Superconductivity depends on the presence of electrons bound together in so-called Cooper pairs. Two electrons are coupled by interactions mediated by vibrations of atoms in a metal and synchronize with each other despite being hundreds of nanometres apart.
As long as this happens below a certain temperature, as cold as near absolute zero, these Cooper pairs act like a liquid that does not lose energy, which means they do not give rise to any resistance to electric current.
“But despite being so close to zero temperature, the superconductors are not completely still and insulated. That’s something that causes disturbances,” says Ville Maisi, assistant professor and principal investigator at NanoLund.
Limits performance
Sometimes a Cooper pair breaks into two quasiparticles – unpaired electrons – which inhibit the performance of superconductors. Researchers still don’t know why they break, but the presence of quasiparticles leads to noise in various types of superconductor-based components, such as quantum bits, the basic building blocks of superconducting quantum computers. This prevents a quantum computer from working flawlessly.
If you could take advantage of the periods of silence and avoid the interference, you could cherry-pick and run an almost perfect quantum computer.
“Even if there is only one quasiparticle per billion Cooper pairs, it limits the performance of quantum bits and prevents a quantum computer from working flawlessly,” says Elsa Mannila, who researched quasiparticles at Aalto University before moving to the Finnish Research Centre for Technology (VTT).
Some theories hypothesize that for example particles from cosmic atmospheric radiation, for example, that cause the interference. But since we don’t know for sure, it would be valuable to know why and how Cooper pairs break down, which could ultimately lead to learning how to build better superconductors that don’t get disturbed so often.
Measures single electrons
Researchers from Lund University, Aalto, and VTT (the Technical Research Centre of Finland) therefore set up an experiment to investigate in real-time when Cooper pairs split.
“It is difficult to measure what happens inside a superconductor, but a new technique simplifies things by opening a ‘tap’ and letting the unbound electrons jump out to a copper conductor, allowing us to count the unbound electrons with a charge detector,” says Peter Samuelsson, professor and principal investigator at NanoLund.
The researchers found that Cooper pairs often break up in bursts – several at a time – leading to very short bursts of quasiparticles followed by long periods of silence, when the superconductor is completely free of quasiparticles.
Fewer bursts over time
The picture that emerges is that there is mostly silence, but occasionally one or more Cooper pairs break up, leading to an explosion of events. The silent periods lasted much longer than the bursts of quasiparticles. In previous experiments, the average number of quasiparticles was measured over time, but it was not known what it looked like at each moment.
“Another finding was that the number of Cooper pairs that broke was much higher at the beginning of the experiment, which lasted 100 days. Over time, the burst rate decreased,” says Ville Maisi.
Why this is the case is not yet known, but it could lead to knowledge about where the energy to break the Cooper pairs comes from. For example, it seems strange if the disruptions were cosmic, as those disruptions should not diminish over time.
“The fact that there is so much going on, in the beginning, maybe due to impurities in the materials. Contaminants cool down very slowly, leading to small changes in the system over time,” says Professor Jukka Pekola at Aalto University.
Taking advantage of periods of silence
Today, there are perturbations and errors in the calculations of quantum computers that require many experiments to be performed because of the perturbations, and these experiments must then be statistically compared to find the most likely result.
“If you could take advantage of the periods of silence and avoid the interference, you could cherry-pick and run an almost perfect quantum computer,” says Peter Samuelsson.