] In an embodiment, the QKD receiver further includes a polarization controller between the input / output port and the polarization beam splitter, a first optical tap before the VOA, and a second optical tap before the second VOA. The polarization controller is operable to control the polarization of the reference optical pulse train so that at least the first optical pulse of the reference optical pulse train is equally split into a first reference optical pulse and a second reference optical pulse by the beam splitter of the interferometer. The beam splitter is configured to route the first reference optical pulse toward the first optical tap and the second reference optical pulse toward the second optical tap.

[0015] In an embodiment, the at least one control signal is configured to cause the VOA to be configured in the second state. The at least one control signal is further configured to enable the second light detector and the third light detector to detect the final light pulse at a predetermined time after receiving the trigger signal. This advantageously ensures that the second light detector and the third light detector are enabled only when the final light pulse is expected to arrive, which can mitigate detection errors caused by background noise.

[0016] In an embodiment, the predetermined time is the time delay between the reference optical pulse train and the signal optical pulse train. This advantageously ensures that the second and third optical detectors are enabled only when the signal optical pulse train is expected to arrive, which can mitigate detection errors due to background noise. The QKD receiver can use the predefined delay to enable gating functionality for precise timing of the second and third optical detectors. This advantageously results in the second and third detectors detecting only the final optical pulse, resulting in significantly enhanced noise reduction.

[0017] Corresponding embodiments and advantages also apply to the automatic compensation quantum key distribution system and method described below.

[0018] In one aspect, a QKD transmitter for an automatically compensated QKD system is provided. The QKD transmitter includes an input / output port configured to receive an initial optical pulse train, a first optical beam splitter, a photodetector, an optical storage line, a second optical beam splitter, an optical routing device, an optical phase modulator, a polarization rotator, and a processing circuit. The first optical beam splitter is configured to split the power of the initial optical pulse train to form a first optical pulse train and a second optical pulse train. The photodetector is configured to detect the optical pulses of the first optical pulse train and output corresponding detection signals. The optical storage line is configured to receive the second optical pulse train. The second optical beam splitter is configured to receive the second optical pulse train output from the storage line. The second optical beam splitter is configured to split the power of the second optical pulse train to form a reference optical pulse train and a third optical pulse train. The optical routing device is configured to route the reference optical pulse train back to the input / output port. The optical phase modulator is configured to phase modulate the optical pulses of the third optical pulse train. The polarization rotator is configured to reflect the third optical pulse train back through the optical phase modulator and the storage line to form a signal optical pulse train. The processing circuit is configured to cause the optical phase modulator to phase modulate the optical pulses of the third optical pulse train in response to receiving the detection signal. The storage line is configured to introduce a time delay τ between the reference optical pulse train and the signal optical pulse train. The input / output port is further configured to output the reference optical pulse train followed by the signal optical pulse train.

[0019] The QKD transmitter enables continuous time alignment of the operation of the QKD transmitter and a QKD receiver during quantum key distribution within an auto-compensated QKD system by forming a reference optical pulse train and a signal optical pulse train. Formation of the reference optical pulse train and the signal optical pulse train mitigates uncertainty in detection timing at the QKD receiver caused by thermomechanical fluctuations in the propagation line between the QKD transmitter and the QKD receiver. This advantageously enables adaptive operation of the QKD receiver without requiring the key transmission to be stopped while time alignment is reestablished, thereby resulting in high availability of the QKD system.

[0020] In an embodiment, the optical storage line is thermally stabilized. This advantageously enables the time delay introduced between the reference optical pulse train and the signal optical pulse train to remain constant and known.

[0021] Corresponding embodiments and advantages also apply to the automatic compensation quantum key distribution system and method described below.

[0022] On the one hand, an automatic compensation quantum key distribution (QKD) system is provided, which includes a QKD transmitter and a QKD receiver.

[0023] The QKD receiver includes an optical source configured to generate an initial optical pulse train, an optical input / output port, an optical tap, a first optical detector, a variable optical attenuator (VOA), an optical phase modulator, a second optical detector, a third optical detector, and processing circuitry. The optical input / output port is configured to output the initial optical pulse train and to receive a reference optical pulse train followed by a signal optical pulse train. The optical tap is configured to tap a portion of the power of the reference optical pulse train to form trigger optical pulses. The first optical detector is configured to output a trigger signal in response to detecting at least one of the trigger optical pulses. The VOA is positioned after the optical tap. The VOA is configurable to block or divert the optical pulse train. The VOA has a first state and a second state. In the first state, the VOA is configured to block the reference optical pulse train, and in the second state, the VOA is configured to divert the signal optical pulse train. The optical phase modulator is configured to phase modulate a portion of the signal optical pulse train. The second and third optical detectors are configured to detect a final optical pulse generated by the signal optical pulse train after being forwarded by the VOA and phase modulated by the optical phase modulator. The processing circuit is configured to receive the trigger signal and generate at least one control signal in response. The at least one control signal is configured to cause the VOA to be in an open state. The at least one control signal is further configured to enable the second and third optical detectors to detect the final optical pulse.

