On-chip quantum key distribution system and method based on heterogeneous integration

Abstract

The present application relates to quantum key distribution, in particular to a quantum key distribution system and method based on heterogeneous integration, an active modulation chip is made of InP material, two independent high-speed traveling wave electrode electro-optic phase modulators are integrated, an encoding electro-optic phase modulator is arranged in a transmitting light path and is used for quantum state encoding, and a decoding electro-optic phase modulator is arranged in a receiving light path and is used for selecting a measurement base vector; a low-loss processing chip is made of SiN material, two asymmetric Mach-Zehnder interferometers are integrated, an encoding Mach-Zehnder interferometer is arranged in the transmitting light path and is used for converting quantum state encoding into phase difference encoding, and a decoding Mach-Zehnder interferometer is arranged in the receiving light path and is used for interference on a measured quantum state, so that phase difference information carried is converted into light intensity probability distribution; the present application can overcome defects that a single material platform cannot simultaneously consider ultra-low transmission loss and high-speed electro-optic phase modulation and a heterogeneous interface coupling loss is high.

Description

Technical Field

[0001] This invention relates to quantum key distribution, and more specifically to an on-chip quantum key distribution system and method based on heterogeneous integration. Background Technology

[0002] Quantum key distribution (QKD) technology, due to its theoretically unconditional security, has become one of the core technologies for ensuring future communication security. However, its practical application and large-scale deployment face key challenges such as complex hardware systems, large size, high cost, and poor environmental stability. Integrating the core optical path of a QKD system onto a chip using photonic integrated circuit technology is an ideal way to solve these problems and also to achieve device miniaturization, stabilization, and mass production.

[0003] Currently, there are two main technical approaches to on-chip QKD technology, but both have inherent limitations:

[0004] 1) Single low-loss platform solution: represented by Si or SiN platforms. These platforms have extremely low waveguide transmission loss (e.g., SiN can be less than 0.1dB / cm), mature technology and CMOS compatibility, making them suitable for building passive structures such as long interferometers. However, they lack efficient electro-optic effects and are difficult to integrate high-speed phase modulators natively. They usually rely on slow, high-power thermo-optic modulators or off-chip discrete modulators, which severely limits the system key generation rate and functional integrity.

[0005] 2) Single-function platform solution: Represented by the InP platform, InP materials can efficiently integrate lasers, high-speed electro-optic phase modulators and single-photon detectors, with comprehensive functions. However, the intrinsic propagation loss of InP waveguides is high (usually >2dB / cm). When constructing long Mach-Zehnder interferometers for quantum decoding, the accumulated loss will drastically reduce the signal-to-noise ratio of the system, thereby limiting the safe transmission distance.

[0006] Furthermore, most existing integrated QKD demonstrations are only "half-systems," either only implementing transmit encoding or only passive reception. A complete QKD protocol (such as the BB84 protocol) requires the receiver to perform fast, random measurement basis selection (active decoding), and integrating this function on a chip in a low-loss, high-speed manner is a technical challenge that current technologies have not yet adequately addressed.

[0007] Recent research has attempted heterogeneous integration, such as combining InP modulators with low-loss waveguides. However, achieving bidirectional collinear operation and coordinated control, while ensuring extremely low coupling loss at the heterogeneous interface to fully leverage the performance advantages of hybrid integration, remains a key technological bottleneck to be overcome. Meanwhile, emerging platforms (such as thin-film lithium niobate) have demonstrated high-speed modulation potential, but they still face challenges in terms of maturity and cost control regarding heterogeneous integration with ultra-low-loss waveguides. Summary of the Invention

[0008] (a) Technical problems to be solved

[0009] In view of the above-mentioned shortcomings of the existing technology, the present invention provides an on-chip quantum key distribution system and method based on heterogeneous integration, which can effectively overcome the shortcomings of the existing technology, such as the inability of a single material platform to achieve both ultra-low transmission loss and high-speed electro-optic phase modulation, as well as the high coupling loss of heterogeneous interfaces.

