CN113972982B - Phase encoding device for quantum key distribution system stabilization - Google Patents
Phase encoding device for quantum key distribution system stabilization Download PDFInfo
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Abstract
A phase coding device for quantum key distribution system stabilization comprises a quantum pulse light source QPS, a double pulse light source DPS, a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM and a single photon detector SPD, wherein a second port and a third port of the first beam splitter BS1 are respectively connected with a second port and a first port of the second beam splitter BS2 through a long arm and a short arm optical fiber to form an unequal arm Mach-Zehnder interferometer; the phase modulator PM is positioned in a long-arm optical fiber of the Mach-Zehnder interferometer, and the invention also provides two phase encoding methods. Compared with the prior art, the phase difference caused by the optical path difference of the long arm and the short arm of the interferometer and the phase drift caused by environmental change can be compensated in real time, stable phase coding is realized, and the stability of the system is improved; the quantum key distribution process does not need to be interrupted, so that the quantum key distribution efficiency is improved; the invention does not need to add an additional pulse phase randomization module, thereby improving the safety of the system.
Description
Technical Field
The invention relates to the technical field of quantum phase encoding, in particular to a stable phase encoding device and a stable phase control method for a quantum key distribution system.
Background
The BB84 protocol quantum key distribution system is mature and is already put into practical use. The encoding modes commonly used by the BB84 protocol are polarization encoding and phase encoding. Due to the birefringence effect of the optical fiber channel, the photon polarization state is easy to change randomly under the influence of the environment, and the polarization encoding mode is not stable enough, so that phase encoding is mostly adopted. The conventional phase coding scheme adopts an unequal arm Mach-Zehnder interferometer or a Faraday Michelson interferometer, and a phase modulator is added into one arm of the unequal arm Mach-Zehnder interferometer or the Faraday Michelson interferometer to randomly modulate the phase difference of a long arm and a short arm, so that 4 quantum states are prepared. Because the lengths of the two arms of the interferometer are different, the interferometer is affected by different environmental temperature changes, mechanical vibration and the like, the phase drift is caused by inconsistent optical path difference changes experienced by photons during the propagation of the two arms, the prepared quantum state is further caused to drift, and the error rate of the system is increased.
Most of the existing interferometer phase compensation techniques adopt an active compensation mode, including discontinuous compensation and discontinuous compensation. The intermittent compensation generally requires interruption of the quantum key distribution process, and phase curve scanning or working point scanning is performed by sending reference light to obtain phase drift parameters of the interferometer, which reduces the duty ratio of the quantum key distribution process, resulting in increased complexity and reduced efficiency of the system. The uninterrupted phase compensation mode can perform phase compensation in the quantum key distribution process, and the generation of the system security key cannot be influenced. The existing uninterrupted phase compensation method adopts temperature measurement on different sites of an interferometer to obtain phase drift parameters, although a relatively accurate result can be obtained, a plurality of high-precision temperature sensors are required to be used to spread over the whole interferometer, and meanwhile, vibration isolation measures are required to be added, so that the complexity and cost of the system are greatly increased, and the conditions of temperature jump and mechanical vibration cannot be met.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a stable phase encoding device and method for a quantum key distribution system, which are used for solving the problems that the duty ratio of the quantum key distribution process is lower in the interrupted compensation in the phase compensation technology of an interferometer, so that the complexity of the system is increased and the efficiency is reduced; the uninterrupted phase compensation has the technical defects of complexity and cost of a system and incapability of coping with the conditions of temperature jump and mechanical vibration.
The invention provides a stable phase encoding device and method for a quantum key distribution system, which comprises the following steps:
the technical scheme of the invention is realized as follows:
a phase coding device for quantum key distribution system stabilization comprises a quantum pulse light source QPS, a double pulse light source DPS, a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM and a single photon detector SPD, wherein a second port and a third port of the first beam splitter BS1 are respectively connected with a second port and a first port of the second beam splitter BS2 through a long arm and a short arm optical fiber to form an unequal arm Mach-Zehnder interferometer; the phase modulator PM is positioned in the long-arm optical fiber of the Mach-Zehnder interferometer and is used for modulating the phase of pulse light in the long-arm optical fiber; the quantum pulse light source QPS and the double-pulse light source DPS are respectively connected with a first port of the first beam splitter BS1 and a fourth port of the second beam splitter BS2 and are respectively used for generating quantum pulse light and double-pulse light which are synchronous and have the same period, the phases of the pulses of the quantum pulse light are random, the overall phase of the double-pulse light is random, the phases of the two sub-pulses of the double-pulse light are the same, and the time difference is equal to the time corresponding to the arm length difference of the unequal arm Mach-Zehnder interferometer; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, the period of a gating signal is the same as that of a quantum pulse light source QPS, and the gating signal is used for detecting an interference result and providing a feedback signal; and the third port of the second beam splitter BS2 is used as an output port of the phase stabilization encoding device, and is used for outputting the phase-encoded quantum state with stable phase difference.
