Modulation Device of Quantum Key Distribution System and Method for Preparing Stable Intensity State

Abstract

The present invention discloses a modulation device for a decoy state quantum key distribution system, comprising: a first beam splitting unit configured to receive an incident optical pulse and split the incident optical pulse into a first pulse and a second pulse; a first modulation unit coupled to the first beam splitting unit and configured to receive the first pulse and generate a first interference pulse after modulation; a second modulation unit coupled to the first beam splitting unit and configured to receive the second pulse and generate a second interference pulse after modulation; and a third modulation unit coupled to the first modulation unit and the second modulation unit and configured to receive the first interference pulse and the second interference pulse and generate a third interference pulse after modulation. The modulation device for a decoy state quantum key distribution system provided by the present invention and the encoding method for preparing a stable intensity state using the same can effectively suppress the encoding memory effect (pattern effect) between adjacent optical pulses and ensure the independence between the encoded pulses.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication technology, and particularly to a modulation device for a decoy state quantum key distribution system and an encoding method for preparing a stable intensity state using the same. Background Art

[0002] Compared with classical communication technology, quantum communication technology based on quantum key distribution (QKD) can ensure the theoretical security of information transmission. However, the prerequisite for realizing quantum key distribution is to have an ideal single-photon source. Due to the lack of an ideal single-photon source, in practical applications, weak coherent light at the single-photon level is usually used instead, but the multi-photon component therein can be exploited by eavesdroppers to create security vulnerabilities. Later, the decoy state method was proposed to perfectly solve this problem.

[0003] The decoy state quantum key distribution system can greatly improve the secure coding rate and transmission distance. Currently, almost all practical quantum key distribution systems adopt the decoy state encoding method. A common decoy state-based quantum key distribution system needs to prepare three weak coherent light pulses with different intensities for encoding. Commercial intensity modulators (IMs) are made based on the Mach-Zehnder interference principle (MZI) of lithium niobate. Utilizing the electro-optic property of lithium niobate, the driving voltage is adjusted to change the phase difference between the two arms to achieve intensity modulation. Its transfer function is as Figure 1 shown, where:

[0004]

[0005] where μ in is the input light intensity, α is the phase difference between the two arms of the Mach-Zehnder interferometer structure. By adjusting the voltage difference applied to the two arms, the refractive index of the lithium niobate medium can be adjusted, thereby adjusting the phase difference between the two arms.

[0006] The decoy state quantum key distribution system usually uses an IM intensity modulator to generate three intensity states, including a signal state (S state), a vacuum state (V state), and a decoy state (D state). Among them, for the signal state (S state) and the vacuum state (V state), the maximum and minimum points of the IM intensity modulator are selected, and the influence of voltage drift on the output light intensity can be ignored. As Figure 1 shown, in the S state, when the electric pulse waveform jitters (ΔV), the change in the output photocurrent intensity is small (ΔI S ). While for the decoy state (D state), only the middle region with a larger slope can be selected, which is easily affected by the distortion of the electric pulse waveform. As Figure 1 shown, in the D state, when the electric pulse waveform jitters (ΔV), the output photocurrent intensity will produce obvious change fluctuations (ΔI D) The fluctuations in the output photocurrent intensity can cause the correlation effect (pattern effect) between adjacent pulses in high-speed modulation applications, seriously violating the requirement of the quantum key distribution system for the independence of encoded pulses.

[0007] To address the defects of the intensity modulator, there are currently three main solutions: post-processing, the Sagnac-effect-based intensity modulator, and the multi-path parallel MZI intensity modulator proposed by the University of Science and Technology of China.

[0008] The post-processing solution decides whether to retain or discard the target pulse according to the correlation state of the front and back pulses. In actual operation, a large number of pulses will be discarded, which inevitably reduces the secure coding rate.

[0009] The Sagnac intensity modulator can only generate two stable intensity modulations and cannot meet the preparation of multi-intensity decoy states. At the same time, the common-path interference mechanism adopted by this intensity modulator has an inherent speed limit, which is a shortcoming of future quantum key distribution systems.

[0010] The multi-path parallel MZI intensity modulator can theoretically generate multiple stable intensity states, but it requires special design of the splitting ratio of the intensity modulator, while commercial intensity modulators usually adopt a symmetric splitting ratio. Secondly, the single-ended microwave injection method used in the multi-path parallel MZI has an obvious chirp effect. At the same time, the modulation voltage is high and the phase encoding is not accurate. For a three-intensity decoy state system, using a two-level voltage encoding, the encoding intensity of the vacuum state is not 0, and the system bit error rate will increase significantly. Summary of the Invention

[0011] In view of at least one defect of the prior art, the present invention provides a modulation device for a decoy state quantum key distribution system, including:

[0012] A first splitting unit configured to receive an incident optical pulse and split the incident optical pulse into a first pulse and a second pulse;

[0013] A first modulation unit coupled to the first splitting unit and configured to modulate the first pulse to generate a first interference pulse;

[0014] A second modulation unit coupled to the first splitting unit and configured to modulate the second pulse to generate a second interference pulse;

[0015] A third modulation unit coupled to the first modulation unit and the second modulation unit and configured to modulate the first interference pulse and the second interference pulse to generate a third interference pulse, and the third interference pulse has four extreme points.

[0016] According to one aspect of the present invention, wherein the first modulation unit includes:

[0017] A second beam splitting unit configured to receive the first pulse and split the first pulse into a third pulse and a fourth pulse;

[0018] A first waveguide unit including two waveguides, coupled to the second beam splitting unit, configured to receive the third pulse and the fourth pulse, and transmit the third pulse and the fourth pulse along one of the two waveguides respectively;

[0019] A first phase modulation unit coupled to the first waveguide unit, configured to modulate the phase difference between the third pulse and the fourth pulse;

[0020] A first coupling unit coupled to the first waveguide unit, configured to receive the modulated third pulse and fourth pulse, and couple the modulated third pulse and fourth pulse to output the first interference pulse.

