263 research outputs found
Photon bunching in parametric down-conversion with continuous wave excitation
The first direct measurement of photon bunching (g2 correlation function) in
one output arm of a spontaneous-parametric-down-conversion source operated with
a continuous pump laser in the single-photon regime is demonstrated. The result
is in agreement with the statistics of a thermal field of the same coherence
length, and shows the feasibility of investigating photon statistics with
compact cw-pumped sources. Implications for entanglement-based quantum
cryptography are discussed.Comment: 7 pages, 4 figures, expanded introduction and experimental details
added. Accepted for publication in Phys.Rev.
Quantum limits on phase-shift detection using multimode interferometers
Fundamental phase-shift detection properties of optical multimode
interferometers are analyzed. Limits on perfectly distinguishable phase shifts
are derived for general quantum states of a given average energy. In contrast
to earlier work, the limits are found to be independent of the number of
interfering modes. However, the reported bounds are consistent with the
Heisenberg limit. A short discussion on the concept of well-defined relative
phase is also included.Comment: 6 pages, 3 figures, REVTeX, uses epsf.st
International benchmarking: Policy responses to biodiversity and climate change in OECD countries
Responding to global environmental issues like biodiversity loss and climate change challenge national governments and intergovernmental bodies. A conventional response has been to set targets. Yet to achieve targets, governments must implement effective policies. Indicators that track policy implementation could provide information on individual country progress towards targets and for international benchmarking. We take up a recommendation from Convention on Biological Diversity mandated experts to develop a policy indicator(s) for biodiversity. This expert group identified four indicator attributes: identification, establishment, strengthening and assessment. We review biodiversity (and climate change) policies implemented in the period 1952–2012 in 54 nations using an Organisation for Economic Co-operation and Development (OECD) database. We find: the number of countries implementing biodiversity policies increased steadily until the end of the 2000s (identification); evidence of continuous innovation in the types of policy instruments implemented (establishment); and evidence of policy revision and shifts in jurisdiction (strengthening). To overcome a lack of data to evaluate policy effectiveness (assessment) we suggest improvements in data collection and the possibility to combine the OECD database with other databases
Simultaneous minimum-uncertainty measurement of discrete-valued complementary observables
We have made the first experimental demonstration of the simultaneous minimum
uncertainty product between two complementary observables for a two-state
system (a qubit). A partially entangled two-photon state was used to perform
such measurements. Each of the photons carries (partial) information of the
initial state thus leaving a room for measurements of two complementary
observables on every member in an ensemble.Comment: 4 pages, 4 figures, REVTeX, submitted to PR
Sub-wavelength lithography over extended areas
We demonstrate a systematic approach to sub-wavelength resolution
lithographic image formation on films covering areas larger than a wavelength
squared. For example, it is possible to make a lithographic pattern with a
feature size resolution of by using a particular -photon, multi-mode entangled state, where , and banks of birefringent
plates. By preparing a statistically mixed such a state one can form any pixel
pattern on a pixel grid occupying a square
with a side of wavelengths. Hence, there is a trade-off between
the exposed area, the minimum lithographic feature size resolution, and the
number of photons used for the exposure. We also show that the proposed method
will work even under non-ideal conditions, albeit with somewhat poorer
performance.Comment: 8 pages, 8 figures, 1 table. Written in RevTe
Distance-based degrees of polarization for a quantum field
It is well established that unpolarized light is invariant with respect to
any SU(2) polarization transformation. This requirement fully characterizes the
set of density matrices representing unpolarized states. We introduce the
degree of polarization of a quantum state as its distance to the set of
unpolarized states. We use two different candidates of distance, namely the
Hilbert-Schmidt and the Bures metric, showing that they induce fundamentally
different degrees of polarization. We apply these notions to relevant field
states and we demonstrate that they avoid some of the problems arising with the
classical definition.Comment: 8 pages, 1 eps figur
Approaching the Heisenberg limit with two mode squeezed states
Two mode squeezed states can be used to achieve Heisenberg limit scaling in
interferometry: a phase shift of can be
resolved. The proposed scheme relies on balanced homodyne detection and can be
implemented with current technology. The most important experimental
imperfections are studied and their impact quantified.Comment: 4 pages, 7 figure
Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ
We study the electronic structure of the quasi-one-dimensional organic
conductor TTF-TCNQ by means of density-functional band theory, Hubbard model
calculations, and angle-resolved photoelectron spectroscopy (ARPES). The
experimental spectra reveal significant quantitative and qualitative
discrepancies to band theory. We demonstrate that the dispersive behavior as
well as the temperature-dependence of the spectra can be consistently explained
by the finite-energy physics of the one-dimensional Hubbard model at metallic
doping. The model description can even be made quantitative, if one accounts
for an enhanced hopping integral at the surface, most likely caused by a
relaxation of the topmost molecular layer. Within this interpretation the ARPES
data provide spectroscopic evidence for the existence of spin-charge separation
on an energy scale of the conduction band width. The failure of the
one-dimensional Hubbard model for the {\it low-energy} spectral behavior is
attributed to interchain coupling and the additional effect of electron-phonon
interaction.Comment: 18 pages, 9 figure
Polarization quantum properties in type-II Optical Parametric Oscillator below threshold
We study the far field spatial distribution of the quantum fluctuations in
the transverse profile of the output light beam generated by a type II Optical
Parametric Oscillator below threshold, including the effects of transverse
walk-off. We study how quadrature field correlations depend on the
polarization. We find spatial EPR entanglement in quadrature-polarization
components: For the far field points not affected by walk-off there is almost
complete noise suppression in the proper quadratures difference of any
orthogonal polarization components. We show the entanglement of the state of
symmetric intense, or macroscopic, spatial light modes. We also investigate
nonclassical polarization properties in terms of the Stokes operators. We find
perfect correlations in all Stokes parameters measured in opposite far field
points in the direction orthogonal to the walk-off, while locally the field is
unpolarized and we find no polarization squeezing.Comment: 16 pages, 18 figure
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