High efficiency tomographic reconstruction of quantum states by quantum nondemolition measurements

We propose a high efficiency tomographic scheme to reconstruct an unknown quantum state of the qubits by using a series of quantum nondemolition (QND) measurements. The proposed QND measurements of the qubits are implemented by probing the the stationary transmissions of the dispersively-coupled resonator. It is shown that only one kind of QND measurements is sufficient to determine all the diagonal elements of the density matrix of the detected quantum state. The remaining non-diagonal elements of the density matrix can be determined by other spectral measurements by beforehand transferring them to the diagonal locations using a series of unitary operations. Compared with the pervious tomographic reconstructions based on the usual destructively projective (DP) measurements (wherein one kind of such measurements could only determine one diagonal element of the density matrix), the present approach exhibits significantly high efficiency for N-qubit (N > 1). Specifically, our generic proposal is demonstrated by the experimental circuit-quantumelectrodynamics (circuit-QED) systems with a few Josephson charge qubits.Comment: 9pages,4figure

Balancing efficiencies by squeezing in realistic eight-port homodyne detection

We address measurements of covariant phase observables (CPOs) by means of realistic eight-port homodyne detectors. We do not assume equal quantum efficiencies for the four photodetectors and investigate the conditions under which the measurement of a CPO may be achieved. We show that balancing the efficiencies using an additional beam splitter allows us to achieve a CPO at the price of reducing the overall effective efficiency, and prove that it is never a smearing of the ideal CPO achievable with unit quantum efficiency. An alternative strategy based on employing a squeezed vacuum as a parameter field is also suggested, which allows one to increase the overall efficiency in comparison to the passive case using only a moderate amount of squeezing. Both methods are suitable for implementantion with current technology.Comment: 8 pages, 5 figures, revised versio

Entanglement and visibility at the output of a Mach-Zehnder interferometer

We study the entanglement between the two beams exiting a Mach-Zehnder interferometer fed by a couple of squeezed-coherent states with arbitrary squeezing parameter. The quantum correlations at the output are function of the internal phase-shift of the interferometer, with the output state ranging from a totally disentangled state to a state whose degree of entanglement is an increasing function of the input squeezing parameter. A couple of squeezed vacuum at the input leads to maximum entangled state at the output. The fringes visibilities resulting from measuring the coincidence counting rate or the squared difference photocurrent are evaluated and compared each other. Homodyne-like detection turns out to be preferable in almost all situations, with the exception of the very low signals regime.Comment: 6 figs, accepted for publication on PRA, see also http://enterprise.pv.infn.it/~pari

Tomographic measurements on superconducting qubit states

We propose an approach to reconstruct any superconducting charge qubit state by using quantum state tomography. This procedure requires a series of measurements on a large enough number of identically prepared copies of the quantum system. The experimental feasibility of this procedure is explained and the time scales for different quantum operations are estimated according to experimentally accessible parameters. Based on the state tomography, we also investigate the possibility of the process tomography.Comment: 12 pages, 4 figure

Linear amplification and quantum cloning for non-Gaussian continuous variables

We investigate phase-insensitive linear amplification at the quantum limit for single- and two-mode states and show that there exists a broad class of non-Gaussian states whose nonclassicality survives even at an arbitrarily large gain. We identify the corresponding observable nonclassical effects and find that they include, remarkably, two-mode entanglement. The implications of our results for quantum cloning outside the Gaussian regime are also addressed.Comment: published version with reference updat

Binary optical communication in single-mode and entangled quantum noisy channels

We address binary optical communication in single-mode and entangled quantum noisy channels. For single-mode we present a systematic comparison between direct photodetection and homodyne detection in realistic conditions, i.e. taking into account the noise that occurs both during the propagation and the detection of the signals. We then consider entangled channels based on twin-beam state of radiation, and show that with realistic heterodyne detection the error probability at fixed channel energy is reduced in comparison to the single-mode cases for a large range of values of quantum efficiency and noise parameters

Cloning of Gaussian states by linear optics

We analyze in details a scheme for cloning of Gaussian states based on linear optical components and homodyne detection recently demonstrated by U. L. Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated for a generic (pure or mixed) Gaussian state taking into account the effect of non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in most quantum information protocols the covariance matrix of the set of input states is not perfectly known, we evaluate the average cloning fidelity for classes of Gaussian states with the degree of squeezing and the number of thermal photons being only partially known.Comment: 8 pages, 7 figure

Generation of arbitrary quantum states of traveling fields

We show that any single-mode quantum state can be generated from the vacuum by alternate application of the coherent displacement operator and the creation operator. We propose an experimental implementation of the scheme for traveling optical fields, which is based on field mixings and conditional measurements in a beam splitter array, and calculate the probability of state generation.Comment: 1 Table and 2 Postscript figures, using Latex; modifications and changes in Figure 2, Table 1 and Eqs. 11-13,17,18,2

Renormalization of the nonequilibrium dynamics of fermions in a flat FRW universe

We derive the renormalized equations of motion and the renormalized energy-momentum tensor for fermions coupled to a spatially homogeneous scalar field (inflaton) in a flat FRW geometry. The fermion back reaction to the metric and to the inflaton field is formulated in one-loop approximation. Having determined the infinite counter terms in an $\bar{MS}$ scheme we formulate the finite terms in a form suitable for numerical computation. We comment on the trace anomaly which is inferred from the standard analysis. We also address the problem of initial singularities and determine the Bogoliubov transformation by which they are removed.Comment: 26 pages, LaTe

Local observables for entanglement witnesses

We present an explicit construction of entanglement witnesses for depolarized states in arbitrary finite dimension. For infinite dimension we generalize the construction to twin-beams perturbed by Gaussian noises in the phase and in the amplitude of the field. We show that entanglement detection for all these families of states requires only three local measurements. The explicit form of the corresponding set of local observables (quorom) needed for entanglement witness is derived.Comment: minor corrections, title change