1,694 research outputs found
Quantum levitation by left-handed metamaterials
Left-handed metamaterials make perfect lenses that image classical
electromagnetic fields with significantly higher resolution than the
diffraction limit. Here we consider the quantum physics of such devices. We
show that the Casimir force of two conducting plates may turn from attraction
to repulsion if a perfect lens is sandwiched between them. For optical
left-handed metamaterials this repulsive force of the quantum vacuum may
levitate ultra-thin mirrors
Optimal cloning of mixed Gaussian states
We construct the optimal 1 to 2 cloning transformation for the family of
displaced thermal equilibrium states of a harmonic oscillator, with a fixed and
known temperature. The transformation is Gaussian and it is optimal with
respect to the figure of merit based on the joint output state and norm
distance. The proof of the result is based on the equivalence between the
optimal cloning problem and that of optimal amplification of Gaussian states
which is then reduced to an optimization problem for diagonal states of a
quantum oscillator. A key concept in finding the optimum is that of stochastic
ordering which plays a similar role in the purely classical problem of Gaussian
cloning. The result is then extended to the case of n to m cloning of mixed
Gaussian states.Comment: 8 pages, 1 figure; proof of general form of covariant amplifiers
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Comment on "Quantum Friction - Fact or Fiction?"
If quantum friction existed [J.B. Pendry, New J. Phys. 12, 033028 (2010)] an
unlimited amount of useful energy could be extracted from the quantum vacuum
and Lifshitz theory would fail. Both are unlikely to be true.Comment: Comment on J.B. Pendry, New J. Phys. 12, 033028 (2010
Measuring Polynomial Invariants of Multi-Party Quantum States
We present networks for directly estimating the polynomial invariants of
multi-party quantum states under local transformations. The structure of these
networks is closely related to the structure of the invariants themselves and
this lends a physical interpretation to these otherwise abstract mathematical
quantities. Specifically, our networks estimate the invariants under local
unitary (LU) transformations and under stochastic local operations and
classical communication (SLOCC). Our networks can estimate the LU invariants
for multi-party states, where each party can have a Hilbert space of arbitrary
dimension and the SLOCC invariants for multi-qubit states. We analyze the
statistical efficiency of our networks compared to methods based on estimating
the state coefficients and calculating the invariants.Comment: 8 pages, 4 figures, RevTex4, v2 references update
Retrodictively Optimal Localisations in Phase Space
In a previous paper it was shown that the distribution of measured values for
a retrodictively optimal simultaneous measurement of position and momentum is
always given by the initial state Husimi function. This result is now
generalised to retrodictively optimal simultaneous measurements of an arbitrary
pair of rotated quadratures x_theta1 and x_theta2. It is shown, that given any
such measurement, it is possible to find another such measurement,
informationally equivalent to the first, for which the axes defined by the two
quadratures are perpendicular. It is further shown that the distribution of
measured values for such a meaurement belongs to the class of generalised
Husimi functions most recently discussed by Wuensche and Buzek. The class
consists of the subset of Wodkiewicz's operational probability distributions
for which the filter reference state is a squeezed vaccuum state.Comment: 11 pages, 2 figures. AMS Latex. Replaced with published versio
Generating entanglement of photon-number states with coherent light via cross-Kerr nonlinearity
We propose a scheme for generating entangled states of light fields. This
scheme only requires the cross-Kerr nonlinear interaction between coherent
light-beams, followed by a homodyne detection. Therefore, this scheme is within
the reach of current technology. We study in detail the generation of the
entangled states between two modes, and that among three modes. In addition to
the Bell states between two modes and the W states among three modes, we find
plentiful new kinds of entangled states. Finally, the scheme can be extend to
generate the entangled states among more than three modes.Comment: 2 figure
Operational Theory of Homodyne Detection
We discuss a balanced homodyne detection scheme with imperfect detectors in
the framework of the operational approach to quantum measurement. We show that
a realistic homodyne measurement is described by a family of operational
observables that depends on the experimental setup, rather than a single field
quadrature operator. We find an explicit form of this family, which fully
characterizes the experimental device and is independent of a specific state of
the measured system. We also derive operational homodyne observables for the
setup with a random phase, which has been recently applied in an ultrafast
measurement of the photon statistics of a pulsed diode laser. The operational
formulation directly gives the relation between the detected noise and the
intrinsic quantum fluctuations of the measured field. We demonstrate this on
two examples: the operational uncertainty relation for the field quadratures,
and the homodyne detection of suppressed fluctuations in photon statistics.Comment: 7 pages, REVTe
Dynamics of light propagation in spatiotemporal dielectric structures
Propagation, transmission and reflection properties of linearly polarized
plane waves and arbitrarily short electromagnetic pulses in one-dimensional
dispersionless dielectric media possessing an arbitrary space-time dependence
of the refractive index are studied by using a two-component, highly symmetric
version of Maxwell's equations. The use of any slow varying amplitude
approximation is avoided. Transfer matrices of sharp nonstationary interfaces
are calculated explicitly, together with the amplitudes of all secondary waves
produced in the scattering. Time-varying multilayer structures and
spatiotemporal lenses in various configurations are investigated analytically
and numerically in a unified approach. Several new effects are reported, such
as pulse compression, broadening and spectral manipulation of pulses by a
spatiotemporal lens, and the closure of the forbidden frequency gaps with the
subsequent opening of wavenumber bandgaps in a generalized Bragg reflector
Measuring the Density Matrix by Local Addressing
We introduce a procedure to measure the density matrix of a material system.
The density matrix is addressed locally in this scheme by applying a sequence
of delayed light pulses. The procedure is based on the stimulated Raman
adiabatic passage (STIRAP) technique. It is shown that a series of population
measurements on the target state of the population transfer process yields
unambiguous information about the populations and coherences of the addressed
states, which therefore can be determined.Comment: 4 pages, 1 figur
Bogoliubov theory of the Hawking effect in Bose-Einstein condensates
Artificial black holes may demonstrate some of the elusive quantum properties
of the event horizon, in particular Hawking radiation. One promising candidate
is a sonic hole in a Bose-Einstein condensate. We clarify why Hawking radiation
emerges from the condensate and how this condensed-matter analog reflects some
of the intriguing aspects of quantum black holes
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