New Highly-Accurate Qubits Herald Advent of Quantum Computers

Australian researchers have developed two new types of 'quantum bits' that process data with more than 99 percent accuracy, overcoming a critical challenge that has held back the realisation of super powerful quantum computers.

Two research teams working in the same laboratories at The University of New South Wales Australia have created quantum bits, or 'qubits' - the building blocks for quantum computers - that each process quantum data with an accuracy above 99 percent.

"For quantum computing to become a reality we need to operate the bits with very low error rates," said Professor Andrew Dzurak, who is Director of the Australian National Fabrication Facility at UNSW, where the devices were made.

"We've now come up with two parallel pathways for building a quantum computer in silicon, each of which shows this super accuracy," added Associate Professor Andrea Morello from UNSW's School of Electrical Engineering and Telecommunications.

The UNSW teams were first in the world to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013.

Now the team led by Dzurak has discovered a way to create an "artificial atom" qubit with a device remarkably similar to the silicon transistors used in consumer electronics.

Meanwhile, Morello's team has been pushing the "natural" phosphorus atom qubit to the extremes of performance. Dr. Juha Muhonen, a post-doctoral researcher and lead author on the natural atom qubit paper, noted: "The phosphorus atom contains in fact two qubits: the electron, and the nucleus. With the nucleus in particular, we have achieved accuracy close to 99.99 percent. That means only one error for every 10,000 quantum operations."

The high-accuracy operations for both natural and artificial atom qubits is achieved by placing each inside a thin layer of specially purified silicon, containing only the silicon-28 isotope.

This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit. The purified silicon was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

The next step for the researchers is to build pairs of highly accurate quantum bits. Large quantum computers are expected to consist of many thousands or millions of qubits and may integrate both natural and artificial atoms.

Morello's research team also established a world-record "coherence time" for a single quantum bit held in solid state.

"Coherence time is a measure of how long you can preserve quantum information before it's lost," Morello said.

The longer the coherence time, the easier it becomes to perform long sequences of operations, and therefore more complex calculations. The team was able to store quantum information in a phosphorus nucleus for more than 30 seconds.

The two findings have been published in the journal Nature Nanotechnology.