A chip-based quantum key distribution system
The quantum key distribution system with chip-based design, employing time-phase encoding and polarization-independent decoding technology, solves the problems of high complexity and poor stability of existing systems, achieving device miniaturization and long-term stable operation.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- BEIJING ZHONGKE GUOGUANG QUANTUM TECH CO LTD
- Filing Date
- 2026-04-07
- Publication Date
- 2026-06-26
AI Technical Summary
Existing quantum key distribution systems suffer from problems such as high complexity and low integration at the transmitter end, polarization sensitivity and poor stability at the receiver end, and asymmetric interferometer structures requiring delay lines, making it difficult to achieve chip-based and mass production.
The device employs a chip-based design, connecting the transmitter and receiver via an optical fiber channel. The transmitter uses an integrated intensity modulation module and an adjustable attenuation module for time-phase encoding, while the receiver uses a decoding chip and a single-photon detector, combined with a passive basis selection and a symmetrical unequal-arm interferometer for polarization-independent decoding.
It achieves miniaturization of the transmitting end and stability of the receiving end, is immune to channel polarization disturbances, improves the system's operational stability and integration, and supports long-term stable operation.
Smart Images
Figure CN122027151B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the fields of optical communication and quantum key distribution technology, and particularly to a chip-based quantum key distribution system. Background Technology
[0002] Quantum key distribution can provide unconditionally secure key distribution for long-distance communication between two parties, with the BB84 quantum key distribution protocol being the most mature currently. However, existing technologies have the following drawbacks:
[0003] 1. High complexity and low integration of the transmitting end: Traditional time-phase coding systems are built with discrete optical components. Intensity modulation, phase modulation and adjustable attenuation are independent of each other. The optical path is complex, the size is large, the cost is high and the consistency is poor, making it difficult to achieve chip-based and mass production.
[0004] 2. Polarization sensitivity and poor stability at the receiver: Optical fiber channels have an inherent birefringence effect, and environmental disturbances can cause random changes in the polarization state of photons. Traditional unequal-arm Mach-Zehnder interferometers are severely affected by polarization, resulting in poor interference stability and the system cannot operate stably for a long time.
[0005] 3. Asymmetrical interferometer structure and need for delay lines: Conventional unequal arm interferometers rely on physical delay lines to achieve time difference, resulting in poor arm length consistency, uneven loss, low interference visibility, and the need for circulators, which further limits miniaturization and integration.
[0006] Therefore, there is an urgent need for a fully chip-based quantum key distribution system with a simplified transmitter, a polarization-independent receiver, and a symmetrical interferometer with no delay line, in order to solve the pain points of existing technologies. Summary of the Invention
[0007] To address the aforementioned shortcomings of existing technologies, this invention proposes a chip-based quantum key distribution system.
[0008] The technical solution of this invention is implemented as follows:
[0009] A chip-based quantum key distribution system includes a transmitter and a receiver connected via an optical fiber channel, wherein the transmitter includes:
[0010] A laser (LD) is used to generate optical pulse signals.
[0011] The encoding chip includes an intensity modulation module and an adjustable attenuation module integrated on the same substrate;
[0012] The intensity modulation module is used to chop the optical pulse signal into two sub-pulses with a time difference of T, and to perform intensity modulation on the two sub-pulses to generate a signal state and a decoy state of time phase encoded quantum state;
[0013] The time-phase encoded quantum state comprises two Z-based quantum states and one X-based quantum state;
[0014] The adjustable attenuation module is used to attenuate the time-phase encoded quantum state to the single-photon level and output it to the fiber optic channel;
[0015] The receiver includes a decoding chip and a first single-photon detector SPD1 and a second single-photon detector SPD2;
[0016] The decoding chip is used to receive quantum states transmitted via optical fiber channels and perform Z-based and X-based decoding; it includes a first beam splitter BS1, a second beam splitter BS2, a first polarization beam splitter PBS1, and an orthogonal polarization delay module;
[0017] BS1 is used to split the received quantum state to achieve passive basis selection; the quantum state emitted from one output port of BS1 enters SPD1 to achieve Z-basis decoding measurement, and the quantum state emitted from its other output port enters the unequal arm interferometer with a symmetrical structure consisting of BS2, PBS1 and orthogonal polarization delay module to achieve polarization-independent X-basis decoding.
[0018] The two output ports of BS2 are connected to the two input ports of PBS1 via optical waveguides of equal length;
[0019] The two output ports of PBS1 are connected to the orthogonal polarization delay module through optical waveguides of equal length, forming a Sagnac ring;
[0020] The delay difference between the orthogonal polarization delay module for TM polarization mode and TE polarization mode is T;
[0021] The interference results from the unequal-arm interferometer are fed into SPD2 for detection.
[0022] Preferably, the intensity modulation module obtains a Z-based quantum state when the intensity of the subsequent sub-pulse or the preceding sub-pulse is modulated to 0. or When the intensities of the two sub-pulses are simultaneously modulated to half, the X-based quantum state is obtained. .
