A plug-and-play time-phase-coded quantum key distribution network

Abstract

The present invention belongs to the field of optical secure communication technology and discloses a plug-and-play time-phase coded quantum key distribution network. The central node includes a multi-wavelength laser, three beam splitters, a wavelength division multiplexer, and two single-photon detection units. Each user node includes an adjustable attenuator and a reflective intensity modulation module. The reflective intensity modulation module is used to modulate the intensity of two sub-pulses arriving at the user node through beam splitting-reflection-interference, thereby randomly generating two time-coded states in the Z basis and one phase-coded state in the X basis, as well as corresponding decoy states, while simultaneously rotating the polarization states of the two sub-pulses by 90 degrees. Compared with the prior art, the present invention adopts a round-trip architecture. Each user node uses the reflective intensity modulation module to implement three-state time-phase coding. This not only enables automatic polarization and phase compensation, but also enables polarization-independent quantum state preparation and decoy state preparation.

Description

Technical Field

[0001] The present invention relates to the technical field of optical security communications, and in particular to a plug-and-play time-phase coded quantum key distribution network. Background Art

[0002] Quantum Key Distribution (QKD) can provide unconditionally secure key distribution for both communicating parties and plays an important role in the field of information security. The most basic structure of QKD is point-to-point application. If multi-user application needs to be expanded, it needs to be based on trusted relay nodes or optical node expansion. The former requires ensuring that the relay nodes are trustworthy and reliable, while the latter does not need to ensure the reliability of the optical nodes. Conventional optical nodes can be implemented through optical splitters, optical switches, wavelength division multiplexers or other optical passive devices. They have the advantages of good security and easy implementation, but they need to divide the optical signal equally according to the number of users, which will seriously affect the single-user key generation rate and transmission distance when the network is expanded.

[0003] Patent CN104092538B proposes a plug-and-play QKD network solution for multi-user wavelength division multiplexing. This solution uses multi-wavelength pulses generated simultaneously by a single-source, multi-wavelength laser as the carrier for multi-user information transmission. Each wavelength pulse is sent to a different legitimate user, making each user relatively independent. This ensures a stable key generation rate for each user, which does not decrease with the increase in users. However, this solution uses phase encoding and decoding, requiring phase modulation decoding at an intermediate node. Since all wavelengths (corresponding to all users) pass through the same phase modulator, the decoded signals for different users are identical, and the information between users is no longer random, compromising the security of the key. Furthermore, each user only performs phase encoding, without modulating the decoy state, making it vulnerable to photon number splitting attacks and significantly reducing security. Patent CN117155562A, which uses phase modulation and polarization measurement, also suffers from the above two issues. Summary of the Invention

[0004] In view of the above defects in the prior art, the present invention proposes a bidirectional intensity modulation device.

[0005] The technical solution of the present invention is achieved as follows:

[0006] A plug-and-play time-phase coded quantum key distribution network includes a central node and multiple user nodes, each of which is connected to the central node via an optical fiber channel.

[0007] The central node includes a multi-wavelength laser LD, a first beam splitter BS1, a second beam splitter BS2, a third beam splitter BS3, a wavelength division multiplexer WDM and two single-photon detection units;

[0008] A multi-wavelength laser LD and a single photon detection unit are respectively connected to two input ports of the first beam splitter BS1;

[0009] The two output ports of the first beam splitter BS1 and the two input ports of the second beam splitter BS2 are connected through polarization-maintaining optical fibers of different lengths to form an unequal-arm interferometer;

[0010] The output port of the second beam splitter BS2 and another single photon detection unit are connected to the two input ports of the third beam splitter BS3 respectively;

[0011] The output port of the third beam splitter BS3 is connected to the common end of the wavelength division multiplexer WDM;

[0012] Each wavelength output port of the wavelength division multiplexer WDM is connected to each user node through an optical fiber channel;

[0013] Each user node includes a connected variable attenuator VOA and a reflective intensity modulation module;

[0014] The multi-wavelength laser LD is used to generate horizontally polarized multi-wavelength optical pulses;

[0015] The unequal-arm interferometer is used to split a multi-wavelength optical pulse into two sub-pulses with a time difference of t;

[0016] The single-photon detection unit can realize the simultaneous detection of multi-wavelength optical signals;

[0017] The reflective intensity modulation module is used to perform intensity modulation on the two sub-pulses arriving at the user node through a splitting-reflection-interference method to randomly generate two time-coded states in the Z basis and one phase-coded state in the X basis, as well as the corresponding decoy states, while rotating the polarization states of the two sub-pulses by 90°.

[0018] The variable attenuator (VOA) is used to decay the time-phase-encoded quantum state to the single-photon level.