[0021] According to one aspect of the present invention, wherein the second modulation unit includes:

[0022] A third beam splitting unit configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse;

[0023] A second waveguide unit including two waveguides, coupled to the third beam splitting unit, configured to receive the fifth pulse and the sixth pulse, and transmit the fifth pulse and the sixth pulse along one of the two waveguides of the second waveguide unit respectively;

[0024] A second phase modulation unit coupled to the second waveguide unit, configured to modulate the phase difference between the fifth pulse and the sixth pulse;

[0025] A second coupling unit coupled to the second waveguide unit, configured to receive the modulated fifth pulse and sixth pulse, and couple the modulated fifth pulse and sixth pulse to output the second interference pulse.

[0026] According to one aspect of the present invention, wherein the third modulation unit includes:

[0027] A third waveguide unit including two waveguides, respectively coupled to the first coupling unit and the second coupling unit, configured to receive the first interference pulse and the second interference pulse, and transmit the first interference pulse and the second interference pulse along one of the two waveguides of the third waveguide unit respectively;

[0028] A third phase modulation unit, coupled to the third waveguide unit, configured to modulate the phase difference between the first interference pulse and the second interference pulse;

[0029] A third coupling unit, coupled to the third waveguide unit, configured to receive the modulated first interference pulse and the modulated second interference pulse, and couple the modulated first interference pulse and the modulated second interference pulse to output the third interference pulse.

[0030] According to one aspect of the present invention, the splitting ratios of the first splitting unit, the second splitting unit, and / or the third splitting unit are all 1:1.

[0031] According to one aspect of the present invention, two waveguides of any one of the first waveguide unit and the second waveguide unit are arranged in parallel and made of lithium niobate material. The first phase modulation unit and the second phase modulation unit respectively include:

[0032] Traveling wave electrodes, configured to inject radio frequency signals into the corresponding two waveguides;

[0033] DC bias electrodes, configured to inject DC voltages into the corresponding two waveguides.

[0034] According to one aspect of the present invention, the first phase modulation unit and the second phase modulation unit are further configured to:

[0035] Inject radio frequency signals into the corresponding two waveguides respectively through the traveling wave electrodes, and the directions of the radio frequency signals injected into the corresponding two waveguides are opposite.

[0036] According to one aspect of the present invention, the first phase modulation unit and the second phase modulation unit are further configured to:

[0037] Inject radio frequency signals into the corresponding two waveguides in a push-pull manner through the traveling wave electrodes.

[0038] According to one aspect of the present invention, two waveguides of the third waveguide unit are arranged in parallel and made of lithium niobate material. The third phase modulation unit includes:

[0039] DC bias electrodes, configured to inject DC voltages into the corresponding two waveguides.

[0040] According to one aspect of the present invention, the third interference pulse has four extreme points, and the first phase modulation unit, the second phase modulation unit, and the third phase modulation unit are configured to:

[0041] By modulating the phase differences between the third pulse and the fourth pulse, the fifth pulse and the sixth pulse, and the first interference pulse and the second interference pulse, the third interference pulse is positioned at the extreme point.

[0042] According to one aspect of the present invention, the first modulation unit, the second modulation unit, and the third modulation unit each include: a Mach-Zehnder interferometer.

[0043] The present invention also provides a method for preparing a stable intensity state using the modulation device as described above, wherein,

[0044] The first modulation unit further includes:

[0045] A second beam splitting unit configured to receive the first pulse and split the first pulse into a third pulse and a fourth pulse;

[0046] A first phase modulation unit configured to modulate the phase difference between the third pulse and the fourth pulse;

[0047] The second modulation unit further includes:

[0048] A third beam splitting unit configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse;

[0049] A second phase modulation unit configured to modulate the phase difference between the fifth pulse and the sixth pulse;

[0050] The third modulation unit further includes:

[0051] A third phase modulation unit configured to modulate the phase difference between the first interference pulse and the second interference pulse;

[0052] The method includes:

[0053] Modulating, by the first phase modulation unit, the phase difference between the third pulse and the fourth pulse to be 0;

[0054] Modulating, by the second phase modulation unit, the phase difference between the fifth pulse and the sixth pulse to be 0;

[0055] Modulating, by the third phase modulation unit, the phase difference between the first interference pulse and the second interference pulse to be 0;

[0056] Outputting the third interference pulse as a signal state output.

[0057] According to one aspect of the present invention, the method further includes:

[0058] Modulate the phase difference between the third pulse and the fourth pulse to be π through the first phase modulation unit;

[0059] Modulate the phase difference between the fifth pulse and the sixth pulse to be 0 through the second phase modulation unit;

[0060] Modulate the phase difference between the first interference pulse and the second interference pulse to be 0 through the third phase modulation unit;

[0061] Output the third interference pulse as the decoy state output.

[0062] According to one aspect of the present invention, the method further includes:

[0063] Modulate the phase difference between the third pulse and the fourth pulse to be 0 through the first phase modulation unit;

[0064] Modulate the phase difference between the fifth pulse and the sixth pulse to be π through the second phase modulation unit;

[0065] Modulate the phase difference between the first interference pulse and the second interference pulse to be 0 through the third phase modulation unit;

[0066] Output the third interference pulse as the decoy state output.

[0067] According to one aspect of the present invention, the method further includes:

[0068] Modulate the phase difference between the third pulse and the fourth pulse to be π through the first phase modulation unit;

[0069] Modulate the phase difference between the fifth pulse and the sixth pulse to be π through the second phase modulation unit;

[0070] Modulate the phase difference between the first interference pulse and the second interference pulse to be 0 through the third phase modulation unit;

[0071] Output the third interference pulse as the vacuum state output.

