A quantum world record has been smashed by researchers, overcoming a key barrier to the creation of ultrafast quantum computers.
An international team was able to maintain the nuclei of phosphorus atoms in silicon in a normally fragile ‘superposition’ state at room temperature for a world record 39 minutes.
In conventional computers data is stored in binary code as a string of 1s and 0s, but in the experiment quantum bits of information, 'qubits', were put into a superposition state in which they can be both 1s and 0s at the same time – enabling them to perform multiple calculations simultaneously.
In the experiment the team raised the temperature of the system from -269°C to 25°C and demonstrated that the superposition states survived for 39 minutes – outside of silicon the previous record for such a state's survival at room temperature was around two seconds.
“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 1 per cent,” said Stephanie Simmons of Oxford University's Department of Materials, an author of the paper.
“Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer.”
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’ – necessary as the optical technique used to read the qubits only works at very low temperatures.
“This opens up the possibility of truly long-term coherent information storage at room temperature,” said Mike Thewalt of Simon Fraser University, Canada, who carried out the test detailed in this week's edition of journal Science along with his colleagues.
The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus and 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 team prepared their sample at just 4°C above absolute zero (-269°C) and placed it in a magnetic field. Additional magnetic field pulses were used to tilt the direction of the nuclear spin and create the superposition states.
When the sample was held at this cryogenic temperature, the nuclear spins of about 37 per cent of the ions – a typical benchmark to measure quantum coherence – remained in their superposition state for three hours. The same fraction survived for 39 minutes when the temperature of the system was raised to 25°C.
“These lifetimes are at least ten times longer than those measured in previous experiments,” said Simmons. “We've managed to identify a system that seems to have basically no noise. They're high-performance qubits.”
But the team says there is still some work ahead before they can carry out large-scale quantum computations. The nuclear spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state, but to run calculations 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,” said Simmons.