300 research outputs found
Teleportation is necessary for faithful quantum state transfer through noisy channels of maximal rank
Quantum teleportation enables deterministic and faithful transmission of
quantum states, provided a maximally entangled state is pre-shared between
sender and receiver, and a one-way classical channel is available. Here, we
prove that these resources are not only sufficient, but also necessary, for
deterministically and faithfully sending quantum states through any fixed noisy
channel of maximal rank, when a single use of the cannel is admitted. In other
words, for this family of channels, there are no other protocols, based on
different (and possibly cheaper) sets of resources, capable of replacing
quantum teleportation.Comment: 4 pages, comments are welcom
Distillation of mixed-state continuous-variable entanglement by photon subtraction
We present a detailed theoretical analysis for the distillation of one copy
of a mixed two-mode continuous-variable entangled state using beamsplitters and
coherent photon-detection techniques, including conventional on-off detectors
and photon number resolving detectors. The initial Gaussian mixed-entangled
states are generated by transmitting a two-mode squeezed state through a lossy
bosonic channel, corresponding to the primary source of errors in current
approaches to optical quantum communication. We provide explicit formulas to
calculate the entanglement in terms of logarithmic negativity before and after
distillation, including losses in the channel and the photon detection, and
show that one-copy distillation is still possible even for losses near the
typical fiber channel attenuation length. A lower bound for the transmission
coefficient of the photon-subtraction beamsplitter is derived, representing the
minimal value that still allows to enhance the entanglement.Comment: 13 pages, 8 figure
Building Gaussian Cluster States by Linear Optics
The linear optical creation of Gaussian cluster states, a potential resource
for universal quantum computation, is investigated. We show that for any
Gaussian cluster state, the canonical generation scheme in terms of QND-type
interactions, can be entirely replaced by off-line squeezers and beam
splitters. Moreover, we find that, in terms of squeezing resources, the
canonical states are rather wasteful and we propose a systematic way to create
cheaper states. As an application, we consider Gaussian cluster computation in
multiple-rail encoding. This encoding may reduce errors due to finite
squeezing, even when the extra rails are achieved through off-line squeezing
and linear optics.Comment: 5 Pages, 3 figure
Practical effects in the preparation of cluster states using weak non-linearities
We discuss experimental effects in the implementation of a recent scheme for
performing bus mediated entangling operations between qubits. Here a bus mode,
a strong coherent state, successively undergoes weak Kerr-type non-linear
interactions with qubits. A quadrature measurement on the bus then projects the
qubits into an entangled state. This approach has the benefit that entangling
gates are non-destructive, may be performed non-locally, and there is no need
for efficient single photon detection. In this paper we examine practical
issues affecting its experimental implementation. In particular, we analyze the
effects of post-selection errors, qubit loss, bus loss, mismatched coupling
rates and mode-mismatch. We derive error models for these effects and relate
them to realistic fault-tolerant thresholds, providing insight into realistic
experimental requirements.Comment: 8 pages, 5 figure
Experiment towards continuous-variable entanglement swapping: Highly correlated four-partite quantum state
We present a protocol for performing entanglement swapping with intense
pulsed beams. In a first step, the generation of amplitude correlations between
two systems that have never interacted directly is demonstrated. This is
verified in direct detection with electronic modulation of the detected
photocurrents. The measured correlations are better than expected from a
classical reconstruction scheme. In the entanglement swapping process, a
four--partite entangled state is generated. We prove experimentally that the
amplitudes of the four optical modes are quantum correlated 3 dB below shot
noise, which is due to the potential four--party entanglement.Comment: 9 pages, 10 figures, update of references 9 and 10; minor
inconsistency in notation removed; format for units in the figures change
Graphical calculus for Gaussian pure states
We provide a unified graphical calculus for all Gaussian pure states,
including graph transformation rules for all local and semi-local Gaussian
unitary operations, as well as local quadrature measurements. We then use this
graphical calculus to analyze continuous-variable (CV) cluster states, the
essential resource for one-way quantum computing with CV systems. Current
graphical approaches to CV cluster states are only valid in the unphysical
limit of infinite squeezing, and the associated graph transformation rules only
apply when the initial and final states are of this form. Our formalism applies
to all Gaussian pure states and subsumes these rules in a natural way. In
addition, the term "CV graph state" currently has several inequivalent
definitions in use. Using this formalism we provide a single unifying
definition that encompasses all of them. We provide many examples of how the
formalism may be used in the context of CV cluster states: defining the
"closest" CV cluster state to a given Gaussian pure state and quantifying the
error in the approximation due to finite squeezing; analyzing the optimality of
certain methods of generating CV cluster states; drawing connections between
this new graphical formalism and bosonic Hamiltonians with Gaussian ground
states, including those useful for CV one-way quantum computing; and deriving a
graphical measure of bipartite entanglement for certain classes of CV cluster
states. We mention other possible applications of this formalism and conclude
with a brief note on fault tolerance in CV one-way quantum computing.Comment: (v3) shortened title, very minor corrections (v2) minor corrections,
reference added, new figures for CZ gate and beamsplitter graph rules; (v1)
25 pages, 11 figures (made with TikZ
Entanglement properties of optical coherent states under amplitude damping
Through concurrence, we characterize the entanglement properties of optical
coherent-state qubits subject to an amplitude damping channel. We investigate
the distillation capabilities of known error correcting codes and obtain upper
bounds on the entanglement depending on the non-orthogonality of the coherent
states and the channel damping parameter. This work provides a first, full
quantitative analysis of these photon-loss codes which are naturally
reminiscent of the standard qubit codes against Pauli errors.Comment: 7 pages, 6 figures. Revised version with small corrections; main
results remain unaltere
Universal Quantum Computation with Continuous-Variable Cluster States
We describe a generalization of the cluster-state model of quantum
computation to continuous-variable systems, along with a proposal for an
optical implementation using squeezed-light sources, linear optics, and
homodyne detection. For universal quantum computation, a nonlinear element is
required. This can be satisfied by adding to the toolbox any single-mode
non-Gaussian measurement, while the initial cluster state itself remains
Gaussian. Homodyne detection alone suffices to perform an arbitrary multi-mode
Gaussian transformation via the cluster state. We also propose an experiment to
demonstrate cluster-based error reduction when implementing Gaussian
operations.Comment: 4 pages, no figure
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