[0072] According to one aspect of the present invention, wherein the first phase modulation unit and the second phase modulation unit respectively include: traveling wave electrodes configured to inject radio frequency signals into the third pulse and the fourth pulse, the fifth pulse and the sixth pulse respectively; the method further includes:

[0073] Load the encoding pulse sequence to be transmitted onto the radio frequency signal.

[0074] The modulation device for the decoy state quantum key distribution system provided by the present invention and the encoding method for preparing a stable intensity state using the same have a transmission curve of the output interference pulse with 4 extreme points, and can prepare at least three stable intensity states. By using the modulation device and the preparation method provided by the present invention, the encoding memory effect (pattern effect) between adjacent optical pulses can be effectively suppressed, ensuring the independence between the encoded pulses. Moreover, in the modulation device and the preparation method provided by the present invention, the radio frequency signal adopts push-pull modulation, suppressing the generation of chirp signals, with a low modulation voltage, and the phase difference between the two interference pulses after modulation remains unchanged, which is more suitable for high-speed decoy state quantum key distribution systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without exceeding the scope of protection required by the present application.

[0076] Figure 1 Shows the output photocurrent curve of a commercial intensity modulator (IM) in the prior art;

[0077] Figure 2 Shows the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0078] Figure 3 Shows the first modulation unit of the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0079] Figure 4 Shows the second modulation unit of the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0080] Figure 5 Shows the third modulation unit of the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0081] Figure 6 Shows the phase modulation unit of the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0082] Figure 7 Shows the injected radio frequency signal of the phase modulation unit of the modulation device for the decoy state quantum key distribution system provided by an embodiment of the present invention;

[0083] Figure 8Shows an experimental setup for verifying the output pulse intensity of a commercial intensity modulator (IM);

[0084] Figure 9A Shows the DC voltage scan results of a commercial intensity modulator (IM);

[0085] Figure 9B Shows the RF signal scan results of a commercial intensity modulator (IM);

[0086] Figure 10 Shows the measurement results of the output pulse intensity of different coded modulation signals of a commercial intensity modulator (IM);

[0087] Figure 11 Shows an experimental setup for verifying the output pulse intensity of a modulation device provided by an embodiment of the present invention;

[0088] Figure 12 Shows the measurement results of the output pulse intensity of different coded modulation signals of a modulation device provided by an embodiment of the present invention;

[0089] Figure 13A Shows the transfer function of a modulation device provided by an embodiment of the present invention;

[0090] Figure 13B Shows the transfer function of a modulation device provided by an embodiment of the present invention;

[0091] Figure 14A Shows a method for preparing a stable intensity state using a modulation device provided by an embodiment of the present invention;

[0092] Figure 14B Shows a method for preparing a stable intensity state using a modulation device provided by an embodiment of the present invention;

[0093] Figure 14C Shows a method for preparing a stable intensity state using a modulation device provided by an embodiment of the present invention;

[0094] Figure 14D Shows a method for preparing a stable intensity state using a modulation device provided by an embodiment of the present invention; Detailed implementation manners

[0095] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0096] The above has introduced the embodiments of the present application in detail. Specific examples are used herein to elaborate on the principle and implementation manner of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application. At the same time, based on the idea of the present application, changes or deformations made by those skilled in the art in the specific implementation manner and application scope of the present application all belong to the protection scope of the present application. In summary, the content of this specification should not be construed as a limitation to the present application.

[0097] The present invention provides an intensity modulator based on the Mach-Zehnder interference principle with a nested structure. The output photocurrent curve of the intensity modulator has four extreme points, and at least three stable weak coherent pulses can be prepared, greatly suppressing the correlation effect between adjacent optical pulses and ensuring the independence of the encoded pulses for the decoy-state quantum key distribution system.

[0098] According to an embodiment of the present invention, as Figure 2 shown, the present invention provides a modulation device 100 for a decoy-state quantum key distribution system, including a first beam splitting unit 110, a first modulation unit 120, a second modulation unit 130, and a third modulation unit 140. Among them:

[0099] The first beam splitting unit 110 is configured to receive an incident optical pulse and split the incident optical pulse into a first pulse and a second pulse. That is, the incident optical pulse enters from the input port of the first beam splitting unit 110, and after passing through the first beam splitting unit 110, it is split into two beams, namely the first pulse and the second pulse, and the first pulse and the second pulse respectively exit from the two output ports of the first beam splitting unit 110.

[0100] The first modulation unit 120 is coupled to the first beam splitting unit 110 and is configured to receive the first pulse and generate a first interference pulse after modulation. The input port of the first modulation unit 120 is coupled to an output port of the first beam splitter 110, so that the first pulse enters the first modulation unit 120 for modulation after exiting.

[0101] The second modulation unit 130 is coupled to the first beam splitting unit 110 and is configured to receive the second pulse and generate a second interference pulse after modulation. The input port of the second modulation unit 130 is coupled to the other output port of the first beam splitter 110, so that the second pulse enters the second modulation unit 130 for modulation after exiting.

[0102] The third modulation unit 140 is coupled to the first modulation unit 120 and the second modulation unit 130, and is configured to receive the first interference pulse and the second interference pulse, and generate a third interference pulse after modulation. The third modulation unit 140 has two input ports, which are respectively coupled to the output port of the first modulation unit 120 and the output port of the second modulation unit 130, so that after the modulated first interference pulse and second interference pulse are emitted, they enter the third modulation unit 140 for modulation, and the third interference pulse after re-modulation is output from the output port of the third modulation unit.

[0103] In the above embodiments of the present invention, the incident optical pulse is first divided into two beams, and phase difference modulation is respectively performed in two groups of modulation units. Optionally, the interference pulse intensity curve output by each group of modulation units has two extreme points. By making the two interference pulses interfere again and adjusting the phase difference of the three interferences, the interference pulse intensity curve of the output can have four extreme points. Using the interference pulse with four extreme points to prepare a stable intensity state can reduce the correlation between adjacent optical pulses.

