8,402 research outputs found
Purification and detection of entangled coherent states
In [J. C. Howell and J. A. Yeazell, Phys. Rev. A 62, 012102 (2000)], a
proposal is made to generate entangled macroscopically distinguishable states
of two spatially separated traveling optical modes. We model the decoherence
due to light scattering during the propagation along an optical transmission
line and propose a setup allowing an entanglement purification from a number of
preparations which are partially decohered due to transmission. A purification
is achieved even without any manual intervention. We consider a nondemolition
configuration to measure the purity of the state as contrast of interference
fringes in a double-slit setup. Regarding the entangled coherent states as a
state of a bipartite quantum system, a close relationship between purity and
entanglement of formation can be obtained. In this way, the contrast of
interference fringes provides a direct means to measure entanglement.Comment: 9 pages, 6 figures, using Revtex
Entanglement purification of multi-mode quantum states
An iterative random procedure is considered allowing an entanglement
purification of a class of multi-mode quantum states. In certain cases, a
complete purification may be achieved using only a single signal state
preparation. A physical implementation based on beam splitter arrays and
non-linear elements is suggested. The influence of loss is analyzed in the
example of a purification of entangled N-mode coherent states.Comment: 6 pages, 3 eps-figures, using revtex
Secure Coherent-state Quantum Key Distribution Protocols with Efficient Reconciliation
We study the equivalence between a realistic quantum key distribution
protocol using coherent states and homodyne detection and a formal entanglement
purification protocol. Maximally-entangled qubit pairs that one can extract in
the formal protocol correspond to secret key bits in the realistic protocol.
More specifically, we define a qubit encoding scheme that allows the formal
protocol to produce more than one entangled qubit pair per coherent state, or
equivalently for the realistic protocol, more than one secret key bit. The
entanglement parameters are estimated using quantum tomography. We analyze the
properties of the encoding scheme and investigate its application to the
important case of the attenuation channel.Comment: REVTeX, 11 pages, 2 figure
Dark Bell states in tunnel-coupled spin qubits
We investigate the dynamical purification of maximally entangled electron
states by transport through coupled quantum dots. Under resonant ac driving and
coherent tunneling, even-parity Bell states perform Rabi oscillations that
decouple from the environment, leading to a dark state. The two electrons
remain spatially separated, one in each quantum dot. We propose configurations
where this effect will prove as antiresonances in transport spectroscopy
experiments.Comment: 5 pages, 4 figures + supplementary information. Published versio
Quantum Information Processing using coherent states in cavity QED
Using the highly detuned interaction between three-level -type atoms
and coherent optical fields, we can realize the C-NOT gates from atoms to
atoms, optical fields to optical fields, atoms to optical fields and optical
fields to atoms. Based on the realization of the C-NOT gates we propose an
entanglement purification scheme to purify a mixed entangled states of coherent
optical fields. The simplicity of the current scheme makes it possible that it
will be implemented in experiment in the near future.Comment: 5 pages, no figur
Test one to test many: a unified approach to quantum benchmarks
Quantum benchmarks are routinely used to validate the experimental
demonstration of quantum information protocols. Many relevant protocols,
however, involve an infinite set of input states, of which only a finite subset
can be used to test the quality of the implementation. This is a problem,
because the benchmark for the finitely many states used in the test can be
higher than the original benchmark calculated for infinitely many states. This
situation arises in the teleportation and storage of coherent states, for which
the benchmark of 50% fidelity is commonly used in experiments, although finite
sets of coherent states normally lead to higher benchmarks. Here we show that
the average fidelity over all coherent states can be indirectly probed with a
single setup, requiring only two-mode squeezing, a 50-50 beamsplitter, and
homodyne detection. Our setup enables a rigorous experimental validation of
quantum teleportation, storage, amplification, attenuation, and purification of
noisy coherent states. More generally, we prove that every quantum benchmark
can be tested by preparing a single entangled state and measuring a single
observable.Comment: 18 pages, 6 figures, updated affiliation
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