A molecular approach to quantum computing
New research demonstrates how the use of molecules in quantum computing
leads to fewer errors

Date:
September 2, 2020
Source:
California Institute of Technology
Summary:
Molecules in quantum superposition could help in the development
of quantum computers.



FULL STORY ==========================================================================
The technology behind the quantum computers of the future is fast
developing, with several different approaches in progress. Many of the strategies, or "blueprints," for quantum computers rely on atoms or
artificial atom-like electrical circuits. In a new theoretical study
in the journal Physical Review X, a group of physicists at Caltech
demonstrates the benefits of a lesser- studied approach that relies not
on atoms but molecules.


==========================================================================
"In the quantum world, we have several blueprints on the table and we are simultaneously improving all of them," says lead author Victor Albert,
the Lee A. DuBridge Postdoctoral Scholar in Theoretical Physics. "People
have been thinking about using molecules to encode information since
2001, but now we are showing how molecules, which are more complex than
atoms, could lead to fewer errors in quantum computing." At the heart
of quantum computers are what are known as qubits. These are similar
to the bits in classical computers, but unlike classical bits they can experience a bizarre phenomenon known as superposition in which they
exist in two states or more at once. Like the famous Schro"dinger's cat
thought experiment, which describes a cat that is both dead and alive
at the same time, particles can exist in multiple states at once. The phenomenon of superposition is at the heart of quantum computing: the
fact that qubits can take on many forms simultaneously means that they
have exponentially more computing power than classical bits.

But the state of superposition is a delicate one, as qubits are prone
to collapsing out of their desired states, and this leads to computing
errors.

"In classical computing, you have to worry about the bits flipping,
in which a '1' bit goes to a '0' or vice versa, which causes errors,"
says Albert. "This is like flipping a coin, and it is hard to do. But in quantum computing, the information is stored in fragile superpositions,
and even the quantum equivalent of a gust of wind can lead to errors."
However, if a quantum computer platform uses qubits made of molecules,
the researchers say, these types of errors are more likely to be prevented
than in other quantum platforms. One concept behind the new research
comes from work performed nearly 20 years ago by Caltech researchers
John Preskill, Richard P.

Feynman Professor of Theoretical Physics and director of the Institute of Quantum Information and Matter (IQIM), and Alexei Kitaev, the Ronald and
Maxine Linde Professor of Theoretical Physics and Mathematics at Caltech,
along with their colleague Daniel Gottesman (PhD '97) of the Perimeter Institute in Ontario, Canada. Back then, the scientists proposed a
loophole that would provide a way around a phenomenon called Heisenberg's uncertainty principle, which was introduced in 1927 by German physicist
Werner Heisenberg. The principle states that one cannot simultaneously
know with very high precision both where a particle is and where it
is going.



========================================================================== "There is a joke where Heisenberg gets pulled over by a police officer
who says he knows Heisenberg's speed was 90 miles per hour, and Heisenberg replies, 'Now I have no idea where I am,'" says Albert.

The uncertainty principle is a challenge for quantum computers because
it implies that the quantum states of the qubits cannot be known
well enough to determine whether or not errors have occurred. However, Gottesman, Kitaev, and Preskill figured out that while the exact position
and momentum of a particle could not be measured, it was possible to
detect very tiny shifts to its position and momentum. These shifts could
reveal that an error has occurred, making it possible to push the system
back to the correct state. This error- correcting scheme, known as GKP
after its discoverers, has recently been implemented in superconducting
circuit devices.

"Errors are okay but only if we know they happen," says Preskill,
a co-author on the Physical Review X paper and also the scientific
coordinator for a new Department of Energy-funded science center called
the Quantum Systems Accelerator. "The whole point of error correction
is to maximize the amount of knowledge we have about potential errors."
In the new paper, this concept is applied to rotating molecules in superposition. If the orientation or angular momentum of the molecule
shifts by a small amount, those shifts can be simultaneously corrected.

"We want to track the quantum information as it's evolving under the
noise," says Albert. "The noise is kicking us around a little bit. But
if we have a carefully chosen superposition of the molecules' states,
we can measure both orientation and angular momentum as long as they
are small enough. And then we can kick the system back to compensate."
Jacob Covey, a co-author on the paper and former Caltech postdoctoral
scholar who recently joined the faculty at the University of Illinois,
says that it might be possible to eventually individually control
molecules for use in quantum information systems such as these. He and
his team have made strides in using optical laser beams, or "tweezers,"
to control single neutral atoms (neutral atoms are another promising
platform for quantum-information systems).

"The appeal of molecules is that they are very complex structures that can
be very densely packed," says Covey. "If we can figure out how to utilize molecules in quantum computing, we can robustly encode information and
improve the efficiency in which qubits are packed." Albert says that the
trio of himself, Preskill, and Covey provided the perfect combination of theoretical and experimental expertise to achieve the latest results. He
and Preskill are both theorists while Covey is an experimentalist.

"It was really nice to have somebody like John to help me with the
framework for all this theory of error-correcting codes, and Jake gave
us crucial guidance on what is happening in labs." Says Preskill,
"This is a paper that no one of the three of us could have written
on our own. What's really fun about the field of quantum information
is that it's encouraging us to interact across some of these divides,
and Caltech, with its small size, is the perfect place to get this done."

========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Original written by Whitney
Clavin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Victor V. Albert, Jacob P. Covey, John Preskill. Robust Encoding
of a
Qubit in a Molecule. Physical Review X, 2020; 10 (3) DOI: 10.1103/
PhysRevX.10.031050 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200902095130.htm

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