[0024] The QKD transmitter includes an input / output port configured to receive the initial optical pulse train, a first optical beam splitter, a photodetector, an optical storage line, a second optical beam splitter, an optical routing device, an optical phase modulator, a polarization rotator, and processing circuitry. The first optical beam splitter is configured to split the power of the initial optical pulse train to form a first optical pulse train and a second optical pulse train. The photodetector is configured to detect the optical pulses of the first optical pulse train and output corresponding detection signals. The optical storage line is configured to receive the second optical pulse train. The second optical beam splitter is configured to receive the second optical pulse train output from the storage line. The second optical beam splitter is configured to split the power of the second optical pulse train to form a reference optical pulse train and a third optical pulse train. The optical routing device is configured to route the reference optical pulse train back to the input / output port. The optical phase modulator is configured to phase modulate the optical pulses of the third optical pulse train. The polarization rotator is arranged to reflect the third optical pulse train back through the optical phase modulator and the storage line to form a signal optical pulse train. The processing circuit is configured to generate a control signal in response to receiving the detection signal. The control signal is configured to cause the optical phase modulator to phase modulate the optical pulses of the third optical pulse train. The storage line is configured to introduce a time delay τ between the reference optical pulse train and the signal optical pulse train. The input / output port is further configured to output the reference optical pulse train followed by the signal optical pulse train.

[0025] The QKD receiver is configured to send the initial optical pulse train to the QKD transmitter and receive the reference optical pulse train followed by the signal optical pulse train from the QKD transmitter. The QKD transmitter is configured to receive the initial optical pulse train from the QKD receiver and send the reference optical pulse train and the signal optical pulse train to the QKD receiver.

[0026] The auto-compensating QKD system enables continuous time alignment of the operation of the QKD receiver and the QKD transmitter during quantum key distribution by using the same initial optical pulse train to form the reference optical pulse train and signal optical pulse train of the quantum bit for key distribution. Therefore, the QKD system mitigates the uncertainty of detection timing caused by thermomechanical fluctuations in the propagation line between the QKD transmitter and the QKD receiver. This advantageously enables adaptive operation without the need to stop key generation when re-establishing time alignment, resulting in high availability of the QKD system. Therefore, the QKD system can be used in network scenarios where continuous key generation is required.

[0027] In an embodiment, the initial optical pulse train has an initial optical pulse power. The first beam splitter at the QKD transmitter is configured with a first splitting ratio for forming the second optical pulse train. The second beam splitter at the QKD transmitter is configured with a second splitting ratio for forming the third optical pulse train. The first splitting ratio and the second splitting ratio are combined so that the optical pulses of the signal optical pulse train include, on average, less than one photon per pulse. This ensures that the quantum bit is transmitted using a signal optical pulse train comprising single-photon pulses, meaning that the quantum bit cannot be intercepted by an eavesdropper.

[0028] In an embodiment, at the QKD receiver, the predetermined time after receiving the trigger signal is at least equal to the time delay τ introduced by the storage line at the QKD transmitter. This advantageously ensures that the second and third photodetectors are enabled only at times when the signal light pulse train is expected to arrive, which can mitigate detection errors due to background noise. The QKD receiver can use the predefined delay to enable gating functionality for precise timing of the second and third photodetectors. This advantageously results in the second and third detectors detecting only the final light pulse, resulting in significantly enhanced noise reduction.

[0029] Corresponding embodiments and advantages also apply to the method described below.

[0030] In one aspect, a method for operating an auto-compensating QKD system including a quantum key distribution (QKD) receiver and a QKD transmitter is provided. The method comprises the following steps: transmitting an initial optical pulse train from the QKD receiver to the QKD transmitter. At the QKD transmitter, splitting the initial optical pulse train to form a reference optical pulse train and a second optical pulse train. At the QKD transmitter, phase modulating and time delaying the second optical pulse train relative to the reference optical pulse train to form a signal optical pulse train. The reference optical pulse train and the signal optical pulse train are transmitted from the QKD transmitter to the QKD receiver. At the QKD receiver, a portion of the power of the reference optical pulse train is tapped to form a trigger optical pulse. At the QKD receiver, after forming the trigger optical pulse, the reference optical pulse train is blocked and the signal optical pulse train to be detected is then forwarded. In response to detecting the trigger optical pulse, phase modulation of a portion of the signal optical pulse train is enabled, and detection of a final optical pulse resulting from the phase modulation and forwarding of the signal optical pulse train is enabled.

[0031] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1This is a schematic diagram of an existing "plug-and-play" auto-compensation QKD system; Figure 2 and Figure 3 is a schematic diagram of an embodiment of a QKD receiver for an automatically compensated QKD system; Figure 4 is a schematic diagram of an embodiment of a QKD transmitter for an automatically compensated QKD system; Figure 5 and Figure 6 is a schematic diagram of an embodiment of an automatically compensated QKD system; and Figure 7 is a flow chart illustrating an embodiment of the method steps. DETAILED DESCRIPTION

[0033] In the different embodiments, the same reference numerals are used for corresponding features.

