4,319 research outputs found
Identifying dynamical systems with bifurcations from noisy partial observation
Dynamical systems are used to model a variety of phenomena in which the
bifurcation structure is a fundamental characteristic. Here we propose a
statistical machine-learning approach to derive lowdimensional models that
automatically integrate information in noisy time-series data from partial
observations. The method is tested using artificial data generated from two
cell-cycle control system models that exhibit different bifurcations, and the
learned systems are shown to robustly inherit the bifurcation structure.Comment: 16 pages, 6 figure
Many-body localization and thermalization in the full probability distribution function of observables
We investigate the relation between thermalization following a quantum quench
and many-body localization in quasiparticle space in terms of the long-time
full distribution function of physical observables. In particular, expanding on
our recent work [E. Canovi {\em et al.}, Phys. Rev. B {\bf 83}, 094431 (2011)],
we focus on the long-time behavior of an integrable XXZ chain subject to an
integrability-breaking perturbation. After a characterization of the breaking
of integrability and the associated localization/delocalization transition
using the level spacing statistics and the properties of the eigenstates, we
study the effect of integrability-breaking on the asymptotic state after a
quantum quench of the anisotropy parameter, looking at the behavior of the full
probability distribution of the transverse and longitudinal magnetization of a
subsystem. We compare the resulting distributions with those obtained in
equilibrium at an effective temperature set by the initial energy. We find
that, while the long time distribution functions appear to always agree {\it
qualitatively} with the equilibrium ones, {\it quantitative} agreement is
obtained only when integrability is fully broken and the relevant eigenstates
are diffusive in quasi-particle space.Comment: 18 pages, 11 figure
A quantum beam splitter for atoms
An interferometric method is proposed to controllably split an atomic
condensate in two spatial components with strongly reduced population
fluctuations. All steps in our proposal are in current use in cold atom
laboratories, and we show with a theoretical calculation that our proposal is
very robust against imperfections of the interferometer.Comment: 6 pages, 3 figures, revtex
Nonclassical correlations of phase noise and photon number in quantum nondemolition measurements
The continuous transition from a low resolution quantum nondemolition
measurement of light field intensity to a precise measurement of photon number
is described using a generalized measurement postulate. In the intermediate
regime, quantization appears as a weak modulation of measurement probability.
In this regime, the measurement result is strongly correlated with the amount
of phase decoherence introduced by the measurement interaction. In particular,
the accidental observation of half integer photon numbers preserves phase
coherence in the light field, while the accidental observation of quantized
values increases decoherence. The quantum mechanical nature of this correlation
is discussed and the implications for the general interpretation of
quantization are considered.Comment: 16 pages, 5 figures, final version to be published in Phys. Rev. A,
Clarifications of the nature of the measurement result and the noise added in
section I
Anomalous Expansion of Attractively Interacting Fermionic Atoms in an Optical Lattice
Strong correlations can dramatically modify the thermodynamics of a quantum
many-particle system. Especially intriguing behaviour can appear when the
system adiabatically enters a strongly correlated regime, for the interplay
between entropy and strong interactions can lead to counterintuitive effects. A
well known example is the so-called Pomeranchuk effect, occurring when liquid
3He is adiabatically compressed towards its crystalline phase. Here, we report
on a novel anomalous, isentropic effect in a spin mixture of attractively
interacting fermionic atoms in an optical lattice. As we adiabatically increase
the attraction between the atoms we observe that the gas, instead of
contracting, anomalously expands. This expansion results from the combination
of two effects induced by pair formation in a lattice potential: the
suppression of quantum fluctuations as the attraction increases, which leads to
a dominant role of entropy, and the progressive loss of the spin degree of
freedom, which forces the gas to excite additional orbital degrees of freedom
and expand to outer regions of the trap in order to maintain the entropy. The
unexpected thermodynamics we observe reveal fundamentally distinctive features
of pairing in the fermionic Hubbard model.Comment: 6 pages (plus appendix), 6 figure
Local asymptotic normality for qubit states
We consider n identically prepared qubits and study the asymptotic properties
of the joint state \rho^{\otimes n}. We show that for all individual states
\rho situated in a local neighborhood of size 1/\sqrt{n} of a fixed state
\rho^0, the joint state converges to a displaced thermal equilibrium state of a
quantum harmonic oscillator. The precise meaning of the convergence is that
there exist physical transformations T_{n} (trace preserving quantum channels)
which map the qubits states asymptotically close to their corresponding
oscillator state, uniformly over all states in the local neighborhood.
