Probing a transmon qubit via the ultra-strong coupling to a Josephson waveguide. (arXiv:1802.00633v1 [cond-mat.mes-hall] CROSS LISTED)

Exploring the quantum world often starts by drawing a sharp boundary between
a microscopic subsystem and the bath to which it is invariably coupled. In most
cases, knowledge of the physical processes occuring in the bath is not required
in great detail. However, recent developments in circuit quantum
electrodynamics are presenting regimes where the actual dynamics of engineered
baths, such as microwave photon resonators, becomes relevant. Here we take a
major technological step forward, by tailoring a centimeter-scale on-chip bath
from a very long metamaterial made of 4700 tunable Josephson junctions. By
monitoring how each measurable bosonic resonance of the circuit acquires a
phase-shift due to its interaction with a transmon qubit, one can indirectly
measure qubit properties, such as transition frequency, linewidth and
non-linearity. This new platform also demonstrates the ultra-strong coupling
regime for the first time in the context of Josephson waveguides. Our device
combines a large number of modes (up to 10 in the present setup) that are
simultaneously hybridised with the two-level system, and a broadening dominated
by the artificial environment that is a sizeable fraction of the qubit
transition frequency. Finally, we provide a quantitative and parameter-free
model of this large quantum system, and show that the finite environment seen
by the qubit is equivalent to a truly macroscopic bath.

Article web page: