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Bulletin of the American Physical Society

Tuning the Dispersive Coupling Rate and Suppressing the Photon Shot Noise in a Superconducting Qubit

Gengyan Zhang, Srikanth J. Srinivasan, Yanbing Liu, Andrew Houck

Published Mar 3, 2014• 0 citations
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Quantum optics with superconducting qubits in the dispersive limit

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Bulletin of the American Physical Society•Mar 23, 2005•0 citations
Abstract We demonstrate the suppression of photon shot noise dephasing in a superconducting qubit by eliminating its dispersive coupling to the readout cavity. This is achieved in a tunable coupling qubit, where the qubit frequency and coupling rate can be controlled independently. We observe that the coherence time approaches twice the relaxation time and becomes less sensitive to thermal photon noise when the dispersive coupling rate is tuned from several MHz to 22 kHz. This work provides a promising building block in circuit quantum electrodynamics that can hold high coherence and be integrated into larger systems.
We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying in situ the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We observe large increases in coherence times as the cavity is decoupled from the environment, and after implementing filtering find that the intrinsic coherence of small Josephson junctions when corrected with a single Hahn echo is greater than several hundred microseconds. Similar filtering and thermalization may be important for other qubit designs in order to prevent photon shot noise from becoming the dominant source of dephasing.
We propose a circuit QED architecture that realizes the full anisotropic Rabi model (ARM) with in-situ tunability of its interaction symmetry. The circuit comprises a superconducting qubit coupled to a resonator through two simultaneous channels: a direct inductive path and a flux-tunable capacitive path mediated by a transmon coupler. By controlling the external magnetic flux, the destructive and constructive interferences between these paths continuously tune the ratio of the Jaynes-Cummings (JC) and anti-Jaynes-Cummings (AJC) couplings, sweeping the system across the entire ARM parameter space---from pure JC, through the standard Rabi regime, to pure AJC---without requiring rotating-wave approximations or external parametric drives. Furthermore, we identify a coherence sweet spot where the dispersive shift and photon shot-noise dephasing are suppressed. In the pure AJC regime, we demonstrate that the vacuum Rabi splitting vanishes and the Purcell decay rate is strongly mitigated while preserving high qubit readout contrast.