[0010] (II) Technical Solution

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] An on-chip quantum key distribution system based on heterogeneous integration includes an active modulation chip and a low-loss processing chip;

[0013] The active modulation chip, based on InP material, integrates two independent high-speed traveling-wave electrode electro-optic phase modulators. The coded electro-optic phase modulator is located in the transmitting optical path for quantum state encoding, and the decoder electro-optic phase modulator is located in the receiving optical path for selecting the measurement basis vector.

[0014] The low-loss processing chip, made of SiN material, integrates two asymmetric Mach-Zehnder interferometers. The coded Mach-Zehnder interferometer is located in the transmitting optical path to convert quantum state encoding into phase difference encoding, while the decoding Mach-Zehnder interferometer is located in the receiving optical path to interfere with the measured quantum state and convert the carried phase difference information into a light intensity probability distribution for single-photon detectors to distinguish.

[0015] Preferably, the coded electro-optic phase modulator adopts a capacitively loaded traveling wave electrode design with a modulation bandwidth greater than 15 GHz and a half-wave voltage design value less than 2.0 V. The coded electro-optic phase modulator applies any precise phase shift of 0, π / 2, π, or 3π / 2 to the input continuous laser pulse according to the BB84 protocol to complete quantum state encoding.

[0016] Preferably, the performance parameters of the decoding electro-optic phase modulator are symmetrical to those of the encoding electro-optic phase modulator. During decoding, the decoding electro-optic phase modulator applies a fast phase shift of 0 or π within the nanosecond range based on the real-time generated random number, thereby dynamically selecting the X-base or Y-base for measurement and realizing active decoding.

[0017] Preferably, the length difference ΔL between the two interferometer arms of the coded Mach-Zehnder interferometer makes the corresponding optical path delay τ equal to the system clock period T. The coded Mach-Zehnder interferometer converts the absolute phase code applied by the coded electro-optic phase modulator into the relative phase difference code between two adjacent time windows, i.e., time-phase code.

[0018] Preferably, the structure of the decoding Mach-Zehnder interferometer is symmetrical to that of the encoding Mach-Zehnder interferometer, and the length difference ΔL between the two interferometer arms is the same. The decoding Mach-Zehnder interferometer interferes with the quantum state modulated by the decoding electro-optic phase modulator, and converts the carried phase difference information into light intensity probability distributions at different output ports or different time windows for single-photon detectors to distinguish.

[0019] Preferably, each interferometer arm of the coded Mach-Zehnder interferometer and the decoded Mach-Zehnder interferometer integrates a thermo-optical phase tuner. The thermo-optical phase tuner is implemented using a Ti or Pt thin film resistor deposited on the surface of a SiN waveguide. It is used to compensate for static phase deviation caused by the manufacturing process and dynamic phase drift caused by ambient temperature drift, thereby stabilizing the operating point of the interferometer near the orthogonal point.

[0020] Preferably, the active modulation chip and the low-loss processing chip are directly bonded together via a precise in-plane edge coupling structure to form a monolithic hybrid photonic integrated circuit, comprising:

[0021] On the end face of the active modulation chip, a three-dimensional mode pattern converter is fabricated by etching: first, a lateral tapered widening is performed, and then a longitudinal mode field matching is performed through a thin waveguide layer to widen the small optical field mode pattern in the InP waveguide and adapt it to the coupling interface.

[0022] On the corresponding end face of the low-loss processing chip, an inverse tapered waveguide is fabricated: the waveguide width is thermally gradually narrowed from the micrometer size to the submicrometer size of the coupling interface, so as to efficiently receive the optical field from the InP waveguide and thermally compress it into the SiN waveguide.

[0023] Using a high-precision flip-chip bonding process based on machine vision, two prepared end faces are aligned and permanently bonded.

[0024] Preferably, the low-loss processing chip also integrates a laser input port, a quantum signal transmission port, and a quantum signal reception port. The laser input port is connected to an external laser source, and the quantum signal transmission port and the quantum signal reception port are connected to an optical fiber channel.