Preferably, the phase stabilization encoding device comprises a first circulator CIR1, the first and second ports of the first circulator CIR1 are respectively connected with the dipulse light source DPS and the third port of the second beam splitter BS2, and the third port of the first circulator CIR1 is used as the second port of the interferometer.
Preferably, the phase stabilization encoding device comprises a first laser L1, a third beam splitter BS3, a first amplitude modulator AM1 and a second amplitude modulator AM2, the first laser L1 is connected to an input port of the third beam splitter BS3, and two output ports of the third beam splitter BS3 are respectively connected to the first amplitude modulator AM1 and the second amplitude modulator AM 2; the first laser L1 is a phase-randomized wide pulse laser, and the first amplitude modulator AM1 modulates each wide pulse of one path of pulse light split by the first laser L1 through the third beam splitter BS3 into a narrow double pulse light to serve as a double pulse light source DPS; the second amplitude modulator AM2 modulates each wide pulse of the other pulse light split by the third beam splitter BS3 from the first laser L1 into a narrow single pulse light, which is used as the quantum pulse light source QPS.
Preferably, the phase stabilization coding device comprises a second laser L2, a third laser L3, a fourth beam splitter BS4, a third amplitude modulator AM3 and a second circulator CIR2, the second laser L2 is connected with an input port of a fourth beam splitter BS4, two output ports of the fourth beam splitter BS4 are respectively connected with first ports of the third amplitude modulator AM3 and the second circulator CIR2, and the third laser L3 is connected with a second port of the second circulator CIR 2; the second laser L2 is a phase-randomized wide pulse laser, and the third amplitude modulator AM3 modulates each wide pulse of one path of pulse light split by the second laser L2 through the fourth beam splitter BS4 into a narrow double pulse light to serve as a double pulse light source DPS; the second laser L2 is split by the fourth beam splitter BS4 to perform injection locking on the third laser L3 by another path of pulsed light, and is used to generate narrow pulsed light with the same period as the second laser L2, and the narrow pulsed light is used as a quantum pulse light source QPS.
Preferably, the phase stabilization encoding device includes a fourth laser L4, a fifth laser L5, a sixth laser L6, a fifth beam splitter BS5, a third circulator CIR3 and a fourth circulator CIR4, the fourth laser L4 is connected to an input port of the fifth beam splitter BS5, two output ports of the fifth beam splitter BS5 are respectively connected to first ports of the third circulator CIR3 and the fourth circulator CIR4, and the fifth laser L5 and the sixth laser L6 are respectively connected to second ports of the third circulator CIR3 and the fourth circulator CIR 4; one path of pulse light split by the fourth laser L4 through the fifth beam splitter BS5 is used for injecting and locking the fifth laser L5, is used for generating double pulse light with the same period as that of the fourth laser L4, and is used as a double pulse light source DPS; the fourth laser L4 injection-locks the sixth laser L6 with the other pulse light split by the fifth beam splitter BS5 to generate a narrow pulse light having the same period as the fourth laser L4 as the quantum pulse light source QPS.
The invention also provides a stable phase encoding method for the quantum key distribution system, which comprises the following steps:
s1: in the quantum key distribution process, 4 different voltage signals are loaded to the phase modulator PM at random, each voltage signal is the sum of radio frequency voltage Vrf and bias voltage Vb and corresponds to modulation phases of 0, pi/2, pi and 3 pi/2, the time for the quantum pulse light and the double pulse light to reach the phase modulator PM is adjusted to be the same, and the quantum pulse light and the double pulse light are modulated by the phase modulator PM to have the same phase;
s2: carrying out statistical modulation on corresponding photon counts C (0), C (pi/2), C (pi) and C (3 pi/2) of a single-photon detector SPD under 4 phases, and calculating bias phase delta = arctan { [ C (pi/2) -C (3 pi/2) ]/[ C (pi) -C (0) ] }dueto long-arm and short-arm optical path difference and phase drift;
s3: and calculating the bias voltage Vb = delta x V pi/pi of the phase modulator PM corresponding to the bias phase delta to be used as the phase modulation voltage compensation quantity in the next quantum key distribution process, and carrying out phase coding in the quantum key distribution process, wherein V pi is the half-wave voltage of the phase modulator PM.
