Cosmic rays may soon stymie quantum computing
Building quantum computers underground or designing radiation-proof
qubits may be needed, researchers find.

Date:
August 26, 2020
Source:
Massachusetts Institute of Technology
Summary:
Infinitesimally low levels of radiation, such as from incoming
cosmic rays, may soon stymie progress in quantum computing.



FULL STORY ========================================================================== [Quantum computing | Credit: (c) vchalup / stock.adobe.com] Quantum
computing abstract concept (stock image).

Credit: (c) vchalup / stock.adobe.com [Quantum computing | Credit:
(c) vchalup / stock.adobe.com] Quantum computing abstract concept
(stock image).

Credit: (c) vchalup / stock.adobe.com Close The practicality of quantum computing hangs on the integrity of the quantum bit, or qubit.


========================================================================== Qubits, the logic elements of quantum computers, are coherent
two-level systems that represent quantum information. Each qubit has
the strange ability to be in a quantum superposition, carrying aspects
of both states simultaneously, enabling a quantum version of parallel computation. Quantum computers, if they can be scaled to accommodate
many qubits on one processor, could be dizzyingly faster, and able to
handle far more complex problems, than today's conventional computers.

But that all depends on a qubit's integrity, or how long it can operate
before its superposition and the quantum information are lost --
a process called decoherence, which ultimately limits the computer
run-time. Superconducting qubits -- a leading qubit modality today --
have achieved exponential improvement in this key metric, from less
than one nanosecond in 1999 to around 200 microseconds today for the best-performing devices.

But researchers at MIT, MIT Lincoln Laboratory, and Pacific Northwest
National Laboratory (PNNL) have found that a qubit's performance will
soon hit a wall.

In a paper published in Nature, the team reports that the low-level,
otherwise harmless background radiation that is emitted by trace elements
in concrete walls and incoming cosmic rays are enough to cause decoherence
in qubits. They found that this effect, if left unmitigated, will limit
the performance of qubits to just a few milliseconds.

Given the rate at which scientists have been improving qubits, they
may hit this radiation-induced wall in just a few years. To overcome
this barrier, scientists will have to find ways to shield qubits -- and
any practical quantum computers -- from low-level radiation, perhaps by building the computers underground or designing qubits that are tolerant
to radiation's effects.

"These decoherence mechanisms are like an onion, and we've been peeling
back the layers for past 20 years, but there's another layer that left
unabated is going to limit us in a couple years, which is environmental radiation," says William Oliver, associate professor of electrical
engineering and computer science and Lincoln Laboratory Fellow at
MIT. "This is an exciting result, because it motivates us to think of
other ways to design qubits to get around this problem." The paper's
lead author is Antti Vepsa"la"inen, a postdoc in MIT's Research Laboratory
of Electronics.



==========================================================================
"It is fascinating how sensitive superconducting qubits are to the weak radiation. Understanding these effects in our devices can also be helpful
in other applications such as superconducting sensors used in astronomy," Vepsa"la"inen says.

Co-authors at MIT include Amir Karamlou, Akshunna Dogra, Francisca
Vasconcelos, Simon Gustavsson, and physics professor Joseph Formaggio,
along with David Kim, Alexander Melville, Bethany Niedzielski, and
Jonilyn Yoder at Lincoln Laboratory, and John Orrell, Ben Loer, and
Brent VanDevender of PNNL.

A cosmic effect Superconducting qubits are electrical circuits made from superconducting materials. They comprise multitudes of paired electrons,
known as Cooper pairs, that flow through the circuit without resistance
and work together to maintain the qubit's tenuous superposition state. If
the circuit is heated or otherwise disrupted, electron pairs can split
up into "quasiparticles," causing decoherence in the qubit that limits
its operation.

There are many sources of decoherence that could destabilize a qubit,
such as fluctuating magnetic and electric fields, thermal energy, and
even interference between qubits.



========================================================================== Scientists have long suspected that very low levels of radiation may
have a similar destabilizing effect in qubits.