[0025] The on-chip quantum key distribution method based on heterogeneous integration includes two stages: encoding and decoding. The encoding stage includes the following steps:

[0026] S1. An external laser source injects continuous laser pulses into the laser input port;

[0027] S2. The laser pulse enters the active modulation chip. The coded electro-optic phase modulator applies any coded phase from 0, π / 2, π, or 3π / 2 to the input laser pulse based on the key information generated in real time by the random number generator. Complete quantum state encoding;

[0028] S3, carrying coded phase The laser pulse returns to the low-loss processing chip and enters the encoded Mach-Zehnder interferometer. Due to the length difference ΔL between the two interferometer arms, the encoded phase... It is converted into the relative phase difference between two adjacent time windows;

[0029] S4. The encoded quantum state is output from the quantum signal transmission port to the optical fiber channel;

[0030] The decoding phase includes the following steps:

[0031] S5. The quantum state from the optical fiber channel enters the low-loss processing chip from the quantum signal receiving port;

[0032] S6. The quantum state enters the active modulation chip, and the decoding electro-optic phase modulator applies a decoding phase of 0 or π based on the random number generated in real time by the random number generator. This completes the active selection of measurement basis vectors;

[0033] S7. The modulated quantum state returns to the low-loss processing chip and enters the decoding Mach-Zehnder interferometer for interference. The interference result consists of the relative phase difference and the decoded phase. Joint decision;

[0034] S8. The interference result is detected by a single-photon detector, and by combining it with the basis information of the publicly compared partial basis vectors, a secure shared key can be extracted.

[0035] (III) Beneficial Effects

[0036] Compared with existing technologies, the on-chip quantum key distribution system and method based on heterogeneous integration provided by this invention have the following advantages:

[0037] 1) Breakthrough performance integration: For the first time, the ultra-low transmission loss of SiN (less than 0.1dB / cm) and the high-speed electro-optic phase modulation characteristics of InP have been successfully integrated on a single chip, solving the long-standing contradiction that a single platform could not achieve both. The measured internal transmission loss of the system does not exceed 7.5dB (as an example loss budget: about 2.0dB for each of the two electro-optic phase modulators, about 0.9dB for each of the two heterogeneous edge coupling interfaces, about 1.0dB for the SiN waveguide transmission loss, and about 0.8dB for end-face coupling and other margins, totaling about 7.6dB), laying the technical foundation for high signal-to-noise ratio and high key rate;

[0038] 2) Functional completeness and system simplification: It pioneered the implementation of a full-featured QKD transceiver system with an active modulation receiver on the chip. The system can complete all optical processing steps of the BB84 protocol on its own, which greatly simplifies the external optical path and control system, and significantly improves the reliability and deployability of the system.

[0039] 3) Excellent secure transmission distance and key rate: Thanks to extremely low transmission loss, the system achieves a secure key rate of up to 1.57Mbps (1GHz clock) with 7dB channel loss (approximately 35km of standard fiber), and can support an extreme secure transmission distance of over 250km (44dB channel loss), placing its performance at an internationally leading level.

[0040] 4) Excellent stability and manufacturability: The inherent high stability of SiN waveguides and CMOS-compatible processes, combined with a precise and controllable heterogeneous integrated edge coupling structure, ensure the consistency and repeatability of system performance, paving the way for its large-scale, low-cost production. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0042] Figure 1 This is a schematic diagram of the system of the present invention;

[0043] Figure 2 This is a performance curve diagram of the present invention under long-term continuous operation. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] The core of this invention lies in providing an "edge-coupled hybrid InP-SiN on-chip QKD transceiver system" that adopts a "dual-material partitioning, three-port collinearity" architecture (e.g.) Figure 1 As shown, the ultra-low transmission loss and interference capability of SiN are seamlessly integrated with the high-speed electro-optic phase modulation capability of InP through high-precision in-plane edge coupling technology.

[0046] The following describes the specific functional modules of the on-chip quantum key distribution system based on heterogeneous integration provided by this invention, using concrete examples (such as...). Figure 1 (as shown) and technical effects.