The invention also provides a stable phase encoding method for the quantum key distribution system, which comprises the following steps:
s1: in the current quantum key distribution process, 4 different voltage signals are loaded to the phase modulator PM at random, each voltage signal is the sum of radio frequency voltage Vrf and bias voltage Vb and corresponds to modulation phases of 0, pi/2, pi and 3 pi/2, the time for quantum pulse light and double pulse light to reach the phase modulator PM is adjusted to be different, and the quantum pulse light is ensured to be modulated by the phase modulator PM, but the double pulse light is not modulated;
s2: and counting photon counts C of the single photon detector SPD in the quantum key distribution process, and adjusting the bias voltage Vb through a PID closed loop feedback control algorithm to keep the photon counts C at a minimum value close to 0.
S3: and (4) performing phase stabilization coding in the quantum key distribution process by taking the bias voltage Vb with the minimum photon count C in S2 as the bias voltage of the phase modulation voltage in the next quantum key distribution process.
Compared with the prior art, the invention has the following beneficial effects:
the stable phase coding device and method for the quantum key distribution system can compensate the phase difference caused by the optical path difference of the long arm and the short arm of the interferometer in real time and the phase drift caused by environmental change, realize stable phase coding and improve the stability of the system; the quantum key distribution process does not need to be interrupted, so that the quantum key distribution efficiency is improved; the invention does not need to add an additional pulse phase randomization module and does not need to transmit reference light to a receiving end, thereby improving the safety of the system.
Drawings
FIG. 1 is a schematic block diagram of a phase-stabilized encoding apparatus for quantum key distribution according to the present invention;
FIG. 2 is a modulation diagram of a first embodiment of a phase control method according to the present invention;
FIG. 3 is a modulation diagram of a second embodiment of the phase control method of the present invention;
FIG. 4 is a schematic block diagram of a first embodiment of a stable phase encoding apparatus according to the present invention;
FIG. 5 is a schematic block diagram of a second embodiment of a stable phase encoding apparatus according to the present invention;
fig. 6 is a schematic block diagram of a third embodiment of the stable phase encoding apparatus of the present invention.
Detailed Description
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.
As shown in fig. 1, a phase-stable encoding device (hereinafter referred to as a phase-stable encoding device) for quantum key distribution includes a quantum pulse light source QPS, a double pulse light source DPS, a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM, and a single photon detector SPD, wherein a second port and a third port of the first beam splitter BS1 are respectively connected to a second port and a first port of a second beam splitter BS2 through a long arm and a short arm optical fiber to form an unequal arm mach-zehnder interferometer; the phase modulator PM is positioned in the long-arm optical fiber of the Mach-Zehnder interferometer and is used for modulating the phase of pulse light in the long-arm optical fiber; the quantum pulse light source QPS and the double-pulse light source DPS are respectively connected with a first port of the first beam splitter BS1 and a fourth port of the second beam splitter BS2 and are respectively used for generating quantum pulse light and double-pulse light which are synchronous and have the same period, the phases of the pulses of the quantum pulse light are random, the overall phase of the double-pulse light is random, the phases of the two sub-pulses are the same, and the time difference is equal to the time corresponding to the arm length difference of the unequal arm Mach-Zehnder interferometer; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, the period of a gating signal is the same as that of a quantum pulse light source QPS, and the gating signal is used for detecting an interference result and providing a feedback signal; and the third port of the second beam splitter BS2 is used as an output port of the phase stabilization encoding device, and is used for outputting the phase-encoded quantum state with stable phase difference.
The specific stable phase encoding process is as follows:
in the kth quantum key distribution process, the quantum pulse light source QPS and the double pulse light source DPS respectively send N periods of pulses, wherein the quantum pulse light source QPS comprises a pulse Q1 in one period, is divided into two pulse components Q11 and Q12 after passing through the unequal arm Mach-Zehnder interferometer, wherein Q11 walks away from the short arm of the interferometer, experiences a phase change of φ S, Q12 walks away from the long arm of the interferometer, experiences a phase change of φ L, is modulated by the phase modulator PM, the phase modulator PM modulates the phase φ 1, when the phase modulator PM loads 4 different voltage signals Vk randomly, each voltage signal is the sum of a radio frequency voltage Vrf and a bias voltage Vbk, the half-wave voltage of the phase modulator PM is V π, the radio frequency voltages Vrf are respectively 0, V π/2, V π, 3V π/2, and correspondingly modulates φ rf =0, π/2, π and 3 π/2, the bias voltage Vbk corresponds to a modulation of phase phib, and thus phi 1= phirf + phib, when the actual phase difference between Q11 and Q12 is delta psi = phirf + phib + phil-phis. When phib + phil-phis =0, the phase is encoded as the exact 4 quantum states, and the phase difference Δ ψ = {0, pi/2, pi, 3 pi/2 }. When there is phase drift, phib + phil-phis = δ is not 0, the phase encoding is inaccurate.
