In a study that could benefit quantum computing, researchers show a
superlattice embedded with nanodots may be immune from dissipating energy to
the environment.
Scientists around the world are developing new hardware for quantum
computers, a new type of device that could accelerate drug design, financial
modeling, and weather prediction. These computers rely on qubits, bits of
matter that can represent some combination of 1 and 0 simultaneously. The
problem is that qubits are fickle, degrading into regular bits when
interactions with surrounding matter interfere. But new research at MIT
suggests a way to protect their states, using a phenomenon called many-body
localization (MBL).
MBL is a peculiar phase of matter, proposed decades ago, that is unlike
solid or liquid. Typically, matter comes to thermal equilibrium with its
environment. That’s why soup cools and ice cubes melt. But in MBL, an object
consisting of many strongly interacting bodies, such as atoms, never reaches
such equilibrium. Heat, like sound, consists of collective atomic vibrations
and can travel in waves; an object always has such heat waves internally.
But when there’s enough disorder and enough interaction in the way its atoms
are arranged, the waves can become trapped, thus preventing the object from
reaching equilibrium.
MBL had been demonstrated in “optical lattices,” arrangements of atoms at
very cold temperatures held in place using lasers. But such setups are
impractical. MBL had also arguably been shown in solid systems, but only
with very slow temporal dynamics, in which the phase’s existence is hard to
prove because equilibrium might be reached if researchers could wait long
enough. The MIT research found a signatures of MBL in a “solid-state” system
— one made of semiconductors — that would otherwise have reached equilibrium
in the time it was watched.
“It could open a new chapter in the study of quantum dynamics,” says Rahul
Nandkishore, a physicist at the University of Colorado at Boulder, who was
not involved in the work.
Mingda Li, the Norman C Rasmussen Assistant Professor Nuclear Science and
Engineering at MIT, led the new study, published in a recent issue of Nano
Letters. The researchers built a system containing alternating semiconductor
layers, creating a microscopic lasagna — aluminum arsenide, followed by
gallium arsenide, and so on, for 600 layers, each 3 nanometers (millionths
of a millimeter) thick. Between the layers they dispersed “nanodots,”
2-nanometer particles of erbium arsenide, to create disorder. The lasagna,
or “superlattice,” came in three recipes: one with no nanodots, one in which
nanodots covered 8 percent of each layer’s area, and one in which they
covered 25 percent.
According to Li, the team used layers of material, instead of a bulk
material, to simplify the system so dissipation of heat across the planes
was essentially one-dimensional. And they used nanodots, instead of mere
chemical impurities, to crank up the disorder.
To measure whether these disordered systems are still staying in
equilibrium, the researchers measured them with X-rays. Using the Advanced
Photon Source at Argonne National Lab, they shot beams of radiation at an
energy of more than 20,000 electron volts, and to resolve the energy
difference between the incoming X-ray and after its reflection off the
sample’s surface, with an energy resolution less than one one-thousandth of
an electron volt. To avoid penetrating the superlattice and hitting the
underlying substrate, they shot it at an angle of just half a degree from
parallel.
Just as light can be measured as waves or particles, so too can heat. The
collective atomic vibration for heat in the form of a heat-carrying unit is
called a phonon. X-rays interact with these phonons, and by measuring how
X-rays reflect off the sample, the experimenters can determine if it is in
equilibrium.
The researchers found that when the superlattice was cold — 30 kelvin, about
-400 degrees Fahrenheit — and it contained nanodots, its phonons at certain
frequencies remained were not in equilibrium.
More work remains to prove conclusively that MBL has been achieved, but
“this new quantum phase can open up a whole new platform to explore quantum
phenomena,” Li says, “with many potential applications, from thermal storage
to quantum computing.”
To create qubits, some quantum computers employ specks of matter called
quantum dots. Li says quantum dots similar to Li’s nanodots could act as
qubits. Magnets could read or write their quantum states, while the
many-body localization would keep them insulated from heat and other
environmental factors.
In terms of thermal storage, such a superlattice might switch in and out of
an MBL phase by magnetically controlling the nanodots. It could insulate
computer parts from heat at one moment, then allow parts to disperse heat
when it won’t cause damage. Or it could allow heat to build up and be
harnessed later for generating electricity.
Conveniently, superlattices with nanodots can be constructed using
traditional techniques for fabricating semiconductors, alongside other
elements of computer chips. According to Li, “It’s a much larger design
space than with chemical doping, and there are numerous applications.”
“I am excited to see that signatures of MBL can now also be found in real
material systems,” says Immanuel Bloch, scientific director at the
Max-Planck-Institute of Quantum Optics, of the new work. “I believe this
will help us to better understand the conditions under which MBL can be
observed in different quantum many-body systems and how possible coupling to
the environment affects the stability of the system. These are fundamental
and important questions and the MIT experiment is an important step helping
us to answer them.”
Reference:
“Signature of Many-Body Localization of Phonons in Strongly Disordered
Superlattices” by Thanh Nguyen, Nina Andrejevic, Hoi Chun Po, Qichen Song,
Yoichiro Tsurimaki, Nathan C. Drucker, Ahmet Alatas, Esen E. Alp, Bogdan M.
Leu, Alessandro Cunsolo, Yong Q. Cai, Lijun Wu, Joseph A. Garlow, Yimei Zhu,
Hong Lu, Arthur C. Gossard, Alexander A. Puretzky, David B. Geohegan,
Shengxi Huang and Mingda Li, 27 July 2021, Nano Letters.
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