[0019] Preferably, the reflective intensity modulation module includes a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first Faraday mirror FM1, a second Faraday mirror FM2 and a first phase modulator PM1.

[0020] Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively;

[0021] Another input port of the first polarization beam splitter PBS1 and another input port of the second polarization beam splitter PBS2 are connected to two ends of the first phase modulator PM1 through polarization-maintaining optical fibers F3 and F4, respectively;

[0022] One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected to the first Faraday mirror FM1 and the second Faraday mirror FM2 through the polarization-maintaining fiber F5 and the polarization-maintaining fiber F6 respectively;

[0023] The other output port of the first polarization beam splitter PBS1 and the other output port of the second polarization beam splitter PBS2 are connected through a polarization-maintaining fiber F7.

[0024] Preferably, the lengths of the polarization-maintaining fiber F3 and the polarization-maintaining fiber F4 are equal, the lengths of the polarization-maintaining fiber F5 and the polarization-maintaining fiber F6 are equal, and the length of the polarization-maintaining fiber F7 is twice that of the polarization-maintaining fiber F5.

[0025] Preferably, the reflective intensity modulation module includes a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a Faraday rotator FR, a second phase modulator PM2 and a third phase modulator PM3.

[0026] Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively;

[0027] One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected via a polarization-maintaining fiber F8. A Faraday rotator FR is provided on F8. The polarization rotation angle of the Faraday rotator FR is 90°, and the polarization directions at both ends are aligned with the slow axis and the fast axis of the polarization-maintaining fiber, respectively.

[0028] Another input port of the first polarization beam splitter PBS1 and another output port of the second polarization beam splitter PBS2 are connected to two ends of the second phase modulator PM2 through polarization-maintaining optical fibers F9 and F10 respectively;

[0029] The other output port of the first polarization beam splitter PBS1 and the other input port of the second polarization beam splitter PBS2 are connected to the third phase modulator PM3 through the polarization-maintaining fiber F11 and the polarization-maintaining fiber F12 respectively.

[0030] Preferably, the length of the polarization-maintaining fiber F10 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F9, and the length of the polarization-maintaining fiber F11 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F12.

[0031] Preferably, the single-photon detection unit includes a wavelength division multiplexer WDM and a plurality of single-photon detectors SPD, and each single-photon detector SPD is used to detect one of the optical signals separated by the wavelength division multiplexer WDM.

[0032] Preferably, the period of the light pulse generated by the multi-wavelength laser LD is 2t, and the gating period of the single photon detector SPD in the single photon detection unit is t.

[0033] Preferably, the splitting ratio of the third beam splitter BS3 is 10:90, wherein 10% of the beam splitting ports are connected to the second beam splitter BS2.

[0034] Preferably, each user node further includes an unbalanced beam splitter and a photodetector, wherein the port of the unbalanced beam splitter with lower beam splitting energy is connected to the variable attenuator VOA, and the port with higher beam splitting energy is connected to the photodetector for security monitoring.

[0035] Preferably, an optical isolator is further provided between the multi-wavelength laser LD and the first beam splitter BS1 for isolating the optical signal returning from the first beam splitter BS1 to the multi-wavelength laser LD.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention proposes a plug-and-play time-phase coded quantum key distribution network, which adopts a round-trip architecture. Each user node uses a reflective intensity modulation module to implement three-state time-phase coding. It can not only realize automatic compensation of polarization and phase, but also realize polarization-independent quantum state preparation and decoy state preparation. The use of passive basis measurement can avoid the problem of the central node decoding the same information for all users. In addition, the structure can support higher rates, greatly improving the stability and practicality of the QKD network. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a schematic diagram of the principle of the plug-and-play time-phase coded quantum key distribution network of the present invention;

[0039] Figure 2 This is a schematic diagram showing the principle of a reflective intensity modulation module according to a first embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram showing the principle of a second embodiment of a reflective intensity modulation module according to the present invention;

[0041] Figure 4 This is a structural diagram of an embodiment of a single-photon detection unit of the present invention. DETAILED DESCRIPTION

[0042] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0043] like Figure 1As shown, a plug-and-play time phase coded quantum key distribution network includes a central node and multiple user nodes, each user node is connected to the central node through an optical fiber channel.

