189 research outputs found
Optimal photonic indistinguishability tests in multimode networks
Particle indistinguishability is at the heart of quantum statistics that
regulates fundamental phenomena such as the electronic band structure of
solids, Bose-Einstein condensation and superconductivity. Moreover, it is
necessary in practical applications such as linear optical quantum computation
and simulation, in particular for Boson Sampling devices. It is thus crucial to
develop tools to certify genuine multiphoton interference between multiple
sources. Here we show that so-called Sylvester interferometers are near-optimal
for the task of discriminating the behaviors of distinguishable and
indistinguishable photons. We report the first implementations of integrated
Sylvester interferometers with 4 and 8 modes with an efficient, scalable and
reliable 3D-architecture. We perform two-photon interference experiments
capable of identifying indistinguishable photon behaviour with a Bayesian
approach using very small data sets. Furthermore, we employ experimentally this
new device for the assessment of scattershot Boson Sampling. These results open
the way to the application of Sylvester interferometers for the optimal
assessment of multiphoton interference experiments.Comment: 9+10 pages, 6+6 figures, added supplementary material, completed and
updated bibliograph
Experimental generalized quantum suppression law in Sylvester interferometers
Photonic interference is a key quantum resource for optical quantum
computation, and in particular for so-called boson sampling machines. In
interferometers with certain symmetries, genuine multiphoton quantum
interference effectively suppresses certain sets of events, as in the original
Hong-Ou-Mandel effect. Recently, it was shown that some classical and
semi-classical models could be ruled out by identifying such suppressions in
Fourier interferometers. Here we propose a suppression law suitable for
random-input experiments in multimode Sylvester interferometers, and verify it
experimentally using 4- and 8-mode integrated interferometers. The observed
suppression is stronger than what is observed in Fourier interferometers of the
same size, and could be relevant to certification of boson sampling machines
and other experiments relying on bosonic interference.Comment: 5 pages, 3 figures + 11 pages, 3 figures Supplementary Informatio
Experimental Scattershot Boson Sampling
Boson Sampling is a computational task strongly believed to be hard for
classical computers, but efficiently solvable by orchestrated bosonic
interference in a specialised quantum computer. Current experimental schemes,
however, are still insufficient for a convincing demonstration of the advantage
of quantum over classical computation. A new variation of this task,
Scattershot Boson Sampling, leads to an exponential increase in speed of the
quantum device, using a larger number of photon sources based on parametric
downconversion. This is achieved by having multiple heralded single photons
being sent, shot by shot, into different random input ports of the
interferometer. Here we report the first Scattershot Boson Sampling
experiments, where six different photon-pair sources are coupled to integrated
photonic circuits. We employ recently proposed statistical tools to analyse our
experimental data, providing strong evidence that our photonic quantum
simulator works as expected. This approach represents an important leap toward
a convincing experimental demonstration of the quantum computational supremacy.Comment: 8 pages, 5 figures (plus Supplementary Materials, 14 pages, 8
figures
Cosmology with variable parameters and effective equation of state for Dark Energy
A cosmological constant, Lambda, is the most natural candidate to explain the
origin of the dark energy (DE) component in the Universe. However, due to
experimental evidence that the equation of state (EOS) of the DE could be
evolving with time/redshift (including the possibility that it might behave
phantom-like near our time) has led theorists to emphasize that there might be
a dynamical field (or some suitable combination of them) that could explain the
behavior of the DE. While this is of course one possibility, here we show that
there is no imperative need to invoke such dynamical fields and that a variable
cosmological constant (including perhaps a variable Newton's constant too) may
account in a natural way for all these features.Comment: LaTeX, 9 pages, 1 figure. Talk given at the 7th Intern. Workshop on
Quantum Field Theory Under the Influence of External Conditions (QFEXT 05
Persistent black holes in bouncing cosmologies
In this paper we explore the idea that black holes can persist in a universe
that collapses to a big crunch and then bounces into a new phase of expansion.
We use a scalar field to model the matter content of such a universe {near the
time} of the bounce, and look for solutions that represent a network of black
holes within a dynamical cosmology. We find exact solutions to Einstein's
constraint equations that provide the geometry of space at the minimum of
expansion and that can be used as initial data for the evolution of
hyperspherical cosmologies. These solutions illustrate that there exist models
in which multiple distinct black holes can persist through a bounce, and allow
for concrete computations of quantities such as the black hole filling factor.
