A team of quantum engineers at UNSW Sydney has developed a method for resetting a quantum computer – ie preparing a quantum bit in the “0” state – with very high reliability, as required for reliable quantum calculations. The method is surprisingly simple: it’s related to the ancient concept of the “Maxwell demon,” an omniscient being that can separate a gas into hot and cold by observing the speed of each molecule.
“Here we used a much more sophisticated ‘demon’ – a fast digital voltmeter – to monitor the temperature of an electron taken at random from a warm electron pool. By doing this, we made it much colder than the pool it came from, and this corresponds to a high degree of certainty that it is in computational state ‘0’,” 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 error. And you can have near-perfect quantum operations, but if the calculation starts with the wrong code, the end result will be wrong too. Our digital ‘Maxwell’s Demon’ gives us a 20x improvement in how accurately we can pinpoint when the calculation begins.”
The study was published in Physical Check Xa journal published by the American Physical Society.
Watching an electron to make it colder
Prof. Morello’s team has pioneered the use of electron spins in silicon to encode and manipulate quantum information, and demonstrate record-breaking fidelity – ie a very low probability of error – when performing quantum operations. The last remaining hurdle to efficient quantum computations with electrons was the accuracy of preparing the electron in a known state as the starting point of the computation.
“The normal way to prepare an electron’s quantum state is to reach extremely low temperatures near absolute zero and hope that all the electrons relax to the low-energy ‘0’ state,” explains Dr. Mark Johnson, the lead experimental author on the paper. “Unfortunately, even with the most powerful refrigerators, we still had a 20 percent chance of accidentally putting the electron in the ‘1’ state. That was unacceptable, we had to do better.”
dr Johnson, an Electrical Engineering graduate from UNSW, 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 further calculations. The effect of this process was to reduce the error probability from 20 percent to 1 percent.
A new twist on an old idea
“As we started writing down our findings and thinking about how best to explain them, we realized that what we had done was a modern twist on the old idea of the ‘Maxwell demon,'” says Prof. Morello.
The concept of the “Maxwell Demon” dates back to 1867 when James Clerk Maxwell envisioned a creature that had the ability to know the speed of every single molecule in a gas. He would take a box full of gas, with a divider in the middle and a door that opens and closes quickly. With his knowledge of the speed of each molecule, the demon can open the door for the slow (cold) molecules to pile up on one side and the fast (hot) molecules pile up on the other.
“The demon was a thought experiment to discuss the possibility of a violation of the second law of thermodynamics, but of course no such demon ever existed,” says Prof. Morello.
“Now, in a sense, we’ve created one with fast digital electronics. We tasked him with just observing one electron and making sure it’s as cold as possible. Here “cold” directly means it’s in the ‘0’ state of the quantum computer 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 failures, but only if they are sufficiently rare. The typical threshold for fault tolerance is around 1 percent. This applies to all errors, including preparation, operation and reading the final result.
This electronic version of a “Maxwell Demon” enabled the UNSW team to reduce preparation errors twenty-fold, from 20 percent to 1 percent.
“Just by using a modern electronic instrument, without additional complexity in the quantum hardware layer, we were able to prepare our electron quantum bits with sufficient accuracy to enable reliable subsequent computation,” says Dr. Johnson.
“This is an important result for the future of quantum computing. And it’s quite odd that it’s also the embodiment of an idea from 150 years ago!’
Mark AI Johnson et al, Beating the Thermal Limit of Qubit Initialization with a Bayesian Maxwell’s Demon, Physical Check X (2022). DOI: 10.1103/PhysRevX.12.041008
Provided by the University of New South Wales
Citation: New quantum computing feat is a modern twist on a 150-year-old thought experiment (2022 November 30) retrieved November 30, 2022 from https://phys.org/news/2022-11-quantum-feat-modern-year-alter -thought.html
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