"I the last five years, the quality of superconducting qubits has become
much better, and now we're within a factor of 10 of where the effects
of radiation are going to matter," adds Kim, a technical staff member
at MIT Lincoln Laboratotry.

So Oliver and Formaggio teamed up to see how they might nail down the
effect of low-level environmental radiation on qubits. As a neutrino
physicist, Formaggio has expertise in designing experiments that shield
against the smallest sources of radiation, to be able to see neutrinos
and other hard-to-detect particles.

"Calibration is key" The team, working with collaborators at Lincoln
Laboratory and PNNL, first had to design an experiment to calibrate
the impact of known levels of radiation on superconducting qubit
performance. To do this, they needed a known radioactive source --
one which became less radioactive slowly enough to assess the impact
at essentially constant radiation levels, yet quickly enough to assess
a range of radiation levels within a few weeks, down to the level of
background radiation.

The group chose to irradiate a foil of high purity copper. When exposed
to a high flux of neutrons, copper produces copious amounts of copper-64,
an unstable isotope with exactly the desired properties.

"Copper just absorbs neutrons like a sponge," says Formaggio, who worked
with operators at MIT's Nuclear Reactor Laboratory to irradiate two
small disks of copper for several minutes. They then placed one of the
disks next to the superconducting qubits in a dilution refrigerator in
Oliver's lab on campus. At temperatures about 200 times colder than outer space, they measured the impact of the copper's radioactivity on qubits' coherence while the radioactivity decreased -- down toward environmental background levels.

The radioactivity of the second disk was measured at room temperature as
a gauge for the levels hitting the qubit. Through these measurements
and related simulations, the team understood the relation between
radiation levels and qubit performance, one that could be used to infer
the effect of naturally occurring environmental radiation. Based on
these measurements, the qubit coherence time would be limited to about
4 milliseconds.

"Not game over" The team then removed the radioactive source and proceeded
to demonstrate that shielding the qubits from the environmental radiation improves the coherence time. To do this, the researchers built a 2-ton
wall of lead bricks that could be raised and lowered on a scissor lift,
to either shield or expose the refrigerator to surrounding radiation.

"We built a little castle around this fridge," Oliver says.

Every 10 minutes, and over several weeks, students in Oliver's lab
alternated pushing a button to either lift or lower the wall, as a
detector measured the qubits' integrity, or "relaxation rate," a measure
of how the environmental radiation impacts the qubit, with and without
the shield. By comparing the two results, they effectively extracted the
impact attributed to environmental radiation, confirming the 4 millisecond prediction and demonstrating that shielding improved qubit performance.

"Cosmic ray radiation is hard to get rid of," Formaggio says. "It's very penetrating, and goes right through everything like a jet stream. If
you go underground, that gets less and less. It's probably not necessary
to build quantum computers deep underground, like neutrino experiments,
but maybe deep basement facilities could probably get qubits operating at improved levels." Going underground isn't the only option, and Oliver
has ideas for how to design quantum computing devices that still work
in the face of background radiation.

"If we want to build an industry, we'd likely prefer to mitigate the
effects of radiation above ground," Oliver says. "We can think about
designing qubits in a way that makes them 'rad-hard,' and less sensitive
to quasiparticles, or design traps for quasiparticles so that even if
they're constantly being generated by radiation, they can flow away
from the qubit. So it's definitely not game-over, it's just the next
layer of the onion we need to address." This research was funded,
in part, by the U.S. Department of Energy Office of Nuclear Physics,
the U.S. Army Research Office, the U.S. Department of Defense, and the
U.S. National Science Foundation.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Antti P. Vepsa"la"inen, Amir H. Karamlou, John L. Orrell,
Akshunna S.

Dogra, Ben Loer, Francisca Vasconcelos, David K. Kim, Alexander J.

Melville, Bethany M. Niedzielski, Jonilyn L. Yoder, Simon
Gustavsson, Joseph A. Formaggio, Brent A. VanDevender, William
D. Oliver. Impact of ionizing radiation on superconducting
qubit coherence. Nature, 2020; 584 (7822): 551 DOI:
10.1038/s41586-020-2619-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200826113716.htm

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