[0034] refer to Figure 2 In one embodiment, a quantum key distribution (QKD) receiver 100 is provided for automatically compensating a QKD system. The QKD receiver includes a light source 102, an optical input / output port 104, an optical tap 106, a first optical detector 108, a variable optical attenuator (VOA) 110, an optical phase modulator 112, a second optical detector 114, a third optical detector 116, and a processing circuit 118.

[0035] The light source 102 is configured to generate an initial light pulse train. A light pulse train is a sequence of light pulses with a defined structure, such as a defined number of pulses, time intervals between pulses, pulses arranged in pairs, time intervals between pulse pairs, and the like.

[0036] Light source 102 includes a laser 120, an optical circulator 130, and an interferometer 132. The interferometer includes a 50:50 optical beam splitter BS 122 and a polarization beam splitter PBS 124, connected by a short arm 128 and a long arm 134. The long arm 134 includes a delay line 126. A phase modulator 112 is disposed in one of the interferometer arms; in this example, the phase modulator is disposed in the long arm, but it could alternatively be disposed in the short arm. The laser generates a seed optical pulse train, which is routed to the base station via optical circulator 130 (in the case of a fiber-based implementation). The seed optical pulses are power-split at the base station, with the corresponding split optical pulses output into the short arm 128 and the long arm 134 of the interferometer. The optical pulses traveling through the long arm are delayed by the delay line, thereby being delayed relative to the optical pulses traveling through the short arm. The short arm is configured to impart a first polarization to the seed pulses, and the long arm is configured to impart a second, orthogonal polarization to the seed pulses. For example, the short arm includes a polarization-maintaining fiber of a first polarization, and the long arm includes a polarization-maintaining fiber of a second orthogonal polarization incident on the PBS. PBS 124 combines the pulses from the short arm with correspondingly delayed orthogonally polarized light pulses from the long arm, so that an initial optical pulse train of paired orthogonally polarized light pulses is formed at the output of PBS 124; the pulses in each pair are separated in time by a time delay added by delay line 126.

[0037] The optical input / output port 104 is used to output the initial optical pulse train and to receive the reference optical pulse train followed by the signal optical pulse train. Like the initial optical pulse train, the signal optical pulse train includes pairs of orthogonally polarized optical pulses, but the pulses of the pulse pairs of the signal optical pulse train have orthogonal polarizations to the pulses of the pulse pairs of the initial optical pulse train.

[0038] The pulses of the received signal optical pulse train are very weak (less than one photon on average). Therefore, the second and third optical detectors need to be gated, that is, enabled only to detect when a signal pulse is expected to be received, to minimize noise errors. The second and third optical detectors are turned off during the round-trip time between sending the initial optical pulse train to the QKD transmitter and receiving the signal optical pulse train to avoid being blinded (unable to detect pulses) when the signal optical pulse train arrives. This is achieved by using a reference optical pulse train to generate control signals for the second and third optical detectors, as shown below.

[0039] The optical tap 106 is configured to tap a portion of the power of the reference optical pulse train to form trigger optical pulses. The first optical detector 108 is configured to output a trigger signal in response to detecting at least one of the trigger optical pulses.

[0040] VOA 110 is disposed between PBS 124 and optical tap 106. The VOA can be configured to block or divert the optical pulse train. The VOA has a first state, in which it is configured to block the reference optical pulse train, and a second state, in which it is configured to divert the signal optical pulse train to the PBS. The VOA 110 also has a third state, in which the VOA 110 is configured to divert the initial optical pulse train to output 104.

[0041] Optical phase modulator 112 is configured to phase modulate a portion of the signal optical pulse train in the arm in which optical phase modulator 112 is located (the long arm in this example). The first and second optical pulses of each pulse pair in the signal optical pulse train received from VOA 110 at PBS 124 are routed to the long and short arms of the interferometer, respectively, based on their polarization. Because the optical pulses of the signal optical pulse train have orthogonal polarizations to those of the initial optical pulse train, the optical pulses of the signal pulse train are routed to the interferometer arm opposite the arm through which the corresponding initial optical pulse traveled. Therefore, both optical pulses of each optical pulse pair pass through delay line 126 (either during the formation of the initial optical pulse train or on their way back within the signal optical pulse train). As a result, they return together to BS 122, where they interfere depending on their respective phases and form a final optical pulse (for each optical pulse pair). The final optical pulse is routed to either second optical detector 114 or third optical detector 116, depending on the relative phases of the optical pulses from which the final optical pulse was formed.

[0042] The second optical detector 114 and the third optical detector 116 are configured to detect final optical pulses generated by the signal optical pulse train that has undergone forwarding by the VOA 110 and phase modulation by the optical phase modulator 112 during transmission back through the interferometer 132 .

[0043] The second photodetector 114 and the third photodetector 116 are single-photon detectors, such as single-photon avalanche diodes (SPADs) or photomultiplier tubes, which have a single-photon detection efficiency of less than 1. Blocking the transmission of the reference optical pulse train by the VOA protects these sensitive photodetectors from the optical pulses of the reference optical pulse train (which are generally stronger than the optical pulses of the signal optical pulse train), thereby ensuring that the second and third photodetectors are not in a blind state when the signal optical pulse train arrives.

