Towards strong coupling of a single spin with a superconducting circuit: Flux qubits in 3D cavities.

The flux qubit is often considered as a major design for the future of quantum integrated circuits and its properties have triggered intense interest in the last decade [1-2]. This superconducting circuit behaves as a two-level system, each level being characterized by the direction of a macroscopic permanent current flowing in the loop of the qubit.  The permanent current, typically of the order of several hundreds of nAs, generates a large magnetic dipole, which offers interesting prospects for hybrid quantum circuits [3]. However, the flux qubit suffers from limited and irreproducible lifetimes which prevent these potential applications. Recently, a novel architecture where qubits are placed in a three dimensional cavity was introduced for transmon qubit [4]. It was shown that coherence properties can be greatly improved.

I will present the first measurements of flux qubits in a three dimensional cavity and show that they can reach long and more reproducible T1. The qubits were fabricated on a sapphire substrate and were measured by coupling them inductively to an on-chip superconducting resonator embedded in a three dimensional copper cavity. All the measured flux qubits exhibit an intrinsic T1 comprised between 5 and 13 us.

These long and reproducible depolarization times are a key element to reach the strong coupling limit with a single spin as suggested in [3]. I will describe our current experimental efforts towards this long term objective.

[1] I. Chiorescu et al., Science, 299, 5614 (2003).

[2] J. Bylander et al., Nature Physics, 7, 565 (2011).

[3] D. Marcos et al., PRL , 105, 210501 (2010).

[4] H. Paik et al., Phys. Rev. Lett., 107, 240501 (2011).