80 research outputs found

### Conditional implementation of asymmetrical universal quantum cloning machine

We propose two feasible experimental implementations of an optimal asymmetric
1->2 quantum cloning of a polarization state of photon. Both implementations
are based on a partial and optimal reverse of recent conditional symmetrical
quantum cloning experiments. The reversion procedure is performed only by a
local measurement of one from the clones and ancilla followed by a local
operation on the other clone. The local measurement consists only of a single
unbalanced beam splitter followed in one output by a single photon detector and
the asymmetry of fidelities in the cloning is controlled by a reflectivity of
the beam splitter.Comment: 5 pages, 3 figures, accepted for pulication in PR

### Collective spin systems in dispersive optical cavity QED: Quantum phase transitions and entanglement

We propose a cavity QED setup which implements a dissipative
Lipkin-Meshkov-Glick model -- an interacting collective spin system. By varying
the external model parameters the system can be made to undergo both first-and
second-order quantum phase transitions, which are signified by dramatic changes
in cavity output field properties, such as the probe laser transmission
spectrum. The steady-state entanglement between pairs of atoms is shown to peak
at the critical points and can be experimentally determined by suitable
measurements on the cavity output field. The entanglement dynamics also
exhibits pronounced variations in the vicinities of the phase transitions.Comment: 19 pages, 18 figures, shortened versio

### Entanglement and bifurcations in Jahn-Teller models

We compare and contrast the entanglement in the ground state of two
Jahn-Teller models. The $E\otimes\beta$ system models the coupling of a
two-level electronic system, or qubit, to a single oscillator mode, while the
$E\otimes\epsilon$ models the qubit coupled to two independent, degenerate
oscillator modes. In the absence of a transverse magnetic field applied to the
qubit, both systems exhibit a degenerate ground state. Whereas there always
exists a completely separable ground state in the $E\otimes\beta$ system, the
ground states of the $E\otimes\epsilon$ model always exhibit entanglement. For
the $E\otimes\beta$ case we aim to clarify results from previous work, alluding
to a link between the ground state entanglement characteristics and a
bifurcation of a fixed point in the classical analogue. In the
$E\otimes\epsilon$ case we make use of an ansatz for the ground state. We
compare this ansatz to exact numerical calculations and use it to investigate
how the entanglement is shared between the three system degrees of freedom.Comment: 11 pages, 9 figures, comments welcome; 2 references adde

### Entanglement generation in persistent current qubits

In this paper we investigate the generation of entanglement between two
persistent current qubits. The qubits are coupled inductively to each other and
to a common bias field, which is used to control the qubit behaviour and is
represented schematically by a linear oscillator mode. We consider the use of
classical and quantum representations for the qubit control fields and how
fluctuations in the control fields tend to suppress entanglement. In
particular, we demonstrate how fluctuations in the bias fields affect the
entanglement generated between persistent current qubits and may limit the
ability to design practical systems.Comment: 7 pages, 4 figures, minor changes in reply to referees comment

### Interference in dielectrics and pseudo-measurements

Inserting a lossy dielectric into one arm of an interference experiment acts
in many ways like a measurement. If two entangled photons are passed through
the interferometer, a certain amount of information is gained about which path
they took, and the interference pattern in a coincidence count measurement is
suppressed. However, by inserting a second dielectric into the other arm of the
interferometer, one can restore the interference pattern. Two of these
pseudo-measurements can thus cancel each other out. This is somewhat analogous
to the proposed quantum eraser experiments.Comment: 7 pages RevTeX 3.0 + 2 figures (postscript). Submitted to Phys. Rev.

### Dissipation-driven quantum phase transitions in collective spin systems

We consider two different collective spin systems subjected to strong
dissipation -- on the same scale as interaction strengths and external fields
-- and show that either continuous or discontinuous dissipative quantum phase
transitions can occur as the dissipation strength is varied. First, we consider
a well known model of cooperative resonance fluorescence that can exhibit a
second-order quantum phase transition, and analyze the entanglement properties
near the critical point. Next, we examine a dissipative version of the
Lipkin-Meshkov-Glick interacting collective spin model, where we find that
either first- or second-order quantum phase transitions can occur, depending
only on the ratio of the interaction and external field parameters. We give
detailed results and interpretation for the steady state entanglement in the
vicinity of the critical point, where it reaches a maximum. For the first-order
transition we find that the semiclassical steady states exhibit a region of
bistability.Comment: 12 pages, 16 figures, removed section on homodyne spectr

### Einstein-Podolsky-Rosen paradox without entanglement

We claim that the nonlocality without entanglement revealed quite recently by
Bennett {\it et al.} [quant-ph/9804053] should be rather interpreted as {\it
Einstein-Podolsky-Rosen paradox without entanglement}. It would be true
nonlocality without entanglement if one knew that quantum mechnics provides the
best possible means for extracting information from physical system i.e. that
it is ``operationally complete''.Comment: RevTeX, 2 page

### Numerical simulation of information recovery in quantum computers

Decoherence is the main problem to be solved before quantum computers can be
built. To control decoherence, it is possible to use error correction methods,
but these methods are themselves noisy quantum computation processes. In this
work we study the ability of Steane's and Shor's fault-tolerant recovering
methods, as well a modification of Steane's ancilla network, to correct errors
in qubits. We test a way to measure correctly ancilla's fidelity for these
methods, and state the possibility of carrying out an effective error
correction through a noisy quantum channel, even using noisy error correction
methods.Comment: 38 pages, Figures included. Accepted in Phys. Rev. A, 200

### On 1-qubit channels

The entropy H_T(rho) of a state rho with respect to a channel T and the
Holevo capacity of the channel require the solution of difficult variational
problems. For a class of 1-qubit channels, which contains all the extremal
ones, the problem can be significantly simplified by associating an Hermitian
antilinear operator theta to every channel of the considered class. The
concurrence of the channel can be expressed by theta and turns out to be a flat
roof. This allows to write down an explicit expression for H_T. Its maximum
would give the Holevo (1-shot) capacity.Comment: 12 pages, several printing or latex errors correcte

### Superbroadcasting of harmonic oscillators mixed states

We consider the problem of broadcasting quantum information encoded in the
displacement parameter for an harmonic oscillator, from N to M>N copies of a
thermal state. We show the Weyl-Heisenberg covariant broadcasting map that
optimally reduces the thermal photon number, and we prove that it minimizes the
noise in conjugate quadratures at the output for general input states. We find
that from two input copies broadcasting is feasible, with the possibility of
simultaneous purification (superbroadcasting).Comment: 9 pages, 1 figure, revtex4, to appear in the Proceedings of ICQO2006,
Minsk, May 200

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