We then consider solutions in flat cosmologies, as well as in
higher-dimensional spaces (with up to nine spatial dimensions). We derive
conditions for the black holes to remain distinct (i.e. avoid merging) and
hence persist into the new expansion phase. Some potentially interesting
consequences of these models are also discussed.Comment: 37 pages, 16 figure
Piecewise Silence in Discrete Cosmological Models
20 pages, 1 figure20 pages, 1 figureWe consider a family of cosmological models in which all mass is confined to a regular lattice of identical black holes. By exploiting the reflection symmetry about planes that bisect these lattices into identical halves, we are able to consider the evolution of a number of geometrically distinguished surfaces that exist within each of them. We find that the evolution equations for the reflection symmetric surfaces can be written as a simple set of Friedmann-like equations, with source terms that behave like a set of interacting effective fluids. We then show that gravitational waves are effectively trapped within small chambers for all time, and are not free to propagate throughout the space-time. Each chamber therefore evolves as if it were in isolation from the rest of the universe. We call this phenomenon "piecewise silence"
From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework
Using the Einstein-Hilbert approximation of asymptotically safe quantum
gravity we present a consistent renormalization group based framework for the
inclusion of quantum gravitational effects into the cosmological field
equations. Relating the renormalization group scale to cosmological time via a
dynamical cutoff identification this framework applies to all stages of the
cosmological evolution. The very early universe is found to contain a period of
``oscillatory inflation'' with an infinite sequence of time intervals during
which the expansion alternates between acceleration and deceleration. For
asymptotically late times we identify a mechanism which prevents the universe
from leaving the domain of validity of the Einstein-Hilbert approximation and
obtain a classical de Sitter era.Comment: 47 pages, 17 figure
Hubble expansion and structure formation in the "running FLRW model" of the cosmic evolution
A new class of FLRW cosmological models with time-evolving fundamental
parameters should emerge naturally from a description of the expansion of the
universe based on the first principles of quantum field theory and string
theory. Within this general paradigm, one expects that both the gravitational
Newton's coupling, G, and the cosmological term, Lambda, should not be strictly
constant but appear rather as smooth functions of the Hubble rate. This
scenario ("running FLRW model") predicts, in a natural way, the existence of
dynamical dark energy without invoking the participation of extraneous scalar
fields. In this paper, we perform a detailed study of these models in the light
of the latest cosmological data, which serves to illustrate the
phenomenological viability of the new dark energy paradigm as a serious
alternative to the traditional scalar field approaches. By performing a joint
likelihood analysis of the recent SNIa data, the CMB shift parameter, and the
BAOs traced by the Sloan Digital Sky Survey, we put tight constraints on the
main cosmological parameters. Furthermore, we derive the theoretically
predicted dark-matter halo mass function and the corresponding redshift
distribution of cluster-size halos for the "running" models studied. Despite
the fact that these models closely reproduce the standard LCDM Hubble
expansion, their normalization of the perturbation's power-spectrum varies,
imposing, in many cases, a significantly different cluster-size halo redshift
distribution. This fact indicates that it should be relatively easy to
distinguish between the "running" models and the LCDM cosmology using realistic
future X-ray and Sunyaev-Zeldovich cluster surveys.Comment: Version published in JCAP 08 (2011) 007: 1+41 pages, 6 Figures, 1
Table. Typos corrected. Extended discussion on the computation of the
linearly extrapolated density threshold above which structures collapse in
time-varying vacuum models. One appendix, a few references and one figure
adde
A Stokes-based spectro-polarimetric analysis of the amplified spontaneous emission in a semiconductor optical amplifier
Semiconductor Optical Amplifiers (SOAs), key devices for future all-optical communication systems, are inherently polarisation-dependent, which is a major drawback for most networks applications. In spite of numerous studies carried out in order to design polarisation-insensitive structures, no complete spectro-polarimetric characterization of a SOA has been published so far. In particular, the spectral and polarimetric behaviour of the Amplified Spontaneous Emission (ASE), acting as a partly polarized broadband source, is of interest, since ASE draws from the same carrier reservoir as the amplified signal. In this paper, we present a full spectro-polarimetric characterization of ASE emitted from a commercial, strained-bulk SOA within the frame of the Stokes formalism. This formalism not only allows a determination of the degree of polarisation (DOP) of ASE directly from its Stokes vector, but also gives access to a full, spectrally resolved characterization of its polarized fraction with respect to the bias current applied to the SOA. The way the state of polarisation of that fraction is governed by the dependence of the material gain upon polarisation is spectrally resolved, quantified, and discussed. The same study is performed when a polarized signal is injected into the SOA
- …