[0023] Preferably, the intensity modulation module is a first Mach-Zehnder modulator MZM1 composed of a third beam splitter BS3 and a fourth beam splitter BS4, wherein a first phase modulator PM1 is provided on one arm of the MZM1.
[0024] Preferably, the intensity modulation module BS4 is provided with an additional output port, which is connected to a photodetector PD for power monitoring.
[0025] Preferably, the intensity modulation module includes a fifth beam splitter BS5 and a second phase modulator PM2. One input port of BS5 is connected to a laser LD, and the other input port is connected to an adjustable attenuation module. The two output ports of BS5 are directly connected through an optical waveguide, and PM2 is set on the optical waveguide.
[0026] Preferably, the adjustable attenuation module is a second Mach-Zehnder modulator MZM2 composed of a sixth beam splitter BS6 and a seventh beam splitter BS7, wherein a first phase shifter PS1 is provided on one arm of the MZM2.
[0027] Preferably, the orthogonal polarization delay module includes a second polarization beam splitter PBS2, a first quarter-wave plate QWP1, a second quarter-wave plate QWP2, a second phase shifter PS2, and a reflective film;
[0028] One input port and one output port of PBS2 are connected to the two output ports of PBS1 through optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip through optical waveguides of equal length, and the end faces of the waveguides at the corresponding positions are coated with reflective films.
[0029] QWP1 and QWP2 are each set on a waveguide to rotate the polarization of the optical signal by 90° when it passes through back and forth.
[0030] PS2 is set on one of the waveguides.
[0031] Preferably, QWP1 and QWP2 are both high birefringence waveguide segments, and the waveguide segments have an asymmetric cross-sectional structure, so that the TE mode and TM mode of the transmitted light generate an effective refractive index difference Δn.
[0032] The length L of the high birefringence waveguide section satisfies: Δn × L = λ0 / 4, where λ0 is the operating wavelength;
[0033] The QWP1 and QWP2 ensure that the total phase delay of the optical signal after passing through them is λ / 2, achieving a polarization rotation of 90° and returning along the original incident light path.
[0034] Preferably, both QWP1 and QWP2 include a waveguide substrate, a waveguide core layer, and a groove structure formed on the waveguide core layer;
[0035] The waveguide core layer is made of a high refractive index material. The groove structure extends along the light transmission direction and penetrates the width or depth direction of the waveguide core layer. The groove is filled with a low refractive index medium. The difference between the refractive index of the low refractive index medium and the refractive index of the waveguide core layer is not less than 1.0.
[0036] The groove structure and the waveguide core layer form an asymmetric transmission structure, which generates an effective refractive index difference Δn of a preset size between the TE polarization mode and the TM polarization mode of the transmitted light.
[0037] The total length L of the waveguide core layer (including the groove structure) along the optical transmission direction satisfies: Δn×L = λ / 4, where λ is the operating wavelength.
[0038] Preferably, the orthogonal polarization delay module includes a second polarization beam splitter PBS2, a first subwavelength grating SWG1, a second subwavelength grating SWG2, and a third phase shifter PS3;
[0039] One input port and one output port of PBS2 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip via optical waveguides of equal length.
[0040] SWG1 and SWG2 are respectively set at the ends of the two waveguides. The period Λ of the grating is much smaller than the working wavelength λ0, satisfying Λ < λ0 / (2n), and only supports zero-order reflection and transmission.
[0041] The subwavelength grating forms a one-dimensional photonic bandgap through periodic refractive index modulation, achieving strong coherent reflection in the working wavelength band;
[0042] The grating structure has birefringence characteristics, which generates a π phase difference for the incident polarized light, causing the polarization of the reflected light to rotate by 90°.
[0043] The PS3 is mounted on one of the waveguides.
[0044] Preferably, the orthogonal polarization delay module includes a third polarization beam splitter PBS3 and a fourth phase shifter PS4;
[0045] One input port and one output port of PBS3 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS3 are directly connected via optical waveguides, and PS4 is set on the optical waveguides.
[0046] Preferably, BS1 is also provided with an additional input port for outputting another interference result of the interferometer, and a third single-photon detector SPD3 is provided for detection.
[0047] Preferably, the beam splitting ratio of BS1 is 10:90.
[0048] Compared with the prior art, the present invention has the following beneficial effects:
[0049] This invention proposes a chip-based quantum key distribution system. By chopping an optical pulse into two sub-pulses and then modulating their intensity separately, time-phase encoding can be achieved, reducing the complexity of the transmitter. The decoding chip employs passive basis selection and a symmetrical unequal-arm interferometer, enabling polarization-independent reception and decoding, thus improving system stability. Therefore, this invention, through integrated chip design, significantly reduces the size of the transmitter and receiver optical systems, achieving device miniaturization, while also possessing immunity to channel polarization perturbations, ensuring long-term system stability. Attached Figure Description
[0050] Figure 1 This is a schematic diagram of the chip-based quantum key distribution system of the present invention;
[0051] Figure 2 This is a schematic diagram of the intensity modulation encoded quantum state of the chip-based quantum key distribution system of the present invention;
[0052] Figure 3 This is a principle block diagram of a first embodiment of the chip-based quantum key distribution system of the present invention;
[0053] Figure 4 This is a principle block diagram of Embodiment 2 of the chip-based quantum key distribution system of the present invention;
[0054] Figure 5 This is a principle block diagram of Embodiment 3 of the chip-based quantum key distribution system of the present invention. Detailed Implementation
[0055] The present invention will now be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention.
