Researchers use Maxwell's demon to prepare a quantum bit

Quantum engineers from 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 computations. The method is related to the old concept of ‘Maxwell’s demon’, an omniscient being that can separate gas into hot and cold by watching the speed of the individual molecules. Professor Andrea Morello from UNSW, who led the team of researchers, said they used a 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, the researchers made it much colder than the pool it came from — this corresponds to a high certainty of it being in the ‘0’ computational state.

“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,” Morello said.

The research was published in Physical Review X, a journal published by the American Physical Society.

Morello’s team studied the use of electron spins in silicon to encode and manipulate quantum information, and demonstrated high fidelity — that is, very low probability of errors — in performing quantum operations. Another hurdle for efficient quantum computations with electrons was the fidelity of preparing the electron in a known state as the starting point of the calculation. Dr Mark Johnson, the lead experimental author on the paper, said that 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 will relax to the low-energy ‘0’ state. “Unfortunately, even using the most powerful refrigerators, we still had a 20% chance of preparing the electron in the ‘1’ state by mistake. That was not acceptable, we had to do better than that,” Johnson said.

Johnson decided to use a very fast digital measurement instrument to ‘watch’ the state of the electron, and use a real-time decision-making processor within the instrument to decide whether to keep that electron and use it for further computations.

“When we started writing up our results and thought about how best to explain them, we realised that what we had done was a modern twist on the old idea of the ‘Maxwell’s demon’,” Morello said.

The concept of ‘Maxwell’s demon’ dates back to 1867, when James Clerk Maxwell imagined a creature with the capacity to know the velocity of each individual molecule in a gas. He would take a box full of gas, with a dividing wall in the middle, and a door that could be opened and closed quickly. With its 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. According to Morello, the demon was a thought experiment, to debate the possibility of violating the second law of thermodynamics — of course, no such demon 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,” Morello said.

The implications of this result are 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 sufficiently rare. The typical threshold for error tolerance is around 1%. This applies to all errors, including preparation, operation and readout of the final result. This electronic version of a ‘Maxwell’s demon’ allowed the UNSW researchers to reduce the preparation errors from 20% to 1%.

“Just by using a modern electronic instrument, with no additional complexity in the quantum hardware layer, we’ve been able to prepare our 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!” Johnson said.

Image caption: Professor Andrea Morello explains how the ‘Maxwell’s demon’ thought experiment was analogous to his team's achievement by selecting only cool electrons for quantum computations. Image credit: Richard Freeman/UNSW.