A team of quantum engineers at UNSW Sydney has developed a method to reset a quantum computer – ie bring a quantum bit to the “0” state – with very high reliability, as is 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 drawn at random from a warm electron pool. By doing this, we made it much colder than the pool it came from, and that’s a high level of certainty that it’s in computational state ‘0’,” says Professor Andrea Morello of UNSW, who led the team.
“Quantum computers only make sense if they come to the end result with a very low probability of error. And you can have near-perfect quantum operations, but if the calculation started with the wrong code, the end result will be wrong too. Our digital “Maxwell Demon” gives us a 20x improvement in the accuracy with which we can determine when the calculation begins.”
The research was published in Physical Review X, a journal of 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 is demonstrating record-breaking accuracy – ie a very low probability of error – in 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.
“Normally, one prepares the quantum state of an electron at extremely low temperatures, close to absolute zero, and hopes that all the electrons will relax to the low-energy “0” state,” explains Dr. Mark Johnson, the lead experimenter author on the paper. “Unfortunately, even with the most powerful refrigerators, we still had a 20 percent chance of accidentally preparing 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 and use that electron it should be used for further calculations. The effect of this process was to reduce the error probability from 20 percent to 1 percent.
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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, with fast digital electronics, we’ve created one in a sense. We tasked it with just observing one electron and making sure it’s as cold as possible. Here ‘cold’ directly means 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 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 allow 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!’