[0056] like Figure 1 As shown, a chip-based quantum key distribution system includes a transmitter and a receiver connected via an optical fiber channel, wherein the transmitter includes:
[0057] A laser (LD) is used to generate optical pulse signals.
[0058] The encoding chip includes an intensity modulation module and an adjustable attenuation module integrated on the same substrate;
[0059] The intensity modulation module is used to chop the optical pulse signal into two sub-pulses with a time difference of T, and to perform intensity modulation on the two sub-pulses to generate a signal state and a decoy state of time phase encoded quantum state;
[0060] The time-phase encoded quantum state comprises two Z-based quantum states and one X-based quantum state;
[0061] The adjustable attenuation module is used to attenuate the time-phase encoded quantum state to the single-photon level and output it to the fiber optic channel;
[0062] The receiver includes a decoding chip and two single-photon detectors, SPD1 and SPD2.
[0063] The decoding chip is used to receive quantum states transmitted via optical fiber channels and perform Z-based and X-based decoding; it includes a first beam splitter BS1, a second beam splitter BS2, a first polarization beam splitter PBS1, and an orthogonal polarization delay module;
[0064] BS1 is used to split the received quantum state to achieve passive basis selection; the quantum state emitted from one output port of BS1 enters SPD1 to achieve Z-basis decoding measurement, and the quantum state emitted from its other output port enters the unequal arm interferometer with a symmetrical structure consisting of BS2, PBS1 and orthogonal polarization delay module to achieve polarization-independent X-basis decoding.
[0065] The two output ports of BS2 are connected to the two input ports of PBS1 via optical waveguides of equal length;
[0066] The two output ports of PBS1 are connected to the orthogonal polarization delay module through optical waveguides of equal length, forming a Sagnac ring;
[0067] The delay difference between the orthogonal polarization delay module for TM polarization mode and TE polarization mode is T;
[0068] The interference results from the unequal-arm interferometer are fed into SPD2 for detection.
[0069] The intensity modulation module obtains Z-based quantum states when the intensity of the subsequent sub-pulse or the preceding sub-pulse is modulated to 0. or When the intensities of the two sub-pulses are simultaneously modulated to half, the X-based quantum state is obtained. .
[0070] The specific work process is as follows:
[0071] The transmitting laser generates an optical pulse signal with a pulse width of W. After entering the encoding chip, it is first chopped into two sub-pulses with a time difference of T by the intensity modulation module, and the intensity of the two sub-pulses is modulated respectively. Then, it is attenuated to the single-photon level by the adjustable attenuation module, and finally the signal state and decoy state of the time phase encoded quantum state are generated.
[0072] like Figure 2 As shown, the optical pulse signal enters the intensity modulation module and is modulated by different driving signals to generate different quantum states. The pulse width of the driving signal in the intensity modulation module is u, the pulse width of the optical pulse signal is W, and the time difference between the two sub-pulses obtained after chopping is T. The following relationship must be satisfied: W = u + T. Therefore, when the amplitude of the driving signal pulse is V and it is located before the optical pulse signal, the intensity of the second sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V and it is located at the end of the optical pulse signal, the intensity of the previous sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V / 2 and it is located at the beginning and end of the optical pulse signal, the intensity of both sub-pulses is modulated to half, and a quantum state can be obtained. By selecting different values for the amplitude of the driving pulse, both the signal state and the decoy state can be obtained.
[0073] The prepared quantum state is transmitted through an optical fiber channel to the receiving end. It first enters the input port of the decoding chip BS1 and is then split. The quantum state exits from one port with a probability P and enters SPD1 for Z-basis measurement. It exits from the other port of BS1 with a probability of 1-P and enters the unequal-arm interferometer for X-basis measurement. The quantum state is first split by BS2, and the components exiting from both ports have equal intensity and the same polarization. One component enters one input port of PBS1 for polarization beam splitting. The TE polarization component, after being polarized by PBS1, still propagates in the optical waveguide along the TE polarization direction, enters one port of the orthogonal polarization delay module, and exits from its other port. It then propagates along the optical waveguide back to PBS1 and exits from its input port, becoming TM polarization mode. The TM polarization component of this component, after being polarized by PBS1, also propagates in the optical waveguide along the TE polarization direction, enters another port of the orthogonal polarization delay module, and exits from one port. It then propagates along the optical waveguide back to PBS1 and exits from its input port, becoming TE polarization mode. It can be seen that after the above process, this component still returns to BS2 along the original path, but the TE polarization and TM polarization have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation.
