Micrograph of NIST ion trap with red dot indicating where a beryllium ion hovers above the chip. The horizontal and vertical lines separate gold electrodes, which are tuned to hold the ion and generate microwave pulses to manipulate it. The chip was used in experiments demonstrating record-low error rates in quantum information processing with a single quantum bit. Image: NIST |
Thanks to advances in experimental design, physicists at NIST
have achieved a record-low probability of error in quantum information
processing with a single quantum bit (qubit)—the first published error rate
small enough to meet theoretical requirements for building viable quantum
computers.
A quantum computer could potentially solve certain problems
that are intractable using today’s technology, even supercomputers. The NIST
experiment with a single beryllium ion qubit, described in a forthcoming paper,
is a milestone for simple quantum logic operations. However, a working quantum
computer also will require two-qubit logic operations with comparably low error
rates.
“One error per 10,000 logic operations is a commonly
agreed upon target for a low enough error rate to use error correction
protocols in a quantum computer,” explains Kenton Brown, who led the
project as a NIST postdoctoral researcher. “It is generally accepted that
if error rates are above that, you will introduce more errors in your
correction operations than you are able to correct. We’ve been able to show
that we have good enough control over our single-qubit operations that our
probability of error is 1 per 50,000 logic operations.”
The NIST experiment was performed on 1,000 unique sequences
of logic operations randomly selected by computer software. Sequences of 10
different lengths, ranging from one to 987 operations, were repeated 100 times
each. The measured results were compared to perfect theoretical outcomes. The
maximum length of the sequences was limited by the hardware used to control the
experiment.
The record low error rate was made possible by two major
changes in the group’s experimental setup. First, scientists manipulated the
ion using microwaves instead of the usual laser beams. A microwave antenna was
incorporated into the ion trap, with the ion held close by, hovering 40
micrometers above the trap surface. The use of microwaves reduced errors caused
by instability in laser beam pointing and power, as well as spontaneous ion
emissions. Second, the ion trap was placed inside a copper vacuum chamber and
cooled to 4.2 K with a helium bath to reduce errors caused by magnetic field
fluctuations in the laboratory.
