Hot and cold: controlling noise in a quantum satellite

Friday, October 29, 2021

The quantum internet is one step closer to reality as researchers have demonstrated an effective regime for controlling noise in the photon detectors of a quantum satellite.“This experiment is an important demonstration of a crucial subsystem for the quantum satellite under the kind of conditions that the satellite will actually face in orbit,” said Brendon Higgins, a Research Associate at the Institute for Quantum Computing (IQC) and the University of Waterloo Department of Physics and Astronomy, and the Science Team Technical Lead of the Quantum EncrYption and Science Satellite (QEYSSat).

Thomas Jennwein's team

Left to right: Ian DSouza, Sascha Agne, Jin Gyu Lim [seated], Thomas Jennewein, Jean-Philippe Bourgoin, Brendon Higgins, and Ramy Tannous.

The QEYSSat mission aims to launch Canada’s first quantum satellite in early 2023 to serve as a technology demonstration for quantum satellite links. A quantum satellite network could secure our most precious data through the fundamental laws of quantum mechanics, using quantum key distribution (QKD).

To make the quantum internet possible, researchers need to be able to send photons carrying quantum information to satellites in orbit, and the satellites need single photon detectors that can receive the message. But proton radiation in space and the very nature of the detectors themselves often contribute to a kind of false positive noise called dark counts.

Combatting dark counts has been one of the key technical challenges the QEYSSat science team led by Thomas Jennewein, a faculty member at IQC and in the University of Waterloo Department of Physics and Astronomy, has been working to address. If they can limit dark counts, the detectors can do their job detecting real incoming quantum keys sent from ground stations on Earth.

The team set out to find the best approach to reduce dark counts and ensure their detectors can pick up a clean quantum signal. There are two main ways to combat dark counts: heating and cooling.

In a previous experiment, the team brought some photon detectors to the TRIUMF particle accelerator in Vancouver and bombarded them with the kind of proton radiation that they would face in space for set intervals of time. They kept the detectors as cool as possible without resorting to large, expensive cryostats that would be a logistical and financial challenge to launch into orbit and found that the lower temperature helped reduce dark counts to a nearly acceptable level.

To repair the damage done by the radiation and reduce the dark counts even further, the team then tried an approach called annealing. Annealing is a process where a material is heated up just enough that the thermal energy helps work out any defects in it. They found that annealing got the dark counts down to the levels necessary to facilitate successful quantum key distribution.

“However, the fact is these detectors will be orbiting in space and continually accumulating proton radiation and the corresponding defects,” said Higgins. “So the key question is: when do you decide to perform the annealing?”

That is what the team set out to discover in their latest publication in EPJ Quantum Technology.

As the satellite bearing the detectors orbits the Earth, it will need to connect with ground stations it passes at certain times, and at other times it will be just travelling. What the team wanted to find out was whether the dark counts were kept under control best by annealing at set periods during the satellite’s orbit, or if the annealing should only take place once the dark counts reach a certain threshold. They also wanted to know if the repetitive exposure to radiation would make the dark counts unmanageable.

To find out, the researchers needed a much more complicated experiment that allowed them to cool the detectors and irradiate them, and then anneal them, and then cool and irradiate them again, repeatedly, better simulating actual operation in orbit.

“We found that we were able to keep the dark counts below the threshold we need to achieve to support the QKD protocol want to use,” said Higgins.

And they also found that whether they annealed on a set schedule or based on the incidence of dark counts didn’t make a significant difference, so for the actual QEYSSat launch, they will err on the side of caution and only anneal when the dark counts exceed a predefined threshold.

There is still much work to be done before Canada launches its first satellite for quantum communication in 2023, but these latest results are an important step towards a functioning quantum internet.

Repeated radiation damage and thermal annealing of avalanche photodiodes was published in EPJ Quantum Technology on May 17, 2021

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