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

Transition Metal Complexes of a-Naphthylamine Dithiocarbamate

a-Naphthylamine dithiocarbamate and its complexes with Co(II), Ni(U), Cu(II), Ru(III) , Rh(III), Pd(II), Pt(IV), Zn(II), Cd(II) and Hg(II) have been prepared and characterized by chemical analysis, IR - and reflectance spectral studies and magnetic susceptibili ty measurements. In all these complexes the dithiocarbamato moiety acts as a chelate. The Ni(II), Cu(II) and Pd(Il) complexes have been found to be square planar while those of Ru(III), Rh(III) and Pt(IV) were proposed to be octahedral in nature. The Co(II) ion seems to have a tetrahedral geometry, unlike the other known square planar dithiocarbamato complexes of Co(II). No definite structure, however, could be proposed for Zn(II), Cd(II) and Hg(II) on the basis of limited studies

An RF-Driven Josephson Bifurcation Amplifier for Quantum Measurements

We have constructed a new type of amplifier whose primary purpose is the readout of superconducting quantum bits. It is based on the transition of an RF-driven Josephson junction between two distinct oscillation states near a dynamical bifurcation point. The main advantages of this new amplifier are speed, high-sensitivity, low back-action, and the absence of on-chip dissipation. Pulsed microwave reflection measurements on nanofabricated Al junctions show that actual devices attain the performance predicted by theory.Comment: 5 Figure

Quantum Fluctuations in the Chirped Pendulum

An anharmonic oscillator when driven with a fast, frequency chirped voltage pulse can oscillate with either small or large amplitude depending on whether the drive voltage is below or above a critical value-a well studied classical phenomenon known as autoresonance. Using a 6 GHz superconducting resonator embedded with a Josephson tunnel junction, we have studied for the first time the role of noise in this non-equilibrium system and find that the width of the threshold for capture into autoresonance decreases as the square root of T, and saturates below 150 mK due to zero point motion of the oscillator. This unique scaling results from the non-equilibrium excitation where fluctuations, both quantum and classical, only determine the initial oscillator motion and not its subsequent dynamics. We have investigated this paradigm in an electrical circuit but our findings are applicable to all out of equilibrium nonlinear oscillators.Comment: 5 pages, 4 figure

Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise

We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^\alpha$ fit to the measured flux noise spectral density below 1 Hz.Comment: 5 pages, 4 figures. Final journal versio

RF bifurcation of a Josephson junction: microwave embedding circuit requirements

A Josephson tunnel junction which is RF-driven near a dynamical bifurcation point can amplify quantum signals. The bifurcation point will exist robustly only if the electrodynamic environment of the junction meets certain criteria. In this article we develop a general formalism for dealing with the non-linear dynamics of 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 need to present a resonance at a particular frequency $\omega_{R}$, with the quality factor $Q$ of the resonance and the participation ratio $p$ of the junction satisfying $Qp\gg 1$, b) the drive frequency should be low frequency detuned away from $\omega_{R}$ by more than $\sqrt{3}\omega_{R}/(2Q)$.Comment: Submitted to Phys. Rev. B, 12 pages, 6 figure

Proposal for generating and detecting multi-qubit GHZ states in circuit QED

We propose methods for the preparation and entanglement detection of multi-qubit GHZ states in circuit quantum electrodynamics. Using quantum trajectory simulations appropriate for the situation of a weak continuous measurement, we show that the joint dispersive readout of several qubits can be utilized for the probabilistic production of high-fidelity GHZ states. When employing a nonlinear filter on the recorded homodyne signal, the selected states are found to exhibit values of the Bell-Mermin operator exceeding 2 under realistic conditions. We discuss the potential of the dispersive readout to demonstrate a violation of the Mermin bound, and present a measurement scheme avoiding the necessity for full detector tomography.Comment: 9 pages, 5 figure