The double pulse light source DPS includes two sub-pulses P1 and P2 in one period, and the time difference between the two is T, which is the same as the time corresponding to the arm length difference of the unequal arm mach-zehnder interferometer. P1 and P2 pass through the unequal arm Mach-Zehnder interferometer in sequence and then are respectively divided into two pulse components P11 and P12, P21 and P22, wherein P12 walks the long arm of the interferometer, the experienced phase change is PhiL, the phase phi 2 is modulated by the phase modulator PM, P21 walks the short arm of the interferometer, the experienced phase change is PhiS, P12 and P21 simultaneously reach the first beam splitter BS1 for interference, the actual phase difference between the two is delta phi = Phi 2+ PhiL-Phi S, and the response count caused by the interference result entering the single-photon detector SPD can be written as:
C(Δφ)=N[1-cos(φ2+φL-φS)]/2.
according to the counting statistical result of the single-photon detector SPD and the phase phi 2, a parameter delta = phi b + phi L-phi S can be obtained, and a voltage drift amount delta Vk of the corresponding phase modulator PM is further obtained to be used as voltage correction of the phase modulator PM on the modulation phase of the quantum pulse light source in the k +1 th quantum key distribution process, namely phase modulation voltage correction of the k +1 th quantum key distribution process is Vk +1= Vk + delta Vk = Vrf + Vbk + delta Vk + Vk. The phase locking can be realized after several iterations of closed loop feedback control, and stable phase encoding is completed. Since the Q1 is obtained by different wide pulse modulation, the phase of the whole pulse is random, and the safety requirement of the BB84 protocol on the randomization of the phase of the optical pulse is met. Meanwhile, since the double pulses P1 and P2 are derived from the same wide pulse and thus have the same phase, the corresponding sub-pulses P12 and P21 can achieve stable interference.
The offset phase of the phase modulator PM can be obtained by two embodiments of the phase stabilization control method provided by the present invention, which are separately described below.
The first embodiment of the phase stabilization control method:
in this embodiment, Q12 is adjusted to arrive at the phase modulator PM at the same time as P12, ensuring that both are modulated by the phase modulator PM at the same phase, i.e., Φ 1= Φ 2= Φ rf + Φ b. As shown in fig. 2, the time relationship between the 4-period pulse and the phase modulation voltage of the phase modulator PM is shown, the first row of fig. 2 is quantum optical pulse, the second row of fig. 2 is double pulses P1 and P2 are sub pulses P12 and P22 in the long arm of the unequal arm mach-zehnder interferometer, the third row of fig. 2 is voltage Vpm = Vfr + Vb loaded by the phase modulator PM, the modulation phases pi, pi/2, 3 pi/2 and 0 are sequentially corresponded from front to back, and the phase modulation voltage has an integral offset Vb.
In the k-th quantum key distribution process, corresponding photon counts C (0), C (pi/2), C (pi) and C (3 pi/2) of a single photon detector SPD under 4 phases modulated by a phase stabilization coding device are counted, bias phases delta k = phib + phiL-phiS = arctan { [ C (pi/2) -C (3 pi/2) ]/[ C (pi) -C (0) ] } caused by long-short arm optical path difference and phase drift are calculated, then phase modulator PM voltages delta Vk = delta k/pi V corresponding to the bias phases delta k are calculated by combining a PID control algorithm and serve as phase modulation voltage compensation quantities of the k + 1-th quantum key distribution process, phase coding of the quantum key distribution process is carried out, and V pi is half-wave voltage of the phase modulator PM.
Phase stabilization control method embodiment two:
in this embodiment, adjusting Q12 differently from P12 for the time of arrival at the phase modulator PM ensures that Q12 loads voltage Vrf + Vb when passing through the phase modulator PM, while dual pulsed light loads voltage Vb when passing through the phase modulator PM, i.e., Φ 1= Φ rf + Φ b, Φ 2= Φ b. As shown in fig. 3, the time relationship between the 4-period pulse and the phase modulation voltage of the phase modulator PM is shown, the first row of fig. 3 is a pulse component Q12 of the quantum optical pulse in the long arm of the unequal arm mach-zehnder interferometer, the second row of fig. 3 is a double pulse P1 and P2 in the sub-pulses P12 and P22 of the long arm of the unequal arm mach-zehnder interferometer, the third row of fig. 3 is a voltage Vpm = Vfr + Vb loaded by the phase modulator PM, the phases pi, pi/2, 3 pi/2 and 0 are correspondingly modulated from front to back, and the phase modulation voltage has an integral bias Vb.
