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Vortex Traps for More Stable Qubits

20 January 2015

Scientists from Jülich and Yale have found a way to make quantum bits, or “qubits” for short, more stable. Qubits store and process information in quantum computers, which are expected to achieve significantly higher computing speeds for certain types of computational tasks than has up to now been possible using conventional digital computers.

Vortex TrapsImages of the qubit (top) and its main element, a Josephson junction, (bottom right). Illustration of the simplified model used in the calculations (bottom left).
Copyright: Forschungszentrum Jülich/Yale University

The qubits studied at Yale are made up of superconducting circuits. Superconducting materials are already employed in many devices, for instance in ultra-powerful electromagnets for use in magnetic resonance tomography and particle accelerators. What makes superconducting materials attractive in the fabrication of qubits is the fact that they dissipate very little energy. Loss of energy makes qubits unstable – they then lose the stored information. This dissipation of energy is caused by so-called quasiparticles, collective excitations of several particles which themselves behave in some aspects like particles.

Researchers have now discovered that quasiparticles can be trapped in magnetic vortices. Magnetic fields are usually excluded from superconductors, but in the form of a vortex, they are able to penetrate them. The magnetic vortices trap the quasiparticles similarly to how a leaf floating on water would be sucked into a whirlpool. The qubits then become much more stable. It was already presumed that this would be the case; researchers at Yale have now provided the experimental proof. Together with his Yale colleagues, Dr. Gianluigi Catelani of the Peter Grünberg Institute in Jülich developed a detailed theoretical model of the qubits used in these experiments, which was necessary for the quantitative analysis of the experimental data.

Original publication:

C. Wang et al.; Measurement and control of quasiparticle dynamics in a superconducting qubit
Nature communications (published online on 18.12.2014),
DOI: 10.1038/ncomms6836