Natural radiation can interfere with quantum computers
Study shows the need to shield qubits from natural radiation, like cosmic
rays from outer space

Date:
August 26, 2020
Source:
DOE/Pacific Northwest National Laboratory
Summary:
Radiation from natural sources in the environment can limit the
performance of superconducting quantum bits, known as qubits. The
discovery has implications for quantum computing and for the search
for dark matter.



FULL STORY ==========================================================================
A multidisciplinary research team has shown that radiation from natural
sources in the environment can limit the performance of superconducting
quantum bits, known as qubits. The discovery, reported today in the
journal Nature, has implications for the construction and operation
of quantum computers, an advanced form of computing that has attracted
billions of dollars in public and private investment globally.


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The collaboration between teams at the U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL) and the Massachusetts Institute
of Technology (MIT), helps explain a mysterious source of interference
limiting qubit performance.

"Our study is the first to show clearly that low-level ionizing radiation
in the environment degrades the performance of superconducting qubits,"
said John Orrell, a PNNL research physicist, senior author of the
study and expert in low-level radiation measurement. "These findings
suggest that radiation shielding will be necessary to attain long-sought performance in quantum computers of this design." Natural radiation
wreaks havoc with computers Computer engineers have known for at least
a decade that natural radiation emanating from materials like concrete
and pulsing through our atmosphere in the form of cosmic rays can cause
digital computers to malfunction. But digital computers aren't nearly
as sensitive as a quantum computer.

"We found that practical quantum computing with these devices will not
be possible unless we address the radiation issue," said PNNL physicist
Brent VanDevender, a co-investigator on the study.



==========================================================================
The researchers teamed up to solve a puzzle that has been vexing efforts
to keep superconducting quantum computers working for long enough to
make them reliable and practical. A working quantum computer would be
thousands of times faster than even the fastest supercomputer operating
today. And it would be able to tackle computing challenges that today's
digital computers are ill- equipped to take on. But the immediate
challenge is to have the qubits maintain their state, a feat called "coherence," said Orrell. This desirable quantum state is what gives
quantum computers their power.

MIT physicist Will Oliver was working with superconducting qubits and
became perplexed at a source of interference that helped push the qubits
out of their prepared state, leading to "decoherence," and making them non-functional. After ruling out a number of different possibilities,
he considered the idea that natural radiation from sources like metals
found in the soil and cosmic radiation from space might be pushing the
qubits into decoherence.

A chance conversation between Oliver, VanDevender, and his long-time collaborator, MIT physicist Joe Formaggio, led to the current project.

It's only natural To test the idea, the research team measured the
performance of prototype superconducting qubits in two different
experiments:
* They exposed the qubits to elevated radiation from copper metal
activated
in a reactor.

* They built a shield around the qubits that lowered the amount
of natural
radiation in their environment.

The pair of experiments clearly demonstrated the inverse relationship
between radiation levels and length of time qubits remain in a coherent
state.



==========================================================================
"The radiation breaks apart matched pairs of electrons that typically
carry electric current without resistance in a superconductor," said VanDevender.

"The resistance of those unpaired electrons destroys the delicately
prepared state of a qubit." The findings have immediate implications for
qubit design and construction, the researchers concluded. For example,
the materials used to construct quantum computers should exclude material
that emits radiation, the researchers said.

In addition, it may be necessary to shield experimental quantum computers
from radiation in the atmosphere.

At PNNL, interest has turned to whether the Shallow Underground
Laboratory, which reduces surface radiation exposure by 99%, could
serve future quantum computer development. Indeed, a recent study by a
European research team corroborates the improvement in qubit coherence
when experiments are conducted underground.

"Without mitigation, radiation will limit the coherence time of
superconducting qubits to a few milliseconds, which is insufficient for practical quantum computing," said VanDevender.

The researchers emphasize that factors other than radiation exposure
are bigger impediments to qubit stability for the moment. Things like microscopic defects or impurities in the materials used to construct
qubits are thought to be primarily responsible for the current performance limit of about one-tenth of a millisecond. But once those limitations are overcome, radiation begins to assert itself as a limit and will eventually become a problem without adequate natural radiation shielding strategies,
the researchers said.

Findings affect global search for dark matter In addition to helping
explain a source of qubit instability, the research findings may also
have implications for the global search for dark matter, which is
thought to comprise just under 85% of the known universe, but which has
so far escaped human detection with existing instruments. One approach
to signals involves using research that depends on superconducting
detectors of similar design to qubits. Dark matter detectors also need
to be shielded from external sources of radiation, because radiation can trigger false recordings that obscure the desirable dark matter signals.

"Improving our understanding of this process may lead to improved designs
for these superconducting sensors and lead to more sensitive dark matter searches," said Ben Loer, a PNNL research physicist who is working
both in dark matter detection and radiation effects on superconducting
qubits. "We may also be able to use our experience with these particle
physics sensors to improve future superconducting qubit designs."

========================================================================== Story Source: Materials provided by
DOE/Pacific_Northwest_National_Laboratory. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Vepsa"la"inen, A.P., Karamlou, A.H., Orrell, J.L. et al. Impact of
ionizing radiation on superconducting qubit coherence. Nature,
2020 DOI: 10.1038/s41586-020-2619-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200826110328.htm

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