[0104] According to an embodiment of the present invention, as Figure 3 shown, the first modulation unit 120 includes a second beam splitting unit 121, a first waveguide unit 122, a first phase modulation unit 123, and a first coupling unit 124. Wherein:

[0105] The second beam splitting unit 121 is configured to receive the first pulse and divide the first pulse into a third pulse and a fourth pulse. That is, the first pulse enters from the input port of the second beam splitting unit 121, and after passing through the second beam splitting unit 121, it is divided into two beams, namely the third pulse and the fourth pulse, and the third pulse and the fourth pulse respectively exit from the two output ports of the second beam splitting unit 121.

[0106] The first waveguide unit 122 includes two waveguides, which are coupled to the second beam splitting unit 121, and the two waveguides are respectively coupled to the two output ports of the second beam splitting unit 121. After the third pulse and the fourth pulse exit from the second beam splitting unit 121, they are respectively transmitted along one of the waveguides of the first waveguide unit 122.

[0107] The first phase modulation unit 123 is coupled to the first waveguide unit 122 and is configured to modulate the phase difference between the third pulse and the fourth pulse. Optionally, the first phase modulation unit 123 changes the optical refractive index in the two waveguides, so that an optical path difference is generated between the third pulse and the fourth pulse during transmission, and thus interference occurs.

[0108] The first coupling unit 124 is coupled to the first waveguide unit 122 and is configured to receive the modulated third pulse and the fourth pulse, couple the modulated third pulse and the fourth pulse, and output the first interference pulse. The first coupling unit 124 has two input ports, which are respectively coupled to two waveguides, so that after the modulated third interference pulse and the fourth interference pulse are emitted, they enter the first coupling unit 124 for coupling and are emitted from the output port of the first coupling unit 124. Since the third pulse and the fourth pulse have a modulation phase difference, the output pulse is the first interference pulse after interference.

[0109] According to an embodiment of the present invention, as Figure 4 shown, the second modulation unit 130 includes a third beam splitting unit 131, a second waveguide unit 132, a second phase modulation unit 133, and a second coupling unit 134. Among them:

[0110] The third beam splitting unit 131 is configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse. That is, the second pulse enters from the input port of the third beam splitting unit 131, and after passing through the third beam splitting unit 131, it is split into two beams, namely the fifth pulse and the sixth pulse, and the fifth pulse and the sixth pulse are respectively emitted from the two output ports of the third beam splitting unit 131.

[0111] The second waveguide unit 132 includes two waveguides, which are coupled to the third beam splitting unit 131, and the two waveguides are respectively coupled to the two output ports of the third beam splitting unit 131. After the fifth pulse and the sixth pulse are emitted from the third beam splitting unit 131, they are respectively transmitted along one of the waveguides of the second waveguide unit 132.

[0112] The second phase modulation unit 133 is coupled to the second waveguide unit 132 and is configured to modulate the phase difference between the fifth pulse and the sixth pulse. Optionally, the second phase modulation unit 133 changes the refractive index of light in the two waveguides, so that the fifth pulse and the sixth pulse generate an optical path difference during transmission, and then interfere.

[0113] The second coupling unit 134 is coupled to the second waveguide unit 132 and is configured to receive the modulated fifth pulse and sixth pulse, couple the modulated fifth pulse and sixth pulse, and output the second interference pulse. The second coupling unit 134 has two input ports, which are respectively coupled to two waveguides, so that after the modulated fifth interference pulse and sixth interference pulse are emitted, they enter the second coupling unit 134 for coupling and are emitted from the output port of the second coupling unit 134. Since the fifth pulse and the sixth pulse have a modulation phase difference, the output pulse is the second interference pulse after interference.

[0114] According to an embodiment of the present invention, as Figure 5 shown, the third modulation unit 140 includes: a third waveguide unit 141, a third phase modulation unit 142, and a third coupling unit 143. Among them:

[0115] The third waveguide unit 141 includes two waveguides, which are coupled to the first coupling unit 124 and the second coupling unit 134. The two waveguides are respectively coupled to the output ports of the first coupling unit 124 and the second coupling unit 134. After the first interference pulse and the second interference pulse are emitted from the first coupling unit 124 and the second coupling unit 134, they are respectively transmitted along one of the waveguides of the third waveguide unit 141.

[0116] The third phase modulation unit 142 is coupled to the third waveguide unit 141 and is configured to modulate the phase difference between the first interference pulse and the second interference pulse. Optionally, the third phase modulation unit 142 changes the optical refractive index in the two waveguides, so that an optical path difference is generated between the first interference pulse and the second interference pulse during transmission, and thus interference occurs.

[0117] The third coupling unit 143 is coupled to the third waveguide unit 141 and is configured to receive the modulated first interference pulse and the second interference pulse, and couple the modulated first interference pulse and the second interference pulse to output the third interference pulse. The third coupling unit 143 has two input ports, which are respectively coupled to the two waveguides, so that after the modulated first interference pulse and the second interference pulse are emitted, they enter the third coupling unit 143 to be coupled, and are emitted from the output port of the third coupling unit 143. Its output pulse is the third interference pulse after interference occurs again.

[0118] According to an embodiment of the present invention, the splitting ratios of the first splitting unit 110, the second splitting unit 121, and the third splitting unit 131 are all 1:1.