[0047] The system's functional modules include: an active modulation chip and a low-loss processing chip;

[0048] The active modulation chip, based on InP material, integrates two independent high-speed traveling-wave electrode electro-optic phase modulators. The coded electro-optic phase modulator is located in the transmitting optical path for quantum state encoding, and the decoder electro-optic phase modulator is located in the receiving optical path for selecting the measurement basis vector.

[0049] The low-loss processing chip, made of SiN material, integrates two asymmetric Mach-Zehnder interferometers. The coded Mach-Zehnder interferometer is located in the transmitting optical path to convert quantum state encoding into phase difference encoding, while the decoding Mach-Zehnder interferometer is located in the receiving optical path to interfere with the measured quantum state and convert the carried phase difference information into a light intensity probability distribution for single-photon detectors to distinguish.

[0050] I. Active Modulation Chip

[0051] The coded electro-optic phase modulator adopts a capacitively loaded traveling wave electrode design with a modulation bandwidth greater than 15 GHz and a half-wave voltage design value of less than 2.0 V. According to the BB84 protocol, the coded electro-optic phase modulator applies any precise phase shift of 0, π / 2, π, or 3π / 2 to the input continuous laser pulse to complete quantum state encoding.

[0052] The performance parameters of the decoding electro-optic phase modulator are symmetrical to those of the encoding electro-optic phase modulator. During decoding, the decoding electro-optic phase modulator applies a fast phase shift of 0 or π within the nanosecond range based on the real-time generated random number, thereby dynamically selecting the X-base or Y-base for measurement and realizing active decoding.

[0053] II. Low-loss processing chip

[0054] The length difference ΔL between the two interferometer arms of the coded Mach-Zehnder interferometer makes the corresponding optical path delay τ equal to the system clock period T (e.g., 1ns corresponds to a 1GHz repetition frequency). The coded Mach-Zehnder interferometer converts the absolute phase code applied by the coded electro-optic phase modulator into the relative phase difference code between two adjacent time windows ("early" pulse and "late" pulse), i.e., time-phase coding.

[0055] The structure of the decoding Mach-Zehnder interferometer is symmetrical to that of the encoding Mach-Zehnder interferometer, and the length difference ΔL between the two interferometer arms is the same. The decoding Mach-Zehnder interferometer interferes with the quantum state modulated by the decoding electro-optic phase modulator, and converts the carried phase difference information into the light intensity probability distribution of different output ports or different time windows for single-photon detectors to distinguish.

[0056] Each interferometer arm of the coded and decoded Mach-Zehnder interferometers integrates a thermo-optical phase tuner. This thermo-optical phase tuner is implemented using Ti or Pt thin-film resistors deposited on the surface of SiN waveguides. It is used to compensate for static phase deviations caused by manufacturing processes and dynamic phase drifts caused by ambient temperature drifts, thereby stabilizing the operating point of the interferometer near the orthogonal point.

[0057] The low-loss processing chip also integrates a laser input port, a quantum signal transmission port, and a quantum signal reception port. The laser input port is connected to an external laser source, and the quantum signal transmission port and the quantum signal reception port are connected to an optical fiber channel.

[0058] III. Edge Coupling Structure

[0059] To achieve efficient transmission between the active modulation chip and the low-loss processing chip, a precise in-plane edge coupling structure is used to directly dock and bond the active modulation chip and the low-loss processing chip, forming a monolithic hybrid photonic integrated circuit, including:

[0060] On the end face of the active modulation chip, a three-dimensional mode converter is fabricated by etching: first, a lateral tapered widening is performed (the tapered length is preferably 200μm~500μm), and then longitudinal mode field matching is performed through a thin waveguide layer to convert the small optical field mode spot (approximately 0.5μm) in the InP waveguide. 2 (In a preferred embodiment, the three-dimensional mode spot converter employs a double-layer conical adiabatic gradient structure to extend the mode spot diameter from approximately 0.8 μm to approximately 2.5 μm to match the mode field distribution of the SiN waveguide.)

[0061] On the corresponding end face of the low-loss processing chip, an inverse tapered waveguide is fabricated: the waveguide width is thermally gradually narrowed from a micrometer size (e.g., 1.5 μm) to a submicrometer size (e.g., 0.3 μm) at the coupling interface, so as to efficiently receive the optical field from the InP waveguide and thermally compress it into the SiN waveguide.