[0044] The central node includes a multi-wavelength laser LD, a first beam splitter BS1, a second beam splitter BS2, a third beam splitter BS3, a wavelength division multiplexer WDM and two single-photon detection units;

[0045] A multi-wavelength laser LD and a single photon detection unit are respectively connected to two input ports of the first beam splitter BS1;

[0046] The two output ports of the first beam splitter BS1 and the two input ports of the second beam splitter BS2 are connected through polarization-maintaining optical fibers of different lengths to form an unequal-arm interferometer;

[0047] The output port of the second beam splitter BS2 and another single photon detection unit are connected to the two input ports of the third beam splitter BS3 respectively;

[0048] The output port of the third beam splitter BS3 is connected to the common end of the wavelength division multiplexer WDM;

[0049] Each wavelength output port of the wavelength division multiplexer WDM is connected to each user node through an optical fiber channel;

[0050] Each user node includes a connected variable attenuator VOA and a reflective intensity modulation module;

[0051] The multi-wavelength laser LD is used to generate horizontally polarized multi-wavelength optical pulses;

[0052] The unequal-arm interferometer is used to split a multi-wavelength optical pulse into two sub-pulses with a time difference of t;

[0053] The single-photon detection unit can realize the simultaneous detection of multi-wavelength optical signals;

[0054] The reflective intensity modulation module is used to perform intensity modulation on the two sub-pulses arriving at the user node through a splitting-reflection-interference method to randomly generate two time-coded states in the Z basis and one phase-coded state in the X basis, as well as the corresponding decoy states, while rotating the polarization states of the two sub-pulses by 90°.

[0055] The variable attenuator (VOA) is used to decay the time-phase-encoded quantum state to the single-photon level.

[0056] The specific working process is as follows:

[0057] The multi-wavelength laser LD at the central node generates a horizontally polarized multi-wavelength light pulse, which enters the unequal-arm interferometer composed of the first beam splitter BS1 and the second beam splitter BS2 and is divided into two sub-pulses with a time difference of t and perpendicular polarizations. Both are horizontally polarized and have equal amplitudes.

[0058] The two sub-pulses enter the wavelength division multiplexer WDM through the third beam splitter BS3 and are divided into multiple groups of twin sub-pulses with different wavelengths. The twin pulses of φi pass through the corresponding optical fiber channel and reach the corresponding user node i, changing into arbitrary polarization states.

[0059] The first sub-pulse enters the corresponding user node i, passes through the variable attenuator (VOA), and then enters the reflective intensity modulation module. Through beam splitting, reflection, and interference, it is modulated to an intensity of k before exiting the reflective intensity modulation module with its polarization state rotated 90°. The second sub-pulse also enters the corresponding user node i after time t, passes through the variable attenuator (VOA), enters the reflective intensity modulation module, and through beam splitting, reflection, and interference, it is modulated to an intensity of j before exiting the reflective intensity modulation module with its polarization state also rotated 90°.

[0060] Because the polarization of both sub-pulses is rotated 90°, acting like a Faraday mirror, the polarization variation of the optical signal can be automatically compensated. Both are then attenuated to the single-photon level by a variable attenuator (VOA), generating three temporally phase-encoded quantum states.

[0061] For the signal state, when k=u, j=0, the quantum state is in the Z basis state; when k=0, j=u, the quantum state is in the Z basis state; when k=u / 2, j=u / 2, the quantum state is in the X basis state; for the decoy state, when k=v, j=0, the quantum state is state; when k=0, j=v, the quantum state is in the Z basis state; when k=v / 2, j=v / 2, the quantum state is in the X basis state.

[0062] The quantum state returns to the central node via the optical fiber channel, where it is combined by the wavelength division multiplexer (WDM) and enters the third beam splitter (BS3). The polarization state is rotated 90° compared to when it exited the central node, automatically compensating for the polarization change. Now, both sub-pulses of the quantum state are horizontally polarized. The quantum state is then split by the third beam splitter (BS3) for passive basis selection. The higher-intensity component directly enters a single-photon detection unit (SPD) capable of simultaneously detecting multi-wavelength optical signals for Z-basis measurement. The detected quantum state is determined based on the response of each SPD in the SPD unit in two preceding and following time windows. The lower-intensity component enters the unequal-arm interferometer in the opposite direction for interference, generating an X-basis decoded optical signal. Since the optical signal passes back and forth through the polarization interferometer, phase differences are automatically compensated. Therefore, the correct decoding result of the X-basis decoded optical signal enters the direction of the multi-wavelength laser (LD). This portion of the decoded optical signal can be isolated by an optical isolator, while the incorrect decoding result enters another SPD unit.

[0063] After the central node and user node i perform basis matching, they use the X-basis count to estimate parameters and use Z to encode. After error correction and confidentiality amplification, both parties can share the same secure quantum key. The process of quantum key distribution for the central node and other user nodes is exactly the same.

[0064] like Figure 2 As shown, Example 1:

[0065] The reflective intensity modulation module includes a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first Faraday mirror FM1, a second Faraday mirror FM2 and a first phase modulator PM1.

