Quantum computer initialization speed increased by 20 times, all because of this demon
Research at the University of New South Wales (UNSW) has shown that resetting a quantum bit to its |0⟩ state (i.e., the initialization of a quantum bit) is 20 times faster using a modern version of the "Maxwell demon". The research results were published in Physical Review X under the title "Beating the thermal limit of quantum bit initialization with Bayesian Maxwell's demon" [1].
01New method: 20 times faster initialization of quantum computers
Professor Andrea Morello explains how Maxwell Demon's thought experiment was an achievement with his team performing quantum computing by selecting only cold electrons.
A team of quantum engineers led by Professor Andrea Morello at the University of New South Wales has developed a method for resetting a quantum computer: that is, resetting a quantum bit to a |0⟩ state - with the very high confidence required for reliable quantum computing.
The method is surprisingly simple: it is related to the old concept of "Maxwell's demon". "Maxwell's demon is an omniscient being that can classify gases as hot or cold by looking at the speed of individual molecules.
"Here, we used a more modern 'demon' - a fast digital voltmeter - to observe the temperature of a randomly drawn electron from a warm pool of electrons. In doing so, we make it much colder than the pool it came from, which corresponds to a high degree of certainty that it is in a |0⟩ computational state." Professor Andrea Morello said [2].
"Quantum computers are only useful if they can reach the final result with a very low probability of error. And one can have near-perfect quantum operations, but if the computation starts with the wrong code, the final result will also be wrong. Our digital 'Maxwell demon' brings a 20-fold improvement, and we can set the start of the calculation exactly."
02Looking at electrons to make them colder
Professor Morello's team has pioneered the use of electron spins in silicon to encode and manipulate quantum information and has demonstrated record high fidelity in performing quantum operations. The final hurdle to efficient quantum computing with electrons is the fidelity of preparing electrons in known states as the starting point for computation.
"The normal way to prepare the quantum state of an electron is to go to extremely low temperatures, close to absolute zero, and expect the electrons to all relax to the low-energy |0⟩ state," explained Dr. Mark Johnson, lead experimental author of the paper, "Unfortunately Even with the most powerful dilution chiller, we still had a 20% probability of incorrectly preparing the electrons in the |1⟩ state. This is unacceptable, and we must do better."
Dr. Johnson, an electrical engineering graduate at UNSW, decided to use a very fast digital measurement instrument to "observe" the state of the electron and use the real-time decision processor inside the instrument to decide whether to keep the electron and use it for further calculations.
The effect of this process was to reduce the probability of error from 20% to 1%.
03"A modern version of "Maxwell's Demon
Professor Morello says, "When we started writing our results and thinking about how best to interpret them, we realized that what we were doing was a modern twist on the old idea of the 'Maxwell demon.'"
The concept of Maxwell's demon dates back to 1867, when James Maxwell imagined a creature with the ability to know the speed of every 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 his knowledge of the speed of each molecule, the demon could open the door and let the slow (cold) molecules pile up on one side and the fast (hot) ones on the other.
Professor Morello says, "This demon is a thought experiment to debate the possibility of violating the second law of thermodynamics, but of course no such demon has ever existed."
"Now, using fast digital electronics, we have in a sense created a Maxwell demon. We gave him the task of monitoring an electron and making sure it was as cold as possible. Here, 'cold' translates directly into it being in the |0⟩ state of the quantum computer we want to build and operate."
The implications of this result are important for the viability of quantum computers. Such a machine could be built with the ability to tolerate some errors, but only if they are rare enough. The typical threshold for a specific tolerance of errors is about 1%, which applies to all errors, including preparation, manipulation and readout of the final result.
This electronic version of Maxwell's Demon enabled the UNSW team to reduce errors in preparation by a factor of 20: from 20% to 1%.
Dr. Johnson said, "Simply by using modern electronic instrumentation, there is no additional complexity in the quantum hardware layer; we have been able to prepare our electronic quantum bits within a good enough precision to allow reliable subsequent calculations."
"This is an important result for the future of quantum computing. And rather peculiarly, it also represents the embodiment of an idea from 150 years ago!"
Reference links:
[1]https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.041008
[2]https://newsroom.unsw.edu.au/news/science-tech/new-quantum-computing-feat-modern-twist-150-year-old-thought-experiment