[0074] The other component enters another input port of PBS1. The TE-polarized component, after being split by PBS1, propagates along the TM polarization direction in the optical waveguide, enters one port of the orthogonal polarization delay module, and exits from its other port after a delay of T. It then propagates along the optical waveguide back to PBS1, exiting from its input port again, still in TM polarization mode. The TM-polarized component of this component, after being split by PBS1, also propagates along the TM polarization direction in the optical waveguide, enters another port of the orthogonal polarization delay module, and exits from one port after a delay of T. It then propagates along the optical waveguide back to PBS1, exiting from its input port again, changing to TE polarization mode. It can be seen that this component returns to BS2 via the same path after the above process, only the TE and TM polarizations have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation. In addition, this component has an increased delay of T compared to the previous component, but the polarization remains the same, achieving polarization-independent interference. The interference result exits from the other input port of BS2 and enters SPD2 for detection, completing the X-based measurement.
[0075] Based on the three-state time-phase encoded quantum key distribution protocol and the measurement results of Z-basis and X-basis, the distribution of secure keys can be completed after post-processing such as basis matching, error correction, and security amplification.
[0076] like Figure 3 As shown, this is Embodiment 1 of the present invention:
[0077] The intensity modulation module is a first Mach-Zehnder modulator MZM1 composed of BS3 and BS4, wherein a first phase modulator PM1 is provided on one arm of MZM1.
[0078] The adjustable attenuation module is a second Mach-Zehnder modulator MZM2 composed of BS6 and BS7, wherein a first phase shifter PS1 is provided on one arm of MZM2.
[0079] The orthogonal polarization delay module includes PBS2, first quarter-wave plates QWP1, QWP2, PS2, and a reflective film;
[0080] One input port and one output port of PBS2 are connected to the two output ports of PBS1 through optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip through optical waveguides of equal length, and the end faces of the waveguides at the corresponding positions are coated with reflective films.
[0081] QWP1 and QWP2 are each set on a waveguide to rotate the polarization of the optical signal by 90° when it passes through back and forth.
[0082] PS2 is set on one of the waveguides.
[0083] The specific working process of Example 1 is as follows:
[0084] The transmitting laser generates an optical pulse signal with a pulse width of W. After entering the encoding chip, it is first chopped into two sub-pulses with a time difference of T by the first Mach-Zehnder modulator MZM1 composed of BS3 and BS4, and the intensity of the two sub-pulses is modulated respectively. Then, it is attenuated to the single-photon level by the second Mach-Zehnder modulator MZM2 composed of BS6 and BS7, and finally the signal state and decoy state of the time phase encoded quantum state are generated.
[0085] like Figure 2As shown, an optical pulse signal enters the MZM1. By controlling the driving voltage of PM1, the intensity of the optical pulse signal can be modulated. Different PM1 driving signals can produce different quantum states. The pulse width of the PM1 driving signal is u, the pulse width of the optical pulse signal is W, and the time difference between the two sub-pulses obtained after chopping is T. The following relationship must be satisfied: W = u + T. Therefore, when the amplitude of the driving signal pulse is V and it is located at the beginning of the optical pulse signal, the intensity of the second sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V and it is located at the end of the optical pulse signal, the intensity of the previous sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V / 2 and it is located at the beginning and end of the optical pulse signal, the intensity of both sub-pulses is modulated to half, and a quantum state can be obtained. By selecting different values for the driving pulse amplitude, signal state and decoy state can be obtained.
[0086] The prepared quantum state is transmitted through an optical fiber channel to the receiving end. It first enters the input port of the decoding chip BS1 and is then split. The quantum state exits from one port with a probability P and enters SPD1 for Z-basis measurement. It exits from the other port of BS1 with a probability of 1-P and enters the unequal-arm interferometer for X-basis measurement. The quantum state is first split by BS2, and the components exiting from both ports have equal intensity and the same polarization. One component enters an input port of PBS1 for polarization beam splitting. The TE polarization component, after being split by PBS1, still propagates in the optical waveguide along the TE polarization direction, enters an input port of PBS2, exits directly from one of its output ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TM polarization mode. The TM polarization component of this component, after being split by PBS1, also propagates in the optical waveguide along the TE polarization direction, enters an output port of PBS2, exits from one of its input ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TE polarization mode. It can be seen that after the above process, this component still returns to PBS2 along the original path, only the TE polarization and TM polarization have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation.
[0087] The other component enters another input port of PBS1. The TE polarization component, after being polarized and split by PBS1, propagates along the TM polarization direction in the optical waveguide and enters one input port of PBS2. It then propagates along the optical waveguide to QWP1, is transformed, reflected by the reflective film, and undergoes another transformation by QWP1. The corresponding transmission matrix is...
