Researchers from SRON Netherlands Institute for Space Research and Delft University of Technology (TUD) have stumbled across a fundamental limit for superconductors in space detectors and quantum computers. During their research on a superconducting aluminium film, the researchers observed a phenomenon that contradicts the prevailing theory: at temperatures just above absolute zero quite a large number of electrons in a superconductor still appear to behave as individual particles. For superconductors these rebel electrons are disastrous. They cause quantum computers to lose their coherency and radiation detectors to be less sensitive. The researchers published their findings on 29 April in Physical Review Letters.
The phenomenon of superconductivity was discovered exactly 100 years ago by the Dutch physicist Heike Kamerlingh-Onnes. In superconducting materials, the electrical resistance theoretically disappears at very low temperatures because the electrons can no longer behave as individual particles. Consequently, superconducting materials can, in principle, be used to perform quantum calculations or be used as extremely sensitive detectors to measure cosmic radiation.
Researchers from SRON and TUD, who worked together on the development of extremely sensitive radiation detectors consisting of a superconductor and a resonant circuit, have now established a fundamental limit for the use of superconductors. They managed to measure the fluctuations in the number of electrons in a superconducting aluminium film and could therefore ‘count’ the number of electrons present. They discovered that just above absolute zero, a significant number of electrons continue to behave as individual particles. This phenomenon goes completely against the theory.
Researcher Pieter de Visser (SRON and TUD): ‘You would expect the electrons to form a Bose-Einstein condensate, which makes it impossible for them to behave as individual particles. However in our study, there is still a relatively large number of unpaired electrons at the lowest temperatures. That has annoying consequences for detectors: they are then less sensitive. Furthermore, due to this phenomenon quantum computers lose their coherence, as a result of which they no longer work.’
Now the question is: what makes these electrons rebel so that they disrupt the superconductivity? Group leader Jochem Baselmans (SRON): ‘The most obvious causes, such as stray light and cosmic background radiation have already been excluded from our experiment. So there is probably a more fundamental reason why the electrons do not pair up. An answer to this mystery is vital for the improvement of quantum bits, the building blocks of a quantum computer, and for the development of the extremely sensitive detectors needed for the next generation of space telescopes.’
The group of Baselmans and De Visser is working on the Kinetic Inductance Detectors (KIDs). These radiation detectors are being developed by the research group for a submillimetre camera of APEX, a test telescope from the ALMA telescope network currently under construction in Chile. A Memorandum of Understanding was signed for this in January by SRON, NOVA, Delft University of Technology and the Max Planck Insti¬tute for Radio Astronomy in Bonn. The new submillimetre camera works at two wavelengths, namely 350 and 850 microns. In principle, the KIDs are also suitable for detectors on space telescopes. For a long time, they were in the race for SAFARI, the European instrument on the Japanese space telescope SPICA (due to be launched in 2018).
The article Number fluctuations of sparse quasiparticles in a superconductor’, was published on 26 April 2011 in Physical Review Letters: Phys. Rev. Lett. 106, 167004 (2011) and can also be found at arXiv:1103.0758v2. The authors are Pieter de Visser (SRON, TUD), Jochem Baselmans (SRON), Pascale Diener (SRON), Stephen Yates (SRON), Akira Endo (TUD) and Teun Klapwijk (TUD).