Quantum Transmitter Reference Pulse Wavefront Correction
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Solution Overview
Problem
In free-space quantum key distribution (QKD) systems, atmospheric turbulences distort optical beams, making it difficult to focus received light efficiently due to the large telescope size required, which exacerbates wavefront distortions compared to classical communication links.
Innovation Solution
An optical transmitter and receiver system that emits a stream of QKD encoded pulses and reference pulses with a predetermined time delay, where the wavelength difference is 5 nm or less, allowing for wavefront correction using a wavefront sensor and adaptive optical element to improve focusing and reduce Qubit error rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large telescope is used to receive quantum signals, then the signal reception capability is improved, but the wavefront distortion becomes more severe
Solution Approach 1:
A reference channel with reference pulses is introduced as an intermediary to measure atmospheric turbulence. The wavefront sensor uses these reference pulses to characterize distortions, which then inform the adaptive optics system to correct the QKD signal wavefront, resolving the contradiction between large aperture reception and wavefront quality
Solution Approach 2:
A feedback loop is established where the wavefront sensor continuously measures distortions using reference pulses, and the adaptive optics system applies real-time corrections based on these measurements. This feedback mechanism maintains wavefront quality despite using a large telescope aperture
2Measurement precision
If reference pulses are emitted with the same wavelength as QKD pulses, then the wavefront measurement accuracy is improved, but the temporal separation between signals becomes difficult
Solution Approach 1:
Reference pulses are emitted periodically at a repetition rate that is a multiple of the QKD pulse rate, creating a time-delayed periodic pattern. This allows temporal separation through timing discrimination while maintaining wavelength similarity for accurate wavefront measurement
Solution Approach 2:
The problem is solved by transitioning from spectral separation to temporal separation. Instead of using different wavelengths (spectral dimension), the system uses time delays (temporal dimension) to distinguish reference pulses from QKD pulses, simplifying the separation mechanism
3Speed
If the repetition rate of reference pulses is increased, then the wavefront correction speed is improved, but the energy per pulse is reduced
Solution Approach 1:
The reference channel operates at a higher repetition rate than the QKD channel (a multiple thereof), providing excessive measurement samples that enable faster wavefront correction. The reduced energy per reference pulse is acceptable because reference pulses serve only for measurement, not information transmission
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 enables efficient wavefront correction of QKD encoded pulses, reducing Qubit error rates and improving data rates by using a reference pulse with more energy to measure distortions, allowing for better focusing and quick adaptation to atmospheric changes.
Implementation Method 1
a wavefront sensor arranged to receive reference pulses emitted by an optical transmitter, in particular by the optical transmitter according to the preceding claims, and configured to measure a wavefront of the reference pulses to provide a wavefront signal
Implementation Method 2
an adaptive optical element arranged upstream of the detection unit and configured to manipulate a wavefront of the QKD encoded pulses; and a receiver controller connected to the wavefront sensor, the adaptive optical element and the detection unit, wherein the receiver controller is configured to: trigger the adaptive optical element to correct the wavefront of the QKD encoded pulses based on the wavefront signal
Implementation Method 3
apply a time gate to the one or more photon counters in the detection unit at the time of arrival of the QKD encoded pulses
Data Source
AI summary
An optical transmitter for quantum communication, including a QKD channel comprising at least QKD light source and configured to emit a stream of QKD encoded pulses; a reference channel including a reference light source and configured to emit a stream of reference pulses; and a control circuit connected to the QKD channel and to the reference light channel. The control circuit is configured to control the QKD channel and the reference light channel to emit the reference pulses with a predetermined time delay to the QKD encoded pulses. A difference of a wavelength of the QKD encoded pulses and a wavelength of the reference pulses is 5 nm or less. A corresponding optical receiver, an optical system and a method for quantum communication are also provided.