Researchers in Spain and Italy have constructed the first-ever quantum phase battery – a device that maintains a phase difference between two points in a superconducting circuit. The battery, which consists of an indium arsenide (InAs) nanowire in contact with aluminium (Al) superconducting leads, could be used in quantum computing circuits. It might also find applications in magnetometry and highly sensitive detectors based on superconductors.
The concept of a quantum phase battery was studied theoretically in 2015 by Sebastián Bergeret of the Material Physics Center (CFM-CSIC) and Ilya Tokatly at the University of the Basque Country in Donostia-San Sebastián, Spain. Their battery design comprised a combination of superconducting and magnetic materials and was based on a Josephson junction – a non-superconducting region through which the Cooper pairs responsible for superconductivity can tunnel. This semiconducting “weak link” provides a persistent phase difference between the superconductors in the circuit, similar to the way that a classical battery provides a persistent voltage drop in an electronic circuit. Thanks to this phase difference, a superconducting current (that is, a current with zero dissipation) flows when the junction is embedded in the superconducting circuit.
The battery works using the so-called anomalous Josephson effect. In a normal Josephson junction, a superconducting current flows whenever there is a phase difference between two ends of the junction. Such a phase difference can be induced, for example, by placing the junction in a superconducting loop and applying a magnetic field
In contrast, a phase battery provides such a phase difference without the need for this external magnetic field. Instead, the difference is induced by a geometrical effect that involves the interplay between three phenomena: superconductivity; spin-orbit coupling (which describes the interaction between the intrinsic spin of an electron in a solid and the magnetic field induced by the motion of electron); and a magnetic exchange field (which acts on magnetic spins just like an ordinary magnetic field)..
While the battery needs to be charged by polarizing the magnetic impurities with an external magnetic field, the researchers say the phase bias between the poles persists even when they later switch the field off. They can control the value of the phase difference either by changing the direction of the polarization of the magnetic impurities or by modifying the length of the semiconducting wires.
The Pisa/San Sebastián research teams are also involved in a European project called SUPERTED that aims to engineer ultrasensitive radiation detectors based on superconductor-ferromagnetic insulator structures, and in particular EuS/Al junctions.