[0088] ,
[0089] Where M and QWP are the transmission matrices for specular reflection and quarter-wave plate, respectively. It can be seen that for a TE-polarized optical signal, its polarization state becomes TM-polarized after reflection by the quarter-wave plate and reflective film, and TM-polarized becomes TE-polarized.
[0090] Therefore, the TM polarization component, after being transformed into TE polarization by QWP1 and the reflective film, returns to its original polarization state. After passing through PBS2, it enters its other output port, and then, after being transformed back into TM polarization by QWP2 and the reflective film, it finally exits from PBS2, becoming TM polarized again. The TM polarization component then becomes TE polarization and returns along the original path. Both polarization components travel back and forth twice through the optical waveguides where QWP1 and QWP2 are located, respectively, and have a delay T compared to the previous component.
[0091] A preferred embodiment of QWP1 and QWP2 is a high birefringence waveguide section with an asymmetric cross-section structure, which creates an effective refractive index difference Δn between the TE mode and TM mode of the transmitted light. Its length L satisfies: Δn × L = λ0 / 4, where λ0 is the operating wavelength. Therefore, when the optical signal passes through it round trip, the total phase delay is λ / 2, achieving a 90° polarization rotation and returning along the original incident light path.
[0092] Another preferred embodiment of QWP1 and QWP2 includes a waveguide substrate, a waveguide core layer, and a groove structure formed on the waveguide core layer. The waveguide core layer is made of a high refractive index material, and the groove structure extends along the light transmission direction, penetrating the width or depth direction of the waveguide core layer. The groove is filled with a low refractive index medium, and the difference between the refractive index of the low refractive index medium and the refractive index of the waveguide core layer is not less than 1.0. The groove structure and the waveguide core layer form an asymmetric transmission structure, so that the TE polarization mode and the TM polarization mode of the transmitted light produce an effective refractive index difference Δn of a preset size. The total length L of the waveguide core layer (including the groove structure) along the light transmission direction satisfies: Δn×L = λ / 4, where λ is the operating wavelength.
[0093] It can be seen that after the above process, this component still returns to BS2 via the original path, only the TE and TM polarizations have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation. Furthermore, this component has an increased delay T compared to the previous component, but the polarization remains the same, achieving polarization-independent interference. The interference result exits from another input port of BS2 and enters SPD2 for detection, completing the X-based measurement. PS2 can compensate for the phase drift caused by the long and short arms in real time.
[0094] Based on the three-state time-phase encoded quantum key distribution protocol and the measurement results of Z-basis and X-basis, the distribution of secure keys can be completed after post-processing such as basis matching, error correction, and security amplification.
[0095] like Figure 4As shown, this is embodiment two of the present invention:
[0096] The intensity modulation module is a first Mach-Zehnder modulator MZM1 composed of BS3 and BS4, wherein a first phase modulator PM1 is provided on one arm of MZM1.
[0097] The adjustable attenuation module is a second Mach-Zehnder modulator MZM2 composed of BS6 and BS7, wherein a first phase shifter PS1 is provided on one arm of MZM2.
[0098] The orthogonal polarization delay module includes PBS2, first subwavelength gratings SWG1, SWG2, and PS3;
[0099] One input port and one output port of PBS2 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip via optical waveguides of equal length.
[0100] SWG1 and SWG2 are respectively set at the ends of the two waveguides. The period Λ of the grating is much smaller than the working wavelength λ0, satisfying Λ < λ0 / (2n). A one-dimensional photonic bandgap is formed by periodic refractive index modulation to achieve strong coherent reflection.
[0101] The grating structure has birefringence characteristics, which generates a π phase difference for the incident polarized light, causing the polarization of the reflected light to rotate by 90°.
[0102] The PS3 is mounted on one of the waveguides.
[0103] The specific working process of Example 2 is as follows:
[0104] The transmitting laser generates an optical pulse signal with a pulse width of W. After entering the encoding chip, it is first chopped into two sub-pulses with a time difference of T by the first Mach-Zehnder modulator MZM1 composed of BS3 and BS4, and the intensity of the two sub-pulses is modulated respectively. Then, it is attenuated to the single-photon level by the second Mach-Zehnder modulator MZM2 composed of BS6 and BS7, and finally the signal state and decoy state of the time phase encoded quantum state are generated.
[0105] like Figure 2 As shown, an optical pulse signal enters the MZM1. By controlling the driving voltage of PM1, the intensity of the optical pulse signal can be modulated. Different PM1 driving signals can produce different quantum states. The pulse width of the PM1 driving signal is u, the pulse width of the optical pulse signal is W, and the time difference between the two sub-pulses obtained after chopping is T. The following relationship must be satisfied: W = u + T. Therefore, when the amplitude of the driving signal pulse is V and it is located at the beginning of the optical pulse signal, the intensity of the second sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V and it is located at the end of the optical pulse signal, the intensity of the previous sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V / 2 and it is located at the beginning and end of the optical pulse signal, the intensity of both sub-pulses is modulated to half, and a quantum state can be obtained. By selecting different values for the driving pulse amplitude, signal state and decoy state can be obtained.
