A team of quantum engineers at UNSW Sydney has developed a method to reset a quantum computer – that is, to prepare a quantum bit in the ‘0’ state
The new quantum computing feat is a modern twist on a 150-year-old thought experiment related to the old concept of ‘Maxwell’s demon’, an omniscient being that can separate a gas into hot and cold by watching the speed of the individual molecules.
Professor Andrea Morello of UNSW, who led the team said, “Here we used a much more modern ‘demon’ – a fast digital voltmeter – to watch the temperature of an electron drawn at random from a warm pool of electrons.”
“In doing so, we made it much colder than the pool it came from, and this corresponds to a high certainty of it being in the ‘0’ computational state,” says Professor Andrea Morello of UNSW, who led the team.”
“Quantum computers are only useful if they can reach the final result with very low probability of errors. And one can have near-perfect quantum operations, but if the calculation started from the wrong code, the final result will be wrong too.”
Our digital ‘Maxwell’s demon’ gives us a 20x improvement in how accurately we can set the start of the computation.” Proessor Morello said.
The research was published in Physical Review X, a journal published by the American Physical Society.
Watching electrons to make them colder
Prof. Morello’s group has pioneered the use of electron spins in silicon to encode and manipulate quantum information, and has demonstrated record-high fidelity—that is, very low probability of errors—in performing quantum operations.
Prior to this breakthrough, the main barrier to efficient quantum computation with electrons was lowering the error rate involved in preparing the electron in a known state as the starting point for the computation.
Lead experimental author on the paper Dr Mark Johnson explained the normal way to prepare the quantum state of an electron is go to extremely low temperatures, close to absolute zero, and hope that the electrons all relax to the low-energy ‘0’ state.
“Unfortunately, even using the most powerful refrigerators, we still had a 20 per cent chance of preparing the electron in the ‘1’ state by mistake. That was not acceptable, we had to do better than that.” Dr Johnson said.
Dr Johnson, a UNSW graduate in Electrical Engineering, decided to use a very fast digital measurement instrument to ‘watch’ the state of the electron, and use real-time decision-making processor within the instrument to decide whether to keep that electron and use it for further computations. The effect of this process was to reduce the probability of error from 20 per cent to 1 per cent.
A new spin, an old idea………..
Prof. Morello said “When we started writing up our results and thought about how best to explain them, we realized that what we had done was a modern twist on the old idea of the ‘Maxwell’s demon.”
“The demon was a thought experiment, to debate the possibility of violating the second law of thermodynamics, but of course no such demon ever existed,” Prof. Morello says.
In his 1867 paper, James Clerk Maxwell imagined a creature that could keep track of the speed of each molecule in a gas. He envisioned a box divided in half with a door that could be opened and closed quickly, filled with gas.
With his knowledge of each molecule’s speed, the demon can open the door to let the slow (cold) molecules pile up on one side, and the fast (hot) ones on the other.
“The demon was a thought experiment, to debate the possibility of violating the second law of thermodynamics, but of course no such demon ever existed,”
“Now, using fast digital electronics, we have in some sense created one. We tasked him with the job of watching just one electron, and making sure it’s as cold as it can be.”
“Here, ‘cold’ translates directly in it being in the ‘0’ state of the quantum computer we want to build and operate.” Prof. Morello says.
Quantum computers are desirable because they can tolerate some errors. However, these errors must be sufficiently uncommon. In such a computer, 1 percent is the typical error threshold which applies to all errors, including preparation, operation, and readout of the final result.
This electronic version of a ‘Maxwell’s demon’ allowed the UNSW team to reduce the preparation errors twenty-fold, from 20 per cent to 1 per cent
About Fault-tolerant quantum computing
Fault-tolerant quantum computing requires initializing the quantum register in a well-defined fiducial state. In solid-state systems, this is typically achieved through thermalization to a cold reservoir, such that the initialization fidelity is fundamentally limited by temperature.
According to Dr Johnson just by using a modern electronic instrument, with no additional complexity in the quantum hardware layer, they had been been able to prepare their electron quantum bits within good enough accuracy to permit a reliable subsequent computation,
“This is an important result for the future of quantum computing. And it’s quite peculiar that it also represents the embodiment of an idea from 150 years ago!” he said.