[0044] Processing circuit 118 is configured to receive the trigger signal and, in response, generate at least one control signal. The at least one control signal is configured to cause the VOA to be configured in an open state. The at least one control signal is further configured to enable the second photodetector and the third photodetector to detect the final optical pulse. Thus, QKD receiver 100 is operable to ensure that only signal optical pulses are forwarded by VOA 110, and that second photodetector 114 and third photodetector 116 are gated to detect the final pulse generated by the forwarded signal optical pulse.

[0045] In an embodiment, laser 120 is configured to generate optical pulses at a 5 MHz pulse rate, i.e., a pulse train with a pulse-to-pulse interval of approximately 200 ns. The seed optical pulse train contains 480 optical pulses, so the initial optical pulse train (and thus each of the first and second optical pulse trains) contains 480 optical pulse pairs. Delay line 126 has a length of approximately 10 m to introduce a delay of approximately 50 ns, so that the optical pulses of the pulse pairs are separated by approximately 50 ns, and the pulse pairs are separated by approximately 200 ns.

[0046] In an embodiment, the at least one control signal is configured to enable the second light detector 114 and the third light detector 116 to detect the final light pulse a predetermined time after receiving the trigger signal.

[0047] In an embodiment, the predetermined time is a time delay between the reference optical pulse train and the signal optical pulse train.

[0048] refer to Figure 3 In one embodiment, a quantum key distribution (QKD) receiver 200 for automatically compensating a QKD system is provided. The QKD receiver 200 includes a light source 102, an optical input / output port 104, a first optical tap 206, a second optical tap 208, a polarization controller PC 204, a first optical detector 108, a first VOA 110, a second VOA 210, an optical phase modulator 112, a second optical detector 114, a third optical detector 116, and a processing circuit 202.

[0049] The light source 102 is configured to generate an initial optical pulse train of paired orthogonally polarized light pulses. The light source 102 includes a laser 120, an optical circulator 130, and an interferometer 132, as described above with reference to FIG. Figure 2 As stated.

[0050] The optical input / output port 104 is used to output an initial optical pulse train and to receive a reference optical pulse train followed by a signal optical pulse train.

[0051] The pulses of the received signal optical pulse train are very weak (less than one photon on average). Therefore, the second and third optical detectors need to be gated, that is, enabled only to detect when a signal pulse is expected to be received, to minimize noise errors. The second and third optical detectors are turned off during the round-trip time between sending the initial optical pulse train to the QKD transmitter and receiving the signal optical pulse train to avoid being blinded (unable to detect pulses) when the signal optical pulse train arrives. This is achieved by using a reference optical pulse train to generate control signals for the second and third optical detectors, as shown below.

[0052] PC 204 is disposed between input / output port 104 and PBS 124. The PC is operable to forward the optical pulse train without performing polarization control, or to perform polarization control on the optical pulse train. The PC has a first state, in which the PC is configured to perform polarization control on the reference optical pulse train, so that at least the first optical pulse of the reference optical pulse train is equally split into a first reference optical pulse and a second reference optical pulse by a beam splitter of the interferometer. The beam splitter is configured to route the first reference optical pulse toward first optical tap 206 and to route the second reference optical pulse train toward second optical tap 208. The PC has a second state, in which the PC is configured to forward the signal optical pulse train without performing polarization control.

[0053] The first optical tap 206 is configured to tap a portion of the power of the first reference optical pulse train to form a first triggering optical pulse, which is routed to the first optical detector 108. The second optical tap 208 is configured to tap a portion of the power of the second reference optical pulse train to form a second triggering optical pulse, which is also routed to the first optical detector 108.

[0054] The first light detector 108 is configured to output a trigger signal in response to detecting at least one of the first trigger light pulse and the second trigger light pulse.

[0055] Optical phase modulator 112 is configured to phase-modulate a portion of the signal optical pulse train in the long arm. The first and second optical pulses of each pulse pair received at PBS 124 are routed to the long and short arms of the interferometer, respectively, depending on their polarization. That is, they are routed to the interferometer arm opposite the arm in which they propagated during the formation of the initial optical pulse train, so that they return together to BS 122, where they interfere depending on their respective phases and form a final optical pulse (for each pulse pair). The final optical pulse is routed toward either first optical tap 206 or second optical tap 208, depending on its phase.

[0056] The first VOA 110 is disposed between the first optical tap 206 and the second optical detector 114. The second VOA 210 is disposed between the second optical tap 208 and the third optical detector 116. Each VOA 110, 210 is configurable to block or redirect optical pulses. Each VOA has a first state, in which it is configured to block the reference optical pulse train, and a second state, in which it is configured to forward the final optical pulse to the corresponding optical detector 114, 116.

[0057] The second optical detector 114 and the third optical detector 116 are configured to detect final optical pulses generated by the signal optical pulse train subjected to phase modulation by the optical phase modulator 112 during transmission back through the interferometer 132 and forwarded by the first VOA 110 or the second VOA 210 , respectively.

[0058] The second photodetector 114 and the third photodetector 116 are single-photon avalanche diode (SPAD) photon counters, which have a single-photon detection efficiency of less than 1. Blocking the transmission of the reference optical pulse train by the VOAs 110 and 210 protects these sensitive photodetectors from the optical pulses of the reference optical pulse train (which are generally stronger than the optical pulses of the signal optical pulse train), thereby ensuring that the second and third photodetectors are not in a blind state when the signal optical pulse train arrives.