In the k-th quantum key distribution process, counting photon counts C of a single photon detector SPD in the phase stabilization coding device, and adjusting the bias voltage to be Vbk + delta Vk through a PID closed loop feedback control algorithm to enable the photon counts C to be maintained at the minimum value close to 0, wherein delta phi = phi bk + phi L-phi S → 0. And delta Vk is used as a phase modulation voltage compensation quantity in the (k + 1) th quantum key distribution process to perform phase coding in the quantum key distribution process, and the bias voltage Vb at the moment compensates the difference of optical paths of the long arm and the short arm and the phase difference caused by phase drift.
As shown in fig. 4, a first embodiment of the phase-stable encoding apparatus of the present invention:
the phase stabilization encoding device has the structure that: the system comprises a first laser L1, a third beam splitter BS3, a first amplitude modulator AM1, a second amplitude modulator AM2, a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM, a single photon detector SPD and a first circulator CIR1, wherein the first laser L1 is connected with an input port of the third beam splitter BS3, and two output ports of the third beam splitter BS3 are respectively connected with the first amplitude modulator AM1 and the second amplitude modulator AM 2; the first laser L1 is a phase-randomized wide pulse laser, and the first amplitude modulator AM1 modulates each wide pulse of one path of pulse light split by the first laser L1 through the third beam splitter BS3 into a narrow double pulse light to serve as a double pulse light source DPS; the second amplitude modulator AM2 modulates each wide pulse of the other path of pulse light split by the third beam splitter BS3 from the first laser L1 into a narrow single pulse light, which is used as a quantum pulse light source QPS; the other end of the second amplitude modulator AM2 is connected to the first port of the first beam splitter BS 1; the second port and the third port of the first beam splitter BS1 are respectively connected with the second port and the first port of the second beam splitter BS2 through long-arm optical fibers and short-arm optical fibers to form an unequal-arm Mach-Zehnder interferometer; the phase modulator PM is positioned in a long-arm optical fiber of the Mach-Zehnder interferometer; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, and the gating signal period is the same as that of the first laser L1; the first port and the second port of the first circulator CIR1 are respectively connected with the third ports of the first amplitude modulator AM1 and the second beam splitter BS2, and the third port of the first circulator CIR1 is used as the second port of the interferometer and is used for outputting phase-encoded quantum states with stable phase difference.
The phase encoding process of the first embodiment of the phase-stabilized encoding apparatus includes:
the wide pulse light P0 is emitted from the first laser L1 and is divided into two paths by the third beam splitter BS3, wherein one path is modulated into double narrow pulses P1 and P2 by the first amplitude modulator AM1 as a double pulse light source DPS, and the other path is modulated into a single narrow pulse Q1 by the second amplitude modulator AM2 as a quantum pulse light source QPS. Q1 passes through the unequal arm mach-zehnder interferometer before being split into two pulse components Q11 and Q12, where Q11 goes through the short arm of the interferometer and experiences a phase change of os, Q12 goes through the long arm of the interferometer and experiences a phase change of pl, and phase 1 is modulated by phase modulator PM, when the phase modulator PM is loaded randomly with 4 different voltage signals, each voltage signal being the sum of a radio frequency voltage Vrf and a bias voltage Vb, the half-wave voltage of the phase modulator PM is V pi, the corresponding modulation phases phi rf =0, pi/2, pi and 3 pi/2 when the radio frequency voltage Vrf is 0, V pi/2, V pi and 3V pi/2 respectively, the corresponding modulation phase of the bias voltage Vb is phi b, thus, Φ 1= Φ rf + Φ b, the actual phase difference between Q11 and Q12 is Δ ψ = Φ rf + Φ b + Φ L- Φ S.
P1 and P2 pass through the unequal arm Mach-Zehnder interferometer in sequence and then are respectively divided into two pulse components P11 and P12, P21 and P22, wherein P12 walks the long arm of the interferometer, the experienced phase change is phi L, the phase phi 2 is modulated by the phase modulator PM, the experienced phase change is phi S when P21 walks the short arm of the interferometer, the P12 and the P21 simultaneously reach the first beam splitter BS1 for interference, the actual phase difference between the two is delta phi = phi 2+ phi L-phi S, and the interference result enters the single-photon detector SPD.
