New quantum computing feat is a modern twist on a 150-year-old thought experiment

A team of quantum engineers at UNSW Sydney have developed a method to reset a quantum computer, that is, to prepare a quantum bit in the ‘0’ state, with very high confidence, as needed for reliable quantum calculations. The method is surprisingly simple: it is related to the ancient concept of ‘Maxwell’s demon’, an omniscient being that 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 look at the temperature of an electron drawn at random from a warm pool of electrons. In doing so, we made it much cooler than the pool it came from, and this corresponds to a high certainty that it is in computational state ‘0’,” says UNSW Professor Andrea Morello, who led the team.

“Quantum computers are only useful if they can reach the end result with very little chance of errors. And one can have near-perfect quantum operations, but if the calculation started with the wrong code, the end result will also be wrong. The ‘demon of Maxwell’ gives us a 20-fold improvement in the precision with which we can set the start of the computation.”

The research was published in Physical exam Xa journal published by the American Physical Society.

Observe an electron to make it cooler

Professor Morello’s team have pioneered the use of electron spins in silicon to encode and manipulate quantum information, and have demonstrated record fidelity, i.e. very low probability of errors, when performing quantum operations. The last remaining obstacle to 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 wait for all the electrons to relax to the low energy ‘0’ state,” explains Dr. Mark Johnson, Lead experimental author on the article. “Unfortunately, even using the most powerful refrigerators, we still had a 20 percent chance of preparing the electron in state ‘1’ by mistake. That was not acceptable, we had to do better than that.”

Dr Johnson, a UNSW Electrical Engineering graduate, decided to use a very fast digital measuring instrument to ‘observe’ the state of the electron, and use a real-time decision processor within the instrument to decide whether to keep that electron and use it for other calculations. The effect of this process was to reduce the probability of error from 20 to 1 percent.

A new twist on an old idea

“When we started to write down our results and thought about how best to explain them, we realized that what we had done was a modern twist on the old ‘Maxwell’s demon’ idea,” says Professor Morello.

The concept of ‘Maxwell’s demon’ dates back to 1867, when James Clerk Maxwell envisioned a creature with the ability to know the speed of each individual molecule in a gas. He would take a box filled with gas, with a dividing wall in the middle and a door that can be opened and closed quickly. With his knowledge of the speed of each molecule, the demon can open the door to let the slow (cold) molecules accumulate 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 there never was such a demon,” says Professor Morello.

“Now, using fast digital electronics, we’ve in a sense created one. We’ve given it the job of observing just one electron and making sure it’s as cold as possible. Here, ‘cold’ translates directly to being in the ‘0’ state. ‘ of the quantum computer we want to build and operate.”

The implications of this result are very important for the feasibility 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 threshold for error tolerance is around 1 percent. This applies to all errors, including preparation, operation and reading the final result.

This electronic version of a ‘Maxwell daemon’ enabled the UNSW team to reduce setup errors twenty-fold, from 20 per cent to 1 per cent.

“By simply using a modern electronic instrument, with no additional complexity in the quantum hardware layer, we have been able to prepare our electron quantum bits with good enough precision to allow reliable subsequent computation,” says Dr. Johnson.

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