[0066] Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively;

[0067] Another input port of the first polarization beam splitter PBS1 and another input port of the second polarization beam splitter PBS2 are connected to two ends of the first phase modulator PM1 through polarization-maintaining optical fibers F3 and F4, respectively;

[0068] One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected to the first Faraday mirror FM1 and the second Faraday mirror FM2 through the polarization-maintaining fiber F5 and the polarization-maintaining fiber F6 respectively;

[0069] The other output port of the first polarization beam splitter PBS1 and the other output port of the second polarization beam splitter PBS2 are connected through a polarization-maintaining fiber F7.

[0070] The lengths of the polarization-maintaining optical fiber F3 and the polarization-maintaining optical fiber F4 are equal, the lengths of the polarization-maintaining optical fiber F5 and the polarization-maintaining optical fiber F6 are equal, and the length of the polarization-maintaining optical fiber F7 is twice that of the polarization-maintaining optical fiber F5.

[0071] like Figure 4 As shown, the single-photon detection unit includes a wavelength division multiplexer WDM and a plurality of single-photon detectors SPD, and each single-photon detector SPD is used to detect one of the optical signals separated by the wavelength division multiplexer WDM.

[0072] The period of the light pulse generated by the multi-wavelength laser LD is 2t, and the gating period of the single photon detector SPD in the single photon detection unit is t.

[0073] The specific working process of embodiment 1 is as follows:

[0074] The multi-wavelength laser LD at the central node generates a horizontally polarized multi-wavelength light pulse, which enters the unequal-arm interferometer composed of the first beam splitter BS1 and the second beam splitter BS2 and is divided into two sub-pulses with a time difference of t and perpendicular polarizations. Both are horizontally polarized and have equal amplitudes.

[0075] The two sub-pulses enter the wavelength division multiplexer WDM through the third beam splitter BS3 and are divided into multiple groups of twin sub-pulses with different wavelengths. The twin pulses of φi pass through the corresponding optical fiber channel and reach the corresponding user node i, changing into arbitrary polarization states.

[0076] The previous sub-pulse enters the corresponding user node i, passes through the variable attenuator (VOA), and enters the fourth beam splitter (BS4), where it is split into a first component and a second component with the same polarization and equal amplitude. The first component travels along optical fiber F1 to an input port of the first polarization beam splitter (PBS1), where it is polarization-split into a first horizontal component and a first vertical component. The first horizontal component passes through the first polarization beam splitter (PBS1) along the slow axis of polarization-maintaining fiber (F7) and reaches the second polarization beam splitter (PBS2). It is reflected into polarization-maintaining fiber (F4), propagates along its fast axis, and after phase modulation by the first phase modulator (PM1), propagates along the fast axis of polarization-maintaining fiber (F3) and enters the first polarization beam splitter (PBS1). After being transmitted by the first phase modulator (PM1), it enters polarization-maintaining fiber (F5), propagates along its fast axis, is reflected by the first Faraday mirror (FM1), and undergoes a 90° polarization rotation. It then travels along the slow axis of polarization-maintaining fiber (F5) and reaches the first polarization beam splitter (PBS1), where it is reflected again. The first vertical component is reflected by the first polarization beam splitter PBS1, propagates along the slow axis of polarization-maintaining fiber F5, and is reflected by the first Faraday mirror FM1, where it undergoes a 90° polarization rotation. It then passes through the fast axis of F5 and reaches the first polarization beam splitter PBS1. After being transmitted, it propagates along the fast axis of polarization-maintaining fiber F3. It then undergoes phase modulation by the first phase modulator PM1, propagates along the fast axis of polarization-maintaining fiber F4, enters the second polarization beam splitter PBS2, is reflected by the second polarization beam splitter, and enters the slow axis of polarization-maintaining fiber F7, where it is transmitted through the first polarization beam splitter PBS1. Because the first horizontal component and the first vertical component travel the exact same optical path, they are imbued with the same phase by the optical fiber. Furthermore, the optical fiber lengths F3 = F4, and 2F5 = F7, satisfying the equation F3 + 2F5 = F4 + F7. Therefore, both components pass through the first phase modulator PM1 in both directions, receiving the same phase modulation, θ1. Finally, both components are simultaneously emitted from the first polarization beam splitter PBS1 and combined into the first reflected component, with the polarization state rotated by 90°, increasing the overall phase θ1. The first reflected component then propagates through the optical fiber F1 and reaches the fourth beam splitter BS4 , and its polarization is rotated by 90° compared to when it exits the fourth beam splitter BS4 .

