The image visualizes the researchers’ method. The stream of blue particles represents pairs of superconducting electrons, whereas the red particles represent unpaired, single electrons. The microwave-driven pump filters out the unpaired electrons, leaving only superconducting pairs in the device. Illustration: Philip Krantz, Krantz NanoArt

New method leads one step closer to quantum computer

Researchers at MIT and Chalmers, with collaborators, have developed a new method to improve superconducting quantum electronic circuits, and thus get one step closer to the future quantum computers. The findings were published on 8 December in the renowned scientific journal Science, with Jonas Bylander, assistant professor at the Quantum Device Physics Laboratory at the Department of Microtechnology and Nanoscience – MC2 – at Chalmers as one of the co-authors.

In their work, the researchers at MIT (Massachusetts Institute of Technology) and Chalmers designed and applied a technique to pump unpaired electrons, called quasiparticles, away from an electronic device. The result is a three-fold improvement in performance and a substantial reduction of noise and time variations.
"We can now pump away the electron charges, quasiparticles, which are loose and disturb our qubit. This makes the qubit very stable, so that it becomes easier to use in a quantum computer. We made the life of a quantum state three times longer by pumping away the quasiparticles", says Jonas Bylander (picture below).

The idea of constructing a quantum computer – which can solve some complex computational problems that a conventional computer cannot – appeared in the early 1980s. A quantum computer uses quantum mechanical interference to perform computations. The basic unit of quantum computers is the quantum bit, or "qubit," which can be both one and zero simultaneously, in a "coherent superposition".

So far, quantum computing has been realized only in systems with a small number of qubits, but over the last ten years there has been a steady increase in the size and complexity of these devices.

Some of the most promising candidates for implementing a quantum computer involve superconducting circuits, which have now been made to serve 100,000 times better than when they were first invented. In a superconductor, cooled below its critical temperature, electrons pair up into so-called Cooper pairs allowing currents to flow without any resistance. This means that the circuits can be driven without any heat dissipation, which is crucial for maintaining quantum coherence.

Up until now, however, the performance of qubits in superconducting circuits has been limited by residual, left-over unpaired electrons (quasiparticles) present in the devices. These non-superconducting electrons jump around, and their charge disrupts the functioning of the circuit. By instead pumping away these quasiparticles, the life of a quantum state can be extended.
"We expect that these results will be important in the understanding of larger-scale superconducting circuits for quantum computing", says Jonas Bylander.

With the project are also researchers from the Peter Grünberg Institute in Germany, the University of California, US, The Institute of Physical and Chemical Research (RIKEN), the Center for Emergent Matter Science (CEMS) and the University of Tokyo in Japan.

Text and photo: Michael Nystås
Illustration: Philip Krantz, Krantz NanoArt

Read the article "Suppressing relaxation in superconducting qubits by quasiparticle pumping" in Science >>>

Published: Fri 09 Dec 2016. Modified: Fri 16 Dec 2016