According to the two embodiments of phase stability control distribution, the phase drift parameters of the interferometer can be obtained by using the interference result of the dual pulse light, and corresponding phase stability control is performed, so that stable phase encoding is realized.
As shown in fig. 5, a second embodiment of the phase-stable encoding apparatus of the present invention:
the phase stabilization encoding device has the structure that: the single-photon detector SPD laser comprises a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM, a single-photon detector SPD, a second laser L2, a third laser L3, a fourth beam splitter BS4, a third amplitude modulator AM3 and a second circulator CIR2, wherein the second laser L2 is connected with an input port of the fourth beam splitter BS4, two output ports of the fourth beam splitter BS4 are respectively connected with first ports of the third amplitude modulator AM3 and the second circulator CIR2, and the third laser L3 is connected with a second port of the second circulator CIR 2; the third port of the second circulator CIR2 is connected to the first port of the first splitter BS 1; the second laser L2 is a phase-randomized wide pulse laser, and the third amplitude modulator AM3 modulates each wide pulse of one path of pulse light split by the second laser L2 through the fourth beam splitter BS4 into a narrow double pulse light, which is used as a double pulse light source DPS and enters the fourth port of the second beam splitter BS 2; the second laser L2 is split by the fourth beam splitter BS4 to perform injection locking on the third laser L3 by another path of pulsed light, and is used to generate narrow pulsed light with the same period as the second laser L2, and the narrow pulsed light is used as a quantum pulse light source QPS. The second port and the third port of the first beam splitter BS1 are respectively connected with the second port and the first port of the second beam splitter BS2 through long-arm optical fibers and short-arm optical fibers to form an unequal-arm Mach-Zehnder interferometer; the phase modulator PM is positioned in the long-arm optical fiber of the Mach-Zehnder interferometer and is used for modulating the phase of pulse light in the long-arm optical fiber; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, the gating signal period is the same as that of the second laser L2, and the single-photon detector SPD is used for detecting an interference result and providing a feedback signal; and the third port of the second beam splitter BS2 is used as an output port of the phase stabilization encoding device, and is used for outputting the phase-encoded quantum state with stable phase difference.
Phase-stable encoding apparatus embodiments the two-phase encoding process comprises:
the wide pulse light P0 is emitted from the second laser L2 and is split into two paths by the fourth beam splitter BS4, one of the two paths is modulated by the third amplitude modulator AM3 into double narrow pulses P1 and P2 as a double pulse light source DPS, and the other path enters the third laser L3 through the second circulator CIR2 to be injected and locked, and a narrow pulse Q1 with the same period as that of the second laser L2 is generated as a quantum pulse light source QPS. Q1 passes through the unequal arm mach-zehnder interferometer before being split into two pulse components Q11 and Q12, where Q11 goes through the short arm of the interferometer and experiences a phase change of os, Q12 goes through the long arm of the interferometer and experiences a phase change of pl, and phase 1 is modulated by phase modulator PM, when the phase modulator PM is loaded randomly with 4 different voltage signals, each voltage signal being the sum of a radio frequency voltage Vrf and a bias voltage Vb, the half-wave voltage of the phase modulator PM is V pi, the corresponding modulation phases phi rf =0, pi/2, pi and 3 pi/2 when the radio frequency voltage Vrf is 0, V pi/2, V pi and 3V pi/2 respectively, the corresponding modulation phase of the bias voltage Vb is phi b, thus, Φ 1= Φ rf + Φ b, the actual phase difference between Q11 and Q12 is Δ ψ = Φ rf + Φ b + Φ L- Φ S.
P1 and P2 pass through the unequal arm Mach-Zehnder interferometer in sequence and then are respectively divided into two pulse components P11 and P12, P21 and P22, wherein P12 walks the long arm of the interferometer, the experienced phase change is phi L, the phase phi 2 is modulated by the phase modulator PM, the experienced phase change is phi S when P21 walks the short arm of the interferometer, the P12 and the P21 simultaneously reach the first beam splitter BS1 for interference, the actual phase difference between the two is delta phi = phi 2+ phi L-phi S, and the interference result enters the single-photon detector SPD.
According to the two embodiments of phase stability control distribution, the phase drift parameters of the interferometer can be obtained by using the interference result of the dual pulse light, and corresponding phase stability control is performed, so that stable phase encoding is realized.
