A group of quantum engineers at the University of New South Wales have developed a method to reset a quantum computer with very high confidence. The method is related to the idea of 'Maxwell's demon', an omnipotent being that can separate a gas into hot and cold by watching the speed of the individual molecule.
To watch the temperature of an electron drawn at random from a warm pool of electrons, we used a modern 'demon'. 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" state.
If the final result is low in probability, quantum computers are useless. The final result will be wrong if the calculation starts from the wrong code. Our digital demon gives us a 20x improvement in how accurate we can be.
The research was published in a journal.
An electron is used to make it cold.
Prof. Morello's team has demonstrated record-high fidelity in performing quantum operations by using electron spins in Silicon. The final hurdle for efficient quantum computations was the fidelity of preparing the electron in a known state as the 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," says Dr. Mark Johnson. Even with the most powerful refrigerators, we still had a small chance of preparing the electron by mistake. We had to do a better job than that.
Dr. 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 if to keep that electron or use it for further computations. The chance of error was reduced from 20 percent to 1 percent.
An old idea is being reinvented.
"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'."
The concept of 'Maxwell's demon' was created by James Clerk Maxwell in 1867. 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. The demon can open a door if he knows the speed of each molecule.
Prof. Morello says that the demon was a thought experiment to discuss the possibility of violating the second law of thermodynamics.
We have created one with fast digital electronics. He was asked to watch just one electron and make sure it was as cold as possible. The '0' state of the quantum computer we want to build and operate is what 'Cold' means.
Implications of this result are important for the future of quantum computers. It is possible to build a machine that can tolerate some errors but only if they are rare. The threshold for error tolerance is low. This applies to all errors.
The preparation errors were reduced from 20 percent to 1 percent thanks to this electronic version of a 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 allow a reliable computation," Dr. Johnson says.
This is a good result for quantum computing. It's odd that it's the embodiment of an idea from 150 years ago.
Mark A. I. Johnson and his co-conspirators beat the thermal limit of Qubit initialization with a computer model. There is a book titled "PhysRevX.12.041008."
Journal information: Physical Review X
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