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|Ames Lab theoretical physicist Viatcheslav Dobrovitski was recently part of a team that produced and controlled rotations of a single quantum spin at rates less than one trillionth of a second.|
For many exciting applications - from someday building quantum computers to developing ultra-precise magnetometry and improving quantum communication across fiber-optic networks - scientists need to better understand really fast and really small quantum systems. Theoretical physicist Viatcheslav Dobrovitski of DOE's Ames Laboratory recently worked with researchers at Lawrence Berkeley National Laboratory and the University of California Santa Barbara to take a significant step forward in the study of quantum processes.
The team produced and controlled coherent rotations of a single quantum spin at rates less than one trillionth of a second. The group's discoveries, which appeared in a recent issue of Science, open the window to studying new, very fast quantum processes that were previously impossible to detect and analyze.
Dobrovitski and his colleagues studied quantum spins in diamond, which can contain imperfections called nitrogen-vacancy, or N-V, centers, using samples grown and characterized by researchers at Lawrence Berkeley National Laboratory. Since atoms in diamond sit very tightly in their positions and respond very weakly to heat or other excitation, the spin of an isolated N-V center can be studied with very little interference from the rest of the world.
Isolating quantum spins is important, because to use spins in applications like quantum communication they must rotate smoothly and predictably to retain their quantum properties. When spins are exposed to outside forces, they can get bumped off their path.
"If we want a spin to go from position ‘a' to position ‘b,' another way for us to prevent interference from the outside world is to induce the spin rotation as fast as possible so it doesn't have time to interact with other forces," says Dobrovitski.
So researchers at UCSB, who performed the optical and magnetic measurements for the project, applied short and extremely strong pulses of magnetic field to the spins. Similar, but weaker, pulses are used in conventional electron and nuclear magnetic resonance.
As power was increased, the spins started to exhibit a pattern of fast rotations and stallings, caused by the fast changes of the magnetic field during the pulse.
In conventional resonance experiments, the power of a pulse begins low for a short time, reaches a higher and consistent, level for the majority of the pulse and then drops down for a short time. In that case, the affect of pulses' "heads" and "tails" is not very noticeable compared to the even body of the pulse.
But when the magnetic resonance pulse is large and lasts less than a nanosecond, as occurred in the research by Dobrovitski and his colleagues, the pulse is mostly made up of heads and tails.
"If we want to rotate a spin very fast, we necessarily have to deal with the heads and tails," says Dobrovitski. "Our short, strong pulses will consist of mostly just edges. There's no time for a body of the pulse."
The finding that very fast rotations could be induced using these short pulses was a significant discovery in itself.
"We showed that controlled, coherent spin rotation is possible outside the standard framework of nicely defined, long pulses," says Dobrovitski.
In principle, scientists can create much shorter pulse. But these pulses cannot induce a smooth, coherent rotation of a single spin. The pulses' large power excites a plethora of different degrees of freedom, and the precious quantum coherence of the spin is lost. Thus, the pulses have to be strong but not too strong. Dobrovitski and his colleagues at UCSB found the "sweet spot" for short, strong pulses.
The research team went on to determine experimentally and theoretically that the spins were most controllable when the short pulses had gradual increases and decreases in power.
"Those kinds of pulses seem to gradually awaken the spin processes," says Dobrovitski. "It's most controllable that way, and we can rotate the spins smoothly and coherently but still do really fast rotations."
Next up for Dobrovitski and the team is studying other fast processes in N-V centers using these fast pulses.
"Before our most recent research, people could not see these processes," says Dobrovitski. "Now that we can see them, it's my job as a theoretical physicist to explain what we see."
About Ames Laboratory
Ames Laboratory is a government-owned, contractor-operated research facility of the U.S. Department of Energy that is run by Iowa State University.
For more than 60 years, the Ames Laboratory has sought solutions to energy-related problems through the exploration of chemical, engineering, materials, mathematical and physical sciences. Established in the 1940s with the successful development of the most efficient process to produce high-purity uranium metal for atomic energy, the Lab now pursues a broad range of scientific priorities.
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