5,243 research outputs found
Quantum copying can increase the practically available information
While it is known that copying a quantum system does not increase the amount
of information obtainable about the originals, it may increase the amount
available in practice, when one is restricted to imperfect measurements. We
present a detection scheme which using imperfect detectors, and possibly noisy
quantum copying machines (that entangle the copies), allows one to extract more
information from an incoming signal, than with the imperfect detectors alone.
The case of single-photon detection with noisy, inefficient detectors and
copiers (single controlled-NOT gates in this case) is investigated in detail.
The improvement in distinguishability between a photon and vacuum is found to
occur for a wide range of parameters, and to be quite robust to random noise.
The properties that a quantum copying device must have to be useful in this
scheme are investigated.Comment: 10 pages, 6 figures, accepted PR
Entanglement Manipulation and Concentration
We introduce a simple, experimentally realisable, entanglement manipulation
protocol for exploring mixed state entanglement. We show that for both
non-maximally entangled pure, and mixed polarisation-entangled two qubit
states, an increase in the degree of entanglement and purity, which we define
as concentration, is achievable.Comment: Accepted as Rapid Communication PR
A near deterministic linear optical CNOT gate
We show how to construct a near deterministic CNOT using several single
photons sources, linear optics, photon number resolving quantum non-demolition
detectors and feed-forward. This gate does not require the use of massively
entangled states common to other implementations and is very efficient on
resources with only one ancilla photon required. The key element of this gate
are non-demolition detectors that use a weak cross-Kerr nonlinearity effect to
conditionally generate a phase shift on a coherent probe, if a photon is
present in the signal mode. These potential phase shifts can then be measured
using highly efficient homodyne detection.Comment: 4 pages, 3 figure
Teleportation using coupled oscillator states
We analyse the fidelity of teleportation protocols, as a function of resource
entanglement, for three kinds of two mode oscillator states: states with fixed
total photon number, number states entangled at a beam splitter, and the
two-mode squeezed vacuum state. We define corresponding teleportation protocols
for each case including phase noise to model degraded entanglement of each
resource.Comment: 21 pages REVTeX, manuscript format, 7 figures postscript, many
changes to pape
Quantum Computation with Coherent States, Linear Interactions and Superposed Resources
We show that quantum computation circuits with coherent states as the logical
qubits can be constructed using very simple linear networks, conditional
measurements and coherent superposition resource states
Entangled Coherent State Qubits in an Ion Trap
We show how entangled qubits can be encoded as entangled coherent states of
two-dimensional centre-of-mass vibrational motion for two ions in an ion trap.
The entangled qubit state is equivalent to the canonical Bell state, and we
introduce a proposal for entanglement transfer from the two vibrational modes
to the electronic states of the two ions in order for the Bell state to be
detected by resonance fluorescence shelving methods.Comment: 4 pages, No figures, accepted to PRA, minor chang
Generating optical nonlinearity using trapped atoms
We describe a scheme for producing an optical nonlinearity using an
interaction with one or more ancilla two-level atomic systems. The
nonlinearity, which can be implemented using high efficiency fluorescence
shelving measurements, together with general linear transformations is
sufficient for simulating arbitrary Hamiltonian evolution on a Fock state
qudit. We give two examples of the application of this nonlinearity, one for
the creation of nonlinear phase shifts on optical fields as required in single
photon quantum computation schemes, and the other for the preparation of
optical Schrodinger cat states.Comment: Substantially extended from quant-ph/020815
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