Ultrastrong coupling regime of cavity QED with phase-biased flux qubits

TL;DR

This paper proposes a superconducting circuit QED setup with flux qubits and coplanar waveguide resonators that achieves ultrastrong coupling, enabling exploration of new quantum phenomena and models like the E x B Jahn-Teller model.

Contribution

It introduces a method to reach ultrastrong coupling in circuit QED using phase-biased flux qubits with large kinetic inductance, surpassing previous coupling strengths.

Findings

Achieves coupling energies of tens of percent of the resonator frequency.

Demonstrates a coupling strength proportional to shared inductance, not qubit area.

Enables implementation of the E x B Jahn-Teller model on-chip.

Abstract

We theoretically study a circuit QED architecture based on a superconducting flux qubit directly coupled to the center conductor of a coplanar waveguide transmission-line resonator. As already shown experimentally [Abdumalikov et al. Phys. Rev. B 78, 180502 (2008)], the strong coupling regime of cavity QED can readily be achieved by optimizing the local inductance of the resonator in the vicinity of the qubit. In addition to yielding stronger coupling with respect to other proposals for flux qubit based circuit QED, this approach leads to a qubit-resonator coupling strength g which does not scale as the area of the qubit but is proportional to the total inductance shared between the resonator and the qubit. Strong coupling can thus be attained while still minimizing sensitivity to flux noise. Finally, we show that by taking advantage of the the large kinetic inductance of a Josephson junction in the center conductor of the resonator can lead to coupling energies of several tens of percent of the resonator frequency, reaching the ultrastrong coupling regime of cavity QED where the rotating-wave approximation breaks down. This should allow an on-chip implementation of the E x B Jahn-Teller model.