Patent Application
20240007183
The present invention relates to a quantum key distribution technology.
In a quantum key distribution (QKD) system attracting attention in recent years, a transmission device gives quantum information to a weak optical pulse and transmits the optical pulse to a reception device. The reception device generally detects the optical pulse by using an avalanche photodiode (APD). In the detection by the APD, the timing of a gate signal applied to the APD is adjusted in accordance with the arrival of the optical pulse.
An optical fiber is used as an optical transmission line between the transmission device and the reception device. In a general optical fiber, a temperature change occurs in the fiber due to radiation of sunlight or the like, thereby causing a change in transmission time of an optical pulse. Therefore, the time required for an optical pulse to arrive at the reception device after leaving the transmission device is not constant, and it is necessary to adjust the timing of a gate signal for each case.
Non Patent Literature 1 discloses a method for enabling stable signal detection by evaluating signal quality when a reception device receives an optical pulse with an index such as a quantum bit error rate (QBER), and adjusting the timing of a gate signal so as to minimize the index.
Non Patent Literature 1: A. R. Dixon, “High speed prototype quantum key distribution system and long term field trial”, OPTICS EXPRESS, Vol. 23, No. 6, pp. 7583-7592, 2015.
In a case where a code such as quantum information is not assigned to an optical pulse, a timing adjustment method disclosed in Non Patent Literature 1 is not applicable because signal quality cannot be measured. In addition, a measuring instrument for measuring signal quality is required in the reception device, which hinders downsizing of the reception device.
An object of the present invention is to provide a technology that enables simplification of a reception device in a QKD system.
A measurement device according to an aspect of the present invention includes: a generation unit that generates first and second optical pulse trains in which a time interval between optical pulses is constant; a transmission unit that transmits the first optical pulse train to a device to be measured; a reception unit that receives the first optical pulse train returning from the device to be measured; a measurement unit that measures the number of optical pulses transmitted by the transmission unit from when the transmission unit transmits a first optical pulse included in the first optical pulse train to when the reception unit receives the first optical pulse; an identification unit that identifies a phase amount corresponding to a phase difference between the received first optical pulse train and the second optical pulse train; and a calculation unit that calculates a propagation delay amount between the measurement device and the device to be measured on the basis of the measured number of optical pulses and the identified phase amount.
According to the present invention, a technology that can simplify a reception device in a QKD system is provided.
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
The transmission device 110 includes a quantum signal transmitter 112, a measurement device 114, and an optical component 116.
The quantum signal transmitter 112 transmits an optical signal as a quantum signal to the reception device 120 in order to generate an encryption key shared between the transmission device 110 and the reception device 120.
The measurement device 114 measures a propagation delay amount between the transmission device 110 (measurement device 114) and the reception device 120. The propagation delay amount between the transmission device 110 and the reception device 120 indicates the time until an optical signal exits from the transmission device 110 and arrives at the reception device 120. The reception device 120 is also referred to as a device to be measured.
The optical component 116 multiplexes an optical signal from the quantum signal transmitter 112 and an optical signal from the measurement device 114 into the optical transmission line 130. Furthermore, the optical component 116 guides the optical signal emitted from the measurement device 114 and returning from the reception device 120 to the measurement device 114. Examples of the optical component 116 include an optical switch, a polarizing beam splitter, a wavelength division multiplexing (WDM) coupler, and the like.
The reception device 120 includes a quantum signal receiver 122, an optical component 124, and a loopback 126.
The quantum signal receiver 122 receives an optical signal as a quantum signal from the transmission device 110 in order to generate an encryption key shared between the transmission device 110 and the reception device 120.
The optical component 124 guides an optical signal from the quantum signal transmitter 112 to the quantum signal receiver 122 and directs an optical signal from the measurement device 114 to the loopback 126. Examples of the optical component 124 include an optical switch, a polarizing beam splitter, a WDM coupler, and the like.