[0077] The second component travels along optical fiber F2 to one input port of the second polarization beam splitter PBS2, where it is polarized and split into a second horizontal component and a second vertical component. The second vertical component passes through the second polarization beam splitter PBS2 along the fast axis of polarization-maintaining fiber F7, reaches the first polarization beam splitter PBS1, is reflected, enters polarization-maintaining fiber F3, propagates along its slow axis, is phase-modulated by the first phase modulator PM1, and then propagates along the slow axis of polarization-maintaining fiber F4, enters the second polarization beam splitter PBS2, is transmitted by the first phase modulator, and then enters polarization-maintaining fiber F6. It propagates along the slow axis to the second Faraday mirror FM2, is reflected, and rotated 90°. It then travels along the fast axis of polarization-maintaining fiber F6 and reaches the second polarization beam splitter PBS2, where it is reflected. The second horizontal component is reflected by the second polarization beam splitter PBS2, propagates along the fast axis of polarization-maintaining fiber F6, is reflected by the second Faraday mirror FM2, and has its polarization rotated 90°. It then passes through the slow axis of F6 and reaches the second polarization beam splitter PBS2. After being transmitted, it propagates along the slow axis of polarization-maintaining fiber F4. It then passes through the first phase modulator PM1, where its phase is modulated, and propagates along the slow axis of polarization-maintaining fiber F3. It then enters the first polarization beam splitter PBS1, where it is reflected and enters the fast axis of polarization-maintaining fiber F7. It then reaches the second polarization beam splitter PBS2 and is transmitted. Because the second horizontal component and the vertical component travel the same optical path, they are imbued with the same phase by the optical fiber. Furthermore, the optical fiber lengths F3 = F4, and 2F6 = F7, satisfying the equation F3 + 2F6 = F4 + F7. Both components pass through the first phase modulator PM1 in both directions, receiving the same phase modulation, θ2. Finally, both components are simultaneously emitted from the first polarization beam splitter PBS1 and combined into the second reflected component. Its polarization state is rotated 90°, increasing the overall phase θ2. The second reflected component then propagates through the optical fiber F2 and reaches the fourth beam splitter BS4, with its polarization rotated by 90° compared to when it exits the fourth beam splitter BS4.

[0078] It can be seen that the first reflected component and the second reflected component arrive at the fourth beam splitter BS4 at the same time, and the two have the same polarization, and the phase difference is Δ1=θ1-θ2. After the two interfere, they are emitted from the input port of the fourth beam splitter BS4. The interference result is

[0079] ,

[0080] After time t, the next sub-pulse also enters the fourth beam splitter BS4 and undergoes the same intensity modulation process as the previous sub-pulse. The light intensity becomes

[0081] ,

[0082] Because the polarization of both sub-pulses is rotated 90°, acting like a Faraday mirror, the polarization variation of the optical signal can be automatically compensated. Both are then attenuated to the single-photon level by a variable attenuator (VOA), generating three temporally phase-encoded quantum states.

[0083] For the signal state, when k=u, j=0, the quantum state is in the Z basis state; when k=0, j=u, the quantum state is in the Z basis state; when k=u / 2, j=u / 2, the quantum state is in the X basis state; for the decoy state, when k=v, j=0, the quantum state is state; when k=0, j=v, the quantum state is in the Z basis state; when k=v / 2, j=v / 2, the quantum state is in the X basis state.

[0084] The quantum state returns to the central node via the optical fiber channel, where it is combined by the wavelength division multiplexer (WDM) and enters the third beam splitter (BS3). The polarization state is rotated 90° compared to when it exited the central node, automatically compensating for the polarization change. Now, both sub-pulses of the quantum state are horizontally polarized. The quantum state is then split by the third beam splitter (BS3) for passive basis selection. The higher-intensity component directly enters a single-photon detection unit (SPD) capable of simultaneously detecting multi-wavelength optical signals for Z-basis measurement. For each wavelength, the quantum state is split by the WDM and then enters a corresponding single-photon detector (SPD). The detected quantum state is determined based on the response in the two preceding and following time windows. The lower-intensity component enters the unequal-arm interferometer in the opposite direction for interference, generating an X-basis decoded optical signal. Since the optical signal passes back and forth through the polarization interferometer, phase differences are automatically compensated. Therefore, the correct decoding result of the X-basis decoded optical signal enters the direction of the multi-wavelength laser LD. This portion of the decoded optical signal can be isolated by an optical isolator, while the incorrect decoding result enters another SPD unit.

[0085] After the central node and user node i perform basis matching, they use the X-basis count to estimate parameters and use Z to encode. After error correction and confidentiality amplification, both parties can share the same secure quantum key. The process of quantum key distribution for the central node and other user nodes is exactly the same.

[0086] like Figure 3 As shown, Example 2:

[0087] The reflective intensity modulation module includes a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a Faraday rotator FR, a second phase modulator PM2 and a third phase modulator PM3.

