From: National University of Science and Technology MISiS
Posted: Thursday, October 21, 2021
The technology of tenfold extension of service life of a single-photon detector for the Earth-orbit quantum communication channel has been developed by an international group of scientists with the participation of NUST MISIS. Heating under certain conditions heals defects in the detector's silicon base that result from the irradiation of the satellite with hard space radiation. This prolongs the life of the satellite by reducing the level of "noise" that disables the quantum communication system. The results of the work have been published in the international scientific journal EPJ Quantum Technology.
The exchange of secret keys between users of a communication network enables a safe way to exchange messages—without eavesdropping and information leakage. If we use these keys in cryptographic protocols, we are almost invulnerable. Conventional cryptosystems use algorithms to ensure data security due to computational complexity. However, a quantum computer is, basically, capable of breaking even such encryption due to the so-called Shor's factoring algorithm.
There is a solution—quantum key distribution (QKD) protocols. They make use of public optical channels to securely distribute keys by exchange of quantum bits, carried by single photons of light. This absolute security is obtained through the no-cloning theorem: any measurement of a quantum bit by an eavesdropper risks changing the state of that bit. This reveals the eavesdropper's presence.
The quantum distribution of keys between two communicating users occurs using photons, which travel through optical fibers or over atmospheric line-of-sight channels. Optical fiber is limited by loss of photons, thus we cannot stretch the quantum communication across the oceans. However orbiting satellites can cover the entire surface of the planet with a quantum communication network.
So, one half of the quantum system will be placed on a satellite orbiting about 500 km above us, and the other half is on the Earth. Twice a day, telescopes on Earth and on the satellite will point at each other and a QKD session will last for 4 minutes, generating a secret random sequence of zeros and ones.
The receiver in the system is a single-photon detector. It is based on silicon avalanche photodiodes. Registration of single photons in a satellite is a crucial element the QKD system. However, detectors have a significant, critical disadvantage—they are very sensitive to solar radiation and are gradually damaged by it. As a result, they produce an ever-increasing amount of noise, which can distort the operation of the system, eventually making QKD impossible. The solution to this problem has been found by Canadian and Russian scientists.
"We have shown that thermal annealing of silicon detector modules is effective in maintaining the parasitic 'noise' at a level suitable for quantum key distribution during the entire period of the satellite's operation," says Professor Vadim Makarov, a co-author of the study, head of the Quantum hacking lab at the NTI Center for Quantum Communications at NUST MISIS.
In this work, we studied two strategies—annealing at fixed time intervals, and annealing only when the thermal "noise" exceeds a certain limit. We find both strategies exhibit acceptable thermal noise at end-of-life, with the latter strategy having a slight overall advantage."
It turned out that silicon lattice defects created by hard in-orbit radiation can be repaired by heating them for a short time—by performing annealing. This is done using the so-called Peltier cooler. It usually works in the detector device, creating a temperature of −80 degrees Celsius. If you turn it on in reverse mode and heat the detector modules to +80 degrees for an hour, the defects are being healed by themselves.
"At the level of semiconductors, the physics of the process is not well understood, but the results of experiments have shown that during heating—annealing—defects are partially "healed". Thus, the amount of parasitic noise in the detector is reduced to an acceptable level," explains Associate Professor Thomas Jennewein, head of Quantum Photonics Laboratory, part of the Institute for Quantum Computing at the University of Waterloo, Canada, where the experiments were conducted.
The task for the satellite is to work in orbit for as long as possible. According to the scientists, in reality, with the existing radiation, it turns out to last only about a year. And if it is periodically annealed when necessary, the service life can be extended up to 10 years. This must be done regularly—a command is remotely sent to the satellite, and the electronics performs the annealing. "We will monitor the noise level and perform annealing when the detectors become too noisy," explains Jennewein.
The research team carried out the radiation testing at a particle accelerator in Vancouver, which simulated a flux of heavy radiation particles, protons, that is accumulated in space over 10 years. In the test, the proton beam was so intense that a two-minute-long burst of radiation simulated several months in space between annealing cycles.
A satellite named QEYSSat with the single-photon detector equipped with this annealing technology is currently under construction in Canada. Its launch is scheduled for 2023.
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