[0106] The prepared quantum state is transmitted through an optical fiber channel to the receiving end. It first enters the input port of the decoding chip BS1 and is then split. The quantum state exits from one port with a probability P and enters SPD1 for Z-basis measurement. It exits from the other port of BS1 with a probability of 1-P and enters the unequal-arm interferometer for X-basis measurement. The quantum state is first split by BS2, and the components exiting from both ports have equal intensity and the same polarization. One component enters an input port of PBS1 for polarization beam splitting. The TE polarization component, after being split by PBS1, still propagates in the optical waveguide along the TE polarization direction, enters an input port of PBS2, exits directly from one of its output ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TM polarization mode. The TM polarization component of this component, after being split by PBS1, also propagates in the optical waveguide along the TE polarization direction, enters an output port of PBS2, exits from one of its input ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TE polarization mode. It can be seen that after the above process, this component still returns to PBS2 along the original path, only the TE polarization and TM polarization have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation.
[0107] The other component enters another input port of PBS1. The TE polarization component, after being polarized and split by PBS1, propagates along the TM polarization direction in the optical waveguide and enters one input port of PBS2. It then propagates along the optical waveguide to SWG1. The period Λ of this grating is much smaller than the working wavelength λ0, satisfying Λ < λ0 / (2n). A one-dimensional photonic bandgap is formed through periodic refractive index modulation, achieving strong coherent reflection. The grating structure has birefringence characteristics, generating a π phase difference in the incident polarized light, causing the reflected light to rotate polarization by 90°. The corresponding transmission matrix is...
[0108] ,
[0109] It can be seen that for TE polarized optical signals, their polarization state will become TM polarized after passing through SWG1, and TM polarization will become TE polarization.
[0110] Therefore, the TM polarization component, after being transformed by SWG1, becomes TE polarization and returns. After passing through PBS2, it enters its other output port, and is then transformed back into TM polarization by SWG2, finally exiting from PBS2 as TM polarization. The TM polarization component then becomes TE polarization and returns along the original path. Both polarization components pass through the optical waveguides where QWP1 and QWP2 are located twice, and have a delay T compared to the previous component.
[0111] It can be seen that after the above process, this component still returns to BS2 via the original path, only the TE and TM polarizations have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation. Furthermore, this component has an increased delay T compared to the previous component, but the polarization remains the same, achieving polarization-independent interference. The interference result exits from another input port of BS2 and enters SPD2 for detection, completing the X-based measurement. PS3 can compensate for the phase drift caused by the long and short arms in real time.
[0112] Based on the three-state time-phase encoded quantum key distribution protocol and the measurement results of Z-basis and X-basis, the distribution of secure keys can be completed after post-processing such as basis matching, error correction, and security amplification.
[0113] like Figure 5 As shown, this is embodiment three of the present invention:
[0114] The intensity modulation module includes BS5 and PM2. One input port of BS5 is connected to the laser, and the other input port is connected to the adjustable attenuation module. The two output ports of BS5 are directly connected through an optical waveguide, and PM2 is placed on the optical waveguide. The orthogonal polarization delay module includes PBS3 and PS4.
[0115] One input port and one output port of PBS3 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS3 are directly connected via optical waveguides, and PS4 is set on the optical waveguides.
[0116] BS1 also has an additional input port for outputting another interference result from the interferometer and setting up SPD3 for detection.
[0117] The specific working process of Example 3 is as follows:
[0118] The transmitting laser generates an optical pulse signal with a pulse width of W. After entering the encoding chip, it first enters one input port of BS5 and is split into two components of equal amplitude. Since the two output ports of BS5 are directly connected through optical waveguides, they propagate in clockwise and counterclockwise directions respectively in the ring structure formed by the optical waveguides and then return to BS5 simultaneously for interference. When the optical pulse passes through PM2 set on the ring structure optical waveguide at different times in opposite directions, it is modulated with different phases, which can realize intensity modulation of the optical pulse. After passing through this structure, the optical pulse is chopped into two sub-pulses with a time difference of T, and the intensity of the two sub-pulses is modulated respectively. Then, it is attenuated to the single-photon level by the second Mach-Zehnder modulator MZM2 composed of BS6 and BS7, finally generating the signal state and decoy state of the time-phase encoded quantum state.
[0119] like Figure 2 As shown, the optical pulse signal enters BS5. By controlling the PM2 driving voltage, the intensity of the optical pulse signal can be modulated. Different PM2 driving signal modulations can produce different quantum states. The PM2 driving signal pulse width is u, the optical pulse signal pulse width is W, and the time difference between the two sub-pulses obtained after chopping is T. The following relationship must be satisfied: W = u + T. Therefore, when the driving signal pulse amplitude is V and it is located at the beginning of the optical pulse signal, the intensity of the second sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V and it is located at the end of the optical pulse signal, the intensity of the previous sub-pulse is modulated to 0, and a quantum state can be obtained. When the amplitude of the driving signal pulse is V / 2 and it is located at the beginning and end of the optical pulse signal, the intensity of both sub-pulses is modulated to half, and a quantum state can be obtained. By selecting different values for the driving pulse amplitude, signal state and decoy state can be obtained.
