Qubit Survives for 39 Minutes

A quantum state, normally a very fragile condition, has been demonstrated to survive at room temperature for an unprecedented 39 minutes. This achievement overcomes a key barrier to building ultrafast quantum computers.

An international team of physicists led by Simon Fraser University professor Mike Thewalt accomplished the feat, which opens the possibility of long-term coherent information storage at room temperature. Other team members included Stephanie Simmons of Oxford University and University College London's John Morton (London Centre for Nanotechnology).

Replacing electronic computer circuits with light-based ones in future computers is seen as a way to surmount limitations to current computer processing technology, because quantum bit, or qubit, can exist as both a “1” and a “0” at the same time, something known as a superposition state. This enables the computers to perform multiple calculations simultaneously, but normally is possible only at very low temperatures. And even then, superposition states are delicate structures that can collapse like a soufflé if nudged by a stray particle, such as an air molecule.

A normally fragile quantum state has been shown to survive at room temperature for a world-record 39 minutes by Oxford University researchers. An artistic rendition of a 'bound exciton' quantum state used to prepare and read out information stored in the form of quantum bits. Courtesy of ©2013 Stef Simmons with CC BY

The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus. Quantum information was encoded in the nuclei of the phosphorus atoms: Each nucleus has an intrinsic quantum property called spin, which acts like a tiny bar magnet when placed in a magnetic field. Spins can be manipulated to point up (0), down (1) or any angle in between, representing a superposition of the two other states.

The investigators raised the temperature of a system from −269 to 25 °C and demonstrated that the superposition states survived at this balmy temperature for 39 minutes; outside of silicon, the previous record for such a state’s survival at room temperature was around two seconds. The team even found that they could manipulate the qubits as the temperature of the system rose, and that they were robust enough for this information to survive being "refrozen" (the optical technique used to read the qubits works only at very low temperatures).

"Thirty-nine minutes may not seem very long, but as it only takes one-hundred-thousandth of a second to flip the nuclear spin of a phosphorus ion — the type of operation used to run quantum calculations — in theory over 20 million operations could be applied in the time it takes for the superposition to naturally decay by one percent. Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer," Simmons said.

“A powerful universal quantum computer would change technology in ways that we already understand, and doubtless in ways we do not yet envisage,” Thewalt said. “It would have a huge impact on security, code-breaking, and the transmission and storage of secure information. It would be able to solve problems which are impossible to solve on any conceivable normal computer. It would be able to model the behavior of quantum systems, a task beyond the reach of normal computers, leading, for example, to the development of new drugs by a deeper understanding of molecular interactions.”

“These lifetimes are at least ten times longer than those measured in previous experiments, ” Simmons said. “We've managed to identify a system that seems to have basically no noise. They're high-performance qubits.”

“Our research extends the demonstrated coherence time in a solid at room temperature by a factor of 100 — and at liquid helium temperature by a factor of 60 (from three minutes to three hours),” Thewalt said. “These are large, significant improvements in what is possible.”

To run calculations, however, physicists will need to place different qubits in different states. "To have them controllably talking to one another – that would address the last big remaining challenge," Simmons said.

The work appears in Science. doi: 10.1126/science.1239584. See also: Studies Boost Quantum Memory Storage .

For more information, visit: www.sfu.ca