As shown in fig. 6, a third embodiment of the phase-stable encoding apparatus of the present invention:
the phase stabilization encoding device has the structure that: the phase modulator comprises a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM, a single photon detector SPD, a fourth laser L4, a fifth laser L5, a sixth laser L6, a fifth beam splitter BS5, a third circulator CIR3 and a fourth circulator CIR4, wherein the fourth laser L4 is connected with an input port of a fifth beam splitter BS5, two output ports of the fifth beam splitter BS5 are respectively connected with first ports of a third circulator CIR3 and a fourth circulator CIR4, and the fifth laser L5 and the sixth laser L6 are respectively connected with second ports of the third circulator CIR3 and the fourth circulator CIR 4; the third ports of the third circulator CIR3 and the fourth circulator CIR4 are respectively connected with the first port of the first beam splitter BS1 and the fourth port of the second beam splitter BS 2; one path of pulse light split by the fourth laser L4 through the fifth beam splitter BS5 is used for injecting and locking the fifth laser L5, is used for generating double pulse light with the same period as that of the fourth laser L4, and is used as a double pulse light source DPS; the fourth laser L4 injection-locks the sixth laser L6 with the other pulse light split by the fifth beam splitter BS5 to generate a narrow pulse light having the same period as the fourth laser L4 as the quantum pulse light source QPS. The second port and the third port of the first beam splitter BS1 are respectively connected with the second port and the first port of the second beam splitter BS2 through long-arm optical fibers and short-arm optical fibers to form an unequal-arm Mach-Zehnder interferometer; the phase modulator PM is positioned in the long-arm optical fiber of the Mach-Zehnder interferometer and is used for modulating the phase of pulse light in the long-arm optical fiber; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, the gating signal period is the same as that of the second laser L2, and the single-photon detector SPD is used for detecting an interference result and providing a feedback signal; and the third port of the second beam splitter BS2 is used as an output port of the phase stabilization encoding device, and is used for outputting the phase-encoded quantum state with stable phase difference.
The three-phase encoding process of the phase stabilization encoding device comprises the following steps:
the wide pulse light P0 is emitted from the fourth laser L4 and divided into two paths by the fifth beam splitter BS5, wherein one path enters the fifth laser L5 through the third circulator CIR3 for injection locking to generate double narrow pulses P1 and P2 as a double pulse light source DPS, the other path enters the third laser L3 through the second circulator CIR2 for injection locking, and the narrow pulse Q1 with the same period as the second laser L2 is generated as a quantum pulse light source QPS. Q1 passes through the unequal arm mach-zehnder interferometer before being split into two pulse components Q11 and Q12, where Q11 goes through the short arm of the interferometer and experiences a phase change of os, Q12 goes through the long arm of the interferometer and experiences a phase change of pl, and phase 1 is modulated by phase modulator PM, when the phase modulator PM is loaded randomly with 4 different voltage signals, each voltage signal being the sum of a radio frequency voltage Vrf and a bias voltage Vb, the half-wave voltage of the phase modulator PM is V pi, the corresponding modulation phases phi rf =0, pi/2, pi and 3 pi/2 when the radio frequency voltage Vrf is 0, V pi/2, V pi and 3V pi/2 respectively, the corresponding modulation phase of the bias voltage Vb is phi b, thus, Φ 1= Φ rf + Φ b, the actual phase difference between Q11 and Q12 is Δ ψ = Φ rf + Φ b + Φ L- Φ S.
P1 and P2 pass through the unequal arm Mach-Zehnder interferometer in sequence and then are respectively divided into two pulse components P11 and P12, P21 and P22, wherein P12 walks the long arm of the interferometer, the experienced phase change is phi L, the phase phi 2 is modulated by the phase modulator PM, the experienced phase change is phi S when P21 walks the short arm of the interferometer, the P12 and the P21 simultaneously reach the first beam splitter BS1 for interference, the actual phase difference between the two is delta phi = phi 2+ phi L-phi S, and the interference result enters the single-photon detector SPD.
According to the two embodiments of phase stability control distribution, the phase drift parameters of the interferometer can be obtained by using the interference result of the dual pulse light, and corresponding phase stability control is performed, so that stable phase encoding is realized.
According to the embodiments of the invention, the stable phase encoding device and method for the quantum key distribution system can compensate the phase difference caused by the optical path difference of the long arm and the short arm of the interferometer in real time and the phase drift caused by the environmental change, realize stable phase encoding and improve the stability of the system; the quantum key distribution process does not need to be interrupted, so that the quantum key distribution efficiency is improved; the invention does not need to add an additional pulse phase randomization module and does not need to transmit reference light to a receiving end, thereby improving the safety of the system.