[0120] The prepared quantum state is transmitted through an optical fiber channel to the receiving end. It first enters the input port of the decoding chip BS1 and is then split. The quantum state exits from one port with a probability P and enters SPD1 for Z-basis measurement. It exits from the other port of BS1 with a probability of 1-P and enters the unequal-arm interferometer for X-basis measurement. The quantum state is first split by BS2, and the components exiting from both ports have equal intensity and the same polarization. One component enters one input port of PBS1 for polarization beam splitting. The TE polarization component, after being polarized by PBS1, still propagates in the optical waveguide along the TE polarization direction, enters one input port of PBS3, exits directly from one of its output ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TM polarization mode. The TM polarization component of this component, after being polarized by PBS1, also propagates in the optical waveguide along the TE polarization direction, enters one output port of PBS3, exits from one of its input ports, propagates along the optical waveguide back to PBS1, and exits again from its input port, becoming TE polarization mode. It can be seen that after the above process, this component still returns to BS2 along the original path, only the TE polarization and TM polarization have been exchanged, and the birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation.
[0121] The other component enters another input port of PBS1. After being split by polarization in PBS1, the TE polarization component propagates along the TM polarization direction in the optical waveguide, enters one input port of PBS3, propagates along the optical waveguide, returns to PBS3, and exits from the original port, becoming TM polarized. The TM polarization component, after being split by polarization in PBS1, also propagates along the TM polarization direction in the optical waveguide, enters another output port of PBS3, propagates in the opposite direction in the optical waveguide, returns to PBS3, and exits from the original port, becoming TE polarized.
[0122] It can be seen that after the above process, this component still returns to BS2 via the original path, only the TE and TM polarizations have been exchanged. The birefringence caused by the two polarization directions cancels each other out due to the round-trip propagation. Furthermore, this component has an increased delay T compared to the previous component, but the polarization remains the same, achieving polarization-independent interference. The interference result exits from another input port of BS2 and enters SPD2 for detection, completing the X-based measurement. PS3 can compensate for the phase drift caused by the long and short arms in real time.
[0123] Based on the three-state time-phase encoded quantum key distribution protocol and the measurement results of Z-basis and X-basis, the distribution of secure keys can be completed after post-processing such as basis matching, error correction, and security amplification.
[0124] As can be seen from the various embodiments of this invention, this invention proposes a chip-based quantum key distribution system. By chopping an optical pulse into two sub-pulses and then performing intensity modulation on each, time-phase encoding can be achieved, reducing the complexity of the transmitting end. The decoding chip employs passive basis selection and a symmetrical unequal-arm interferometer, enabling polarization-independent reception and decoding, thus improving system stability. Therefore, this invention, through integrated chip design, significantly reduces the size of the transmitting and receiving optical systems, achieving device miniaturization, while also possessing immunity to channel polarization perturbations, ensuring long-term system stability.
Claims
1. A chip-based quantum key distribution system, comprising a transmitter and a receiver connected via an optical fiber channel, characterized in that, The sending end includes: A laser (LD) is used to generate optical pulse signals. The encoding chip includes an intensity modulation module and an adjustable attenuation module integrated on the same substrate; The intensity modulation module is used to chop the optical pulse signal into two sub-pulses with a time difference of T, and to perform intensity modulation on the two sub-pulses to generate a signal state and a decoy state of time phase encoded quantum state; The time-phase encoded quantum state comprises two Z-based quantum states and one X-based quantum state; The adjustable attenuation module is used to attenuate the time-phase encoded quantum state to the single-photon level and output it to the fiber optic channel; The receiver includes a decoding chip and a first single-photon detector SPD1 and a second single-photon detector SPD2; The decoding chip is used to receive quantum states transmitted via optical fiber channels and perform Z-based and X-based decoding; it includes a first beam splitter BS1, a second beam splitter BS2, a first polarization beam splitter PBS1, and an orthogonal polarization delay module; BS1 is used to split the received quantum state to achieve passive basis selection; the quantum state emitted from one output port of BS1 enters SPD1 to achieve Z-basis decoding measurement, and the quantum state emitted from its other output port enters the unequal arm interferometer with a symmetrical structure consisting of BS2, PBS1 and orthogonal polarization delay module to achieve polarization-independent X-basis decoding. The two output ports of BS2 are connected to the two input ports of PBS1 via optical waveguides of equal length; The two output ports of PBS1 are connected to the orthogonal polarization delay module through optical waveguides of equal length, forming a Sagnac ring; The delay difference between the orthogonal polarization delay module for TM polarization mode and TE polarization mode is T; The interference results from the unequal-arm interferometer are fed into SPD2 for detection.
