The new feat of quantum computing is a modern take on a 150-year-old thought experiment

A team of quantum engineers from UNSW Sydney have developed a method to reset a quantum computer, i.e. prepare a quantum bit to the “0” state, with very high confidence, necessary to reliable quantum calculations. The method is surprisingly simple: it is related to the old concept of “Maxwell’s demon”, an omniscient being who can separate a gas into hot and cold by observing the speed of individual molecules.

“Here we used a much more modern ‘demon’ – a fast digital voltmeter – to monitor the temperature of an electron drawn at random from a hot pool of electrons. In doing so, we made it much cooler than the basin it came from, and that corresponds to a high certainty that it is in the ‘0’ computational state,” says Professor Andrea Morello of UNSW, who led the team.

“Quantum computers are only useful if they can achieve the end result with a very low probability of errors. And you can have almost perfect quantum operations, but if the calculation started from the wrong code, the end result will also be wrong. “Maxwell’s Demon” allows us to multiply by 20 the precision with which we can define the beginning of the calculation.”

The research has been published in Physical examination Xa journal published by the American Physical Society.

Look at an electron to make it colder

Professor Morello’s team was the first to use electron spins in silicon to encode and manipulate quantum information, and demonstrated record fidelity, i.e. a very low probability of errors, in performing quantum operations. The last remaining hurdle for efficient quantum calculations with electrons was the fidelity of preparing the electron in a known state as the starting point of the calculation.

“The normal way to prepare the quantum state of an electron is to go to extremely low temperatures, close to absolute zero, and hope that the electrons all relax to the low-energy “0” state” says Dr. Mark Johnson, the lead experimental author on the paper. “Unfortunately, even using the most powerful refrigerators, we still had a 20% chance of preparing the electron to state ‘1’ by mistake. This was not acceptable, we had to do better than that.”

Dr Johnson, an electrical engineering graduate from UNSW, decided to use a very fast digital measuring instrument to “monitor” the state of the electron and to use a real-time decision-making processor in the instrument to decide to keep this electron and use it for other calculations. The effect of this process was to reduce the probability of error from 20% to 1%.

A new twist on an old idea

“As we started writing up our results and thinking about how best to explain them, we realized what we had done was a modern take on the old ‘Maxwell’s Demon’ idea,” Prof Morello says.

The concept of “Maxwell’s demon” dates back to 1867, when James Clerk Maxwell imagined a creature capable of knowing the speed of each individual molecule in a gas. He would take a box full of gas, with a partition in the middle, and a door that opens and closes quickly. Knowing the speed of each molecule, 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 such a demon never existed,” says Professor Morello.

“Now, using fast digital electronics, we’ve kind of created one. We’ve tasked it with monitoring a single electron and making sure it’s as cold as possible. Here, ‘cold’ translates directly by the fact that it is in the ‘0’ state of the quantum computer that we want to build and operate.”

The implications of this result are very important for the viability of quantum computers. Such a machine can be built with the ability to tolerate some errors, but only if they are rare enough. The typical error tolerance threshold is around 1%. This applies to all errors, including preparation, use and reading of the end result.

This electronic version of a “Maxwell daemon” enabled the UNSW team to reduce picking errors twenty-fold, from 20% to 1%.

“Just by using a modern electronic instrument, with no additional complexity in the quantum material layer, we were able to prepare our electron quantum bits with sufficient precision to enable reliable subsequent computation,” says Dr Johnson.

“This is an important result for the future of quantum computing. And it’s quite special that it also represents the embodiment of an idea from 150 years ago!”