In circuit quantum electrodynamics (cQED), microwave resonators are used to isolate, couple and readout multiple superconducting quantum bits. The traditional cQED architecture is a planar integrated circuit with resonators built from terminated coplanar-waveguide transmission lines. The realization of simple quantum algorithms and, very recently, basic quantum error correction using planar cQED attest to its versatility. However, a major challenge moving forward is extending the coherence time of quantum bits (qubits) in planar cQED, at present a few microseconds and the bottleneck to fidelity of quantum gate operations. Recently, the Schoelkopf group at Yale has demonstrated order-of-magnitude extensions in qubit coherence by replacing the coplanar-waveguide resonator with a three-dimensional superconducting cavity. This new approach, here dubbed cQ3D, minimizes the participation ratio of lossy surfaces and interfaces by storing most electromagnetic energy in vacuum. I will present our adoption of cQ3D at TU Delft with the long-run objective of realizing quantum feedback schemes with applications in quantum computing (for example, entanglement by measurement). I will present our ongoing characterization of two- and three-transmon-qubit cQ3D devices. Highlights include a maximum qubit relaxation time of 85 microseconds, a coherence time of 94 microseconds achieved by dynamical decoupling, and 93% single-shot readout.
Kavli Institute of Nanoscience, Delft (the Netherlands)