[0059] Processing circuit 202 is configured to receive the trigger signal and generate at least one control signal in response. The at least one control signal is configured to cause PC 204 to be in the second state. The at least one control signal is further configured to cause first VOA 110 and second VOA 210 to be in the open state. The at least one control signal is further configured to enable the second photodetector and the third photodetector to detect the final light pulse.

[0060] Thus, the QKD receiver 200 is operable to ensure that only signal light pulses are forwarded by the VOAs 110, 210, and the second and third light detectors 114, 116 are gated to detect the final pulses generated by the forwarded signal light pulses.

[0061] In an embodiment, laser 120 is configured to generate optical pulses at a 5 MHz pulse rate, i.e., a pulse train with a pulse-to-pulse interval of approximately 200 ns. The seed optical pulse train contains 480 optical pulses, so the initial optical pulse train (and thus each of the first and second optical pulse trains) contains 480 optical pulse pairs. Delay line 126 has a length of approximately 10 m to introduce a delay of approximately 50 ns, so that the optical pulses of the pulse pairs are separated by approximately 50 ns, and the pulse pairs are separated by approximately 200 ns.

[0062] In an embodiment, the at least one control signal is configured to enable the second light detector 114 and the third light detector 116 to detect the final light pulse a predetermined time after receiving the trigger signal.

[0063] In an embodiment, the predetermined time is a time delay between the reference optical pulse train and the signal optical pulse train.

[0064] refer to Figure 4 In one embodiment, a QKD transmitter 300 for an automatically compensated QKD system is provided. The QKD transmitter 300 includes an input / output port 302, a first optical beam splitter 304, a photodetector 320, an optical storage line 308, a second optical beam splitter 310, an optical routing device 312, an optical phase modulator PM 314, a polarization rotator 316, and a processing circuit 318.

[0065] Input / output port 302 is configured to receive an initial optical pulse train. First optical beam splitter 304 is configured to split the power of the initial optical pulse train to form a first optical pulse train and a second optical pulse train. Photodetector 320 is configured to detect the optical pulses of the first optical pulse train and output corresponding detection signals to a processing circuit. Optical storage line 308 is configured to receive the second optical pulse train and apply a time delay τ to the second optical pulse train.

[0066] The second optical beam splitter 310 is configured to receive the second optical pulse train output from the storage line. The second optical beam splitter 310 is configured to split the power of the second optical pulse train to form a reference optical pulse train and a third optical pulse train. The second optical beam splitter is configured to route the reference optical pulse train toward the optical routing device 312, which is configured to route the reference optical pulse train back to the input / output port 302.

[0067] The second optical beam splitter is configured to route the third optical pulse train to the optical phase modulator 314 , which is configured to phase-modulate the optical pulses of the third optical pulse train.

[0068] Polarization rotation mirror 316 (e.g., Faraday mirror FM) is arranged to reflect the third optical pulse train back toward the optical phase modulator, causing the third optical pulse train to return through the optical phase modulator and the storage line to form a signal optical pulse train. Input / output port 302 is also configured to output a reference optical pulse train followed by a signal optical pulse train.

[0069] The processing circuit 318 is configured to cause the optical phase modulator 314 to phase-modulate the optical pulses of the third optical pulse train in response to receiving the detection signal.

[0070] In an embodiment, the initial optical pulse train is a pair of orthogonally polarized optical pulses. The control signal generated by processing circuit 318 is configured to cause PM 314 to phase modulate one optical pulse in each pair of orthogonally polarized optical pulses in the third optical pulse train. By phase modulating only one optical pulse in a pulse pair, the QKD transmitter uses the phase difference between the optical pulses in the pulse pair to prepare the original key qubit.

[0071] In an embodiment, QKD transmitter 300 further includes a variable optical attenuator (VOA) 306 disposed between BS 304 and PM 314 (e.g., between BS and storage line 308). The VOA can be configured to apply attenuation to the third optical pulse train. This can enable the signal optical pulse train to be formed from single-photon pulses.

[0072] In an embodiment, the second optical beam splitter 310 is configured to split the optical power of the second optical pulse train into the reference optical pulse train within a range of 50% to 90%. Therefore, a linear photodetector can be used at the QKD receiver to detect the reference optical pulse train.

[0073] In an embodiment, the optical storage line 308 is thermally stable.

[0074] refer to Figure 5 , an embodiment provides an automatic compensation quantum key distribution QKD system 400, which includes the QKD transmitter 300 and the QKD receiver 100 as described above.

[0075] The QKD receiver 100 is configured to send an initial optical pulse train to the QKD transmitter 300. The QKD receiver 100 is configured to receive a reference optical pulse train followed by a signal optical pulse train from the QKD transmitter.

[0076] The QKD transmitter 300 is configured to receive an initial optical pulse train from a QKD receiver and transmit a reference optical pulse train and a signal optical pulse train to the QKD receiver.