[0119] The first splitting unit 110 equally divides the incident optical pulse into a first pulse and a second pulse with equal pulse intensities. The first pulse enters the first modulation unit 120 and is divided by the second splitting unit 121 into a third pulse and a fourth pulse with equal pulse intensities. The second pulse enters the second modulation unit 130 and is divided by the third splitting unit 131 into a fifth pulse and a sixth pulse with equal pulse intensities. Among them, the third pulse and the fourth pulse that enter the first modulation unit 120 modulate their phase difference through the first phase modulation unit 123. Let the phase difference between the third pulse and the fourth pulse be α I . The fifth pulse and the sixth pulse that enter the second modulation unit 130 modulate their phase difference through the second phase modulation unit 133. Let the phase difference between the fifth pulse and the sixth pulse be α QThe first modulation unit 120 outputs the interference pulse of the third pulse and the fourth pulse as the first interference pulse, and the second modulation unit 130 outputs the interference pulse of the fifth pulse and the sixth pulse as the second interference pulse. The first interference pulse and the second interference pulse enter the third modulation unit 140, and the phase difference is modulated by the third phase modulation unit 142. Let the phase difference between the first interference pulse and the second interference pulse be α G .

[0120] The relationship between the pulse intensity of the output third interference pulse and the pulse intensity of the incident optical pulse is:

[0121]

[0122] where η is the coefficient introduced by device differences, and μin is the pulse intensity of the incident optical pulse. It can be seen that the pulse intensity of the output third interference pulse:

[0123] I out ∝(α I , α Q , α G )

[0124] Taking the derivative of the above transfer function formula, it can be seen that when α I , α Q , α G ∈ {0, π}, the function takes the extreme point. By modulating the phase difference between the first interference pulse and the second interference pulse through the third phase modulation unit 140, making α G = 0, then:

[0125]

[0126] When (α I , α Q ) = (0, 0), the maximum value is output as the signal state S:

[0127]

[0128] When (α I , α Q ) = (π, 0), the intermediate value is taken as the decoy state D1:

[0129]

[0130] When (α I , α Q ) = (0, π), the intermediate value is taken as the decoy state D2:

[0131]

[0132] When (α I , α Q) When = (π, π), the minimum value is taken as the vacuum state V:

[0133] I V =0

[0134] Therefore, by analyzing the transfer function, it can be seen that the modulation device 100 provided by the present invention can have 4 stable output intensities, and at the same time, at the extreme points, it can theoretically suppress the coding memory effect (pattern effect) caused by electrical signal distortion.

[0135] According to an embodiment of the present invention, two waveguides of the first waveguide unit 122 and the second waveguide unit 132 are arranged in parallel, made of lithium niobate material, and the first phase modulation unit 123 and the second phase modulation unit 133 respectively include (as Figure 6 shown, taking the first waveguide unit 122 and the first phase modulation unit 123 as an example):

[0136] Traveling wave electrodes 1231, configured to inject radio frequency signals into the two waveguides;

[0137] DC bias electrodes 1232, configured to inject DC voltages into the two waveguides.

[0138] Figure 6 Shows an implementation manner of the first waveguide unit 122 and the first phase modulation unit 123. The first waveguide unit 122 and the first phase modulation unit 123, as well as the second waveguide unit 132 and the second phase modulation unit 133, can all adopt this implementation manner to form the transmission waveguide of the optical pulse and its modulation electrodes. Or, one of the first waveguide unit 122 and the first phase modulation unit 123, as well as the second waveguide unit 132 and the second phase modulation unit 133, adopts this implementation manner to form the transmission waveguide of the optical pulse and its modulation electrodes.

[0139] According to an embodiment of the present invention, as Figure 6 shown, wherein the first phase modulation unit 123 and the second phase modulation unit 133 respectively inject radio frequency signals into the two waveguides through the traveling wave electrodes, and the directions of the radio frequency signals injected into the two waveguides are opposite. Taking the first waveguide unit 122 and the first phase modulation unit 123 as an example: The first phase modulation unit 123 injects radio frequency signals into the two waveguides of the first waveguide unit 122 through the traveling wave electrodes 1231 arranged on the two waveguides of the first waveguide unit 122, and injects the radio frequency signal U1 into the upper waveguide in Figure 6 , and injects the radio frequency signal U2 into the lower waveguide in Figure 6 , and the directions of U1 and U2 are opposite.

[0140] According to an embodiment of the present invention, as Figure 6 、 Figure 7As shown, the first phase modulation unit 123 and the second phase modulation unit 133 alternately inject radio frequency signals into the two waveguides through the traveling wave electrodes. Still taking the first waveguide unit 122 and the first phase modulation unit 123 as an example: The first phase modulation unit 123 alternately injects radio frequency signals into the two waveguides of the first waveguide unit 122 through the traveling wave electrodes 1231 provided on the two waveguides of the first waveguide unit 122. As Figure 7 shown, the radio frequency signal U1 injected by the first phase modulation unit 123 into the upper waveguide in Figure 6 is the upper half cycle of a sine wave signal, and the radio frequency signal U2 injected into the lower waveguide in Figure 6 is the lower half cycle of a sine wave signal.

[0141] In the above embodiment of the present invention, the first phase modulation unit 123 and the second phase modulation unit 133 respectively inject radio frequency signals into the first waveguide unit 122 and the second waveguide unit 132. The radio frequency signal adopts push-pull modulation, which suppresses the generation of chirp signals. At the same time, the modulation voltage is low, and the original phase difference between the first interference pulse and the second interference pulse output by the first modulation unit 120 and the second modulation unit 130 remains unchanged after modulation.

[0142] According to an embodiment of the present invention, the two waveguides of the third waveguide unit 141 are arranged in parallel and made of lithium niobate material. The third phase modulation unit 142 includes:

[0143] A DC bias electrode configured to inject a DC voltage into the two waveguides.

[0144] The first phase modulation unit 123, the second phase modulation unit 133, and the third phase modulation unit 142 all include DC bias electrodes, which respectively provide DC bias voltages for the two waveguides of the first waveguide unit 122, the second waveguide unit 132, and the third waveguide unit 141. By changing the optical refractive index of the lithium niobate waveguide through the DC bias voltage, the optical path of the optical pulse transmitted in the lithium niobate waveguide is adjusted. After adding the DC bias voltage, the V π of the intensity modulator can be found by sweeping the frequency, that is, the voltage difference corresponding to the two optical pulses from constructive interference to destructive interference. Under the bias of this DC voltage, the output optical extinction ratio is the largest when the input radio frequency signals are encoded as 1 and 0.

