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New Technique to Drive Single-Quantum Spin Transition of Electrons

Electrons are highly susceptible to magnetic fields. Researchers can use magnetic fields to control the electrons and their angular momentum, or their “spin.”

In 2013, a team of Cornell researchers led by Greg Fuchs, assistant professor of applied and engineering physics in the College of Engineering, found a new technique to achieve this control by using acoustic waves produced by mechanical resonators.

Through that technique, they were able to control transitions in the spin of electrons (also called spin resonance) that would otherwise be impossible through traditional magnetic behavior.

That invention proved to beneficial for anyone seeking to develop quantum sensors similar to those used in mobile navigation devices. But such devices still needed a magnetic control field—and thus a heavy magnetic antenna—to power specific spin transitions.

At present, the research team led by Fuchs has demonstrated that it is possible to drive these transitions solely by using acoustics. This avoids the need for the magnetic antenna, thereby allowing engineers to develop smaller, more power-efficient acoustic sensors that could be packed more tightly on a single device.

The study by researchers, titled “Acoustically Driving the Single Quantum Spin Transition of Diamond Nitrogen-Vacancy Centers,” was published in the Physical Review Applied journal on May 27th, 2020.

You can use a magnetic field to drive these spin transitions, but a magnetic field is actually a very extended, big object. In contrast, acoustic waves can be very confined. So if you’re thinking about controlling different regions of spins inside your chip, locally and independently, then doing it with acoustic waves is a sensible approach.

Greg Fuchs, Assistant Professor of Applied and Engineering Physics, College of Engineering, Cornell University

Fuchs and Huiyao Chen ’20, the lead author of the study, initiated the electron spin transitions by using nitrogen-vacancy (NV) centers—defects found in the crystal lattice of a diamond.

The acoustic resonators are microelectromechanical systems (MEMS) devices provided with a transducer. Upon applying voltage, the device vibrates and sends acoustic waves of 2 to 3 GHz into the crystal. Due to these frequencies, stress and strain appear in the defect, leading to the electron spin resonance.

However, there is one problem: Since this process also activates the magnetic field, the researchers have never been fully assured of the mechanical vibration effect versus the magnetic oscillation effect. Therefore, Fuchs and Chen endeavored to meticulously quantify the coupling between the spin transition and the acoustic waves, and compare it with the calculations put forward by theoretical physicists.

We were able to separately establish the magnetic part and the acoustic part, and thereby measure that unknown coefficient that determines how strongly the single quantum transition couples to acoustic waves.

Greg Fuchs, Assistant Professor of Applied and Engineering Physics, College of Engineering, Cornell University

Fuchs added, “The answer was, to our surprise and delight, that it’s an order of magnitude larger than predicted. That means that you can indeed design fully acoustic spin resonance devices that would make excellent magnetic field sensors, for instance, but you don’t need a magnetic control field to run them.”

Fuchs has been collaborating with Cornell’s Center for Technology Licensing to patent the invention, which could find crucial applications in navigation technology.

There’s a significant effort nationwide to make highly stable magnetic field sensors with diamond NV centers. People are already building these devices based on conventional magnetic resonance using magnetic antennas. I think our discovery is going to have tremendous benefit in terms of how compact you can make it and the ability to make independent sensors that are closely spaced.

Greg Fuchs, Assistant Professor of Applied and Engineering Physics, College of Engineering, Cornell University

Sunil Bhave, professor of electrical and computer engineering at Purdue University, also contributed to the study.

The study was financially supported by the Defense Advanced Research Projects Agency and the Office of Naval Research.

Journal Reference:

Chen, H. Y., et al. (2020) Acoustically Driving the Single-Quantum Spin Transition of Diamond Nitrogen-Vacancy Centers. Physical Review Applied.


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