36,324 research outputs found
A simple formula for pooling knowledge about a quantum system
When various observers obtain information in an independent fashion about a
classical system, there is a simple rule which allows them to pool their
knowledge, and this requires only the states-of-knowledge of the respective
observers. Here we derive an equivalent quantum formula. While its realm of
applicability is necessarily more limited, it does apply to a large class of
measurements, and we show explicitly for a single qubit that it satisfies the
intuitive notions of what it means to pool knowledge about a quantum system.
This analysis also provides a physical interpretation for the trace of the
product of two density matrices.Comment: 5 pages, Revtex
Quantum-dot-spin single-photon interface
Using background-free detection of spin-state-dependent resonance
fluorescence from a single-electron charged quantum dot with an efficiency of
0:1%, we realize a single spin-photon interface where the detection of a
scattered photon with 300 picosecond time resolution projects the quantum dot
spin to a definite spin eigenstate with fidelity exceeding 99%. The bunching of
resonantly scattered photons reveals information about electron spin dynamics.
High-fidelity fast spin-state initialization heralded by a single photon
enables the realization of quantum information processing tasks such as
non-deterministic distant spin entanglement. Given that we could suppress the
measurement back-action to well below the natural spin-flip rate, realization
of a quantum non-demolition measurement of a single spin could be achieved by
increasing the fluorescence collection efficiency by a factor exceeding 20
using a photonic nanostructure
Entanglement Witnesses from Single-Particle Interference
We describe a general method of realizing entanglement witnesses in terms of
the interference pattern of a single quantum probe. After outlining the
principle, we discuss specific realizations both with electrons in mesoscopic
Aharonov-Bohm rings and with photons in standard Young's double-slit or
coherent-backscattering interferometers.Comment: 5 pages, 3 figures, epl2, uses pstricks.st
Charge qubits and limitations of electrostatic quantum gates
We investigate the characteristics of purely electrostatic interactions with
external gates in constructing full single qubit manipulations. The quantum bit
is naturally encoded in the spatial wave function of the electron system.
Single-electron{transistor arrays based on quantum dots or insulating
interfaces typically allow for electrostatic controls where the inter-island
tunneling is considered constant, e.g. determined by the thickness of an
insulating layer. A representative array of 3x3 quantum dots with two mobile
electrons is analyzed using a Hubbard Hamiltonian and a capacitance matrix
formalism. Our study shows that it is easy to realize the first quantum gate
for single qubit operations, but that a second quantum gate only comes at the
cost of compromising the low-energy two-level system needed to encode the
qubit. We use perturbative arguments and the Feshbach formalism to show that
the compromising of the two-level system is a rather general feature for
electrostatically interacting qubits and is not just related to the specific
details of the system chosen. We show further that full implementation requires
tunable tunneling or external magnetic fields.Comment: 7 pages, 5 figures, submitted to PR
Spotlighting quantum critical points via quantum correlations at finite temperatures
We extend the program initiated in [T. Werlang et al., Phys. Rev. Lett. 105,
095702 (2010)] in several directions. Firstly, we investigate how useful
quantum correlations, such as entanglement and quantum discord, are in the
detection of critical points of quantum phase transitions when the system is at
finite temperatures. For that purpose we study several thermalized spin models
in the thermodynamic limit, namely, the XXZ model, the XY model, and the Ising
model, all of which with an external magnetic field. We compare the ability of
quantum discord, entanglement, and some thermodynamic quantities to spotlight
the quantum critical points for several different temperatures. Secondly, for
some models we go beyond nearest-neighbors and also study the behavior of
entanglement and quantum discord for second nearest-neighbors around the
critical point at finite temperature. Finally, we furnish a more quantitative
description of how good all these quantities are in spotlighting critical
points of quantum phase transitions at finite T, bridging the gap between
experimental data and those theoretical descriptions solely based on the
unattainable absolute zero assumption.Comment: 11 pages, 12 figures, RevTex4-1; v2: published versio
Generation of Werner states via collective decay of coherently driven atoms
We show deterministic generation of Werner states as a steady state of the
collective decay dynamics of a pair of neutral atom coupled to a leaky cavity
and strong coherent drive. We also show how the scheme can be extended to
generate -particle analogue of the bipartite Werner states.Comment: 4 pages, 1 figur
Quantum discord and non-Markovianity of quantum dynamics
The problem of recognizing (non-)Markovianity of a quantum dynamics is
revisited through analyzing quantum correlations. We argue that
instantaneously-vanishing quantum discord provides a necessary and sufficient
condition for Markovianity of a quantum map. This is used to introduce a
measure of non-Markovianity. This measure, however, requires demanding
knowledge about the system and the environment. By using a quantum correlation
monogamy property and an ancillary system, we propose a simplified measure with
less requirements. Non-Markovianity is thereby decided by quantum state
tomography of the system and the ancilla.Comment: 5 pages, 3 figure
Cavity-mediated long-range interaction for fast multiqubit quantum logic operations
Interactions among qubits are essential for performing two-qubit quantum
logic operations. However, nature gives us only nearest neighbor interactions
in simple and controllable settings. Here we propose a strategy to induce
interactions among two atomic entities that are not necessarily neighbors of
each other through their common coupling with a cavity field. This facilitates
fast multiqubit quantum logic operations through a set of two-qubit operations.
The ideas presented here are applicable to various quantum computing proposals
for atom based qubits such as, trapped ions, atoms trapped in optical cavities
and optical lattices.Comment: 10 pages, 3 figure
Coherent control of atomic spin currents in a double well
We propose an experimental feasible method for controlling the atomic
currents of a two-component Bose-Einstein condensate in a double well by
applying an external field to the atoms in one of the potential wells. We study
the ground-state properties of the system and show that the directions of spin
currents and net-particle tunneling can be manipulated by adiabatically varying
the coupling strength between the atoms and the field. This system can be used
for studying spin and tunneling phenomena across a wide range of interaction
parameters. In addition, spin-squeezed states can be generated. It is useful
for quantum information processing and quantum metrology.Comment: 6 pages, 7 figures, minor revisio
Optimal estimation of one parameter quantum channels
We explore the task of optimal quantum channel identification, and in
particular the estimation of a general one parameter quantum process. We derive
new characterizations of optimality and apply the results to several examples
including the qubit depolarizing channel and the harmonic oscillator damping
channel. We also discuss the geometry of the problem and illustrate the
usefulness of using entanglement in process estimation.Comment: 23 pages, 4 figures. Published versio
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