[0145] According to an embodiment of the present invention, the third interference pulse has four extreme points. The first phase modulation unit 123, the second phase modulation unit 133, and the third phase modulation unit 142 are configured as:

[0146] By modulating the phase differences between the third pulse and the fourth pulse, the fifth pulse and the sixth pulse, and the first interference pulse and the second interference pulse, the third interference pulse is positioned at the extreme point.

[0147] According to an embodiment of the present invention, the first modulation unit 120, the second modulation unit 130, and the third modulation unit 140 each include: a Mach-Zehnder interferometer.

[0148] The technical effects of the invention are verified through experiments below.

[0149] First, the coding memory effect (Pattern effect) of a commercial IM intensity modulator (FTM7920FBA) is measured as a reference, and the experimental setup is as Figure 8 shown. This IM intensity modulator has a DC interface and an RF interface, which are respectively connected to a DC bias voltage signal and an RF signal.

[0150] By scanning the DC bias voltage, the change in the output power of the commercial IM intensity modulator is measured, as Figure 9A shown. It can be determined that the V π of the commercial IM intensity modulator is approximately 2.96V. The adjacent pulses of the RF signal V pp of the intensity modulator are encoded as 0 and 1 by an arbitrary waveform generator, and the dependence of the adjacent pulse intensity on the DC bias voltage is measured by an oscilloscope, changing V pp . As Figure 9B shown, when V pp = 500mV, the adjacent pulses exhibit the maximum output and the minimum output, and this voltage intensity is the maximum value of the RF signal of the intensity modulator. The DC bias voltage is fixed at 4.5V, and V pp is set to 500mV, and the commercial IM intensity modulator is encoded. The applied RF signal is encoded by an arbitrary waveform generator, where the constructive interference output is used as the signal state (S state), the destructive interference output is used as the vacuum state (V state), and the selected decoy state (D state) is 1 / 4 of the signal state intensity.

[0151] Figure 10 Shows the measurement results of different encoded modulation signals. When the current pulse is the decoy state (D state), the intensity of the subsequent pulse is significantly affected. When encoded as DDS and DDV, due to the influence of the previous pulse decoy state (D state), the intensities of the two decoy states are significantly different. This is consistent with the theoretical analysis results, and the decoy state (D state) will cause an obvious correlation effect on the adjacent subsequent pulse.

[0152] Next, measure the coding memory effect (Pattern effect) of the modulation device 100 provided by one or more of the above embodiments of the present invention. The experimental device is as follows Figure 11 shown. Suppose the first modulation unit 120 of the modulation device 100 is I-channel modulation, the second modulation unit 130 is Q-channel modulation, and the third modulation unit 140 is DP-channel modulation. The modulation device 100 includes an IQ modulator.

[0153] As Figure 11 shown, where the DC power supplies 1 and 2 are used to adjust the DC bias voltages of the I-channel, Q-channel, and DP-channel and the drive voltage of the RF amplifier. The modulation signal generated by an arbitrary waveform generator (AWG) drives a laser through an RF amplifier to achieve a pulsed laser output of 1.25 GHz. The encoding of α I , α Q ∈{0, π} is achieved through the RF interfaces of the I-channel and Q-channel. Finally, the output optical signal is connected to an oscilloscope through an optoelectronic converter for signal acquisition, and a power meter is used to monitor the output signal intensity.

[0154] First, measure the transfer function of the modulation device 100. Adjust the DC bias voltage of the DP-channel to achieve constructive interference (α G = 0). From theoretical analysis, it can be known that by adjusting the DC voltage U I of the I-channel such that the output and input intensities satisfy I out = μ in / 4, then cosα I = -1 can be obtained, and I out = μ in (1 + cosα Q ) / 8. Then, adjust U G such that the output I out = 0, and (U I , U G ) = (8.4, 8.4) is obtained. Close one arm and measure the transfer curve of the other arm. It can be obtained that the transfer curves of the I-channel and Q-channel are almost the same. From the transfer function, the DC half-wave voltage is about V π = 8.5 V.

[0155] Subsequently, close the I-channel (destructive interference). Use an arbitrary waveform generator to encode 0 and 1 for adjacent pulses of the RF signals of the I-channel and Q-channel. Measure the dependence of the adjacent pulse intensity on the DC bias voltage through an oscilloscope, and change V pp . When the adjacent pulses show the maximum output and minimum output, this intensity is the RF V π of the intensity modulator. Then, close the Q-channel and measure the RF V π of the I-channel. Finally, determine the V pp of both.It is approximately 480 mV. Then, the DC voltages of the I path, Q path, and DP path are set to 8.5 V, 8.5 V, and 13 V respectively, and the RF voltages of the I path and Q path are set to 480 mV. The IQ intensity modulator is encoded by an arbitrary waveform generator, and the measurement results are as Figure 12 shown. It can be seen from Figure 12 that when DD coding occurs, the output intensity of the post-pulse is not significantly affected, suppressing the coding memory effect.

[0156] Compared with a commercial IM intensity modulator, for the modulation device 100 (IQ intensity modulator) provided by the present invention, when DD coding occurs, the output optical pulse is not significantly affected, and their intensities are almost the same. This shows that the modulation device 100 provided by the present invention can suppress the coding memory effect. The following table (Table 1) summarizes the simulation measurement results of the commercial IM intensity modulator and the modulation device 100 (IQ intensity modulator) provided by the present invention. By comparison, it can be determined that the modulation device 100 provided by the present invention can greatly suppress the correlation between adjacent pulses.

