In a new study, Stanford researchers demonstrate how to manipulate atoms so
they interact with an unprecedented degree of control. Using precisely
delivered light and magnetic fields, the researchers programmed a straight
line of atoms into treelike shapes, a twisted loop called a Möbius strip and
other patterns.
These shapes were produced not by physically moving the atoms, but by
controlling the way atoms exchange particles and "sync up" to share certain
properties. By carefully manipulating these interactions, researchers can
generate a vast range of geometries. Importantly, they found that atoms at
the far ends of the straight line could be programmed to interact just as
strongly as the atoms located right next to each other at the center of the
line. To the researchers' knowledge, the ability to program nonlocal
interactions to this degree, irrespective of the atoms' actual spatial
locations, had never been demonstrated before.
The findings could prove a key step forward in the development of advanced
technologies for computation and simulation based on the laws of quantum
mechanics—the mathematical description of how particles move and interact on
the atomic scale.
"In this paper, we've demonstrated a whole new level of control over the
programmability of interactions in a quantum mechanical system," said study
senior author Monika Schleier-Smith, the Nina C. Crocker Faculty Scholar and
associate professor in the Department of Physics in Stanford's School of
Humanities and Sciences. "It's an important milestone that we've long been
working towards, while at the same time it's a starting point for new
opportunities."
The study published Dec. 22 in the journal Nature.
Two graduate students, Avikar Periwal and Eric Cooper, as well as a
postdoctoral scholar, Philipp Kunkel, are co-lead authors of the paper.
Periwal, Cooper and Kunkel are researchers in Schleier-Smith's lab at
Stanford.
"Avikar, Eric and Philipp worked tremendously well together as a team in
running the experiments, devising clever ways of analyzing and visualizing
the data and developing the theoretical models," said Schleier-Smith. "We're
all very excited about these results."
"We chose some simple geometries, like rings and disconnected chains, just
as proof of principle, but we also formed more complex geometries including
ladder-like structures and treelike interactions, which have applications to
open problems in physics," Periwal, Cooper and Kunkel said in a group
statement.
Syncing up atoms on command
Periwal, Cooper, Kunkel and colleagues performed experiments for the study
on apparatuses known as optical tables, a pair of which dominate the
floorspace in Schleier-Smith's lab. The tables are inset with intricate
arrays of electronic components strung together by multicolored wires. At
the heart of one optical table is a vacuum chamber, consisting of a metallic
cylinder studded with porthole windows. A pump expels all air from this
chamber so that no other atoms can disturb the small bunches of rubidium
atoms carefully placed inside it. The Stanford researchers beamed lasers
into this airless chamber to trap the rubidium atoms, slowing the atoms'
movement and cooling them down to within whiskers of absolute zero—the
lowest temperature theoretically possible where particle movement comes to a
virtual standstill. The extremely cold realm just above absolute zero is
where quantum mechanical effects can dominate over those of classical
physics, and thus where the atoms can be quantum mechanically manipulated.
Shining light through the bunches of atoms in this way also serves as a
means of getting the atoms to "talk" to each other. As the light strikes
each atom, it conveys information between them, generating patterns called
"correlations" wherein every atom shares a certain desired quantum
mechanical property. An example of a quantum mechanical property is the
total angular momentum, known as the spin of an atom and which can have
values of, for example, +1, 0 or –1.
Researchers at Stanford and elsewhere have correlated atomic networks before
using systems of laser-cooled atoms, but, until recently, only two basic
kinds of atomic networks could be made. In one, called an all-to-all
network, every atom talks to every other atom. The second kind of network
operates on what's known as a nearest neighbor principle, where
laser-suspended atoms interact most strongly with adjacent atoms.
In this new study, the Stanford researchers debut a far more dynamic method
that conveys information over specific distances between discrete groups of
atoms. This way, spatial location does not matter, and a vastly richer set
of correlations can be programmed.
"With an all-to-all network, it's like I'm sending a worldwide bulletin to
everyone, while in a nearest-neighbor network, it's like I'm only talking to
the person who lives next door," said Schleier-Smith. "With the
programmability that we have now demonstrated in our lab, it's like I'm
picking up a phone and dialing the exact person I want to talk to located
anywhere in the world."
The researchers succeeded in creating these nonlocal interactions and
correlations by controlling the frequencies of light shone at the trapped
bunches of rubidium atoms and varying the strength of an applied magnetic
field in the optical table. As the magnetic field strengthened in intensity
from one end of the vacuum chamber to the other, it caused each bunch of
atoms along the line to spin a bit faster than the prior, neighboring bunch.
Although each atomic bunch had a unique rotation rate, every so often,
certain bunches would nonetheless periodically arrive at the same
orientation—rather like how a row of clocks with progressively
faster-spinning hands will still momentarily read off the same times. The
researchers used light to selectively enable and measure interactions
between these momentarily synced-up atomic clouds. Overall, using a straight
line of 18 clouds of atoms, the researchers could generate interactions
between clouds at any specified set of distances along the line.
"The ability to generate and control these kinds of nonlocal interactions is
powerful," Schleier-Smith added. "It fundamentally changes the way
information can travel and the quantum systems we can engineer."
Benefitting from versatile control
One of the many applications of the Stanford team's work is the crafting of
optimization algorithms for quantum computers—machines that rely on the laws
of quantum mechanics for crunching numbers. Quantum computing has
applications in artificial intelligence, machine learning, cybersecurity,
financial modeling, drug development, climate change forecasting, logistics
and scheduling optimization. For example, quantum computer-tailored
algorithms could efficiently solve scheduling problems by finding the
shortest possible routes for deliveries, or optimal scheduling of university
classes so the greatest number of students can attend.
Another highly promising application is testing out theories of quantum
gravity. The treelike shapes in this study were expressly designed for this
purpose—they serve as basic models of space-time curved by a hypothetical
new concept of gravity based on quantum mechanical principles that would
revamp our understanding of gravity as described in Albert Einstein's theory
of relativity. A similar approach can also be applied to investigate the
light-trapping, ultra-dense cosmic objects called black holes.
Schleier-Smith and colleagues are now working on showing that their
experiments can produce quantum entanglement, where quantum states among
atoms are correlated in a manner that can be harnessed for applications
ranging from ultraprecise sensors to quantum computation.
"We made a lot of progress with this study and we're looking to build on
it," said Schleier-Smith. "Our work demonstrates a new level of control that
can help bridge the gap, in several areas of physics, between elegant
theoretical ideas and actual experiments."
Reference:
Avikar Periwal et al, Programmable interactions and emergent geometry in an
array of atom clouds, Nature (2021).
DOI: 10.1038/s41586-021-04156-0
Tags:
Physics