721 research outputs found
Homodyne detection for measuring internal quantum correlations of optical pulses
A new method is described for determining the quantum correlations at
different times in optical pulses by using balanced homodyne detection. The
signal pulse and sequences of ultrashort test pulses are superimposed, where
for chosen distances between the test pulses their relative phases and
intensities are varied from measurement to measurement. The correlation
statistics of the signal pulse is obtained from the time-integrated difference
photocurrents measured.Comment: 7 pages, A4.sty include
Orbital reconstruction and two-dimensional electron gas at the LaAlO3/SrTiO3 interface
Conventional two-dimensional electron gases are realized by engineering the
interfaces between semiconducting compounds. In 2004, Ohtomo and Hwang
discovered that an electron gas can be also realized at the interface between
large gap insulators made of transition metal oxides [1]. This finding has
generated considerable efforts to clarify the underlying microscopic mechanism.
Of particular interest is the LaAlO3/SrTiO3 system, because it features
especially striking properties. High carrier mobility [1], electric field
tuneable superconductivity [2] and magnetic effects [3], have been found. Here
we show that an orbital reconstruction is underlying the generation of the
electron gas at the LaAlO3/SrTiO3 n-type interface. Our results are based on
extensive investigations of the electronic properties and of the orbital
structure of the interface using X-ray Absorption Spectroscopy. In particular
we find that the degeneracy of the Ti 3d states is fully removed, and that the
Ti 3dxy levels become the first available states for conducting electrons.Comment: 12 pages, 3 figure
Direct sampling of the Susskind-Glogower phase distributions
Coarse-grained phase distributions are introduced that approximate to the
Susskind--Glogower cosine and sine phase distributions. The integral relations
between the phase distributions and the phase-parametrized field-strength
distributions observable in balanced homodyning are derived and the integral
kernels are analyzed. It is shown that the phase distributions can be directly
sampled from the field-strength distributions which offers the possibility of
measuring the Susskind--Glogower cosine and sine phase distributions with
sufficiently well accuracy. Numerical simulations are performed to demonstrate
the applicability of the method.Comment: 10 figures using a4.st
Mode structure and photon number correlations in squeezed quantum pulses
The question of efficient multimode description of optical pulses is studied.
We show that a relatively very small number of nonmonochromatic modes can be
sufficient for a complete quantum description of pulses with Gaussian
quadrature statistics. For example, a three-mode description was enough to
reproduce the experimental data of photon number correlations in optical
solitons [S. Spalter et al., Phys. Rev. Lett. 81, 786 (1998)]. This approach is
very useful for a detailed understanding of squeezing properties of soliton
pulses with the main potential for quantum communication with continuous
variables. We show how homodyne detection and/or measurements of photon number
correlations can be used to determine the quantum state of the multi-mode
field. We also discuss a possible way of physical separation of the
nonmonochromatic modes.Comment: 14 pages, 4 figures; minor revisions of the text, new references; to
appear in the Phys. Rev.
Direct sampling of exponential phase moments of smoothed Wigner functions
We investigate exponential phase moments of the s-parametrized
quasidistributions (smoothed Wigner functions). We show that the knowledge of
these moments as functions of s provides, together with photon-number
statistics, a complete description of the quantum state. We demonstrate that
the exponential phase moments can be directly sampled from the data recorded in
balanced homodyne detection and we present simple expressions for the sampling
kernels. The phase moments are Fourier coefficients of phase distributions
obtained from the quasidistributions via integration over the radial variable
in polar coordinates. We performed Monte Carlo simulations of the homodyne
detection and we demonstrate the feasibility of direct sampling of the moments
and subsequent reconstruction of the phase distribution.Comment: RevTeX, 8 pages, 6 figures, accepted Phys. Rev.
Renormalized quantum tomography
The core of quantum tomography is the possibility of writing a generally
unbounded complex operator in form of an expansion over operators that are
generally nonlinear functions of a generally continuous set of spectral
densities--the so-called "quorum" of observables. The expansion is generally
non unique, the non unicity allowing further optimization for given criteria.