[0157]

[0158] Table 1

[0159] Figure 13A and Figure 13B show the simulation diagrams of the transfer function of the modulation device 100 provided in the above embodiments (a three-dimensional diagram and a planar diagram respectively), where Figure 13A is the case where the I path and Q path are exactly the same, Figure 13B is the case where there are differences in transmission losses between the I path and Q path. In both cases, the transfer function of the modulation device 100 has 9 extreme points. The difference is that when the losses are different, the intensities of the two states D(π, 0) and (0, π) are different. And the intensity of the vacuum state I(π, π) is always 0. Therefore, there are two coding situations for the device: 1: S(0, 0), D(π, 0), and V(π, π); 2: S(0, 0), D(0, π), and V(π, π). Both situations meet the extreme point conditions and the intensity of the vacuum state is 0. At the same time, two-level coding is adopted, greatly reducing the complexity.

[0160] The present invention also provides a method 10 for preparing a stable intensity state using the modulation device 100 as described above. Among them, the first modulation unit 120 further includes: a second beam splitting unit 121 configured to receive the first pulse and split the first pulse into a third pulse and a fourth pulse; a first phase modulation unit 123 configured to modulate the phase difference between the third pulse and the fourth pulse; the second modulation unit 130 further includes: a third beam splitting unit 131 configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse; a second phase modulation unit 133 configured to modulate the phase difference between the fifth pulse and the sixth pulse; the third modulation unit 140 further includes: a third phase modulation unit 142 configured to modulate the phase difference between the first interference pulse and the second interference pulse; as Figure 14A shown, the method 10 for preparing a stable intensity state includes steps S101 - S104.

[0161] In step S101, the phase difference between the third pulse and the fourth pulse is modulated to 0 by the first phase modulation unit;

[0162] In step S102, the phase difference between the fifth pulse and the sixth pulse is modulated to 0 by the second phase modulation unit;

[0163] In step S103, the phase difference between the first interference pulse and the second interference pulse is modulated to 0 by the third phase modulation unit;

[0164] In step S104, the third interference pulse is output as the signal state output.

[0165] According to an embodiment of the present invention, as Figure 14B shown, the method 10 for preparing a stable intensity state further includes steps S105 - S108.

[0166] In step S105, the phase difference between the third pulse and the fourth pulse is modulated to π by the first phase modulation unit;

[0167] In step S106, the phase difference between the fifth pulse and the sixth pulse is modulated to 0 by the second phase modulation unit;

[0168] In step S107, the phase difference between the first interference pulse and the second interference pulse is modulated to 0 by the third phase modulation unit;

[0169] In step S108, the third interference pulse is output as the decoy state output.

[0170] According to an embodiment of the present invention, as Figure 14CAs shown, the method 10 for preparing a stable intensity state further includes steps S109 - S112.

[0171] In step S109, the phase difference between the third pulse and the fourth pulse is modulated to 0 by the first phase modulation unit;

[0172] In step S110, the phase difference between the fifth pulse and the sixth pulse is modulated to π by the second phase modulation unit;

[0173] In step S111, the phase difference between the first interference pulse and the second interference pulse is modulated to 0 by the third phase modulation unit;

[0174] In step S112, the third interference pulse is output as a decoy state output.

[0175] According to an embodiment of the present invention, as Figure 14D shown, the method 10 for preparing a stable intensity state further includes steps S113 - S116.

[0176] In step S113, the phase difference between the third pulse and the fourth pulse is modulated to π by the first phase modulation unit;

[0177] In step S114, the phase difference between the fifth pulse and the sixth pulse is modulated to π by the second phase modulation unit;

[0178] In step S115, the phase difference between the first interference pulse and the second interference pulse is modulated to 0 by the third phase modulation unit;

[0179] In step S116, the third interference pulse is output as a vacuum state.

[0180] According to an embodiment of the present invention, the first phase modulation unit 123 and the second phase modulation unit 133 respectively include: traveling - wave electrodes configured to inject radio - frequency signals into the third pulse and the fourth pulse, the fifth pulse and the sixth pulse respectively; the method 10 for preparing a stable intensity state further includes:

[0181] Loading the encoded pulse sequence to be transmitted onto the radio - frequency signal.

[0182] The modulation device for a decoy state quantum key distribution system and the method for preparing a stable intensity state using the same provided by the present invention have a pulse intensity curve of the output interference pulse with 4 extreme points and can prepare at least three stable intensity states. By using the modulation device and the preparation method provided by the present invention, the coding memory effect (pattern effect) between adjacent optical pulses can be effectively suppressed, ensuring the independence between the coded pulses. Moreover, in the modulation device and the preparation method provided by the present invention, the radio frequency signal uses push-pull modulation, suppressing the generation of chirp signals, with a low modulation voltage, and the phase difference between the two interference pulses after modulation remains unchanged, making it more suitable for high-speed decoy state quantum key distribution systems.

Claims

1. A modulation device for a decoy state quantum key distribution system, characterized in that Comprising: A first beam splitting unit configured to receive an incident optical pulse and split the incident optical pulse into a first pulse and a second pulse; A first modulation unit coupled to the first beam splitting unit and configured to modulate the first pulse to generate a first interference pulse; A second modulation unit coupled to the first beam splitting unit and configured to modulate the second pulse to generate a second interference pulse; A third modulation unit coupled to the first modulation unit and the second modulation unit and configured to modulate the first interference pulse and the second interference pulse to generate a third interference pulse, the third interference pulse having four extreme points, wherein The third modulation unit comprises: A third waveguide unit including two waveguides respectively coupled to the first modulation unit and the second modulation unit and configured to receive the first interference pulse and the second interference pulse and cause the first interference pulse and the second interference pulse to respectively propagate along one of the two waveguides of the third waveguide unit; A third phase modulation unit coupled to the third waveguide unit and configured to modulate the phase difference between the first interference pulse and the second interference pulse; A third coupling unit coupled to the third waveguide unit and configured to receive the modulated first interference pulse and the second interference pulse and couple the modulated first interference pulse and the second interference pulse to output the third interference pulse.

