Phase Polarization QKD Network System
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Solution Overview
Problem
Current quantum key distribution (QKD) systems face challenges in maintaining stability and efficiency, especially in long-distance transmission and multi-user scenarios, due to environmental influences and low utilization of quantum secret communication photons, which affect the key generation rate and security.
Innovation Solution
A phase and polarization multi-degree-of-freedom modulated QKD network system using a single-light-source and multi-wavelength laser device, with differential phase encoding and polarization encoding, allows for one-to-many communication, increasing the key generation rate and stability by utilizing multi-wavelength pulses and reducing bit error rates through wavelength division multiplexing and advanced modulation techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If polarization modulation is used to encode quantum information, then the encoding capability is provided, but the polarization states are easily affected by stress birefringence and polarization mode dispersion in optical fiber, requiring polarization compensation devices that increase system complexity
Solution Approach 1:
The patent combines phase modulation and polarization modulation into a unified quantum encoding system. The phase modulator and polarization controller work together to encode quantum information, where phase modulation provides robustness against fiber disturbances while polarization modulation enables multi-degree-of-freedom encoding. This merging eliminates the need for separate polarization compensation devices while maintaining encoding capability.
Solution Approach 2:
The patent changes the encoding parameter from purely polarization-based to a combination of phase and polarization parameters. By using phase difference between pulses as the primary encoding parameter (which is insensitive to polarization changes), the system achieves immunity to polarization mode dispersion and stress birefringence, eliminating the requirement for polarization compensation equipment.
2Device complexity
If single-degree-of-freedom modulation is used for quantum signal encoding, then the system is simple to implement, but the photon utilization ratio is low and key generation rate is limited
Solution Approach 1:
The patent transitions from single-degree-of-freedom (polarization only) to multi-degree-of-freedom modulation by adding phase modulation as another dimension. The quantum state is encoded using both polarization state and phase difference, creating a two-dimensional encoding space that doubles the information capacity per photon and significantly increases the key generation rate while maintaining implementation feasibility.
3Reliability
If BB84 or B92 protocol is used with M-Z interferometer, then quantum key distribution is achieved, but the code generation rate is low due to the requirement for comparing encoding base with measuring base
Solution Approach 1:
The patent extracts and eliminates the base comparison step from the quantum key distribution process by using differential phase encoding. Instead of requiring Alice and Bob to compare their chosen bases (which reduces efficiency), the system directly encodes information in the phase difference between consecutive pulses, allowing Bob to measure the phase difference directly without base comparison, thereby maintaining security while dramatically increasing code generation rate.
4Stability of the object's composition
If double asymmetric M-Z interferometer is used to improve interference stability, then the time disturbance on overlaid optical fiber affects both pulses equally, but small changes in arm length still reduce interference contrast ratio
Solution Approach 1:
The patent introduces dynamic adjustment capability through polarization controllers that can adaptively compensate for arm length changes in the M-Z interferometer. The system continuously monitors and adjusts the polarization states to maintain optimal interference contrast ratio despite environmental variations, transforming the static interferometer into a dynamically adaptable system that maintains both stability and precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves stable and efficient key distribution among multiple users with increased photon utilization, improved interference stability, and enhanced resistance to environmental interference, effectively defending against eavesdropping attacks.
Implementation Method 1
The firstly-proposed phase modulation scheme is based on a single M-Z interferometer with two symmetric arms. Due to environment influences, the length difference of two arms may be instable, the phase difference may also drift, thus, the interference effect may be seriously affected
Implementation Method 2
The polarization states of the photon are easily affected by factors such as stress birefringence and polarization mode dispersion in an optical fiber
Implementation Method 3
Phase modulation refers to information encoding using phase shifting of light. In principle, the phase modulation of quantum communication is mainly based on a Mach-Zehnder interferometer, and a phase modulator is used as a core device
Data Source
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AI summary
The present invention relates to a phase and polarization multi-degree-of-freedom modulated quantum key distribution (QKD) network system and method. The system includes an Alice transmitting terminal, a wavelength division multiplexing (WDM) unit and a plurality of Bob terminals for receiving a plurality of photons of different wavelengths; the Alice terminal is connected with the plurality of Bob terminals by the WDM unit; the Alice terminal includes a multi-wavelength laser generation device, an attenuator, a first polarization beam splitter, a first beam combiner, a phase modulator, a first polarization controller and a second polarization controller; the WDM unit includes a wavelength selection device; each Bob terminal includes a second polarization controller, a third polarization controller, a fourth polarization controller, a third polarization beam splitter, a fourth polarization beam splitter, a second beam combiner, a third beam combiner, a first photon detector, a second photon detector, a third photon detector and a fourth photon detector.