[0062] By using a high-precision flip-chip bonding process based on machine vision, two prepared end faces are aligned and permanently bonded. After optimization, the single-interface insertion loss of this edge-coupled structure can be stably controlled below 0.9dB.

[0063] Based on the above-disclosed on-chip quantum key distribution system based on heterogeneous integration, this invention also discloses an on-chip quantum key distribution method based on heterogeneous integration, comprising two stages: encoding and decoding. The encoding stage includes the following steps:

[0064] S1. An external laser source injects continuous laser pulses into the laser input port;

[0065] S2. The laser pulse enters the active modulation chip. The coded electro-optic phase modulator applies any coded phase from 0, π / 2, π, or 3π / 2 to the input laser pulse based on the key information generated in real time by the random number generator. Complete quantum state encoding;

[0066] S3, carrying coded phase The laser pulse returns to the low-loss processing chip and enters the encoded Mach-Zehnder interferometer. Due to the length difference ΔL between the two interferometer arms, the encoded phase... It is converted into the relative phase difference (0 or π) between two adjacent time windows;

[0067] S4. The encoded quantum state (manifested as a pair of laser pulses with a specific relative phase difference) is output from the quantum signal transmission port to the optical fiber channel;

[0068] The decoding phase includes the following steps:

[0069] S5. The quantum state from the optical fiber channel enters the low-loss processing chip from the quantum signal receiving port;

[0070] S6. The quantum state enters the active modulation chip, and the decoding electro-optic phase modulator applies a decoding phase of 0 or π based on the random number generated in real time by the random number generator. This completes the active selection of measurement basis vectors;

[0071] S7. The modulated quantum state returns to the low-loss processing chip and enters the decoding Mach-Zehnder interferometer for interference. The interference result (which output port / time window the photon appears at) is determined by the relative phase difference and the decoding phase. Joint decision;

[0072] S8. The interference result is detected by a single-photon detector, and by combining it with the basis information of the publicly compared partial basis vectors, a secure shared key can be extracted.

[0073] To better illustrate the technical solution of this application, two examples are provided below for detailed explanation.

[0074] Example 1: System Preparation and Basic Performance Testing

[0075] 1) System Preparation

[0076] • SiN low-loss processing chip: A 160nm thick SiN film is grown on a thermally oxidized silicon substrate using plasma-enhanced chemical vapor deposition. A waveguide structure is formed by 248nm deep ultraviolet lithography and reactive ion etching. The waveguide cross-section is 850nm (width) × 160nm (height), with a reverse tapered end width of 320nm and a length of 200μm. Subsequently, Ti / Pt thin film resistors (10nm / 100nm thickness) are deposited and patterned to form a thermo-optical phase tuner.

[0077] •InP active modulation chip: On an n-type InP substrate, a multi-quantum well waveguide structure is epitaxially grown by metal-organic chemical vapor deposition. Ridge waveguides and three-dimensional mode converters are fabricated by a combination of wet etching and dry etching. Traveling wave electrodes adopt a coplanar waveguide design with impedance matching up to 50Ω and electrode length of 1.5mm.

[0078] • Heterogeneous integration: Using a high-precision flip-chip bonding machine and with the assistance of an infrared imaging alignment system, the InP active modulation chip and the SiN low-loss processing chip are aligned end-to-end. Permanent bonding is performed using UV-curable optical adhesive. Constant pressure is applied during the curing process to control the adhesive layer thickness, and the alignment accuracy is better than ±0.5μm.

[0079] 2) Actual measurement of key parameters

[0080] Table 1. Technical Parameters of Core Structure

[0081]

[0082] 3) System-level performance testing

[0083] The test system was built in a constant temperature (25.0±0.1°C) shielded environment, using an external DFB laser (linewidth <100kHz) and a superconducting nanowire single-photon detector (detection efficiency of about 85%).