The loopback 126 loops back an optical signal. An optical signal emitted from the measurement device 114 propagates through the optical transmission line 130 to the reception device 120, turns back at the loopback 126 of the reception device 120, and propagates through the optical transmission line 130 to the measurement device 114.
As illustrated in
The light source 202 generates optical pulses at regular intervals. In one example, the light source 202 may generate linearly polarized optical pulses. The light source 202 can be, but is not limited to, a laser diode. The light source 202 is driven by a control signal applied by the control circuit 208. The control signal is, for example, a voltage signal having a predetermined frequency (e.g., 1 GHz), whereby the light source 202 generates optical pulses at time intervals (e.g., 1 nanosecond intervals) corresponding to the above frequency. The light source 202 outputs an optical pulse train in which the time interval between pulses is constant.
The modulator 204 modulates the phase of each optical pulse included in the optical pulse train output from the light source 202. Specifically, the control circuit 208 randomly selects the phase shift of either 0 or n for each optical pulse, and the modulator 204 applies the phase shift selected by the control circuit 208 to the optical pulse. The control circuit 208 records phase modulation data indicating the phase shift applied to each optical pulse.
The attenuator 206 attenuates the optical pulses such that the average number of photons per pulse is less than 1. The attenuator 206 transmits the attenuated optical pulse to the optical transmission line 130.
The quantum signal receiver 122 includes an interferometer 252, detectors 262 and 264, and a control circuit 266. The interferometer 252 is an asymmetric Mach-Zehnder interferometer including a beam splitter 254 and a coupler 256. The optical transmission line 130 is connected to an input port of the beam splitter 254. A first output port of the beam splitter 254 is connected to a first input port of the coupler 256 by a waveguide 258, and a second output port of the beam splitter 254 is connected to a second input port of the coupler 256 by a waveguide 260. The optical path length of the waveguide 260 is longer than the optical path length of the waveguide 258. A first output port of the coupler 256 is connected to the detector 262, and a second output port of the coupler 256 is connected to the detector 264.
The beam splitter 254 splits each optical pulse of the optical pulse train incident on the quantum signal receiver 122, and guides a part of the optical pulse to the waveguide 258 and the remaining part of the optical pulse to the waveguide 260. Typically, the splitting ratio of the beam splitter 254 is 1:1. The waveguide 260 delays an optical pulse by a predetermined delay time with respect to an optical pulse moving through the waveguide 258. The predetermined delay time is equal to the time interval between the pulses. The coupler 256 multiplexes an optical pulse train moving through the waveguide 258 and an optical pulse train moving through the waveguide 260. Adjacent optical pulses interfere at the coupler 256, and as a result of the interference, a photon is detected at either of the detectors 262 and 264. For example, if the phase difference between adjacent pulses is 0, the detector 262 detects a photon, and if the phase difference between adjacent pulses is n, the detector 264 detects a photon.
The detectors 262 and 264 may be single photon detectors, such as avalanche photodiodes (APD). When APD is used, gate operation may be applied to reduce after-pulse noise. The gate operation causes the APD to go into Geiger mode for a short time in accordance with the time at which detection of a photon is predicted. The control circuit 266 applies a gate signal for operating in Geiger mode to the APD. In a case where an optical fiber is used as the optical transmission line 130, a propagation delay amount changes due to a factor such as a temperature change occurring in the fiber. Therefore, it is necessary to adjust the timing of the gate signal according to the propagation delay amount.
The transmission device 110 and the reception device 120 generate an encryption key by the following procedure.
First, after receiving an optical pulse train, the quantum signal receiver 122 notifies the quantum signal transmitter 112 of the photon detection time. Subsequently, the quantum signal transmitter 112 finds out which one of the detectors 262 and 264 has detected a photon from the notified photon detection time and phase modulation data. In the quantum signal transmitter 112 and the quantum signal receiver 122, an event in which a photon is detected by the detector 262 is set to a bit “1”, and an event in which a photon is detected by the detector 264 is set to a bit “0”.
