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 CBA's 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.Comment: 6 pages, 5 figures To be published in Physical Review

Circuit QED and engineering charge based superconducting qubits

The last two decades have seen tremendous advances in our ability to generate and manipulate quantum coherence in mesoscopic superconducting circuits. These advances have opened up the study of quantum optics of microwave photons in superconducting circuits as well as providing important hardware for the manipulation of quantum information. Focusing primarily on charge-based qubits, we provide a brief overview of these developments and discuss the present state of the art. We also survey the remarkable progress that has been made in realizing circuit quantum electrodynamics (QED) in which superconducting artificial atoms are strongly coupled to individual microwave photons.Comment: Proceedings of Nobel Symposium 141: Qubits for Future Quantum Informatio

Detecting charge noise with a Josephson junction: A problem of thermal escape in presence of non-Gaussian fluctuations

Motivated by several experimental activities to detect charge noise produced by a mesoscopic conductor with a Josephson junction as on-chip detector, the switching rate out of its zero-voltage state is studied. This process is related to the fundamental problem of thermal escape in presence of non-Gaussian fluctuations. In the relevant case of weak higher than second order cumulants, an effective Fokker-Planck equation is derived, which is then used to obtain an explicit expression for the escape rate. Specific results for the rate asymmetry due to the third moment of current noise allow to analyse experimental data and to optimize detection circuits.Comment: 4 pages, 1 figure; minor typos corrected, some revisions in the tex

Influence of Magnetic Field on Effective Electron-Electron Interactions in a Copper Wire

We have measured in a copper wire the energy exchange rate between quasiparticles as a function of the applied magnetic field. We find that the effective electron-electron interaction is strongly modified by the magnetic field, suggesting that magnetic impurities play a role on the interaction processes.Comment: latex anthore.tex, 8 files, 6 figures, 7 pages in: Proceedings of the XXXVIth Rencontres de Moriond `Electronic Correlations: From Meso- to Nano-physics' Les Arcs, France January 20-27, 2001 [SPEC-S01/027

Symmetries and collective excitations in large superconducting circuits

The intriguing appeal of circuits lies in their modularity and ease of fabrication. Based on a toolbox of simple building blocks, circuits present a powerful framework for achieving new functionality by combining circuit elements into larger networks. It is an open question to what degree modularity also holds for quantum circuits -- circuits made of superconducting material, in which electric voltages and currents are governed by the laws of quantum physics. If realizable, quantum coherence in larger circuit networks has great potential for advances in quantum information processing including topological protection from decoherence. Here, we present theory suitable for quantitative modeling of such large circuits and discuss its application to the fluxonium device. Our approach makes use of approximate symmetries exhibited by the circuit, and enables us to obtain new predictions for the energy spectrum of the fluxonium device which can be tested with current experimental technology

Introduction to Quantum Noise, Measurement and Amplification

The topic of quantum noise has become extremely timely due to the rise of quantum information physics and the resulting interchange of ideas between the condensed matter and AMO/quantum optics communities. This review gives a pedagogical introduction to the physics of quantum noise and its connections to quantum measurement and quantum amplification. After introducing quantum noise spectra and methods for their detection, we describe the basics of weak continuous measurements. Particular attention is given to treating the standard quantum limit on linear amplifiers and position detectors using a general linear-response framework. We show how this approach relates to the standard Haus-Caves quantum limit for a bosonic amplifier known in quantum optics, and illustrate its application for the case of electrical circuits, including mesoscopic detectors and resonant cavity detectors.Comment: Substantial improvements over initial version; include supplemental appendices

Charge sensitivity of the Inductive Single-Electron Transistor

We calculate the charge sensitivity of a recently demonstrated device where the Josephson inductance of a single Cooper-pair transistor is measured. We find that the intrinsic limit to detector performance is set by oscillator quantum noise. Sensitivity better than $10^{-6}$e$/\sqrt{\mathrm{Hz}}$ is possible with a high $Q$-value $\sim 10^3$, or using a SQUID amplifier. The model is compared to experiment, where charge sensitivity $3 \times 10^{-5}$e$/\sqrt{\mathrm{Hz}}$ and bandwidth 100 MHz are achieved.Comment: 3 page