[0077] In an embodiment, the initial optical pulse train has an initial optical pulse power. The first beam splitter 304 at the QKD transmitter is configured with a first splitting ratio for forming the second optical pulse train. The second beam splitter 310 at the QKD transmitter is configured with a second splitting ratio for forming the third optical pulse train. The first splitting ratio and the second splitting ratio are combined to cause the optical pulses of the signal optical pulse train to include, on average, less than one photon per pulse.

[0078] In an embodiment, at the QKD receiver, the predetermined time after receipt of the trigger signal is at least equal to the time delay τ introduced by the storage line at the QKD transmitter.

[0079] In this embodiment, the predetermined time is the time delay between the reference optical pulse train and the signal optical pulse train. This delay is primarily caused by the storage line 308 at the QKD transmitter, as the signal optical pulse train passes through this storage line 308 twice, while the reference optical pulse train passes through this storage line 308 only once. However, strictly speaking, the time delay is caused by the difference in optical path length between the reference optical pulse train traveling from the second optical splitter 310 to the first optical splitter 304 and the third optical pulse train / signal optical pulse train returning from the second optical splitter 310 to the first optical splitter 304. The reference optical pulse train path length travels from the second optical splitter to the first optical splitter 304 via the optical circulator 312 and the connecting optical fiber / waveguide. The third optical pulse train / signal optical pulse train path length travels from the second optical splitter 310 through the PM 314 to the FM 316, and from the FM through the PM, the second optical splitter, the storage line, and the connecting optical fiber / waveguide back to the first optical splitter.

[0080] refer to Figure 6 , an embodiment provides an automatic compensation QKD system 500, which includes the QKD transmitter 300 and the QKD receiver 200 as described above.

[0081] The QKD receiver 200 is configured to send an initial optical pulse train to the QKD transmitter 300. The QKD receiver 100 is configured to receive a reference optical pulse train followed by a signal optical pulse train from the QKD transmitter.

[0082] The QKD transmitter 300 is configured to receive an initial optical pulse train from a QKD receiver and transmit a reference optical pulse train and a signal optical pulse train to the QKD receiver.

[0083] refer to Figure 5 Another embodiment provides an auto-compensating QKD system 500 for optical QKD scenarios requiring less than one photon per pulse. The envelope of the pulse represents a qubit. System 500 enables time synchronization between two communicating parties (QKD transmitter 300 and QKD receiver 100).

[0084] Continuous synchronization of pulse generation and detection at the QKD receiver with phase modulation at the QKD transmitter is achieved by using most of the power of the initial optical pulse train transmitted by the QKD receiver to form a timing reference (reference optical pulse train), while the remaining power of the initial optical pulse train is used to form the signal optical pulse train used for key distribution. Thus, continuous synchronization is achieved by detecting the reference optical pulse train at the QKD receiver, thereby avoiding the need for more complex optimization of QKD quality over a regular basis, as required by the prior art.

[0085] The pulse pairs of the initial optical pulse train generated at the QKD receiver 100 pass through the optical fiber connecting the QKD transmitter and the QKD receiver. They then pass through the 90:10 beam splitter BS 304 at the QKD transmitter. The phase modulation of the optical phase modulator PM 314 at the QKD transmitter is triggered by the detection signal of the linear photodiode PD 320, thereby indicating the signal of the 90% output from BS 304. For the timing of the phase modulation at the QKD transmitter, the same method as described in WO 2022 / 135704 A1 is used. The optical pulses of the 10% output leaving BS 304 pass through the storage line SL 310 and arrive at the 90:10 BS 310, which splits the original signal into a reference optical pulse train and a third optical pulse train to be formed into a signal optical pulse train carrying quantum bits for key distribution.

[0086] The reference optical pulse train is guided back to the QKD receiver unit via circulator 312 without any manipulation. When the reference optical pulse train returns to the QKD receiver, 1% of its optical power is tapped off using a 1:99 beam splitter 106 and sent to a linear detector PD before polarization beam splitter PBS 124 can affect its path. This eliminates the need for any polarization manipulation of the reference optical pulse train.

[0087] Although the reference optical pulse train is attenuated by 20 dB at the 1:99 beam splitter, the optical power of the tapped reference optical pulse is high enough to be detected by the linear photodetector PD 108. In this way, the signal optical pulse train will suffer minimal attenuation, thereby maximizing the SNR (signal-to-noise ratio) for its detection at the single-photon avalanche detectors SPADs 114, 116. While the 1:99 beam splitter allows most of the reference optical pulse train to travel toward the SPAD, the VOA 110 is set to suppress the remaining 99% of the power of the reference optical pulse train to avoid damaging the SPAD.

[0088] When the tapped reference light pulse is detected at PD 108, a detection electrical signal is output. Using the first detection signal as the first trigger signal, the processing circuit sets the precise timing for strobing the SPAD for detection. To do this, we only need to measure the optical length of the QKD transmitter once during the installation process. Starting the timer from the first trigger signal from PD 108, we can generate a control signal to enable the SPAD to detect strobing only when the signal light pulse train arrives. Since we can generate and detect a reference light pulse train for each signal light pulse train sent from the QKD transmitter to the QKD receiver in this way, any time fluctuations in the propagation line between the QKD transmitter and the QKD receiver are automatically compensated. In this way, we can solve the above-mentioned problem of realigning the detection strobe with a solution that is resistant to environmental fluctuations.