A few consequences of the main result are derived. We show that the optimal
joint measurement in the Bayesian set-up is also optimal within the pointwise
approach. Moreover, this measurement converges to the heterodyne measurement
which is the optimal joint measurement of position and momentum for the quantum
oscillator. A problem of local state discrimination is solved using local
asymptotic normality.Comment: 16 pages, 3 figures, published versio
The dynamics and prethermalization of one dimensional quantum systems probed through the full distributions of quantum noise
Quantum noise correlations have been employed in several areas in physics
including condensed matter, quantum optics and ultracold atom to reveal
non-classical states of the systems. So far, such analysis mostly focused on
systems in equilibrium. In this paper, we show that quantum noise is also a
useful tool to characterize and study the non-equilibrium dynamics of one
dimensional system. We consider the Ramsey sequence of one dimensional,
two-component bosons, and obtain simple, analytical expressions of time
evolutions of the full distribution functions for this strongly-correlated,
many-body system. The analysis can also be directly applied to the evolution of
interference patterns between two one dimensional quasi-condensates created
from a single condensate through splitting. Using the tools developed in this
paper, we demonstrate that one dimensional dynamics in these systems exhibits
the phenomenon known as "prethermalization", where the observables of {\it
non-equilibrium}, long-time transient states become indistinguishable from
those of thermal {\it equilibrium} states.Comment: 22 pages, 11 figures+appendi
Antiferromagnetism of SrFe2As2 studied by Single-Crystal 75As-NMR
We report results of 75As nuclear magnetic resonance (NMR) experiments on a
self-flux grown high-quality single crystal of SrFe2As2. The NMR spectra
clearly show sharp first-order antiferromagnetic (AF) and structural
transitions occurring simultaneously. The behavior in the vicinity of the
transition is compared with our previous study on BaFe2As2. No significant
difference was observed in the temperature dependence of the static quantities
such as the AF splitting and electric quadrupole splitting. However, the
results of the NMR relaxation rate revealed difference in the dynamical spin
fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear
to be more anisotropic in BaFe2As2 than in SrFe2As2.Comment: 4 pages, 5 figures; discussion revised; accepted for publication in
J. Phys. Soc. Jp
Paramagnetic Phase of a Heavy-Fermion Compound, CeFePO, Probed by 57Fe M\"{o}ssbauer Spectroscopy
57Fe M\"{o}ssbauer spectroscopy was applied to an iron-based layered compound
CeFePO. At temperatures from 9.4 to 293 K, no magnetic splitting was observed
in the M\"ossbauer spectra of CeFePO indicating a paramagnetic phase of the Fe
magnetic sublattice. All the spectra were fitted with a small quadrupole
splitting, and the Debye temperature of CeFePO was found to be \sim448 K. The
isomer shift at room temperature, 0.32 mm/s, was almost equal to those of
LnFeAsO (Ln = La, Ce, Sm). Comparing s-electron density using the isomer shifts
and unit cell volumes, it was found that the Fe of CeFePO has a similar valence
state to other layered iron-based quaternary oxypnictides except LaFePO
Understanding the dynamics of triple helix interactions. The case of English Higher Education Institutions
This paper examines the evolution of the dynamics of the triple helix interactions exemplified by the case of Higher Education Institutions (HEIs) in England. Results highlight the persisting heterogeneity between HEIs in their combination, geography and evolution of triple helix interactions, particularly between research oriented universities and newer universities with strong teaching orientations
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