A Digital Maxwell’s Demon For Qubit Initialization

By Geraint Lewis •  Updated: 12/01/22 •  5 min read

A team of quantum engineers at the University of New South Wales has developed a method to reset a quantum computer, initializing a quantum bit in the “0” state with very high confidence, a requirement for reliable quantum computations.

The method is surprisingly simple: it is based on the old concept of ‘Maxwell’s demon,’ an omniscient being capable of separating a gas into hot and cold states by measuring the speed of individual molecules.

“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,”

said team leader Professor Andrea Morello.

Electron Spins In Silicon

Quantum computers are only useful if the final result is reached with a very low probability of error. We can have near-perfect quantum operations, but if the calculation begins with the incorrect code, the final result will be incorrect as well. This digital ‘Maxwell’s demon’ improves theability to set the start of the computation by a factor of 20.

The team of Prof. Morello has pioneered the use of electron spins in silicon to encode and manipulate quantum information, and has demonstrated record-high fidelity — that is, a very low probability of errors when performing quantum operations. The last remaining obstacle for efficient quantum computations with electrons was the accuracy of preparing the electron in a known state to serve as the calculation’s starting point.

“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 percent chance of preparing the electron in the ‘1’ state by mistake. That was not acceptable, we had to do better than that,”

explained lead experimental author Dr. Mark Johnson.

Maxwell’s Demon

Maxwell's Demon Qubit Initialization Device

False-colored micrograph of a device from the same batch as the one used in this paper.
The SET for electron spin readout and initialization is shown on the right in green. The microwave antenna, used for ESR and NMR, is shown on the left in pink. Pulsing gates DG3, DG4, and SPL (identified by white dots) form a virtual gate (VG).
Credit: Mark A. I. Johnson et al CC-BY

Dr. Johnson, a graduate of UNSW in Electrical Engineering, decided to use a very fast digital measurement instrument to ‘observe’ the state of the electron and a real-time decision-making processor within the instrument to determine whether to retain the electron and use it for further computations. The result of this procedure was a reduction in error probability from 20% to 1%.

“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’,”

Prof. Morello said.

The idea of a ‘Maxwell’s demon’ dates back to 1867, when James Clerk Maxwell imagined a creature that could calculate the velocity of each individual molecule in a gas. He’d take a gas-filled box with a dividing wall in the middle and a quick-opening and closing door.

With his understanding of each molecule’s speed, the demon can open the door, allowing the slow (cold) molecules to pile up on one side and the fast (hot) molecules to pile up on the other. The demon was a thought experiment to debate the possibility of breaking the second law of thermodynamics, but 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 said.

Qubit Initialization

The implications of this finding are crucial to the future of quantum computing. It is possible to create a machine that can tolerate some errors, but only if they occur infrequently enough.

Around 1% is typically the threshold for error tolerance. This holds true for all mistakes, including those made during setup, use, and readout of the outcome. The team was able to cut the preparation errors in half, from 20% to 1%, using this electronic “Maxwell’s demon.”

“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,”

Dr. Johnson said.

Reference: Mark A. I. Johnson, Mateusz T. Mądzik, Fay E. Hudson, Kohei M. Itoh, Alexander M. Jakob, David N. Jamieson, Andrew Dzurak, and Andrea Morello. Beating the Thermal Limit of Qubit Initialization with a Bayesian Maxwell’s Demon. Physical Review X, 12, 041008 DOI: 10.1103/PhysRevX.12.041008

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