Entanglement of a qubit with a single oscillator mode

We solve a model of a qubit strongly coupled to a massive environmental oscillator mode where the qubit backaction is treated exactly. Using a Ginzburg-Landau formalism, we derive an effective action for this well known localization transition. An entangled state emerges as an instanton in the collective qubit-environment degree of freedom and the resulting model is shown to be formally equivalent to a Fluctuating Gap Model (FGM) of a disordered Peierls chain. Below the transition, spectral weight is transferred to an exponentially small energy scale leaving the qubit coherent but damped. Unlike the spin-boson model, coherent and effectively localized behaviors may coexist.Comment: 4 pages, 1 figure; added calculation of entanglement entrop

Towards non-classical light storage via atomic-vapor Raman scattering

We present experimental work that investigates whether quantum information carried by light can be stored via reversible mapping of the quantum state of such light onto a collective atomic coherence. Such a quantum memory could be utilized to allow quantum communication over long, lossy channels. Current efforts concentrate on writing a photon-number-squeezed state of light onto a collective coherence between the ground-state hyperfine levels of 87Rb atoms in a warm vapor cell, and subsequent on-demand retrieval of this light. In this approach, intensity squeezing between the written and retrieved photon fields provides evidence for storage of a photon-number-squeezed state of light. The scheme is based on spontaneous Raman transitions that create the atomic coherence, and at the same time convert control fields into signal fields that propagate under conditions of electromagnetically induced transparency. We present experimental results demonstrating the storage and retrieval of light using this method, and discuss techniques for measuring intensity squeezing between these photon fields

An asymptotical von-Neumann measurement strategy for solid-state qubits

A measurement on a macroscopic quantum system does in general not lead to a projection of the wavefunction in the basis of the detector as predicted by von-Neumann's postulate. Hence, it is a question of fundametal interest, how the preferred basis onto which the state is projected is selected out of the macroscopic Hilbert space of the system. Detector-dominated von-Neumann measurements are also desirable for both quantum computation and verification of quantum mechanics on a macroscopic scale. The connection of these questions to the predictions of the spin-boson modelis outlined. I propose a measurement strategy, which uses the entanglement of the qubit with a weakly damped harmonic oscillator. It is shown, that the degree of entanglement controls the degree of renormalization of the qubit and identify, that this is equivalent to the degree to which the measurement is detector-dominated. This measurement very rapidly decoheres the initial state, but the thermalization is slow. The implementation in Josephson quantum bits is described and it is shown that this strategy also has practical advantages for the experimental implementation.Comment: 4 pages, 3 figures, accepted for publication as a rapid communication in Phys. Rev.

Decoherence by a nonlinear environment: canonical vs. microcanonical case

We compare decoherence induced in a simple quantum system (qubit) for two different initial states of the environment: canonical (fixed temperature) and microcanonical (fixed energy), for the general case of a fully interacting oscillator environment. We find that even a relatively compact oscillator bath (with the effective number of degrees of freedom of order 10), initially in a microcanonical state, will typically cause decoherence almost indistinguishable from that by a macroscopic, thermal environment, except possibly at singularities of the environment's specific heat (critical points). In the latter case, the precise magnitude of the difference between the canonical and microcanonical results depends on the critical behavior of the dissipative coefficient, characterizing the interaction of the qubit with the environment.Comment: 18 pages, revtex, 2 figures; minor textual changes, corrected typo in eq. (53) (v2); textual changes, mostly in the introduction (v3

Engineering the quantum measurement process for the persistent current qubit

The SQUID used to measure the flux state of a superconducting flux-based qubit interacts with the qubit and transmits its environmental noise to the qubit, thus causing the relaxation and dephasing of the qubit state. The SQUID–qubit system is analyzed and the effect of the transmittal of environmental noise is calculated. The method presented can also be applied to other quantum systems

Coherent dynamics of a Josephson charge qubit

We have fabricated a Josephson charge qubit by capacitively coupling a single-Cooper-pair box (SCB) to an electrometer based upon a single-electron transistor configured for radio-frequency readout (RF-SET). Charge quantization of 2e is observed and microwave spectroscopy is used to extract the Josephson and charging energies of the box. We perform coherent manipulation of the SCB by using very fast DC pulses and observe quantum oscillations in time of the charge that persist to ~=10ns. The observed contrast of the oscillations is high and agrees with that expected from the finite E_J/E_C ratio and finite rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100% initial charge state polarization. We also present a method to determine the relaxation time T_1 when it is shorter than the measurement time T_{meas}.Comment: accepted for publication in Phys. Rev.

Decoherence of a Superposition of Macroscopic Current States in a SQUID

We show that fundamental conservation laws mandate parameter-free mechanisms of decoherence of quantum oscillations of the superconducting current between opposite directions in a SQUID -- emission of phonons and photons at the oscillation frequency. The corresponding rates are computed and compared with experimental findings. The decohering effects of external mechanical and magnetic noise are investigated