Resonant approach to quantum problem

Microwave waveguides on a diamond-based chip can generate a magnetic field large enough to change the quantum state of an atomic-scale defect in less than one billionth of a second, using a process that is similar to the one in magnetic resonance imaging and potentially leading to a new approach to quantum computing.

Physicists at the University of California at Santa Barbara and Iowa State University have taken another step forward in research aimed at finding a way to electrically control the quantum states of electrons. Greg Fuchs, a postdoctoral researcher in UCSB's Center for Spintronics and Quantum Computation working with David Awschalom, together with David Toyli and Joseph Heremans, and Slava Dobrovitski of the Ames Laboratory and Iowa State, in Ames, published details in the latest issue of the online version of the journal Science Express.

As computer science approaches the physical limits of size and speed for conventional semiconductor components, the notion that quantum computing might displace everyday computing for certain kinds of problem is becoming increasingly attractive. A quantum computer will exploit the peculiar phenomena of quantum mechanics, such as superposition and entanglement, to perform operations on data that will allow solutions to be found for complicated and non-linear problems that are inaccessible even to the most powerful conventional supercomputers.

The basic principle behind quantum computation is that quantum properties can be used to represent data and perform operations on these data. However, building devices in which individual quantum states adopt the role of logic units in classical computing is proving more difficult than computer scientists first anticipated. A quantum computer has the potential to solve problems in which guessing answers repeatedly and checking them is the best approach and each check takes the same amount of time irrespective of whether it is a right or wrong answer.

For instance, solving the "travelling salesman" problem, predicting the weather or stockmarket crashes, are examples of problems that conventional computing finds impossible to number crunch, whereas the parallel power of quantum superposition could ultimately solve much faster.

Important steps have already been taken towards such control though. Now, Fuchs and colleagues have demonstrated that they can manipulate electrically, at gigahertz rates, the quantum states of electrons trapped on individual defects in diamond crystals.

Two-level systems are at the core of numerous real-world technologies such as magnetic
resonance imaging (MRI) and atomic clocks," the team explains. "The conventional approach to control the state of a two-level quantum system is to apply an oscillating field.  The rate of control is determined by the amplitude of that field.  If you keep increasing the amplitude the 'strong-driving' regime emerges where many of the conventional rules don't apply.  The behaviour of the system becomes more complex, but it can be driven much faster than expected normally." The team has employed a single spin as such a two-level system and has measured the room-temperature "strong-driving" dynamics of a single nitrogen vacancy centre in diamond. They were thus able to observe dynamics on the sub-nanosecond timescale.

This approach, they, say, could help in the development of quantum computers that use electron spin, a quantum "bit", to carry out computation.

"From an information technology standpoint, there is still a lot to learn about controlling quantum systems," explains Awschalom, principal investigator and professor of physics, electrical and computer engineering at UCSB. "Still, it's exciting to stand back and realize that we can already electrically control the quantum state of just a few atoms at gigahertz rates - speeds comparable to what you might find in your computer at home."

The research offers an alternative perspective on how manipulation can be performed. "We set out to see if there is a practical limit to how fast we can manipulate these quantum states in diamond," explains lead author Fuchs. "Eventually, we reached the point where the standard assumptions of magnetic resonance no longer hold, but to our surprise we found that we actually gained an increase in operation speed by breaking the conventional assumptions."