Through the above operation, the quantum signal transmitter 112 and the quantum signal receiver 122 obtain the same bit string. Information disclosed to the outside is only the photon detection time, and bit information is not disclosed. Therefore, the transmission device 110 and the reception device 120 use the bit string as an encryption key.
As illustrated in
The generation unit 302 generates three optical pulse trains having a constant time interval between optical pulses. For example, the generation unit 302 includes a light source and two beam splitters. The light source generates optical pulses at regular intervals. A time interval at which the light source generates optical pulses is denoted as TP. The time interval TP may be, for example, 1 nanosecond. As the light source, for example, an active mode locking laser can be used. An active mode locking laser is a laser that repeatedly generates and outputs a pulse by synchronizing an optical transmission distance between mirrors and a modulation frequency of an optical pulse and forcibly modulating light. The optical pulse train generated by the light source is divided into three by two beam splitters, thereby generating three optical pulse trains with a constant time interval between the optical pulses. The optical pulse trains are synchronously emitted from the generation unit 302. Each of the optical pulse trains is supplied to the change unit 304, the measurement unit 310, and the evaluation unit 314. The generation unit 302 may use the light source 202 illustrated in
Hereinafter, an optical pulse train headed toward the change unit 304 is also referred to as a target optical pulse train, and an optical pulse included in a target optical pulse train is also referred to as a target optical pulse. An optical pulse train headed toward the evaluation unit 314 is also referred to as a reference optical pulse train, and an optical pulse included in a reference optical pulse train is also referred to as a reference optical pulse.
The change unit 304 changes at least one of the target optical pulses included in the target optical pulse train so that the target optical pulse is identifiable. For example, the change unit 304 changes a feature of at least one target optical pulse. Examples of the feature include polarization, amplitude, intensity, and pulse width. In the present embodiment, the change unit 304 changes one target optical pulse so that the target optical pulse is identifiable, and sends a notification signal indicating execution of the change processing to the measurement unit 310.
In one example, the change unit 304 may be an encoder that encodes information (“0” or “1”) into optical pulses. In a scheme of encoding information into a polarization state of an optical pulse, the change unit 304 modulates polarization of the target optical pulse. For example, in a case where the generation unit 302 generates an S-polarized optical pulse, the change unit 304 modulates polarization of the target optical pulse to P-polarization.
In another example, the change unit 304 adjusts the amplitude of the target optical pulse. For example, when the generation unit 302 generates an optical pulse having a first amplitude, the change unit 304 adjusts the amplitude of the target optical pulse to a second amplitude. The second amplitude may be larger or smaller than the first amplitude as long as the second amplitude is different from the first amplitude.
The transmission unit 306 transmits the target optical pulse train that has passed through the change unit 304 to the reception device 120. The target optical pulse train emitted from the measurement device 114 propagates through the optical transmission line 130 to the reception device 120, turns back at the loopback 126 of the reception device 120, and propagates through the optical transmission line 130 to the measurement device 114. The reception unit 308 receives the target optical pulse train returning from the reception device 120, and guides the received target optical pulse train to the measurement unit 310 and the adjustment unit 312.
The measurement unit 310 measures the number of optical pulses transmitted by the transmission unit 306 from when the transmission unit 306 transmits a certain target optical pulse to when the reception unit 308 receives the target optical pulse. For example, the measurement unit 310 includes a photodetector, and uses the photodetector to count the optical pulses incident from the generation unit 302 from the time when the transmission unit 306 transmits the identifiable target optical pulse (target optical pulse whose feature has been changed) to the time when the reception unit 308 receives the identifiable target optical pulse. The measurement unit 310 may recognize the time when a notification signal is received from the change unit 304 as the time when the transmission unit 306 transmits the identifiable target optical pulse. The measurement unit 310 may identify the identifiable target optical pulse, and recognize the time when the identifiable target optical pulse is received as the time when the reception unit 308 receives the identifiable target optical pulse. In an example in which the change unit 304 modulates polarization of the target optical pulse to P-polarization, the measurement unit 310 may further include a polarizing beam splitter and another photodetector. The polarizing beam splitter is provided to selectively guide optical pulses of P-polarization to the other photodetector. The measurement unit 310 may recognize the time when the other photodetector detects the optical pulse as the time when the reception unit 308 receives the identifiable target optical pulse.