[0084] • Performance benchmark test: Using an adjustable optical attenuator to simulate channel loss, when the total link loss is 10.5dB, the system operates at a clock frequency of 1GHz, and the measured average quantum bit error rate is only 0.61%. Through secure key post-processing analysis, the secure key rate reaches 2.01Mbps.

[0085] • Stability Test: Under 50 hours of continuous operation, the quantum bit error rate (QBER) remained consistently within the range of 0.66% ± 0.07% (e.g., ...). Figure 2 As shown in the figure, no performance drift was observed, proving that the system has extremely high long-term stability.

[0086] • Limit Transmission Distance Verification: A one-way transmission experiment was conducted using a 275km long ultra-low loss single-mode fiber (channel loss of approximately 46dB). Under the most stringent security parameters considering the finite code length effect, a positive asymptotic security key rate could still be calculated, verifying the system's ability to operate under extreme loss conditions.

[0087] Table 2 System performance under different channel losses

[0088]

[0089] Example 2: An Alternative Heterogeneous Integration Solution Based on Micro-Transfer Printing

[0090] As an alternative to edge-coupled structures, the heterogeneous integration of this invention can also be achieved using "micro-transfer printing" technology, which is more suitable for wafer-level large-scale manufacturing.

[0091] 1) Electro-optic phase modulator arrays are mass-produced on InP wafers and released from the substrate by selective etching to form thin-film microdevices (called "stamps") with a thickness of about 5 μm.

[0092] 2) Using a flexible stamp made of polydimethylsiloxane, multiple electro-optic phase modulator "stamps" are picked up at once by van der Waals forces;

[0093] 3) The stamp is precisely aligned and pressed onto the target silicon wafer that has completed SiN waveguide processing. The "stamp" is transferred and fixed to the predetermined position by intermolecular forces.

[0094] 4) Finally, the metal interconnects between the electro-optic phase modulator “stamp” and the pre-fabricated pads on the SiN wafer are fabricated through evaporation, photolithography and lift-off processes to complete the electrical connection.

[0095] The advantage of the "micro-transfer printing" solution is that it can integrate a large number of devices in parallel, resulting in high production capacity, and avoids the manual alignment of individual chips. However, it has extremely high requirements for the accuracy and reliability of the stamp.

[0096] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An on-chip quantum key distribution system based on heterogeneous integration, characterized in that: This includes active modulation chips and low-loss processing chips; The active modulation chip, based on InP material, integrates two independent high-speed traveling-wave electrode electro-optic phase modulators. The coded electro-optic phase modulator is located in the transmitting optical path for quantum state encoding, and the decoder electro-optic phase modulator is located in the receiving optical path for selecting the measurement basis vector. The low-loss processing chip, made of SiN material, integrates two asymmetric Mach-Zehnder interferometers. The coded Mach-Zehnder interferometer is located in the transmitting optical path to convert quantum state encoding into phase difference encoding, while the decoding Mach-Zehnder interferometer is located in the receiving optical path to interfere with the measured quantum state and convert the carried phase difference information into a light intensity probability distribution for single-photon detectors to distinguish.

2. The on-chip quantum key distribution system based on heterogeneous integration according to claim 1, characterized in that: The coded electro-optic phase modulator adopts a capacitively loaded traveling wave electrode design with a modulation bandwidth greater than 15 GHz and a half-wave voltage design value less than 2.0 V. According to the BB84 protocol, the coded electro-optic phase modulator applies any precise phase shift of 0, π / 2, π, or 3π / 2 to the input continuous laser pulse to complete quantum state encoding.

3. The on-chip quantum key distribution system based on heterogeneous integration according to claim 2, characterized in that: The performance parameters of the decoding electro-optic phase modulator are symmetrical to those of the encoding electro-optic phase modulator. During decoding, the decoding electro-optic phase modulator applies a fast phase shift of 0 or π within the nanosecond range based on the real-time generated random number, thereby dynamically selecting the X-base or Y-base for measurement and realizing active decoding.

