Long-lived Qubits Survive for Record Times

Maggie McKee

It may not sound long but, to a quantum physicist, 39 minutes is a veritable eternity. That’s the length of time for which fragile quantum states have now been shown to survive in a solid material at room temperatures in the lab. This record-breaking feat -- carried out by an international collaboration, including Stephanie Simmons at Oxford University -- overcomes a key barrier to building practical quantum computers. Their study is published today in Science.

A quantum bit stored in the heart of phosphorus atom can survive even at room temperature, when the surrounding lattice of silicon atoms form a furiously vibrating 'noisy' environment. Image © 2013 Karl G. Nyman with CC BY license.

A quantum bit stored in the heart of phosphorus atom can survive even at room temperature, when the surrounding lattice of silicon atoms form a furiously vibrating 'noisy' environment. Image © 2013 Karl G. Nyman with CC BY license.

"This opens up the possibility of truly long-term coherent information storage at room temperature," says quantum physicist Mike Thewalt, who performed the test at Simon Fraser University, in Burnaby, British Columbia, Canada, with colleagues.

Quantum computers promise to significantly outperform today’s machines at certain tasks, by exploiting the strange properties of subatomic particles. Conventional computers process data stored as strings of 1s or 0s. But quantum objects are not constrained to the either/or nature of binary bits. Instead, each quantum bit, or “qubit”, can be put into a "superposition" of both 1 and 0 at the same time, enabling them to perform multiple calculations simultaneously. This ability to multitask could allow quantum computers to crack seemingly secure encryption codes, for instance.  "A rather ambitious supercomputer would be able to obtain a result in a million years, whereas a moderate quantum computer would be able to do it in an hour," says Simmons.

The problem with attempts to build these extraordinary number-crunchers, however, is that superposition states are delicate structures that can collapse like a soufflé if nudged by a stray particle, such as an air molecule. To minimize this unwanted process, physicists often cool their qubit systems to almost absolute zero (-273 °C) and manipulate them in a vacuum. But such setups are finicky to maintain and, ultimately, it would be advantageous  for quantum computers to operate robustly at everyday temperatures and pressures.

Now Thewalt, Simmons and colleagues have made a significant step towards this goal of bringing quantum computers into the mainstream. They began with a sliver of  silicon -- around the size of a pencil eraser -- doped with small amounts of other elements, including phosphorus. The quantum information is 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. 

A crucial step to help reduce decoherence was to ionize the phosphorus, removing one electron from each atom with a laser. The reason for this is that phosphorus atoms have five outer electrons, but only four of them bind to the silicon atoms that surround them in a crystal lattice. That leaves the extra electron free to orbit the nucleus and electrically tug on the nuclear spin, disrupting its superposition. Once this electron has been excised, the system is far less likely to decohere.

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.  

The team appears to have set records for coherence within a solid material at any temperature, says Thaddeus Ladd (http://www.thaddeusladd.com/), a quantum physicist at HRL Laboratories, LLC, a research firm in Malibu,   California, who was not involved with the study. "We've managed to engineer a system that seems to have basically no noise," Simmons adds. "They're high-performance qubits."

Though 39 minutes may seem short, it takes only one-hundred-thousandth of a second to flip the nuclear spin of a phosphorus ion – the type of operation that would be used to run quantum calculations. So physicists could perform more than 200 million operations before a qubit's superposition caved in. 

"What is perhaps most important is that this is silicon," notes Ladd. Since silicon already forms the backbone of today’s electronics industry, it is an ideal material to use with an eye to fabricating chips with both conventional and quantum processors, enabling computers in the future to use whichever would be most efficient for a given task, suggests Simmons.

But there is still some work ahead before the team can carry out actual 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. 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," says Simmons.

This story has been picked up by various media outlets, including the BBC.

Posted on November 15, 2013 .