[0088] Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively;

[0089] One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected via a polarization-maintaining fiber F8. A Faraday rotator FR is provided on F8. The polarization rotation angle of the Faraday rotator FR is 90°, and the polarization directions at both ends are aligned with the slow axis and the fast axis of the polarization-maintaining fiber, respectively.

[0090] Another input port of the first polarization beam splitter PBS1 and another output port of the second polarization beam splitter PBS2 are connected to two ends of the second phase modulator PM2 through polarization-maintaining optical fibers F9 and F10 respectively;

[0091] The other output port of the first polarization beam splitter PBS1 and the other input port of the second polarization beam splitter PBS2 are connected to the third phase modulator PM3 through the polarization-maintaining fiber F11 and the polarization-maintaining fiber F12 respectively.

[0092] The length of the polarization-maintaining fiber F10 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F9, and the length of the polarization-maintaining fiber F11 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F12.

[0093] like Figure 4 As shown, the single-photon detection unit includes a wavelength division multiplexer WDM and a plurality of single-photon detectors SPD, and each single-photon detector SPD is used to detect one of the optical signals separated by the wavelength division multiplexer WDM.

[0094] The period of the light pulse generated by the multi-wavelength laser LD is 2t, and the gating period of the single photon detector SPD in the single photon detection unit is t.

[0095] The specific working process of Example 2 is as follows:

[0096] The multi-wavelength laser LD at the central node generates a horizontally polarized multi-wavelength light pulse, which enters the unequal-arm interferometer composed of the first beam splitter BS1 and the second beam splitter BS2 and is divided into two sub-pulses with a time difference of t and perpendicular polarizations. Both are horizontally polarized and have equal amplitudes.

[0097] The two sub-pulses enter the wavelength division multiplexer WDM through the third beam splitter BS3 and are divided into multiple groups of twin sub-pulses with different wavelengths. The twin pulses of φi pass through the corresponding optical fiber channel and reach the corresponding user node i, changing into arbitrary polarization states.

[0098] The previous sub-pulse enters the corresponding user node i, passes through the variable attenuator (VOA), and enters the fourth beam splitter (BS4), where it is split into a first component and a second component with the same polarization and equal amplitude. The first component travels along optical fiber F1 to an input port of the first polarization beam splitter (PBS1), where it is polarization-split into a first horizontal component and a first vertical component. The first horizontal component passes through the first polarization beam splitter (PBS1) along the slow axis of polarization-maintaining fiber (F8) and reaches the Faraday rotator (FR). After polarization rotation by 90°, it propagates along the fast axis of F8 and reaches the second polarization beam splitter (PBS2). It is reflected into polarization-maintaining fiber (F12), propagates along its slow axis, and after phase modulation by the third phase modulator (PM3), it propagates along the slow axis of polarization-maintaining fiber (F11), enters the first polarization beam splitter (PBS1), and is reflected. The first vertical component is reflected by the first polarization beam splitter PBS1 and propagates along the slow axis of the polarization-maintaining fiber F11. It then undergoes phase modulation by the third phase modulator PM3 and propagates along the slow axis of the polarization-maintaining fiber F12. It then enters the second polarization beam splitter PBS2, where it is reflected and enters the fast axis of the polarization-maintaining fiber F8. It then undergoes a 90° polarization rotation by the Faraday rotator FR, propagates along the slow axis of the polarization-maintaining fiber F8, and is transmitted through the first polarization beam splitter PBS1. Because the first horizontal component and the first vertical component travel the exact same optical path length, they are imbued with the same phase by the optical fiber, and the fiber length satisfies F8 + F12 = F11. Both components simultaneously pass through the third phase modulator PM3 in both directions, receiving the same phase θ1. Finally, they are simultaneously emitted from the first polarization beam splitter PBS1 and combined into the first reflected component, with its polarization state rotated by 90°, increasing the overall phase θ1. The first reflected component then propagates through the optical fiber F1 and reaches the fourth beam splitter BS4, its polarization rotated by 90° compared to when it exited BS4.