The adjustment unit 312 and the evaluation unit 314 correspond to an identification unit 315 that identifies a phase amount corresponding to the phase difference between a target optical pulse train received by the reception unit 308 and a reference optical pulse train. The phase amount is a time within a range from 0 seconds to the time TP. When the phase difference is represented as θ and the phase amount is represented as TD, TD=(θ/2n) TP.
The adjustment unit 312 adjusts the phase of a target optical pulse train received by the reception unit 308. The evaluation unit 314 evaluates the correlation between the reference optical pulse train and the target optical pulse train whose phase has been adjusted. The evaluation unit 314 sends a control signal for controlling the phase shift amount to the adjustment unit 312, and the adjustment unit 312 adjusts the phase of the target optical pulse train according to the phase shift amount indicated by the control signal. The evaluation unit 314 evaluates the correlation while sequentially changing the phase shift amount. The evaluation unit 314 identifies the phase shift amount with the highest correlation as the phase amount. In other words, the evaluation unit 314 identifies, as the phase amount, the phase shift amount when the reference optical pulse train is synchronized with the target optical pulse train whose phase has been adjusted.
In one example, the evaluation unit 314 may include a coupler that multiplexes the reference optical pulse train and the target optical pulse train whose phase has been adjusted, and a measuring instrument that measures the amplitude of the optical pulse train obtained by the coupler. The highest correlation occurs when the phase of the target optical pulse train whose phase has been adjusted coincides with the phase of the reference optical pulse train. In this case, as illustrated in
In one example, the adjustment unit 312 includes a variable delay line and delays the target optical pulse train received by the reception unit 308 using the variable delay line. The evaluation unit 314 sends a control signal for controlling the delay time to the adjustment unit 312, and the adjustment unit 312 delays the target optical pulse train by the delay time indicated by the control signal. The evaluation unit 314 evaluates the correlation while sequentially changing the delay time. The evaluation unit 314 identifies the delay time with the highest correlation. The evaluation unit 314 calculates the phase amount from the identified delay time. Specifically, the evaluation unit 314 obtains the phase amount by subtracting the identified delay time from the time TP.
The calculation unit 316 calculates a propagation delay amount between the measurement device 114 and the reception device 120 on the basis of the number of optical pulses measured by the measurement unit 310 and the phase amount identified by the evaluation unit 314. For example, a propagation delay amount T between the measurement device 114 and the reception device 120 is calculated by the following Formula (1).
T=(NP·TP+TD)/2 (1)
Here, NP represents the number of optical pulses measured by the measurement unit 310, TP represents the interval between optical pulses, and TD represents the phase amount identified by the evaluation unit 314. 2T represents the time required for the optical pulse to travel back and forth between the measurement device 114 and the reception device 120.
The notification unit 318 notifies the reception device 120 of the propagation delay amount calculated by the calculation unit 316. The notification is sent on a classical channel. The control circuit 266 (
In one example, the quantum signal transmitter 112 and the measurement device 114 may emit optical pulse trains of different polarizations. For example, the quantum signal transmitter 112 emits an S-polarized optical pulse train, and the measurement device 114 emits a P-polarized optical pulse train. In this case, a polarizing beam splitter can be used as the optical components 116 and 124.
