Studies Boost Quantum Memory Storage

Two independent studies have stretched the manipulation times of quantum bits (qubits) of information, and these longer lifetimes suggest that spin-based qubit systems are a feasible foundation for realizing a practical quantum computer.

Despite the many advances announced on a regular basis, quantum computers right now exist pretty much in physicists' concepts and in theoretical research. There are some basic quantum computers in existence, but nobody yet can build a truly practical one — or really knows how. Such computers will harness the powers of atoms and subatomic particles (ions, photons, electrons) to perform memory and processing tasks, thanks to strange subatomic properties that allow the quantum spin state of an electron to be both a "1" and a "0" simultaneously. In theory, such a computer could complete in minutes calculations that might take today's supercomputers years.

SFU physicist Mike Thewalt and grad student Kamyar Saeedi with a sample of highly isotopically enriched silicon; its unique properties could advance quantum computing. (Photo: SFU Public Affairs and Media Relations)

Physicist Mike Thewalt of Simon Fraser University in British Columbia, working with colleagues at Oxford University and in Germany, developed a special highly enriched, highly purified silicon, dubbed "28Silicon," that allows quantum processes to be observed and measured in a solid state thought previously to require a near-perfect vacuum. Using the material, they extended to three minutes the time in which scientists can manipulate, observe and measure the information stored, something that normally lasts only a matter of seconds.

"What we have found, and what wasn't anticipated, are the sharp spectral lines (optical qualities) in the 28Silicon we have been testing. It's so pure, and so perfect. There's no other material like it," Thewalt said. "It's by far a record in solid-state systems. If you'd asked people a few years ago if this was possible, they'd have said no. It opens new ways of using solid-state semiconductors such as silicon as a base for quantum computing."

Using synthetic diamond, Element Six and Harvard University have set a new room-temperature quantum information storage record of more than one second – 1000 times longer than previously recorded. These findings may lead to extremely powerful quantum computers in the future, and novel to sensors based on quantum processes in the near term. (Image: Element Six)

Says Thewalt: "A classical 1/0 bit can be thought of as a person being either at the North or South Pole, whereas a qubit can be anywhere on the surface of the globe — its actual state is described by two parameters similar to latitude and longitude," Thewalt said.

Physicists at Harvard University, the Max Planck Institute for Quantum Optics in Germany and the California Institute of Technology used a single-crystal synthetic diamond grown by chemical vapor deposition to demonstrate the capability of quantum bit memory to exceed one second at room temperature, a new quantum information record.

The diamond was made by England-based Element Six and was produced with essentially no spin impurities other than a very specific defect called the N-V (nitrogen vacancy) center (a vacancy next to a nitrogen atom in the diamond lattice). Because of its very specific properties, this N-V center can be spin polarized using a simple green light source, in this case a green laser.

This study involved a sample of the most perfect enriched silicon in existence, obtained from the Avogadro crystal, shown here immediately after its growth in a floating-zone furnace. This image relates to the paper by Dr. Steger and colleagues. (Image: Leibniz Institute for Crystal Growth)

Their study demonstrated the ability of synthetic diamond to provide the readout of a quantum bit that had preserved its spin polarization for several minutes and its memory coherence for more than a second. This is the first time that such long memory times have been reported for a material at room temperature, giving synthetic diamond a significant advantage over rival materials and technologies that require complex infrastructure that necessitates, for example, cryogenic cooling.

In combination with this method, the Harvard team used a sequence of radio-frequency pulses to suppress interactions with other carbon nuclei in the synthetic diamond. As a result, the researchers were able to store quantum information at room temperatures for nearly two seconds, which was significantly more than was anticipated. Previous experiments in quantum information have generally demonstrated single-qubit memory storage times to be in microseconds.

“The demonstration of a single-qubit quantum memory with seconds of storage time at room temperature is a very exciting development, which combines the four key requirements of initialization, memory, control and measurement. These findings might one day lead to novel quantum communication and computation technologies, but in the nearer term may enable a range of novel and disruptive quantum sensor technologies, such as those being targeted to image magnetic fields on the nanoscale for use in imaging chemical and biological processes,” said Harvard University physics professor Mikhail Lukin.

Thewalt cautions that the world is still a long way from practical quantum computers.

Funding for some of this research was provided by the DARPA QuASAR program; both studies appear in Science.

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