[0099] The second component travels along optical fiber F2 and reaches an input port of the second polarization beam splitter PBS2, where it is polarized and split into a second horizontal component and a second vertical component. The second horizontal component passes through the second polarization beam splitter PBS2 and propagates along the slow axis of polarization-maintaining fiber F8. After being polarized by the Faraday rotator FR and rotated 90°, it propagates along the fast axis of polarization-maintaining fiber F8. It then reaches the first polarization beam splitter PBS1, where it is reflected and enters the polarization-maintaining fiber F9. It then propagates along its slow axis. After being phase-modulated by the second phase modulator PM2, it propagates along the slow axis of polarization-maintaining fiber F10 and enters the second polarization beam splitter PBS2, where it is reflected. The second vertical component is reflected by the second polarization beam splitter PBS2 and propagates along the slow axis of polarization-maintaining fiber F10. After being phase-modulated by the second phase modulator PM2, it propagates along the slow axis of polarization-maintaining fiber F9 and enters the first polarization beam splitter PBS1. It is then reflected and enters the fast axis of polarization-maintaining fiber F8. After being polarized by the Faraday rotator FR and rotated 90°, it propagates along the slow axis of polarization-maintaining fiber F8 and enters the second polarization beam splitter PBS2, where it is transmitted. Because the second horizontal component and the second vertical component follow exactly the same optical path and are imbued with the same phase by the optical fiber, and the optical fiber length satisfies F8 + F9 = F10, both components pass through the second phase modulator PM2 simultaneously in two directions, modulating the components to the same phase θ2. Finally, both components are simultaneously emitted from the second polarization beam splitter PBS2 and combined into the second reflected component, with the polarization state rotated 90°, increasing the overall phase θ2. The second reflected component then propagates through optical fiber F2 to reach the fourth beam splitter BS4, with its polarization rotated 90° compared to when it exited BS4.

[0100] It can be seen that the first reflected component and the second reflected component arrive at the fourth beam splitter BS4 at the same time, and the two have the same polarization, and the phase difference is Δ1=θ1-θ2. After the two interfere, they are emitted from the input port of the fourth beam splitter BS4. The interference result is

[0101] ,

[0102] After time t, the next sub-pulse also enters the fourth beam splitter BS4 and undergoes the same intensity modulation process as the previous sub-pulse. The light intensity becomes

[0103] ,

[0104] Because the polarization of both sub-pulses is rotated 90°, acting like a Faraday mirror, the polarization variation of the optical signal can be automatically compensated. Both are then attenuated to the single-photon level by a variable attenuator (VOA), generating three temporally phase-encoded quantum states.

[0105] For the signal state, when k=u, j=0, the quantum state is in the Z basis state; when k=0, j=u, the quantum state is in the Z basis state; when k=u / 2, j=u / 2, the quantum state is in the X basis state; for the decoy state, when k=v, j=0, the quantum state is state; when k=0, j=v, the quantum state is in the Z basis state; when k=v / 2, j=v / 2, the quantum state is in the X basis state.

[0106] The quantum state returns to the central node via the optical fiber channel, where it is combined by the wavelength division multiplexer (WDM) and enters the third beam splitter (BS3). The polarization state is rotated 90° compared to when it exited the central node, automatically compensating for the polarization change. Now, both sub-pulses of the quantum state are horizontally polarized. The quantum state is then split by the third beam splitter (BS3) for passive basis selection. The higher-intensity component directly enters a single-photon detection unit (SPD) capable of simultaneously detecting multi-wavelength optical signals for Z-basis measurement. For each wavelength, the quantum state is split by the WDM and then enters a corresponding single-photon detector (SPD). The detected quantum state is determined based on the response in the two preceding and following time windows. The lower-intensity component enters the unequal-arm interferometer in the opposite direction for interference, generating an X-basis decoded optical signal. Since the optical signal passes back and forth through the polarization interferometer, phase differences are automatically compensated. Therefore, the correct decoding result of the X-basis decoded optical signal enters the direction of the multi-wavelength laser LD. This portion of the decoded optical signal can be isolated by an optical isolator, while the incorrect decoding result enters another SPD unit.

[0107] After the central node and user node i perform basis matching, they use the X-basis count to estimate parameters and use Z to encode. After error correction and confidentiality amplification, both parties can share the same secure quantum key. The process of quantum key distribution for the central node and other user nodes is exactly the same.

[0108] From the embodiments of the present invention, it can be seen that the present invention proposes a plug-and-play time-phase coded quantum key distribution network, which adopts a round-trip architecture. Each user node uses a reflective intensity modulation module to implement three-state time-phase coding, which can not only realize automatic compensation of polarization and phase, but also realize polarization-independent quantum state preparation and decoy state preparation. The use of passive basis measurement can avoid the problem of the central node decoding the same information for all users. In addition, the structure can support higher rates, greatly improving the stability and practicality of the QKD network.

