Irfan Siddiqi

Measuring the decoherence of a quantronium qubit with the cavity bifurcation amplifier

Dispersive readouts for superconducting qubits have the advantage of speed and minimal invasiveness. We have developed such an amplifier, the Cavity Bifurcation Amplifier (CBA) [10], and applied it to the readout of the quantronium qubit [2]. It consists of a Josephson junction embedded in a microwave on-chip resonator. In contrast with the Josephson bifurcation amplifier [17], which has an on-chip capacitor shunting a junction, the resonator is based on a simple coplanar waveguide imposing a pre-determined frequency and whose other RF characteristics like the quality factor are easily controlled and optimized. Under proper microwave irradiation conditions, the CBA has two metastable states. Which state is adopted by the CBA depends on the state of a quantronium qubit coupled to the CBAs junction. Due to the MHz repetition rate and large signal to noise ratio we can show directly that the coherence is limited by 1/f gate charge noise when biased at the sweet spot - a point insensitive to first order gate charge fluctuations. This architecture lends itself to scalable quantum computing using a multi-resonator chip with multiplexed readouts.

Microwave bifurcation of a Josephson junction: Embedding-circuit requirements

A Josephson tunnel junction which is rf driven near a dynamical bifurcation point can amplify quantum signals. However, the bifurcation point will exist robustly only if the electrodynamic environment of the junction meets certain criteria. We develop a general formalism for dealing with the nonlinear dynamics of a Josephson junction embedded in an arbitrary microwave circuit. We find sufficient conditions for the existence of the bifurcation regime: (a) the embedding impedance of the junction needs to present a resonance at a particular frequency

Aluminum hot-electron bolometer mixers at submillimeter wavelengths

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Noise and conversion efficiency of aluminum superconducting hot-electron bolometer mixer

, with the quality factor

Flux trapping experiments in single flux quantum shift registers

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Heterodyne mixing in diffusion-cooled superconducting aluminum hot-electron bolometers

of the resonance and the participation ratio

Integrated heterodyne array receivers for submillimeter astronomy

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Symmetric reflection line resonator and its quality factor modulation by a two-dimensional electron gas

of the junction satisfying

Nb–Au bilayer hot-electron bolometers for low-noise THz heterodyne detection

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The Josephson Bifurcation Amplifier for Quantum Measurements

, and (b) the drive frequency should be low frequency detuned away from