[0089] Regarding the signal light pulse train, this is processed in the same manner as described in WO 2022 / 135704 A1 to form a quantum bit.

[0090] The overall purpose of a QKD system is to support the security of networked communication systems and services, regardless of the time and location of sensitive data transmission. Some communication scenarios impose the requirement for continuous key generation, and thus QKD systems face difficulties caused by harsh environmental conditions that cause fluctuations in the propagation time of light pulses each time a pulse train passes through the system. The aforementioned QKD system enables real-time fluctuation compensation without the need to repeatedly initiate a new initialization phase, thus enabling continuous key generation.

[0091] Detection is expected to have a significant impact on reducing noise in the system, and this is a fundamental issue with current detection methods in QKD devices. In the aforementioned QKD system, the QKD receiver uses a predefined delay to enable the precisely timed gating functionality of the SPAD. This allows only the signal light pulse train to be detected, resulting in significantly enhanced noise reduction.

[0092] refer to Figure 7 , an embodiment provides a method 600 of operating an auto-compensating QKD system including a QKD receiver and a QKD transmitter.

[0093] The method includes transmitting 602 an initial optical pulse train from a QKD receiver to a QKD transmitter. At the QKD transmitter, the initial optical pulse train is split 604 to form a reference optical pulse train and a second optical pulse train. At the QKD transmitter, the second optical pulse train is phase modulated and time delayed 606 to form a signal optical pulse train; the second optical pulse train is time delayed relative to the reference optical pulse train. The reference optical pulse train and the signal optical pulse train are then transmitted 608 from the QKD transmitter to the QKD receiver. At the QKD receiver, a portion of the power of the reference optical pulse train is tapped 610 to form a trigger optical pulse. At the QKD receiver, the reference optical pulse train is blocked after forming the trigger optical pulse, and the signal optical pulse train is then forwarded for detection. In response to detecting the trigger optical pulse 612, phase modulation of a portion of the signal optical pulse train is enabled at the QKD receiver, and detection of a final optical pulse resulting from the phase modulation and forwarding of the signal optical pulse train is also enabled at the QKD receiver.

Claims

1. A QKD receiver (100, 200) for an automatically compensated quantum key distribution (QKD) system, the QKD receiver comprising: - a light source (102) configured to generate an initial train of light pulses; - an optical input / output port (104) for outputting the initial optical pulse train and for receiving a reference optical pulse train followed by a signal optical pulse train; - an optical tap (106, 206, 208) configured to tap a portion of the power of the reference optical pulse train to form a trigger optical pulse; - a first light detector (108) configured to output a trigger signal in response to detecting at least one of the trigger light pulses; - a variable optical attenuator (VOA) (110, 210) disposed after the optical tap, the VOA being configurable to block or divert the optical pulse train, wherein the VOA has a first state and a second state, in the first state the VOA is configured to block the reference optical pulse train, and in the second state the VOA is configured to divert optical pulses from the signal optical pulse train; - an optical phase modulator (112), configured to phase modulate a portion of the signal optical pulse train; - a second optical detector (114) and a third optical detector (116) configured to detect a final optical pulse generated by the signal optical pulse train subjected to forwarding by the VOA and phase modulation by the optical phase modulator; and - a processing circuit (118, 202) configured to receive the trigger signal and generate at least one control signal in response, the at least one control signal configured to cause the VOA to be configured in an open state, and the at least one control signal configured to enable the second light detector and the third light detector to detect a final light pulse.

2. The QKD receiver according to claim 1, wherein The light source comprises: - a laser (120) configured to generate a seed light pulse train of seed light pulses; and - Optical interferometer (121), comprising: A beam splitter (122) and a polarization beam splitter (124) connected via a short arm and a long arm, the long arm comprising a delay line (126) and the optical phase modulator (112), wherein the beam splitter is configured to split the seed pulse power into the short arm and the long arm, wherein the VOA (110) is arranged between the polarization beam splitter and the optical tap, and in the second state, the VOA is configured to forward the signal optical pulse train to the polarization beam splitter, wherein the optical phase modulator is configured to phase modulate a portion of the signal optical pulse train in the long arm, and wherein the short arm is configured to impart a first polarization to a seed pulse and the long arm is configured to impart a second orthogonal polarization to the seed pulse to form the initial optical pulse train comprising paired orthogonally polarized optical pulses.

3. The QKD receiver according to claim 1, wherein The light source comprises: - a laser (120) configured to generate a seed light pulse train of seed light pulses; and - an optical interferometer comprising a beam splitter (122) and a polarization beam splitter (124) connected by a short arm and a long arm, the long arm comprising a delay line (126) and the optical phase modulator (112), wherein the beam splitter is configured to split the seed pulse power into the short arm and the long arm, wherein the optical phase modulator is configured to phase-modulate a portion of the signal optical pulse train in the long arm, and the optical phase modulator is further configured to phase-modulate the seed pulse in the long arm to form the initial optical pulse train comprising a pair of orthogonally polarized optical pulses, and The VOA (110) is arranged in front of the second optical detector (114), and in the second state, the VOA is configured to forward the final optical pulse to the second optical detector, and the QKD receiver further includes a second VOA (210) arranged in front of the third optical detector (116), and the second VOA can be configured to block or forward the optical pulse train, wherein the second VOA has a first state and a second state, in the first state, the second VOA is configured to block the reference optical pulse train, and in the second state, the second VOA is configured to forward the final optical pulse to the third optical detector.