2. The modulation device according to claim 1, wherein the first modulation unit comprises: A second beam splitting unit configured to receive the first pulse and split the first pulse into a third pulse and a fourth pulse; A first waveguide unit including two waveguides coupled to the second beam splitting unit and configured to receive the third pulse and the fourth pulse and cause the third pulse and the fourth pulse to respectively propagate along one of the two waveguides; A first phase modulation unit coupled to the first waveguide unit and configured to modulate the phase difference between the third pulse and the fourth pulse; A first coupling unit coupled to the first waveguide unit and configured to receive the modulated third pulse and the fourth pulse and couple the modulated third pulse and the fourth pulse to output the first interference pulse.

3. The modulation device according to claim 2, wherein the second modulation unit comprises: A third beam splitting unit configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse; A second waveguide unit including two waveguides coupled to the third beam splitting unit and configured to receive the fifth pulse and the sixth pulse and cause the fifth pulse and the sixth pulse to respectively propagate along one of the two waveguides of the second waveguide unit; A second phase modulation unit coupled to the second waveguide unit and configured to modulate the phase difference between the fifth pulse and the sixth pulse; A second coupling unit coupled to the second waveguide unit and configured to receive the modulated fifth pulse and the sixth pulse and couple the modulated fifth pulse and the sixth pulse to output the second interference pulse.

4. The modulation device according to claim 3, wherein the splitting ratios of the first splitting unit, the second splitting unit, and / or the third splitting unit are all 1:

1.

5. The modulation device according to claim 3, wherein two waveguides of any one of the first waveguide unit and the second waveguide unit are arranged in parallel and made of lithium niobate material, and the first phase modulation unit and the second phase modulation unit respectively include: Traveling wave electrodes configured to inject radio frequency signals into corresponding two waveguides; DC bias electrodes configured to inject DC voltages into corresponding two waveguides.

6. The modulation device according to claim 5, wherein the first phase modulation unit and the second phase modulation unit are further configured to: Inject radio frequency signals into corresponding two waveguides respectively through the traveling wave electrodes, and the directions of the radio frequency signals injected into the corresponding two waveguides are opposite.

7. The modulation device according to claim 6, wherein the first phase modulation unit and the second phase modulation unit are further configured to: Inject radio frequency signals into corresponding two waveguides in a push-pull manner through the traveling wave electrodes.

8. The modulation device according to any one of claims 1-7, wherein two waveguides of the third waveguide unit are arranged in parallel and made of lithium niobate material, and the third phase modulation unit includes: DC bias electrodes configured to inject DC voltages into corresponding two waveguides.

9. The modulation device according to any one of claims 3-7, wherein the first phase modulation unit, the second phase modulation unit, and the third phase modulation unit are configured to: By modulating the phase differences between the third pulse and the fourth pulse, the fifth pulse and the sixth pulse, and the first interference pulse and the second interference pulse, make the third interference pulse located at the extreme point.

10. The modulation device according to any one of claims 3-7, wherein the first modulation unit, the second modulation unit, and the third modulation unit respectively include: Mach-Zehnder interferometer.

11. A method for preparing a stable intensity state using the modulation device according to any one of claims 1-10, characterized in that The first modulation unit further includes: A second splitting unit configured to receive the first pulse and split the first pulse into a third pulse and a fourth pulse; A first phase modulation unit configured to modulate the phase difference between the third pulse and the fourth pulse; The second modulation unit further includes: A third splitting unit configured to receive the second pulse and split the second pulse into a fifth pulse and a sixth pulse; A second phase modulation unit configured to modulate the phase difference between the fifth pulse and the sixth pulse; The third modulation unit further includes: A third phase modulation unit configured to modulate the phase difference between the first interference pulse and the second interference pulse; The method includes: Modulating the phase difference between the third pulse and the fourth pulse to be 0 by the first phase modulation unit; Modulating the phase difference between the fifth pulse and the sixth pulse to be 0 by the second phase modulation unit; Modulating the phase difference between the first interference pulse and the second interference pulse to be 0 by the third phase modulation unit; Outputting the third interference pulse as a signal state output.

12. A method as claimed in claim 11, further comprising: modulating, by means of the first phase modulation unit, the phase difference between the third pulse and the fourth pulse to be π; modulating, by means of the second phase modulation unit, the phase difference between the fifth pulse and the sixth pulse to be 0; modulating, by means of the third phase modulation unit, the phase difference between the first interference pulse and the second interference pulse to be 0; outputting the third interference pulse as a decoy state output.

13. A method as claimed in claim 11 or 12, further comprising: modulating, by means of the first phase modulation unit, the phase difference between the third pulse and the fourth pulse to be 0; modulating, by means of the second phase modulation unit, the phase difference between the fifth pulse and the sixth pulse to be π; modulating, by means of the third phase modulation unit, the phase difference between the first interference pulse and the second interference pulse to be 0; outputting the third interference pulse as a decoy state output.

14. A method as claimed in claim 11 or 12, further comprising: modulating, by means of the first phase modulation unit, the phase difference between the third pulse and the fourth pulse to be π; modulating, by means of the second phase modulation unit, the phase difference between the fifth pulse and the sixth pulse to be π; modulating, by means of the third phase modulation unit, the phase difference between the first interference pulse and the second interference pulse to be 0; outputting the third interference pulse as a vacuum state output.

15. The method according to claim 11 or 12, wherein the first phase modulation unit and the second phase modulation unit respectively comprise: A traveling-wave electrode configured to inject radio frequency signals into the third pulse and the fourth pulse, and the fifth pulse and the sixth pulse respectively; The method further comprises: loading a coded pulse sequence to be transmitted onto the radio frequency signal.

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