Claims

1. A plug-and-play time-phase coded quantum key distribution network, characterized in that: It includes a central node and multiple user nodes. Each user node is connected to the central node through a fiber optic channel. The central node includes a multi-wavelength laser LD, a first beam splitter BS1, a second beam splitter BS2, a third beam splitter BS3, a wavelength division multiplexer WDM and two single-photon detection units; A multi-wavelength laser LD and a single photon detection unit are respectively connected to two input ports of the first beam splitter BS1; The two output ports of the first beam splitter BS1 and the two input ports of the second beam splitter BS2 are connected through polarization-maintaining optical fibers of different lengths to form an unequal-arm interferometer; The output port of the second beam splitter BS2 and another single photon detection unit are connected to the two input ports of the third beam splitter BS3 respectively; The output port of the third beam splitter BS3 is connected to the common end of the wavelength division multiplexer WDM; Each wavelength output port of the wavelength division multiplexer WDM is connected to each user node through an optical fiber channel; Each user node includes a connected variable attenuator VOA and a reflective intensity modulation module; The multi-wavelength laser LD is used to generate horizontally polarized multi-wavelength optical pulses; The unequal-arm interferometer is used to split a multi-wavelength optical pulse into two sub-pulses with a time difference of t; The single-photon detection unit can realize the simultaneous detection of multi-wavelength optical signals; The reflective intensity modulation module is used to perform intensity modulation on the two sub-pulses arriving at the user node through a splitting-reflection-interference method to randomly generate two time-coded states in the Z basis and one phase-coded state in the X basis, as well as the corresponding decoy states, while rotating the polarization states of the two sub-pulses by 90°. The variable attenuator (VOA) is used to decay the time phase encoded quantum state to the single photon level. The reflective intensity modulation module includes a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first Faraday mirror FM1, a second Faraday mirror FM2 and a first phase modulator PM1. Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively; Another input port of the first polarization beam splitter PBS1 and another input port of the second polarization beam splitter PBS2 are connected to two ends of the first phase modulator PM1 through polarization-maintaining optical fibers F3 and F4, respectively; One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected to the first Faraday mirror FM1 and the second Faraday mirror FM2 through the polarization-maintaining fiber F5 and the polarization-maintaining fiber F6 respectively; The other output port of the first polarization beam splitter PBS1 and the other output port of the second polarization beam splitter PBS2 are connected through a polarization-maintaining fiber F7.

2. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: The lengths of the polarization-maintaining optical fiber F3 and the polarization-maintaining optical fiber F4 are equal, the lengths of the polarization-maintaining optical fiber F5 and the polarization-maintaining optical fiber F6 are equal, and the length of the polarization-maintaining optical fiber F7 is twice that of the polarization-maintaining optical fiber F5.

3. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: The reflective intensity modulation module may have the following structure: comprising a fourth beam splitter BS4, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a Faraday rotator FR, a second phase modulator PM2 and a third phase modulator PM3, Two output ports of the fourth beam splitter BS4 are connected to one input port of the first polarization beam splitter PBS1 and one input port of the second polarization beam splitter PBS2 through optical fibers F1 and F2 of equal length, respectively; One output port of the first polarization beam splitter PBS1 and one output port of the second polarization beam splitter PBS2 are connected via a polarization-maintaining fiber F8. A Faraday rotator FR is provided on F8. The polarization rotation angle of the Faraday rotator FR is 90°, and the polarization directions at both ends are aligned with the slow axis and the fast axis of the polarization-maintaining fiber, respectively. Another input port of the first polarization beam splitter PBS1 and another output port of the second polarization beam splitter PBS2 are connected to two ends of the second phase modulator PM2 through polarization-maintaining optical fibers F9 and F10 respectively; The other output port of the first polarization beam splitter PBS1 and the other input port of the second polarization beam splitter PBS2 are connected to the third phase modulator PM3 through the polarization-maintaining fiber F11 and the polarization-maintaining fiber F12 respectively.

4. The plug-and-play time-phase coded quantum key distribution network according to claim 3, characterized in that: The length of the polarization-maintaining fiber F10 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F9, and the length of the polarization-maintaining fiber F11 is the sum of the lengths of the polarization-maintaining fiber F8 and the polarization-maintaining fiber F12.

5. The plug-and-play time phase coded quantum key distribution network according to claim 1, characterized in that: The single-photon detection unit includes a wavelength division multiplexer WDM and a plurality of single-photon detectors SPD, each single-photon detector SPD is used to detect one of the optical signals separated by the wavelength division multiplexer WDM.

6. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: The period of the light pulse generated by the multi-wavelength laser LD is 2t, and the gating period of the single photon detector SPD in the single photon detection unit is t.

7. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: The splitting ratio of the third beam splitter BS3 is 10:90, wherein 10% of the beam splitting ports are connected to the second beam splitter BS2.

8. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: Each user node also includes an unbalanced beam splitter and a photodetector. The port with lower beam splitting energy of the unbalanced beam splitter is connected to the variable attenuator VOA, and the port with higher beam splitting energy is connected to the photodetector for security monitoring.

9. The plug-and-play time-phase coded quantum key distribution network according to claim 1, characterized in that: An optical isolator is further provided between the multi-wavelength laser LD and the first beam splitter BS1 for isolating the optical signal returning from the first beam splitter BS1 to the multi-wavelength laser LD.

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