Eliminating the Purcell Effect in Circuit QED

In circuit QED, it is desirable to have both a long qubit coherence time and a short microwave cavity lifetime in order to perform a high fidelity qubit measurement [1]. However, when a qubit is strongly coupled to a fast cavity, its lifetime is limited by spontaneous emission due to the multi-mode Purcell effect [2]. We present measurements of a new device in which the normal rate of spontaneous emission is reduced by more than an order of magnitude over a wide range of qubit frequencies. The single-shot readout fidelity of the transmon qubit in this device, which is strongly coupled (g/2$\pi \sim $300MHz) to a fast cavity ($\kappa $/2$\pi \sim $20MHz), will also be discussed. 1. Gambetta, Jay \textit{et al.} \textit{Phys. Rev. A }76, 012325 (2007). 2. Houck, A. A. \textit{et al}. \textit{Phys. Rev. Lett. }101, 080502 (2008).