Acoustics put a fresh spin on electron transitions

Date:
June 10, 2020
Source:
Cornell University
Summary:
Electrons are very much at the mercy of magnetic fields, which
scientists can manipulate to control the electrons and their
angular momentum - - i.e. their 'spin.'


FULL STORY ========================================================================== Electrons are very much at the mercy of magnetic fields, which scientists
can manipulate to control the electrons and their angular momentum --
i.e. their "spin."

==========================================================================
A Cornell team led by Greg Fuchs, assistant professor of applied and engineering physics in the College of Engineering, in 2013 invented
a new way to exert this control by using acoustic waves generated by
mechanical resonators. That approach enabled the team to control electron
spin transitions (also known as spin resonance) that otherwise wouldn't
be possible through conventional magnetic behavior.

The finding was a boon for anyone looking to build quantum sensors of
the sort used in mobile navigation devices. However, such devices still required a magnetic control field -- and therefore a bulky magnetic
antenna -- to drive certain spin transitions.

Now, Fuchs's group has shown that these transitions can be driven solely
by acoustics. This eliminates the need for the magnetic antenna, enabling engineers to build smaller, more power-efficient acoustic sensors that
can be packed more tightly on a single device.

The team's paper, "Acoustically Driving the Single Quantum Spin Transition
of Diamond Nitrogen-Vacancy Centers," published May 27 in Physical
Review Applied.

"You can use a magnetic field to drive these spin transitions, but a
magnetic field is actually a very extended, big object," Fuchs said. "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."
In order to drive the electron spin transitions, Fuchs and Huiyao Chen
'20, the paper's lead author, used nitrogen-vacancy (NV) centers,
which are defects in the crystal lattice of a diamond. The acoustic
resonators are microelectromechanical systems (MEMS) devices equipped
with a transducer. When voltage is applied, the device vibrates, sending acoustic waves of 2 to 3 gigahertz into the crystal. These frequencies
cause strain and stress in the defect, which results in the electron
spin resonance.

One complication: This process also excites the magnetic field, so the researchers have never been entirely sure of the effect of the mechanical vibrations versus the effect of the magnetic oscillations. So Fuchs and
Chen set out to painstakingly measure the coupling between the acoustic
waves and the spin transition, and compare it to the calculations proposed
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,"
Fuchs said. "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 is working with Cornell's
Center for Technology Licensing to patent the discovery, which could
have important applications in navigation technology.

"There's a significant effort nationwide to make highly stable magnetic
field sensors with diamond NV centers," Fuchs said. "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." The research was supported by Defense Advanced Research Projects Agency and the Office of Naval Research.


========================================================================== Story Source: Materials provided by Cornell_University. Original written
by David Nutt. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. H. Y. Chen, S. A. Bhave, G. D. Fuchs. Acoustically Driving the
Single-
Quantum Spin Transition of Diamond Nitrogen-Vacancy
Centers. Physical Review Applied, 2020; 13 (5) DOI:
10.1103/PhysRevApplied.13.054068 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200610135020.htm

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