Vladimir Manucharyan

We harness the phenomenon of kinetic inductance of a superconductor for the purposes of quantum information processing with superconducting circuits. In the present work, kinetic inductance of an array of Josephson tunnel junctions with carefully chosen parameters exceeds its geometric (magnetic) inductance by four orders of magnitude. Using such inductance, one can construct electrical circuits, in which quantum electrodynamics of charges and fluxes is governed by an effective fine structure constant value over a unity. We refer to this fundamentally new quantum circuit element as superinductance.

 

In order to experiment with superinductance we use it to shunt a small-capacitance Josephson tunnel junction to form a new superconducting artificial atom, dubbed fluxonium. We tune this atom using magnetic flux threading the fluxonium loop, and communicate with it by coupling the small junction capacitively to a microwave cavity, following a well-established approach of circuit quantum electrodynamics. With an adequate choice of junction parameters, the low energy spectrum of fluxonium is quite unique: it almost corresponds to the inductive energy of the array charged with an integer number of flux quanta. From the measurement of the transition spectrum of fluxonium we established that the junction array indeed behaves as a linear inductance of quoted magnitude for a range of frequencies exceeding 10 GHz. The quality factor of fluxonium transitions reaches 10 5, and our analysis show that it is likely not limited at present by the losses in the superinductance. Finally, inhomogeneous broadening of fluxonium transitions revealed the presence of coherent quantum phase-slip across the Josephson junction array. This phenomenon limits the lower operating frequency of a superinductance; in the present experiment this frequency was below 1 MHz and according to our analysis could easily be suppressed by several orders of magnitude after a small adjustment of the array junction parameters.