Quantum memory

A fragile quantum memory state has been held stable at room temperature for a "world record" 39 minutes - overcoming a key barrier to ultrafast computers.

"Qubits" of information encoded in a silicon system persisted for almost 100 times longer than ever before.

Quantum systems are notoriously fickle to measure and manipulate, but if harnessed could transform computing.

The new benchmark was set by an international team led by Mike Thewalt of Simon Fraser University, Canada.

"This opens the possibility of truly long-term storage of quantum information at room temperature," said Prof Thewalt, whose achievement is detailed in the journal Science.

In conventional computers, "bits" of data are stored as a string of 1s and 0s.

But in a quantum system, "qubits" are stored in a so-called "superposition state" in which they can be both 1s and 0 at the same time - enabling them to perform multiple calculations simultaneously.

The trouble with qubits is their instability - typical devices "forget" their memories in less than a second.

There is no Guinness Book of quantum records. But unofficially, the previous best for a solid state system was 25 seconds at room temperature, or three minutes under cryogenic conditions.

In this new experiment, scientists encoded information into the nuclei of phosphorus atoms held in a sliver of purified silicon.

Magnetic field pulses were used to tilt the spin of the nuclei and create superposition states - the qubits of memory.

The team prepared the sample at -269C, close to absolute zero - the lowest temperature possible.

When they raised the system to room temperature (just above 25C) the superposition states survived for 39 minutes.

What's more, they found they could manipulate the qubits as the temperature of the system rose and fell back towards absolute zero.

At cryogenic temperatures, their quantum memory system remained coherent for three hours.

"Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer," said co-author Stephanie Simmons of Oxford University's department of materials.

"39 minutes may not seem very long. But these lifetimes are many times longer than previous experiments.

"We've managed to identify a system that seems to have basically no noise."

However she cautions there are still many hurdles to overcome before large-scale quantum computations can be performed.

For one thing, their memory device was built with a highly purified form of silicon - free from the magnetic isotopes which interfere with the spin of nuclei.

For another, the spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state.

Whereas to run calculations, physicists will need to place different qubits in different states - and control how they couple and interact.

"To have them controllably talking to one another - that would address the last big remaining challenge," said Dr Simmons.

Independent experts in the quantum field said the new record was an "exciting breakthrough" - which had long been predicted but never fulfilled.

"This result represents an important step towards realising quantum devices," said David Awschalom, professor in Spintronics and Quantum Information, at the University of Chicago.

"However a number of intriguing challenges still remain. For instance - will it be possible to precisely control the local electron-nuclear interaction to enable initialisation, storage, and readout of the nuclear spin states?"

The previous "world record" for a solid state quantum system at room temperature - 25 seconds - was held by Dr Thaddeus Ladd, formerly of Stanford University's Quantum Information Science unit, now working for HRL Laboratories.

"It's remarkable that these coherence states could be held for so long in a measurable system - as measurement normally introduces noise,"

"It's also a nice surprise that nothing goes wrong warming up and cooling the sample again - from an experimental point of view that's pretty remarkable.

"What is perhaps most important is that this is silicon. The global investment in this particular material means that it has a lot of potential for engineering."