132 research outputs found
Entanglement and four wave mixing effects in the dissipation free nonlinear interaction of two photons at a single atom
We investigate the nonlinear interaction between two photons in a single
input pulse at an atomic two level nonlinearity. A one dimensional model for
the propagation of light to and from the atom is used to describe the precise
spatiotemporal coherence of the two photon state. It is shown that the
interaction generates spatiotemporal entanglement in the output state similar
to the entanglement observed in parametric downconversion. A method of
generating photon pairs from coherent pump light using this quantum mechanical
four wave mixing process is proposed.Comment: 10 pages, including 3 figures, correction in eq.(7), updated
references, final version for publication in PR
Uncertainty characteristics of generalized quantum measurements
The effects of any quantum measurement can be described by a collection of
measurement operators {M_m} acting on the quantum state of the measured system.
However, the Hilbert space formalism tends to obscure the relationship between
the measurement results and the physical properties of the measured system. In
this paper, a characterization of measurement operators in terms of measurement
resolution and disturbance is developed. It is then possible to formulate
uncertainty relations for the measurement process that are valid for arbitrary
input states. The motivation of these concepts is explained from a quantum
communication viewpoint. It is shown that the intuitive interpretation of
uncertainty as a relation between measurement resolution and disturbance
provides a valid description of measurement back action. Possible applications
to quantum cryptography, quantum cloning, and teleportation are discussed.Comment: 8 pages, small additions on cloning and on definitions of delta A_mf,
et
Continuous variable teleportation of single photon states
The properties of continuous variable teleportation of single photon states
are investigated. The output state is different from the input state due to the
non-maximal entanglement in the EPR beams. The photon statistics of the
teleportation output are determined and the correlation between the field
information beta obtained in the teleportation process and the change in photon
number is discussed. The results of the output photon statistics are applied to
the transmission of a qbit encoded in the polarization of a single photon.Comment: 14 pages, including 6 figure
Quantum Maxwell-Bloch equations for spatially inhomogeneous semiconductor lasers
We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous
semiconductor laser devices. The QMBE are derived from fully quantum mechanical
operator dynamics describing the interaction of the light field with the
quantum states of the electrons and the holes near the band gap. By taking into
account field-field correlations and field-dipole correlations, the QMBE
include quantum noise effects which cause spontaneous emission and amplified
spontaneous emission. In particular, the source of spontaneous emission is
obtained by factorizing the dipole-dipole correlations into a product of
electron and hole densities. The QMBE are formulated for general devices, for
edge emitting lasers and for vertical cavity surface emitting lasers, providing
a starting point for the detailed analysis of spatial coherence in the near
field and far field patterns of such laser diodes. Analytical expressions are
given for the spectra of gain and spontaneous emission described by the QMBE.
These results are applied to the case of a broad area laser, for which the
frequency and carrier density dependent spontaneous emission factor beta and
the evolution of the far field pattern near threshold are derived.Comment: 22 pages RevTex and 7 figures, submitted to Phys.Rev.A, revisions in
abstract and in the discussion of temporal coherenc
Information and noise in quantum measurement
Even though measurement results obtained in the real world are generally both
noisy and continuous, quantum measurement theory tends to emphasize the ideal
limit of perfect precision and quantized measurement results. In this article,
a more general concept of noisy measurements is applied to investigate the role
of quantum noise in the measurement process. In particular, it is shown that
the effects of quantum noise can be separated from the effects of information
obtained in the measurement. However, quantum noise is required to ``cover up''
negative probabilities arising as the quantum limit is approached. These
negative probabilities represent fundamental quantum mechanical correlations
between the measured variable and the variables affected by quantum noise.Comment: 16 pages, short comment added in II.B., final version for publication
in Phys. Rev.
Nonclassical correlations of photon number and field components in the vacuum state
It is shown that the quantum jumps in the photon number n from zero to one or
more photons induced by backaction evasion quantum nondemolition measurements
of a quadrature component x of the vacuum light field state are strongly
correlated with the quadrature component measurement results. This correlation
corresponds to the operator expectation value which is equal to one
fourth for the vacuum even though the photon number eigenvalue is zero. Quantum
nondemolition measurements of a quadrature component can thus provide
experimental evidence of the nonclassical operator ordering dependence of the
correlations between photon number and field components in the vacuum state.Comment: 13 pages, 3 figures, corrections of omissions in equations (6) and
(25). To be published in Phys. Rev.
Causality in quantum teleportation: information extraction and noise effects in entanglement distribution
Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.Comment: 14 pages, including three figures, discussion of fidelity adde
Input states for quantum gates
We examine three possible implementations of non-deterministic linear optical
cnot gates with a view to an in-principle demonstration in the near future. To
this end we consider demonstrating the gates using currently available sources
such as spontaneous parametric down conversion and coherent states, and current
detectors only able to distinguish between zero or many photons. The
demonstration is possible in the co-incidence basis and the errors introduced
by the non-optimal input states and detectors are analysed
Teleportation improvement by conditional measurements on the two-mode squeezed vacuum
We show that by making conditional measurements on the Einstein-Podolsky-Rosen (EPR) squeezed vacuum [T. Opatrny, G. Kurizki, and D.-G. Welsch, Phys. Rev. A 61, 032302 (2000)], one can improve the efficacy of teleportation for both the position-difference, momentum-sum, and number-difference, phase-sum continuous variable teleportation protocols. We investigate the relative abilities of the standard and conditional EPR states, and show that by conditioning we can improve the fidelity of teleportation of coherent states from below to above the (F) over bar =2/3 boundary, thereby achieving unambiguously quantum teleportation
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