The mathematical problem of tomography is thus the classification of all such
operator expansions for given (suitably closed) linear spaces of unbounded
operators--e.g. Banach spaces of operators with an appropriate norm. Such
problem is a difficult one, and remains still open, involving the theory of
general basis in Banach spaces, a still unfinished chapter of analysis. In this
paper we present new nontrivial operator expansions for the quorum of
quadratures of the harmonic oscillator, and introduce a first very preliminary
general framework to generate new expansions based on the Kolmogorov
construction. The material presented in this paper is intended to be helpful
for the solution of the general problem of quantum tomography in infinite
dimensions, which corresponds to provide a coherent mathematical framework for
operator expansions over functions of a continuous set of spectral densities.Comment: 23 pages, no figure
Numerical simulations of the Warm-Hot Intergalactic Medium
In this paper we review the current predictions of numerical simulations for
the origin and observability of the warm hot intergalactic medium (WHIM), the
diffuse gas that contains up to 50 per cent of the baryons at z~0. During
structure formation, gravitational accretion shocks emerging from collapsing
regions gradually heat the intergalactic medium (IGM) to temperatures in the
range T~10^5-10^7 K. The WHIM is predicted to radiate most of its energy in the
ultraviolet (UV) and X-ray bands and to contribute a significant fraction of
the soft X-ray background emission. While O VI and C IV absorption systems
arising in the cooler fraction of the WHIM with T~10^5-10^5.5 K are seen in
FUSE and HST observations, models agree that current X-ray telescopes such as
Chandra and XMM-Newton do not have enough sensitivity to detect the hotter
WHIM. However, future missions such as Constellation-X and XEUS might be able
to detect both emission lines and absorption systems from highly ionised atoms
such as O VII, O VIII and Fe XVII.Comment: 18 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 14; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Multi-triangulations as complexes of star polygons
Maximal -crossing-free graphs on a planar point set in convex
position, that is, -triangulations, have received attention in recent
literature, with motivation coming from several interpretations of them.
We introduce a new way of looking at -triangulations, namely as complexes
of star polygons. With this tool we give new, direct, proofs of the fundamental
properties of -triangulations, as well as some new results. This
interpretation also opens-up new avenues of research, that we briefly explore
in the last section.Comment: 40 pages, 24 figures; added references, update Section
Identification of morphological biosignatures in martian analogue field specimens using in situ planetary instrumentation
We have investigated how morphological biosignatures (i.e., features related to life) might be identified with an array of viable instruments within the framework of robotic planetary surface operations at Mars. This is the first time such an integrated lab-based study has been conducted that incorporates space-qualified instrumentation designed for combined in situ imaging, analysis, and geotechnics (sampling). Specimens were selected on the basis of feature morphology, scale, and analogy to Mars rocks. Two types of morphological criteria were considered: potential signatures of extinct life (fossilized microbial filaments) and of extant life (crypto-chasmoendolithic microorganisms). The materials originated from a variety of topical martian analogue localities on Earth, including impact craters, high-latitude deserts, and hydrothermal deposits. Our in situ payload included a stereo camera, microscope, Mössbauer spectrometer, and sampling device (all space-qualified units from Beagle 2), and an array of commercial instruments, including a multi-spectral imager, an X-ray spectrometer (calibrated to the Beagle 2 instrument), a micro-Raman spectrometer, and a bespoke (custom-designed) X-ray diffractometer. All experiments were conducted within the engineering constraints of in situ operations to generate realistic data and address the practical challenges of measurement
Quantum feedback with weak measurements
The problem of feedback control of quantum systems by means of weak
measurements is investigated in detail. When weak measurements are made on a
set of identical quantum systems, the single-system density matrix can be
determined to a high degree of accuracy while affecting each system only
slightly. If this information is fed back into the systems by coherent
operations, the single-system density matrix can be made to undergo an
arbitrary nonlinear dynamics, including for example a dynamics governed by a
nonlinear Schr\"odinger equation. We investigate the implications of such
nonlinear quantum dynamics for various problems in quantum control and quantum
information theory, including quantum computation. The nonlinear dynamics
induced by weak quantum feedback could be used to create a novel form of
quantum chaos in which the time evolution of the single-system wave function
depends sensitively on initial conditions.Comment: 11 pages, TeX, replaced to incorporate suggestions of Asher Pere
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