661 research outputs found
Versatile Wideband Balanced Detector for Quantum Optical Homodyne Tomography
We present a comprehensive theory and an easy to follow method for the design
and construction of a wideband homodyne detector for time-domain quantum
measurements. We show how one can evaluate the performance of a detector in a
specific time-domain experiment based on electronic spectral characteristic of
that detector. We then present and characterize a high-performance detector
constructed using inexpensive, commercially available components such as
low-noise high-speed operational amplifiers and high-bandwidth photodiodes. Our
detector shows linear behavior up to a level of over 13 dB clearance between
shot noise and electronic noise, in the range from DC to 100 MHz. The detector
can be used for measuring quantum optical field quadratures both in the
continuous-wave and pulsed regimes with pulse repetition rates up to about 250
MHz.Comment: 11 pages, 8 figures, 1 tabl
The curious nonexistence of Gaussian 2-designs
2-designs -- ensembles of quantum pure states whose 2nd moments equal those
of the uniform Haar ensemble -- are optimal solutions for several tasks in
quantum information science, especially state and process tomography. We show
that Gaussian states cannot form a 2-design for the continuous-variable
(quantum optical) Hilbert space L2(R). This is surprising because the affine
symplectic group HWSp (the natural symmetry group of Gaussian states) is
irreducible on the symmetric subspace of two copies. In finite dimensional
Hilbert spaces, irreducibility guarantees that HWSp-covariant ensembles (such
as mutually unbiased bases in prime dimensions) are always 2-designs. This
property is violated by continuous variables, for a subtle reason: the
(well-defined) HWSp-invariant ensemble of Gaussian states does not have an
average state because the averaging integral does not converge. In fact, no
Gaussian ensemble is even close (in a precise sense) to being a 2-design. This
surprising difference between discrete and continuous quantum mechanics has
important implications for optical state and process tomography.Comment: 9 pages, no pretty figures (sorry!
Measuring the elements of the optical density matrix
Most methods for experimentally reconstructing the quantum state of light
involve determining a quasiprobability distribution such as the Wigner
function. In this paper we present a scheme for measuring individual density
matrix elements in the photon number state representation. Remarkably, the
scheme is simple, involving two beam splitters and a reference field in a
coherent state.Comment: 6 pages and 1 figur
Conditional generation of sub-Poissonian light from two-mode squeezed vacuum via balanced homodyne detection on idler mode
A simple scheme for conditional generation of nonclassical light with
sub-Poissonian photon-number statistics is proposed. The method utilizes
entanglement of signal and idler modes in two-mode squeezed vacuum state
generated in optical parametric amplifier. A quadrature component of the idler
mode is measured in balanced homodyne detector and only those experimental runs
where the absolute value of the measured quadrature is higher than certain
threshold are accepted. If the threshold is large enough then the conditional
output state of signal mode exhibits reduction of photon-number fluctuations
below the coherent-state level.Comment: 7 pages, 6 figures, REVTe
Quantum inference of states and processes
The maximum-likelihood principle unifies inference of quantum states and
processes from experimental noisy data. Particularly, a generic quantum process
may be estimated simultaneously with unknown quantum probe states provided that
measurements on probe and transformed probe states are available. Drawbacks of
various approximate treatments are considered.Comment: 7 pages, 4 figure
Conditional generation of N-photon entangled states of light
We propose a scheme for conditional generation of two-mode N-photon
path-entangled states of traveling light field. These states may find
applications in quantum optical lithography and they may be used to improve the
sensitivity of interferometric measurements. Our method requires only
single-photon sources, linear optics (beam splitters and phase shifters), and
photodetectors with single photon sensitivity.Comment: 4 pages, 2 figures, RevTeX
Linear optics substituting scheme for multi-mode operations
We propose a scheme allowing a conditional implementation of suitably
truncated general single- or multi-mode operators acting on states of traveling
optical signal modes. The scheme solely relies on single-photon and coherent
states and applies beam splitters and zero- and single-photon detections. The
signal flow of the setup resembles that of a multi-mode quantum teleportation
scheme thus allowing the individual signal modes to be spatially separated from
each other. Some examples such as the realization of cross-Kerr nonlinearities,
multi-mode mirrors, and the preparation of multi-photon entangled states are
considered.Comment: 11 pages, 4 eps-figures, using revtex
Conditional generation of arbitrary multimode entangled states of light with linear optics
We propose a universal scheme for the probabilistic generation of an
arbitrary multimode entangled state of light with finite expansion in Fock
basis. The suggested setup involves passive linear optics, single photon
sources, strong coherent laser beams, and photodetectors with single-photon
resolution. The efficiency of this setup may be greatly enhanced if, in
addition, a quantum memory is available.Comment: 7 pages, 5 figure
Brain charts for the human lifespan
Over the past few decades, neuroimaging has become a ubiquitous tool in basic research and clinical studies of the human brain. However, no reference standards currently exist to quantify individual differences in neuroimaging metrics over time, in contrast to growth charts for anthropometric traits such as height and weight1. Here we assemble an interactive open resource to benchmark brain morphology derived from any current or future sample of MRI data (http://www.brainchart.io/). With the goal of basing these reference charts on the largest and most inclusive dataset available, acknowledging limitations due to known biases of MRI studies relative to the diversity of the global population, we aggregated 123,984 MRI scans, across more than 100 primary studies, from 101,457 human participants between 115 days post-conception to 100 years of age. MRI metrics were quantified by centile scores, relative to non-linear trajectories2 of brain structural changes, and rates of change, over the lifespan. Brain charts identified previously unreported neurodevelopmental milestones3, showed high stability of individuals across longitudinal assessments, and demonstrated robustness to technical and methodological differences between primary studies. Centile scores showed increased heritability compared with non-centiled MRI phenotypes, and provided a standardized measure of atypical brain structure that revealed patterns of neuroanatomical variation across neurological and psychiatric disorders. In summary, brain charts are an essential step towards robust quantification of individual variation benchmarked to normative trajectories in multiple, commonly used neuroimaging phenotypes
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
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