40,373 research outputs found
Error Filtration and Entanglement Purification for Quantum Communication
The key realisation which lead to the emergence of the new field of quantum
information processing is that quantum mechanics, the theory that describes
microscopic particles, allows the processing of information in fundamentally
new ways. But just as in classical information processing, errors occur in
quantum information processing, and these have to be corrected. A fundamental
breakthrough was the realisation that quantum error correction is in fact
possible. However most work so far has not been concerned with technological
feasibility, but rather with proving that quantum error correction is possible
in principle. Here we describe a method for filtering out errors and
entanglement purification which is particularly suitable for quantum
communication. Our method is conceptually new, and, crucially, it is easy to
implement in a wide variety of physical systems with present day technology and
should therefore be of wide applicability.Comment: 23 pages (latex) and 4 postscript figure
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
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
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
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
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
Confinement, Crossing Symmetry, and Glueballs
We suggest that the quark-confining force is related by crossing symmetry to
a color-singlet glueball which is well described as a loop of one
quantum of color magnetic flux. Electron pair annihilation as high as above the mass could produce accompanied by or one of its excited states.Comment: LaTeX, 12 pages, no figures, Los Alamos preprint LA-UR-94-263
Information-theoretic temporal Bell inequality and quantum computation
An information-theoretic temporal Bell inequality is formulated to contrast
classical and quantum computations. Any classical algorithm satisfies the
inequality, while quantum ones can violate it. Therefore, the violation of the
inequality is an immediate consequence of the quantumness in the computation.
Furthermore, this approach suggests a notion of temporal nonlocality in quantum
computation.Comment: v2: 5 pages, refereces added, discussion slightly revised, main
result unchanged. v3: typos correcte
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
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