2. The chip-based quantum key distribution system according to claim 1, characterized in that, The intensity modulation module obtains Z-based quantum states when the intensity of the subsequent sub-pulse or the preceding sub-pulse is modulated to 0. or When the intensities of the two sub-pulses are simultaneously modulated to half, the X-based quantum state is obtained. .
3. The chip-based quantum key distribution system according to claim 2, characterized in that, The intensity modulation module is a first Mach-Zehnder modulator MZM1 composed of a third beam splitter BS3 and a fourth beam splitter BS4, wherein a first phase modulator PM1 is provided on one arm of MZM1.
4. The chip-based quantum key distribution system according to claim 3, characterized in that, The intensity modulation module BS4 is equipped with an additional output port, which is connected to the photodetector PD for power monitoring.
5. The chip-based quantum key distribution system according to claim 2, characterized in that, The intensity modulation module includes a fifth beam splitter BS5 and a second phase modulator PM2. One input port of BS5 is connected to a laser LD, and the other input port is connected to an adjustable attenuation module. The two output ports of BS5 are directly connected through an optical waveguide, and PM2 is set on the optical waveguide.
6. The chip-based quantum key distribution system according to any one of claims 1-5, characterized in that, The adjustable attenuation module is a second Mach-Zehnder modulator MZM2 consisting of a sixth beam splitter BS6 and a seventh beam splitter BS7, wherein a first phase shifter PS1 is provided on one arm of MZM2.
7. The chip-based quantum key distribution system according to any one of claims 1-5, characterized in that, The orthogonal polarization delay module includes a second polarization beam splitter PBS2, a first quarter-wave plate QWP1, a second quarter-wave plate QWP2, a second phase shifter PS2, and a reflective film; One input port and one output port of PBS2 are connected to the two output ports of PBS1 through optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip through optical waveguides of equal length, and the end faces of the waveguides at the corresponding positions are coated with reflective films. QWP1 and QWP2 are respectively set on one of their own waveguides to rotate the polarization of the optical signal by 90° when it passes through back and forth. PS2 is set on one of the waveguides.
8. The chip-based quantum key distribution system according to claim 7, characterized in that, QWP1 and QWP2 are both high birefringence waveguide segments. The waveguide segments have an asymmetric cross-sectional structure, which makes the TE mode and TM mode of the transmitted light generate an effective refractive index difference Δn. The length L of the high birefringence waveguide section satisfies: Δn × L = λ0 / 4, where λ0 is the operating wavelength; The QWP1 and QWP2 ensure that the total phase delay of the optical signal after passing through them is λ / 2, achieving a polarization rotation of 90° and returning along the original incident light path.
9. The chip-based quantum key distribution system according to claim 7, characterized in that, Both QWP1 and QWP2 include a waveguide substrate, a waveguide core layer, and a groove structure formed on the waveguide core layer. The waveguide core layer is made of a high refractive index material. The groove structure extends along the light transmission direction and penetrates the width or depth direction of the waveguide core layer. The groove is filled with a low refractive index medium. The difference between the refractive index of the low refractive index medium and the refractive index of the waveguide core layer is not less than 1.
0. The groove structure and the waveguide core layer form an asymmetric transmission structure, which generates an effective refractive index difference Δn of a preset size between the TE polarization mode and the TM polarization mode of the transmitted light. The total length L of the waveguide core along the optical transmission direction satisfies: Δn×L = λ / 4, where λ is the operating wavelength.
10. The chip-based quantum key distribution system according to any one of claims 1-5, characterized in that, The orthogonal polarization delay module includes a second polarization beam splitter PBS2, a first subwavelength grating SWG1, a second subwavelength grating SWG2, and a third phase shifter PS3; One input port and one output port of PBS2 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS2 are connected to the edge of the decoding chip via optical waveguides of equal length. SWG1 and SWG2 are respectively set at the ends of the two waveguides. The period Λ of the grating is much smaller than the working wavelength λ0, satisfying Λ < λ0 / (2n), where n is the refractive index. It only supports zero-order reflection and transmission. The subwavelength grating forms a one-dimensional photonic bandgap through periodic refractive index modulation, achieving strong coherent reflection in the working wavelength band; The grating structure has birefringence characteristics, which generates a π phase difference for the incident polarized light, causing the polarization of the reflected light to rotate by 90°. The PS3 is mounted on one of the waveguides.
11. The chip-based quantum key distribution system according to any one of claims 1-5, characterized in that, The orthogonal polarization delay module includes a third polarization beam splitter PBS3 and a fourth phase shifter PS4; One input port and one output port of PBS3 are connected to the two output ports of PBS1 via optical waveguides of equal length; the other input port and the other output port of PBS3 are directly connected via optical waveguides, and PS4 is set on the optical waveguides.
12. The chip-based quantum key distribution system according to claim 1, characterized in that, BS1 also has an additional input port for outputting another interference result from the interferometer, and is equipped with a third single-photon detector SPD3 for detection.
13. The chip-based quantum key distribution system according to claim 1, characterized in that, The beam splitting ratio of BS1 is 10:90.