400 research outputs found
Quantum metrology with nonclassical states of atomic ensembles
Quantum technologies exploit entanglement to revolutionize computing,
measurements, and communications. This has stimulated the research in different
areas of physics to engineer and manipulate fragile many-particle entangled
states. Progress has been particularly rapid for atoms. Thanks to the large and
tunable nonlinearities and the well developed techniques for trapping,
controlling and counting, many groundbreaking experiments have demonstrated the
generation of entangled states of trapped ions, cold and ultracold gases of
neutral atoms. Moreover, atoms can couple strongly to external forces and light
fields, which makes them ideal for ultra-precise sensing and time keeping. All
these factors call for generating non-classical atomic states designed for
phase estimation in atomic clocks and atom interferometers, exploiting
many-body entanglement to increase the sensitivity of precision measurements.
The goal of this article is to review and illustrate the theory and the
experiments with atomic ensembles that have demonstrated many-particle
entanglement and quantum-enhanced metrology.Comment: 76 pages, 40 figures, 1 table, 603 references. Some figures bitmapped
at 300 dpi to reduce file siz
Phase diagram of incoherently driven strongly correlated photonic lattices
We explore theoretically the nonequilibrium photonic phases of an array of
coupled cavities in presence of incoherent driving and dissipation. In
particular, we consider a Hubbard model system where each site is a Kerr
nonlinear resonator coupled to a two-level emitter, which is pumped
incoherently. Within a Gutzwiller mean-field approach, we determine the
steady-state phase diagram of such a system. We find that, at a critical value
of the inter-cavity photon hopping rate, a second-order nonequilibrium phase
transition associated with the spontaneous breaking of the symmetry
occurs. The transition from an incompressible Mott-like photon fluid to a
coherent delocalized phase is driven by commensurability effects and not by the
competition between photon hopping and optical nonlinearity. The essence of the
mean-field predictions is corroborated by finite-size simulations obtained with
matrix product operators and corner-space renormalization methods.Comment: 12 pages, 9 figure
Keldysh Field Theory for Driven Open Quantum Systems
Recent experimental developments in diverse areas - ranging from cold atomic
gases over light-driven semiconductors to microcavity arrays - move systems
into the focus, which are located on the interface of quantum optics, many-body
physics and statistical mechanics. They share in common that coherent and
driven-dissipative quantum dynamics occur on an equal footing, creating genuine
non-equilibrium scenarios without immediate counterpart in condensed matter.
This concerns both their non-thermal flux equilibrium states, as well as their
many-body time evolution. It is a challenge to theory to identify novel
instances of universal emergent macroscopic phenomena, which are tied
unambiguously and in an observable way to the microscopic drive conditions. In
this review, we discuss some recent results in this direction. Moreover, we
provide a systematic introduction to the open system Keldysh functional
integral approach, which is the proper technical tool to accomplish a merger of
quantum optics and many-body physics, and leverages the power of modern quantum
field theory to driven open quantum systems.Comment: 73 pages, 13 figure
Quantum fluids of light
This article reviews recent theoretical and experimental advances in the
fundamental understanding and active control of quantum fluids of light in
nonlinear optical systems. In presence of effective photon-photon interactions
induced by the optical nonlinearity of the medium, a many-photon system can
behave collectively as a quantum fluid with a number of novel features stemming
from its intrinsically non-equilibrium nature. We present a rich variety of
photon hydrodynamical effects that have been recently observed, from the
superfluid flow around a defect at low speeds, to the appearance of a
Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of
topological excitations such as quantized vortices and dark solitons at the
surface of large impenetrable obstacles. While our review is mostly focused on
a class of semiconductor systems that have been extensively studied in recent
years (namely planar semiconductor microcavities in the strong light-matter
coupling regime having cavity polaritons as elementary excitations), the very
concept of quantum fluids of light applies to a broad spectrum of systems,
ranging from bulk nonlinear crystals, to atomic clouds embedded in optical
fibers and cavities, to photonic crystal cavities, to superconducting quantum
circuits based on Josephson junctions. The conclusive part of our article is
devoted to a review of the exciting perspectives to achieve strongly correlated
photon gases. In particular, we present different mechanisms to obtain
efficient photon blockade, we discuss the novel quantum phases that are
expected to appear in arrays of strongly nonlinear cavities, and we point out
the rich phenomenology offered by the implementation of artificial gauge fields
for photons.Comment: Accepted for publication on Rev. Mod. Phys. (in press, 2012
Quantum Metrology with Cold Atoms
Quantum metrology is the science that aims to achieve precision measurements
by making use of quantum principles. Attribute to the well-developed techniques
of manipulating and detecting cold atoms, cold atomic systems provide an
excellent platform for implementing precision quantum metrology. In this
chapter, we review the general procedures of quantum metrology and some
experimental progresses in quantum metrology with cold atoms. Firstly, we give
the general framework of quantum metrology and the calculation of quantum
Fisher information, which is the core of quantum parameter estimation. Then, we
introduce the quantum interferometry with single and multiparticle states. In
particular, for some typical multiparticle states, we analyze their ultimate
precision limits and show how quantum entanglement could enhance the
measurement precision beyond the standard quantum limit. Further, we review
some experimental progresses in quantum metrology with cold atomic systems.Comment: 53 pages, 9 figures, revised versio
Strong quantum correlation in a pair hybrid optomechanical cavities
We show the quantum correlation between two coupled hybrid optomechanical
cavities by quantifying the non-classical correlation using Gaussian quantum
discord. This involves analyzing and solving Heisenberg Langevin equations to
obtain the (12*12)dimensional covariance matrix of this system. Based on the
experimentalist conditions, we simulate quantum correlation of bipartite
steady-state with continuous conditions using Guassian quantum discord. We know
that the generation of quantum correlation and its robustness essentially
depend on the physical parameters of the system. We provide the stability
analysis by means of the RuthsHurwitz criterion to confirm the choices made
during the analysis of quantum discord dynamics
Perturbative Field-Theoretical Renormalization Group Approach to Driven-Dissipative Bose-Einstein Criticality
The universal critical behavior of the driven-dissipative non-equilibrium
Bose-Einstein condensation transition is investigated employing the
field-theoretical renormalization group method. Such criticality may be
realized in broad ranges of driven open systems on the interface of quantum
optics and many-body physics, from exciton-polariton condensates to cold atomic
gases. The starting point is a noisy and dissipative Gross-Pitaevski equation
corresponding to a complex valued Landau-Ginzburg functional, which captures
the near critical non-equilibrium dynamics, and generalizes Model A for
classical relaxational dynamics with non-conserved order parameter. We confirm
and further develop the physical picture previously established by means of a
functional renormalization group study of this system. Complementing this
earlier numerical analysis, we analytically compute the static and dynamical
critical exponents at the condensation transition to lowest non-trivial order
in the dimensional epsilon expansion about the upper critical dimension d_c =
4, and establish the emergence of a novel universal scaling exponent associated
with the non-equilibrium drive. We also discuss the corresponding situation for
a conserved order parameter field, i.e., (sub-)diffusive Model B with complex
coefficients.Comment: 17 pages, 6 figures, to appear in Phys. Rev. X (2014
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