Enhanced Estimation of a Noisy Quantum Channel Using Entanglement

We discuss the estimation of channel parameters for a noisy quantum channel - the so-called Pauli channel - using finite resources. It turns out that prior entanglement considerably enhances the fidelity of the estimation when we compare it to an estimation scheme based on separable quantum states.Comment: 4 pages, 2 figure

Limits on Flavor Changing Neutral Currents in D^0 Meson Decays

Using the CLEO II detector at the Cornell Electron Storage Ring, we have searched for flavor changing neutral currents and lepton family number violations in D^0 meson decays. The upper limits on the branching fractions for D^0→ℓ^+ℓ^- and D^0→X^0ℓ^+ℓ^- are in the range 10^(-5) to 10^(-4), where X^0 can be a π^0, K_s^0, η, ρ^0, ω, K̅^(*0), or φ meson, and the ℓ^+ℓ^- pair can be e^+e^-, μ^+μ^-, or e^±μ^∓. Although these limits are above the theoretical predictions, most are new or an order of magnitude lower than previous limits

Conditional probabilities for a single photon at a beam splitter

Published versio

Enhanced Quantum Estimation via Purification

We analyze the estimation of a finite ensemble of quantum bits which have been sent through a depolarizing channel. Instead of using the depolarized qubits directly, we first apply a purification step and show that this improves the fidelity of subsequent quantum estimation. Even though we lose some qubits of our finite ensemble the information is concentrated in the remaining purified ones.Comment: 6 pages, including 3 figure

Quantum Nondemolition State Measurement via Atomic Scattering in Bragg Regime

We suggest a quantum nondemolition scheme to measure a quantized cavity field state using scattering of atoms in general Bragg regime. Our work extends the QND measurement of a cavity field from Fock state, based on first order Bragg deflection [9], to any quantum state based on Bragg deflection of arbitrary order. In addition a set of experimental parameters is provided to perform the experiment within the frame work of the presently available technology.Comment: 11 pages text, 4 eps figures, to appear in letter section of journal of physical society of Japa

Engineering Entanglement between two cavity modes

We present scheme for generation of entanglement between different modes of radiation field inside high-Q superconducting cavities. Our scheme is based on the interaction of a three-level atom with the cavity field for pre-calculated interaction times with each mode. This work enables us to generate complete set of Bell basis states and GHZ state

s-ordered phase-sum and phase-difference distribuitons of entangled coherent states

The $s$-ordered phase-sum and phase-difference distributions are considered for Bell-like superpositions of two-mode coherent states. The distributions are sensitive, respectively, to the sum and difference of the phases of the entangled coherent states. They show loss of information about the entangled state and may take on negative values for some orderings $s$.Comment: 8 pages, 2 figures, iopart. accepted for publication in J. Opt. B: Quantum Semiclass Op

Fresnel Representation of the Wigner Function: An Operational Approach

We present an operational definition of the Wigner function. Our method relies on the Fresnel transform of measured Rabi oscillations and applies to motional states of trapped atoms as well as to field states in cavities. We illustrate this technique using data from recent experiments in ion traps [D. M. Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)] and in cavity QED [B. Varcoe et al., Nature 403, 743 (2000)]. The values of the Wigner functions of the underlying states at the origin of phase space are W(0)=+1.75 for the vibrational ground state and W(0)=-1.4 for the one-photon number state. We generalize this method to wave packets in arbitrary potentials.Comment: 4 pages include 4 figures, submitted to PR

Controlling quantum entanglement through photocounts

We present a protocol to generate and control quantum entanglement between the states of two subsystems (the system ${\cal S}$) by making measurements on a third subsystem (the monitor ${\cal M}$), interacting with ${\cal S}$. For the sake of comparison we consider first an ideal, or instantaneous projective measurement, as postulated by von Neumann. Then we compare it with the more realistic or generalized measurement procedure based on photocounting on ${\cal M}$. Further we consider that the interaction term (between ${\cal S}$ and ${\cal M}$) contains a quantum nondemolition variable of ${\cal S}$ and discuss the possibility and limitations for reconstructing the initial state of ${\cal S}$ from information acquired by photocounting on ${\cal M}$.Comment: 12 pages, 3 figures, accepted for publication in Phys. Rev

Engineering cavity-field states by projection synthesis

We propose a reliable scheme for engineering a general cavity-field state. This is different from recently presented strategies,where the cavity is supposed to be initially empty and the field is built up photon by photon through resonant atom-field interactions. Here, a coherent state is previously injected into the cavity. So, the Wigner distribution function of the desired state is constructed from that of the initially coherent state. Such an engineering process is achieved through an adaptation of the recently proposed technique of projection synthesis to cavity QED phenomena.Comment: 5 ps pages plus 3 included figure