QKD Phase Modulator Timing Alignment Using Interferometer Feedback
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Solution Overview
Problem
Existing Quantum Key Distribution (QKD) systems face challenges in aligning the phase modulator with the pulse generator due to unknown delays and latencies, leading to high Quantum Bit Error Rates (QBER) and failure in generating identical keys at both nodes.
Innovation Solution
An apparatus and method that adjusts the phase modulator's modulating signal time reference using a processing device, beam splitter, two-arm interferometer, and photodetectors to detect and compensate for delays, eliminating the need for phase modulator characterization and RF cable length fixation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the phase modulator and pulse generator use the same time reference from the same device, then the alignment between modulating signal and optical pulses should be improved, but unknown delays and latencies in modulators and system-dependent electrical and optical path delays still cause improper alignment
Solution Approach 1:
The patent replaces manual characterization and fixed cable length configurations with an automated feedback control system. The processing device automatically adjusts the time reference of the phase modulator's modulating signal by detecting alignment errors and compensating for delays, eliminating the need for mechanical adjustments and manual calibration.
Solution Approach 2:
The patent implements a feedback mechanism where the processing device continuously monitors the alignment between optical pulses and modulating signal, detects delays, and automatically adjusts the time reference to compensate for misalignment. This closed-loop feedback system ensures proper alignment is maintained despite variations in cable length or phase delay.
2Measurement precision
If manual characterization of phase modulator or fixation of RF cable length is used, then alignment can be achieved under specific conditions, but the system lacks adaptability when cable length or phase delay changes
Solution Approach 1:
The patent transforms the static alignment configuration into a dynamic system. Instead of fixing the RF cable length or relying on manual characterization, the processing device dynamically adjusts the time reference of the modulating signal in real-time, allowing the system to adapt to changes in cable length, phase delay, or other environmental factors automatically.
Solution Approach 2:
The patent changes the time reference parameter of the phase modulator's modulating signal dynamically. By adjusting this temporal parameter based on detected alignment errors, the system compensates for variations in cable length and phase delay, maintaining proper synchronization between optical pulses and modulating signal under varying conditions.
3Device complexity
If phase modulator alignment is not properly adjusted, then the system structure remains simple, but Quantum Bit Error Rate increases and key generation fails
Solution Approach 1:
The patent implements a self-aligning system where the processing device automatically detects and corrects alignment errors without external intervention. The system monitors its own performance, identifies misalignment conditions, and self-corrects by adjusting the time reference, eliminating the need for manual alignment procedures and reducing operational complexity.
Solution Approach 2:
The patent replaces complex manual alignment procedures and fixed hardware configurations with an automated electronic adjustment system. The processing device uses electronic signal processing and feedback control to achieve and maintain proper alignment, simplifying the overall system while improving reliability through automatic compensation for misalignment.
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
Achieves accurate alignment of the phase modulator with the pulse generator, reducing QBER, and automatically re-aligns the modulating signal if cable length or phase delay changes, ensuring proper modulation and key generation.
Implementation Method 1
The two-arm interferometer is configured to receive the modulated pulses through the second path and provide demodulated pulses having the pre-defined time interval (T) between adjacent pulses at the output legs
Implementation Method 2
a first photodetector connected to a first output leg of the interferometer and configured to receive the demodulated pulses after constructive interference from the interferometer and generate a corresponding digital output
Data Source
AI summary
The present disclosure relates to the field Quantum Key distribution (QKD) and discloses an apparatus (100) and method (300) for adjusting the phase modulator's modulating signal time reference in a phase-based QKD system. The QKD system comprises a pulse generator (10) that generates optical pulses and a phase modulator (20) that modulates the phase of each of the pulses. The apparatus (100) comprises abeam splitter/tap (112), a 1-bit delay interferometer (110), two photodetectors (106,108), and a processing device (104). The splitter feeds the phase modulated optical pulses to the interferometer (110). Two photo detectors are connected to the constructive and destructive output legs (110a,110b) of the interferometer (110). The photo detectors' output are then converted to digital signals and fed to the processing device (104). The processing device (104) measures the average power at both the legs of the interferometer (110) to detect delay between optical signal and modulating signal and adjusts the delay to accurately align the phase modulator (20) with the pulse generator (10).