YSM Issue 96.3

FEATURE Physics
THE
PHONON PHENOMENON
Harnessing Photon-Phonon Coupling to Advance Quantum Computing
BY ANNLI ZHU AND LEA PAPA
Communication is a natural part of
life. Humans talk, birds chirp, and
even trees interact through their root
networks. To maintain this essential aspect
of life, we adapt our methods to overcome
communication challenges: a team meeting
that once had to be held in a boardroom
can now be effectively held on a Zoom
call. For quantum computers—a system
that looks to advance past the capabilities
of classical computing—communication
occurs by leveraging quantum particles and
their properties, components of sub-atomic
interactions that have historically been
challenging to harness.
Recently, physicist Mo Li and his colleagues
at the University of Washington were able to
overcome one such challenge: dealing with
unpredictable photon emitters. They achieved
this by designing a deterministic emitter —one
where they can determine where the photon
is emitted—and in doing so, they discovered
that their emitter produced a strong
interaction between two important quantum
quasiparticles: photons and phonons. Now, Li
is hopeful that further research can use this
interaction to advance communication in
quantum computing systems and overcome
some challenges in the field.
In classical computers, information
is stored in bits: either 0 or 1. Quantum
computers use quantum bits—
called “qubits”—which can exist in a
“superposition” state of being both 0 and 1
at the same time, like Schrödinger’s cat. This
allows them to consider many possibilities
simultaneously. Through a process called
entanglement, qubits can be connected
in a way such that the state of one qubit
instantly influences the state of another,
no matter how far apart they are, enabling
quantum computers to perform complex
calculations literally faster than light can
travel. This means quantum computers
have the potential to revolutionize fields
like cryptography, drug discovery, and
more. However, because they are highly
sensitive to environmental conditions,
require extremely low temperatures, and
use extensive space, they are expensive to
build and difficult to scale.
Although many subatomic particles
can be used for quantum computers,
scientists prefer to use photons—tiny,
massless particles of light—to transmit
quantum information because they travel
at, well, the speed of light. But photons are
difficult to reliably produce, control, and
capture. Traditional methods of photon
generation—through so-called “quantum
emitters”—involve taking advantage of
defects in various atomic lattices, which
are patterned arrays of bound atoms.
However, these defects often emit photons
unpredictably, which is undesirable for
highly precise quantum computers.
To address this problem, the team of
scientists at the University of Washington
set out to build a “deterministic” quantum
emitter. “We want to engineer it in such a
way that we can say ‘we want an emitter here’
and it indeed emits there,” said Li, Professor
of Electrical & Computer Engineering and
Physics and leader of the research team.
To achieve this goal, the team used two
single-atom layers of tungsten and selenium,
similar to existing quantum emitters. Then,
they draped these layers over hundreds of
nanoscopic pillars, creating tiny bumps in
the 2D lattice that isolated the target regions.
By shining a precise pulse of laser light at an
electron in the material, they were able to free
it for a very short period of time. Each time
an electron returned to its place, it emitted
a single photon encoded with quantum
information—a successful quantum emitter.
Amidst their successes with the
deterministic emitter, Li and his colleagues
noticed something
intriguing in their
data. “The emitter
ideally is supposed
to generate a very
sharp peak in energy
at one wavelength
28 Yale Scientific Magazine September 2023 www.yalescientific.org