Claims (5)
1. A stable phase coding device for a quantum key distribution system is characterized by comprising a quantum pulse light source QPS, a double-pulse light source DPS, a first beam splitter BS1, a second beam splitter BS2, a phase modulator PM and a single-photon detector SPD, wherein a second port and a third port of the first beam splitter BS1 are respectively connected with a second port and a first port of the second beam splitter BS2 through long-arm optical fibers and short-arm optical fibers to form an unequal-arm Mach-Zehnder interferometer; the phase modulator PM is positioned in the long-arm optical fiber of the Mach-Zehnder interferometer and is used for modulating the phase of pulse light in the long-arm optical fiber; the quantum pulse light source QPS and the double-pulse light source DPS are respectively connected with a first port of the first beam splitter BS1 and a fourth port of the second beam splitter BS2 and are respectively used for generating quantum pulse light and double-pulse light which are synchronous and have the same period, the phases of the pulses of the quantum pulse light are random, the overall phase of the double-pulse light is random, the phases of the two sub-pulses of the double-pulse light are the same, and the time difference is equal to the time corresponding to the arm length difference of the unequal arm Mach-Zehnder interferometer; the single-photon detector SPD is connected with a fourth port of the first beam splitter BS1, the period of a gating signal is the same as that of a quantum pulse light source QPS, and the gating signal is used for detecting an interference result and providing a feedback signal; and the third port of the second beam splitter BS2 is used as an output port of the phase stabilization encoding device, and is used for outputting the phase-encoded quantum state with stable phase difference.
2. The phase encoding apparatus for quantum key distribution system stabilization of claim 1, wherein the phase stabilization encoding apparatus comprises a first circulator CIR1, the first and second ports of the first circulator CIR1 are respectively connected to the dipulse light source DPS and the third port of the second beam splitter BS2, and the third port of the first circulator CIR1 serves as the second port of the interferometer.
3. The phase encoding apparatus for quantum key distribution system stabilization of claim 2, wherein the phase stabilization encoding apparatus comprises a first laser L1, a third beam splitter BS3, a first amplitude modulator AM1 and a second amplitude modulator AM2, the first laser L1 is connected to an input port of a third beam splitter BS3, two output ports of the third beam splitter BS3 are connected to the first amplitude modulator AM1 and the second amplitude modulator AM2, respectively; the first laser L1 is a phase-randomized wide pulse laser, and the first amplitude modulator AM1 modulates each wide pulse of one path of pulse light split by the first laser L1 through the third beam splitter BS3 into a narrow double pulse light to serve as a double pulse light source DPS; the second amplitude modulator AM2 modulates each wide pulse of the other pulse light split by the third beam splitter BS3 from the first laser L1 into a narrow single pulse light, which is used as the quantum pulse light source QPS.
4. The phase encoding apparatus for quantum key distribution system stabilization of claim 1, wherein the phase stabilization encoding apparatus comprises a second laser L2, a third laser L3, a fourth beam splitter BS4, a third amplitude modulator AM3, and a second circulator CIR2, the second laser L2 is connected to an input port of a fourth beam splitter BS4, two output ports of the fourth beam splitter BS4 are connected to first ports of the third amplitude modulator AM3 and the second circulator CIR2, respectively, the third laser L3 is connected to a second port of the second circulator CIR 2; the second laser L2 is a phase-randomized wide pulse laser, and the third amplitude modulator AM3 modulates each wide pulse of one path of pulse light split by the second laser L2 through the fourth beam splitter BS4 into a narrow double pulse light to serve as a double pulse light source DPS; the second laser L2 is split by the fourth beam splitter BS4 to perform injection locking on the third laser L3 by another path of pulsed light, and is used to generate narrow pulsed light with the same period as the second laser L2, and the narrow pulsed light is used as a quantum pulse light source QPS.
5. Phase encoding apparatus for quantum key distribution system stabilization according to claim 1, characterized in that the phase stabilization encoding apparatus comprises a fourth laser L4, a fifth laser L5, a sixth laser L6, a fifth beam splitter BS5, a third circulator CIR3 and a fourth circulator CIR4, the fourth laser L4 is connected to an input port of a fifth beam splitter BS5, two output ports of the fifth beam splitter BS5 are connected to first ports of a third circulator CIR3 and a fourth circulator CIR4, respectively, the fifth laser L5, the sixth laser L6 are connected to second ports of a third circulator CIR3 and a fourth circulator CIR4, respectively; one path of pulse light split by the fourth laser L4 through the fifth beam splitter BS5 is used for injecting and locking the fifth laser L5, is used for generating double pulse light with the same period as that of the fourth laser L4, and is used as a double pulse light source DPS; the fourth laser L4 injection-locks the sixth laser L6 with the other pulse light split by the fifth beam splitter BS5 to generate a narrow pulse light having the same period as the fourth laser L4 as the quantum pulse light source QPS.
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