George A. Keefe

Superconducting qubit in a waveguide cavity with a coherence time approaching 0.1 ms

We report a superconducting artificial atom with a coherence time of

Improved superconducting qubit coherence using titanium nitride

{T}_{2}^{*}=92

50 Ω characteristic impedance low-pass metal powder filters

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High-T/sub c/ SQUID gradiometer for mobile magnetic anomaly detection

s and energy relaxation time

Bulk and surface loss in superconducting transmon qubits

{T}_{1}=70

Low-bandwidth control scheme for an oscillator stabilized Josephson qubit

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Quantum information storage using tunable flux qubits

s. The system consists of a single Josephson junction transmon qubit on a sapphire substrate embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.

Readout for phase qubits without Josephson junctions

We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ∼60 μs, by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared to qubits made with lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that surface losses from two-level system (TLS) defects residing at or near interfaces contribute to decoherence. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators, which is inconsistent with the predicted TLS model.