4. The QKD receiver according to claim 3, further comprising a polarization controller (204) between the input / output port (104) and the polarization beam splitter (124), a first optical tap (206) before the VOA (110), and a second optical tap (208) before the second VOA (210), wherein The polarization controller is operable to control the polarization of the reference optical pulse train so that the first optical pulse of the reference optical pulse train is equally power-split into a first replica optical pulse and a second replica optical pulse by the beam splitter of the interferometer, and wherein the beam splitter (122) is configured to route the first replica optical pulse toward the first optical tap and to route the second replica optical pulse toward the second optical tap.

5. A QKD receiver according to any one of the preceding claims, wherein: The at least one control signal is configured to cause the VOA (110, 210) to be configured in the second state and to enable the second optical detector (114) and the third optical detector (116) to detect a final optical pulse at a predetermined time after receiving the trigger signal.

6. The QKD receiver according to claim 5, wherein: The predetermined time is a time delay between the reference optical pulse train and the signal optical pulse train.

7. A QKD transmitter (300) for an automatically compensated QKD system, the QKD transmitter comprising: An input / output port (302) configured to receive an initial optical pulse train; A first optical beam splitter (304) is configured to split the power of the initial optical pulse train to form a first optical pulse train and a second optical pulse train; a photodetector (320) configured to detect light pulses of the first light pulse train and output a corresponding detection signal; an optical storage line (308) configured to receive the second optical pulse train; A second optical beam splitter (310) is configured to receive the second optical pulse train output from the storage line and to perform power splitting on the second optical pulse train to form a reference optical pulse train and a third optical pulse train; an optical routing device (312) configured to route the reference optical pulse train back to the input / output port; an optical phase modulator (314) configured to perform phase modulation on the optical pulses of the third optical pulse train; a polarization rotation mirror (316) arranged to reflect the third optical pulse train back through the optical phase modulator and the storage line to form a signal optical pulse train; as well as A processing circuit (318) is configured to cause the optical phase modulator to phase-modulate the optical pulses of the third optical pulse train in response to receiving the detection signal, wherein the storage line is configured to introduce a time delay τ between the reference optical pulse train and the signal optical pulse train, and wherein the input / output port is further configured to output the reference optical pulse train followed by the signal optical pulse train.

8. The QKD transmitter according to any one of claims 7, wherein: The optical storage line (308) is thermally stable.

9. An automatic compensation quantum key distribution (QKD) system (400, 500), comprising: A QKD transmitter (300) as claimed in any one of claims 7 to 8; as well as A QKD receiver (100, 200) as claimed in any one of claims 1 to 6, wherein the QKD receiver is configured to send the initial optical pulse train to the QKD transmitter and receive the reference optical pulse train followed by the signal optical pulse train from the QKD transmitter, and wherein the QKD transmitter is configured to receive the initial optical pulse train from the QKD receiver and send the reference optical pulse train and the signal optical pulse train to the QKD receiver.

10. The system according to claim 9, wherein: The initial optical pulse train has an initial optical pulse power, the first beam splitter (304) at the QKD transmitter is configured with a first splitting ratio for forming the second optical pulse train, and the second beam splitter (310) at the QKD transmitter is configured with a second splitting ratio for forming the third optical pulse train, and wherein the first splitting ratio and the second splitting ratio are combined so that the optical pulses of the signal optical pulse train include less than one photon per pulse on average.

11. The system according to any one of claims 9 or 10, wherein: At the QKD receiver, the predetermined time after receipt of the trigger signal is at least equal to the time delay τ introduced by the storage line at the QKD transmitter.

12. A method (600) of operating an auto-compensating quantum key distribution (QKD) system comprising a QKD receiver and a QKD transmitter, the method comprising the steps of: sending (602) an initial optical pulse train from the QKD receiver to the QKD transmitter; At the QKD transmitter, splitting (604) the initial optical pulse train to form a reference optical pulse train and a second optical pulse train; At the QKD transmitter, phase modulating and time delaying the second optical pulse train relative to the reference optical pulse train (606) to form a signal optical pulse train; transmitting (608) the reference optical pulse train and the signal optical pulse train from the QKD transmitter to the QKD receiver; At the QKD receiver, tapping (610) a portion of the power of the reference optical pulse train to form a triggering optical pulse; At the QKD receiver, blocking the reference optical pulse train after forming the trigger optical pulse, and then forwarding the signal optical pulse train to be detected; In response to detecting a trigger optical pulse (612), phase modulation of a portion of the signal optical pulse train is enabled, and detection of a final optical pulse resulting from the phase modulation and forwarding of the signal pulse train is enabled.

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