Lasers lengthen quantum-bit memory life by several hundred times

Physicists have found a way to drastically prolong the shelf life of quantum bits, the 0s and 1s of quantum computers. The quantum bits, formed in this case by arrays of semiconductor quantum dots containing a single extra electron, are easily perturbed by magnetic-field fluctuations from the nuclei of the atoms creating the quantum dot. This perturbation causes the bits to essentially forget the piece of information they were tasked with storing.

The scientists, including the University of Michigan's Duncan Steel, used lasers to elicit a previously undiscovered natural feedback reaction that stabilizes the quantum dot's magnetic field, lengthening the stable existence of the quantum bit by several orders of magnitude, or more than 1,000 times.¹

Because of their ability to represent multiple states simultaneously, quantum computers could theoretically factor numbers dramatically faster and with smaller computers than conventional computers. For this reason, they could vastly improve computer security.

"In our approach, the quantum bit for information storage is an electron spin confined to a single dot in a semiconductor like indium arsenide. Rather than representing a 0 or a 1 as a transistor does in a classical computer, a quantum bit can be a linear combination of 0 and 1. It's sort of like hitting two piano keys at the same time," said Duncan Steel, a professor at the University of Michigan (Ann Arbor, MI) and one of the researchers. "One of the serious problems in quantum computing is that anything that disturbs the phase of one of these spins relative to the other causes a loss of coherence and destroys the information that was stored. It is as though one of the two notes on the piano is silenced, leaving only the other note."

A major cause of information loss in III/V semiconductor quantum dots is the interaction of the electron (the quantum bit) with the nuclei of the atoms in the quantum dot holding the electron. Trapping the electron in a particular spin, as is necessary in quantum computers, gives rise to a small magnetic field that couples with the magnetic field in the nuclei and breaks down the memory in a few billionths of a second.

Blocking the magnetic-field interaction

By exciting the quantum dot with two narrow-linewidth continuous-wave lasers (a pump and a probe), the scientists were able to block the interaction of these magnetic fields. The laser light caused an electron in the quantum dot to jump to a higher energy level, leaving behind a charged hole in the electron cloud. The hole also had a magnetic field due to the collective spin of the remaining electron cloud. It turns out that the hole acts directly with the nuclei and controls its magnetic field without any intervention from outside except the fixed excitation by the lasers to create the hole.

"This discovery was quite unexpected," Steel said. "Naturally occurring, nonlinear feedback in physical systems is rarely observed. We found a remarkable piece of physics in nature. We still have other major technical obstacles, but our work shows that one of the major hurdles to quantum computers that we thought might be a show-stopper isn't one," Steel said.

In addition to the University of Michigan, the other research groups are with the Naval Research Laboratory, the University of California San Diego, and the University of Hong Kong. The research is funded by the U.S. Army Research Office, the Air Force Office of Scientific Research, the Office of Naval Research, The National Security Agency's Laboratory for Physical Sciences, the Intelligence Advanced Research Projects Agency and the National Science Foundation.

REFERENCE:

1. Xiaodong Xu et al., Nature 459, 1105-1109 (25 June 2009).