4. The on-chip quantum key distribution system based on heterogeneous integration according to claim 3, characterized in that: The length difference ΔL between the two interferometer arms of the coded Mach-Zehnder interferometer makes the corresponding optical path delay τ equal to the system clock period T. The coded Mach-Zehnder interferometer converts the absolute phase code applied by the coded electro-optic phase modulator into the relative phase difference code between two adjacent time windows, i.e., time-phase code.

5. The on-chip quantum key distribution system based on heterogeneous integration according to claim 4, characterized in that: The structure of the decoding Mach-Zehnder interferometer is symmetrical to that of the encoding Mach-Zehnder interferometer, and the length difference ΔL between the two interferometer arms is the same. The decoding Mach-Zehnder interferometer interferes with the quantum state modulated by the decoding electro-optic phase modulator, and converts the carried phase difference information into the light intensity probability distribution of different output ports or different time windows for single-photon detectors to distinguish.

6. The on-chip quantum key distribution system based on heterogeneous integration according to claim 5, characterized in that: Each interferometer arm of the coded Mach-Zehnder interferometer and the decoded Mach-Zehnder interferometer integrates a thermo-optical phase tuner. The thermo-optical phase tuner is implemented using a Ti or Pt thin film resistor deposited on the surface of a SiN waveguide. It is used to compensate for static phase deviations caused by the manufacturing process and dynamic phase drifts caused by ambient temperature drifts, thereby stabilizing the operating point of the interferometer near the orthogonal point.

7. The on-chip quantum key distribution system based on heterogeneous integration according to claim 1, characterized in that: The active modulation chip and the low-loss processing chip are directly bonded together via a precise in-plane edge coupling structure to form a monolithic hybrid photonic integrated circuit, comprising: On the end face of the active modulation chip, a three-dimensional mode pattern converter is fabricated by etching: first, a lateral tapered widening is performed, and then a longitudinal mode field matching is performed through a thin waveguide layer to widen the small optical field mode pattern in the InP waveguide and adapt it to the coupling interface. On the corresponding end face of the low-loss processing chip, an inverse tapered waveguide is fabricated: the waveguide width is thermally gradually narrowed from the micrometer size to the submicrometer size of the coupling interface, so as to efficiently receive the optical field from the InP waveguide and thermally compress it into the SiN waveguide. Using a high-precision flip-chip bonding process based on machine vision, two prepared end faces are aligned and permanently bonded.

8. The on-chip quantum key distribution system based on heterogeneous integration according to claim 1, characterized in that: The low-loss processing chip also integrates a laser input port, a quantum signal transmission port, and a quantum signal receiving port. The laser input port is connected to an external laser source, and the quantum signal transmission port and the quantum signal receiving port are connected to an optical fiber channel.

9. An on-chip quantum key distribution method based on heterogeneous integration, applicable to the on-chip quantum key distribution system based on heterogeneous integration as described in claim 1, characterized in that: It includes two stages: encoding and decoding. The encoding stage includes the following steps: S1. An external laser source injects continuous laser pulses into the laser input port; S2. The laser pulse enters the active modulation chip. The coded electro-optic phase modulator applies any coded phase from 0, π / 2, π, or 3π / 2 to the input laser pulse based on the key information generated in real time by the random number generator. Complete quantum state encoding; S3, carrying coded phase The laser pulse returns to the low-loss processing chip and enters the encoded Mach-Zehnder interferometer. Due to the length difference ΔL between the two interferometer arms, the encoded phase... It is converted into the relative phase difference between two adjacent time windows; S4. The encoded quantum state is output from the quantum signal transmission port to the optical fiber channel; The decoding phase includes the following steps: S5. The quantum state from the optical fiber channel enters the low-loss processing chip from the quantum signal receiving port; S6. The quantum state enters the active modulation chip, and the decoding electro-optic phase modulator applies a decoding phase of 0 or π based on the random number generated in real time by the random number generator. This completes the active selection of measurement basis vectors; S7. The modulated quantum state returns to the low-loss processing chip and enters the decoding Mach-Zehnder interferometer for interference. The interference result consists of the relative phase difference and the decoded phase. Joint decision; S8. The interference result is detected by a single-photon detector, and by combining it with the basis information of the publicly compared partial basis vectors, a secure shared key can be extracted.

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