Fig. 1: Cavityless cQED with fluxonium qubits.
From: Electron shelving of a superconducting artificial atom
a Electron microscope image of fluxonium’s superconducting loop, made of one smaller-area Josephson junction and an array of larger-area junctions. Top inset: fluxonium’s effective three elements circuit model. Bottom inset: the smaller-area junction is connected to bow-tie antenna capacitance (optical image). b Effective potential seen by the phase-difference ϕ across the smaller-area junction (black), energy levels (horizontal black lines), and wavefunctions (gray) at the half-integer external flux through the loop, ϕext = π. The qubit levels are \(\left|0\right\rangle\) and \(\left|1\right\rangle\), the readout transition is \(\left|0\right\rangle -\left|3\right\rangle\), and even transitions are dipole-forbidden. c Setup for a broadband wireless coupling of fluxonium circuit to an external coaxial cable (not shown). d Transition frequencies and corresponding matrix elements of the Cooper pair number operator −i∂ϕ (colored stars). The solid line shows microwave transmission through the enclosure measured using a symmetric two-port configuration. Single-port configuration is used for all other measurements, as shown in Fig. 1c.