In one example, the quantum signal transmitter 112 and the measurement device 114 may emit optical pulse trains at different wavelengths. In other words, a first wavelength used when the target optical pulse train is transmitted to the reception device 120 may be different from a second wavelength used when the quantum signal is transmitted to the reception device 120. In this case, a WDM coupler can be used as the optical components 116 and 124. The calculation unit 316 corrects the propagation delay amount on the basis of the difference between the first wavelength and the second wavelength. For example, the calculation unit 316 calculates a propagation delay amount T′ related to the second wavelength by the following Formula (2).
Here, ΔT represents a propagation delay difference, D represents wavelength dispersion of an optical fiber used as the optical transmission line 130, Δλ represents a wavelength difference, and L represents a distance of the optical transmission line 130. The wavelength difference Δλ is a value obtained by subtracting the second wavelength from the first wavelength. The wavelength dispersion D of the optical fiber is a value determined by a total value of dispersion amounts caused by the structure and material of the optical fiber. As a unit of the wavelength dispersion D, “ps/nm/km” is usually used. This refers to a group delay time difference (ps) generated between components having wavelengths different by 1 nm when an optical wave propagates by 1 km. For example, in a single mode fiber which is one type of generally used optical fibers, it is known that the wavelength dispersion D at a wavelength around 1.55 μm having the smallest propagation loss is about 17 ps/nm/km. The notification unit 318 notifies the reception device 120 of the propagation delay amount T′ obtained by the correction.
In step S501 of
In step S502, the change unit 304 changes a target optical pulse included in a target optical pulse train that is an optical pulse train output from the generation unit 302, so that the target optical pulse is identifiable. For example, the change unit 304 adjusts the amplitude of the target optical pulse. For example, the generation unit 302 generates optical pulses having a first amplitude, and the change unit 304 adjusts one of the optical pulses to a second amplitude. The change unit 304 notifies the measurement unit 310 that the amplitude has been adjusted.
In step S503, the transmission unit 306 transmits the target optical pulse train that has passed through the change unit 304 to the reception device 120. In step S504, the reception unit 308 receives the target optical pulse train returning from the reception device 120.
In step S505, the measurement unit 310 measures the number of optical pulses transmitted by the transmission unit 306 from when the transmission unit 306 transmits the target optical pulse to when the reception unit 308 receives the target optical pulse. For example, the measurement unit 310 starts counting the optical pulses incident from the generation unit 302 when receiving the notification from the change unit 304, and ends the counting when detecting the target optical pulse having the second amplitude.
In step S506, the identification unit 315 identifies the phase amount corresponding to the phase difference between the target optical pulse train received by the reception unit 308 and the reference optical pulse train. For example, the adjustment unit 312 adjusts the phase of the target optical pulse train received by the reception unit 308 according to the phase shift amount indicated by a control signal received from the evaluation unit 314. The evaluation unit 314 evaluates the correlation between the reference optical pulse train and the target optical pulse train whose phase has been adjusted. The evaluation unit 314 identifies the phase shift amount with the highest correlation as the phase amount.
In step S507, the calculation unit 316 calculates the propagation delay amount between the measurement device 114 and the reception device 120 on the basis of the number of optical pulses obtained by the measurement unit 310 and the phase amount identified by the identification unit 315. For example, the calculation unit 316 calculates the propagation delay amount according to the above-described Formula (1).
In step S508, the notification unit 318 notifies the reception device 120 of the propagation delay amount calculated by the calculation unit 316.
The optical circuit 608 includes the generation unit 302, the change unit 304, the transmission unit 306, the reception unit 308, the measurement unit 310, the adjustment unit 312, and the evaluation unit 314 illustrated in
The processor 602 includes a general-purpose circuit such as a central processing unit (CPU). The RAM 604 is used as a working memory by the processor 602. The RAM 604 includes a volatile memory such as a synchronous dynamic random access memory (SDRAM). The program memory 606 stores programs executed by the processor 602, such as a propagation delay amount measurement program. The programs include computer-executable instructions. For example, a read-only memory (ROM) is used as the program memory 606.
The processor 602 develops the programs stored in the program memory 606 in the RAM 604, and interprets and executes the programs. When executed by the processor 602, the propagation delay amount measurement program causes the processor 602 to perform control of the optical circuit 608 and the communication interface 610, the processing described in regard to the calculation unit 316, and the like. The control of the optical circuit 608 includes generation of a control signal for controlling the phase shift amount.
The communication interface 610 is an interface for communicating with an external device on a classical channel. The communication interface 610 is used to transmit a signal indicating a propagation delay amount to the reception device 120.
At least a part of the processing including the control of the optical circuit 608 and the communication interface 610 and the processing described in regard to the calculation unit 316 may be implemented by a dedicated circuit such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC).
As described above, in the measurement device 114, the generation unit 302 generates the target optical pulse train and the reference optical pulse train, the transmission unit 306 transmits the target optical pulse train to the reception device 120, the reception unit 308 receives the target optical pulse train returning from the reception device 120, the measurement unit 310 measures the number of optical pulses transmitted by the transmission unit 306 from when the transmission unit 306 transmits a certain optical pulse to when the reception unit 308 receives the optical pulse, the identification unit identifies the phase amount corresponding to the phase difference between the target optical pulse train received by the reception unit 308 and the reference optical pulse train, and the calculation unit 316 calculates the propagation delay amount between the measurement device 114 and the reception device 120 on the basis of the measured number of optical pulses and the identified phase amount. According to this configuration, the transmission device 110 can measure the propagation delay amount. Therefore, it is not necessary to provide a device such as a measuring instrument in the reception device 120. As a result, the reception device 120 can be simplified. This enables downsizing of the reception device 120. In a P-to-MP configuration in which the transmission device 110 is connected to a plurality of reception devices, simplification of the reception device is effective in reducing the cost of the entire system.
The measurement device 114 may change at least one target optical pulse so that the target optical pulse is identifiable, and transmit a target optical pulse train including the identifiably changed target optical pulse to the reception device 120. For example, the measurement device 114 may encode information into at least one target optical pulse. The measurement device 114 may adjust the amplitude of at least one target optical pulse. By identifiably changing at least one target optical pulse, it becomes easy to determine a period for measuring the number of optical pulses.
The measurement device 114 adjusts the phase of the received target optical pulse train and evaluates the correlation between the target optical pulse train whose phase has been adjusted and the reference optical pulse train. The measurement device 114 identifies the phase shift amount with the highest correlation as the phase amount. As a result, the propagation delay amount can be measured accurately.
The measurement device 114 notifies the reception device 120 of the calculated propagation delay amount. As a result, the timing of the gate signal applied to the APD can be adjusted appropriately in the reception device 120.
In a case where a first wavelength used when the target optical pulse train is transmitted to the reception device 120 is different from a second wavelength used when a quantum signal is transmitted to the reception device 120, the measurement device 114 corrects the propagation delay amount on the basis of the difference between the first wavelength and the second wavelength. As a result, it is possible to measure the time required for the quantum signal to leave the transmission device 110 and arrive at the reception device 120 accurately.
The present invention is not limited to the example described above.
In the example illustrated in
In the example illustrated in
The change unit 304 may be eliminated. In this case, for example, the measurement unit 310 may count the optical pulses incident from the generation unit 302 from the timing when the generation unit 302 starts outputting the optical pulse train to the timing when the measurement unit 310 receives the first optical pulse via the reception unit 308.
Note that the present invention is not limited to the foregoing embodiments and various modifications can be made in the implementation stage without departing from the gist of the invention. In addition, the embodiments may be implemented in appropriate combination, and in this case, combined effects can be obtained. Furthermore, the above embodiments include various inventions, and various inventions can be extracted by combinations selected from the plurality of disclosed components. For example, in a case where the problem can be solved and the effects can be obtained even if some components are deleted from all the components described in the embodiments, a configuration from which the components are eliminated can be extracted as an invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/042705 | 11/17/2020 | WO |
