Moments of silence point the way towards better superconductors
Temporal patterns could show us how to reduce noise in superconductor
devices

Date:
December 20, 2021
Source:
Aalto University
Summary:
High-precision measurements have provided important clues about
processes that impair the efficiency of superconductors. Future
work building on this research could offer improvements in a range
of superconductor devices, such quantum computers and sensitive
particle detectors.



FULL STORY ========================================================================== High-precision measurements have provided important clues about processes
that impair the efficiency of superconductors. Future work building
on this research could offer improvements in a range of superconductor
devices, such quantum computers and sensitive particle detectors.


========================================================================== Superconductivity depends on the presence of electrons bound together in
a Cooper pair. Two electrons become coupled because of interactions with
the metal lattice, synchronizing with each other despite being hundreds
of nanometres apart. Below a critical temperature, these Cooper pairs act
as a fluid which doesn't dissipate energy, thus providing no resistance
to electrical current.

But Cooper pairs sometimes break, dissipating into two quasiparticles -
- unpaired electrons -- that hamper the performance of superconductors.

Scientists still don't know why Cooper pairs break, but the presence
of quasiparticles introduces noise into technologies based on
superconductors.

'Even if there was only one quasiparticle per billion Cooper pairs,
that would limit the performance of quantum bits and prevent a quantum
computer from operating flawlessly,' says Elsa Mannila, who researched quasiparticles at Aalto University before moving to the VTT Technical
Research Centre of Finland.

'If there are more unpaired particles, the lifetime of qubits is also
shorter,' she adds.

Long silences Understanding the origin of these quasiparticles -- in
other words, knowing why Cooper pairs break -- would be a step towards improving the performance of superconductors and the many technologies
that rely on them. To answer that question, researchers at Aalto precisely measured the dynamics of Cooper pair breaking in a superconductor.



========================================================================== 'People usually measure the average number of quasiparticles, so they
don't know what the sequence is like over time. We wanted to find out
exactly when Cooper pairs break and how many pairs break at the same
time,' explains Professor Jukka Pekola of Aalto University.

Together with researchers from Lund University and VTT, the team at
Aalto set up an experiment to detect small numbers of quasiparticles
in real-time. The apparatus consisted of a micron-scale aluminium superconductor separated from a normal conductor -- metallic copper -- by
a thin insulating layer. When Cooper pairs in the superconductor broke,
the quasiparticles would tunnel through the insulation to the copper,
where the researchers observed them with a charge detector.

'The challenge was really in getting many things to work together,'
says Mannila. The analysis depended on having only a small number of quasiparticles, which meant the experiment at Aalto's OtaNano facility
had to be shielded from radiation and external disturbance as well as
being cooled to nearly absolute zero. The researchers also needed to
detect tunnelling events in real-time with a resolution of microseconds,
which they accomplished with an ultra-low-noise superconducting amplifier developed by Quantum Technology Finland and VTT.

Bursts of noise The researchers found that Cooper pairs break in bursts,
with long periods of silence interrupted by very short flurries of quasiparticles. 'The picture that emerged is that there is mostly silence
and then sometimes one or more Cooper pairs breaks, and that leads to
a burst of tunnelling,' says Mannila. 'So a single breaking event might
break more than one Cooper pair at a time.'


==========================================================================
The silent periods were several orders of magnitude longer than the
bursts. The superconductor was entirely free of quasiparticles for seconds
at a time, which is much longer than required for a qubit operation. 'One always wants to get rid of quasiparticles,' says Pekola. 'Our study marks
an important step towards building ideally functioning superconducting devices.' Traces in time 'What on Earth makes Cooper pairs break? That's actually the key question,' says Pekola. The energy to break a Cooper
pair has to come from somewhere, and the dynamics the researchers observed provide an important clue.

Over the course of about 100 days, the researchers found
that quasiparticles bursts became less frequent in their
experiment. 'Time-dependent Cooper pair breaking hasn't been observed
before, so that was interesting and surprising,' says Mannila.

An even more interesting result appeared when they reset the apparatus
and tried again. 'When the experiment was started over, everything began
from scratch,' says Pekola. 'The rate at which quasiparticles appear
depends on how much time has passed since we cooled the system to its
lowest temperature.' These dynamics narrow the range of explanations
for Cooper pair breaking. Any external source, like cosmic rays and other radiation sources, would have to become less common over time and reset
after about 100 days to match the changes seen in the experiment.

'This rules out many or most things which has been proposed,' says
Mannila.

'We've shown that something is going on which has these long time delays,
and that isn't something people would usually look for. Now that the
idea is out there, people can look at these time scales in different
systems for an explanation.' To Pekola, the fact that the rate of quasiparticle events decreases with time but not in an exponential
manner is an important clue about the source of energy to break Cooper
pairs. 'The restlessness at the beginning might stem from impurities
in the materials. These impurities cool down much more slowly than
the device,' he says. These small differences within the system could
result in the release of enough energy to break Cooper pairs, though
this remains speculation.

Pekola plans to continue with experiments using two or more detectors to
pin down the source of these quasiparticles. By looking for correlations between quasiparticle bursts in several devices, he hopes to get more
clues about precisely where the processes driving Cooper pair breakage
happen.

The research was carried out using OtaNano, a national open access
research infrastructure. Aalto research group is also part of InstituteQ,
the Finnish quantum institute.

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


========================================================================== Journal Reference:
1. E. T. Mannila, P. Samuelsson, S. Simbierowicz, J. T. Peltonen, V.

Vesterinen, L. Gro"nberg, J. Hassel, V. F. Maisi, J. P. Pekola. A
superconductor free of quasiparticles for seconds. Nature Physics,
2021; DOI: 10.1038/s41567-021-01433-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211220120621.htm

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