Quantum Transmitter Pulse Modulation for Signal Stability
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Solution Overview
Problem
Existing quantum cryptographic communication systems face challenges in maintaining signal-to-noise ratio (SNR) and signal output stability, particularly in optical transmission lines with environmental susceptibility, such as free space, where attenuation rates vary due to factors like water vapor and temperature changes, and current solutions do not effectively address these issues at the transmitter side.
Innovation Solution
A quantum cryptographic communication system where a transmitter modulates the pulse intensity and width of coherent light transmission based on detected attenuation rates using probe light, ensuring the signal and reference lights are transmitted with adjusted intensities and pulse widths to maintain energy consistency and compensate for environmental changes, thereby stabilizing the signal output and improving SNR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the laser output power is increased to compensate for signal level attenuation, then the signal level is improved, but the equipment size increases, durability decreases, and security is reduced
Solution Approach 1:
The patent introduces an optical amplifier as an intermediary device placed in the transmission line to amplify the local oscillator light. This mediator compensates for signal level attenuation without requiring the transmitter to output excessive power, thus maintaining security and durability while improving the signal level at the receiver.
Solution Approach 2:
The patent segments the compensation function from the transmitter to the receiver side. Instead of having the transmitter compensate for all attenuation, the system divides the compensation task: the transmitter sends at normal power levels, and the receiver side (via optical amplifier) compensates for the attenuation of the local oscillator light separately.
2Illumination intensity
If the optical amplifier is installed on the transmission line to amplify signal light, then the signal level is improved, but the cryptographic key information is affected
Solution Approach 1:
The patent applies local quality by making the optical amplifier selective in what it amplifies. The amplifier is configured to amplify only the local oscillator light (reference light) while leaving the signal light unchanged. This selective amplification improves the signal level for detection without affecting the quantum states of the signal light that carry cryptographic key information.
3Adaptability or versatility
If the attenuation rate varies due to environmental changes in free space transmission, then the transmission adaptability is improved, but the signal output stability deteriorates
Solution Approach 1:
The patent implements feedback control where the receiver measures the attenuation rate of the transmission line based on received signal characteristics, then feeds this information back to control the optical amplifier's gain. This feedback mechanism allows the system to adapt to environmental changes while maintaining stable signal output by dynamically adjusting the amplification level.
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
This approach effectively stabilizes the signal output and enhances the SNR in homodyne detection even in environments with high attenuation rates, ensuring reliable quantum cryptographic communication across varying environmental conditions.
Implementation Method 1
generate signal light and reference light from transmission light pulse of coherent light
Implementation Method 2
The homodyne detection enables the extraction of the phase information of the signal light that has been phase-modulated at the transmitter
Implementation Method 3
generate a signal output by interfering the received reference light with the received signal light
Implementation Method 4
the propagation characteristics of light changing due to changes of environment... attenuation rates vary due to factors like water vapor and temperature changes
Data Source
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AI summary
An illustrative transmitter can achieve improvement in SN ratio and stability of the signal output under control at the transmission side, even in the case of an optical transmission line having the propagation characteristics susceptible to environmental changes. The transmitter includes: an optical transmission section configured to generate weak signal light and reference light from a transmission light pulse, and transmit the signal light and the reference light to the optical transmission line; and a controller configured to modulate pulse intensity and pulse width of the transmission light pulse according to a transmission line state of probe light having propagated through the optical transmission line.