313 research outputs found
Loops and Strings in a Superconducting Lattice Gauge Simulator
We propose an architecture for an analog quantum simulator of
electromagnetism in 2+1 dimensions, based on an array of superconducting
fluxonium devices. The encoding is in the integer (spin-1 representation of the
quantum link model formulation of compact U(1) lattice gauge theory. We show
how to engineer Gauss' law via an ancilla mediated gadget construction, and how
to tune between the strongly coupled and intermediately coupled regimes. The
witnesses to the existence of the predicted confining phase of the model are
provided by nonlocal order parameters from Wilson loops and disorder parameters
from 't Hooft strings. We show how to construct such operators in this model
and how to measure them nondestructively via dispersive coupling of the
fluxonium islands to a microwave cavity mode. Numerical evidence is found for
the existence of the confined phase in the ground state of the simulation
Hamiltonian on a ladder geometry.Comment: 17 pages, 5 figures. Published versio
Strongly correlated 2D quantum phases with cold polar molecules: controlling the shape of the interaction potential
We discuss techniques to tune and shape the long-range part of the
interaction potentials in quantum gases of polar molecules by dressing
rotational excitations with static and microwave fields. This provides a novel
tool towards engineering strongly correlated quantum phases in combination with
low dimensional trapping geometries. As an illustration, we discuss a 2D
crystalline phase, and a superfluid-crystal quantum phase transition.Comment: 4 pages, 3 figure
Effects of random localizing events on matter waves: formalism and examples
A formalism is introduced to describe a number of physical processes that may
break down the coherence of a matter wave over a characteristic length scale l.
In a second-quantized description, an appropriate master equation for a set of
bosonic "modes" (such as atoms in a lattice, in a tight-binding approximation)
is derived. Two kinds of "localizing processes" are discussed in some detail
and shown to lead to master equations of this general form: spontaneous
emission (more precisely, light scattering), and modulation by external random
potentials. Some of the dynamical consequences of these processes are
considered: in particular, it is shown that they generically lead to a damping
of the motion of the matter-wave currents, and may also cause a "flattening" of
the density distribution of a trapped condensate at rest.Comment: v3; a few corrections, especially in Sections IV and
Interlayer superfluidity in bilayer systems of fermionic polar molecules
We consider fermionic polar molecules in a bilayer geometry where they are
oriented perpendicularly to the layers, which permits both low inelastic losses
and superfluid pairing. The dipole-dipole interaction between molecules of
different layers leads to the emergence of interlayer superfluids. The
superfluid regimes range from BCS-like fermionic superfluidity with a high
to Bose-Einstein (quasi-)condensation of interlayer dimers, thus
exhibiting a peculiar BCS-BEC crossover. We show that one can cover the entire
crossover regime under current experimental conditions.Comment: 4 pages, 4 figure
Dispersion interactions and reactive collisions of ultracold polar molecules
Progress in ultracold experiments with polar molecules requires a clear
understanding of their interactions and reactivity at ultra-low collisional
energies. Two important theoretical steps in this process are the
characterization of interaction potentials between molecules and the modeling
of reactive scattering mechanism. Here, we report on the {\it abinitio}
calculation of isotropic and anisotropic van der Waals interaction potentials
for polar KRb and RbCs colliding with each other or with ultracold atoms. Based
on these potentials and two short-range scattering parameters we then develop a
single-channel scattering model with flexible boundary conditions. Our
calculations show that at low temperatures (and in absence of an external
electric field) the reaction rates between molecules or molecules with atoms
have a resonant character as a function of the short-range parameters. We also
find that both the isotropic and anisotropic van der Waals coefficients have
significant contributions from dipole coupling to excited electronic states.
Their values can differ dramatically from those solely obtained from the
permanent dipole moment. A comparison with recently obtained reaction rates of
fermionic KRb shows that the experimental data can not be
explained by a model where the short-range scattering parameters are
independent of the relative orbital angular momentum or partial wave.Comment: 15 pages, 12 figure
Conductivity in organic semiconductors hybridized with the vacuum field
Organic semiconductors have generated considerable interest for their
potential for creating inexpensive and flexible devices easily processed on a
large scale [1-11]. However technological applications are currently limited by
the low mobility of the charge carriers associated with the disorder in these
materials [5-8]. Much effort over the past decades has therefore been focused
on optimizing the organisation of the material or the devices to improve
carrier mobility. Here we take a radically different path to solving this
problem, namely by injecting carriers into states that are hybridized to the
vacuum electromagnetic field. These are coherent states that can extend over as
many as 10^5 molecules and should thereby favour conductivity in such
materials. To test this idea, organic semiconductors were strongly coupled to
the vacuum electromagnetic field on plasmonic structures to form polaritonic
states with large Rabi splittings ca. 0.7 eV. Conductivity experiments show
that indeed the current does increase by an order of magnitude at resonance in
the coupled state, reflecting mostly a change in field-effect mobility as
revealed when the structure is gated in a transistor configuration. A
theoretical quantum model is presented that confirms the delocalization of the
wave-functions of the hybridized states and the consequences on the
conductivity. While this is a proof-of-principle study, in practice
conductivity mediated by light-matter hybridized states is easy to implement
and we therefore expect that it will be used to improve organic devices. More
broadly our findings illustrate the potential of engineering the vacuum
electromagnetic environment to modify and to improve properties of materials.Comment: 16 pages, 13 figure
Topological p_x+ip_y Superfluid Phase of Fermionic Polar Molecules
We discuss the topological p_x+ip_y superfluid phase in a 2D gas of
single-component fermionic polar molecules dressed by a circularly polarized
microwave field. This phase emerges because the molecules may interact with
each other via a potential V_0(r) that has an attractive dipole-dipole 1/r^3
tail, which provides p-wave superfluid pairing at fairly high temperatures. We
calculate the amplitude of elastic p-wave scattering in the potential V_0(r)
taking into account both the anomalous scattering due to the dipole-dipole tail
and the short-range contribution. This amplitude is then used for the
analytical and numerical solution of the renormalized BCS gap equation which
includes the second order Gor'kov-Melik-Barkhudarov corrections and the
correction related to the effective mass of the quasiparticles. We find that
the critical temperature T_c can be varied within a few orders of magnitude by
modifying the short-range part of the potential V_0(r). The decay of the system
via collisional relaxation of molecules to dressed states with lower energies
is rather slow due to the necessity of a large momentum transfer. The presence
of a constant transverse electric field reduces the inelastic rate, and the
lifetime of the system can be of the order of seconds even at 2D densities ~
10^9 cm^{-2}. This leads to T_c of up to a few tens of nanokelvins and makes it
realistic to obtain the topological p_x+ip_y phase in experiments with
ultracold polar molecules.Comment: 15 pages, 9 figures, published versio
Realization of an Excited, Strongly-Correlated Quantum Gas Phase
Ultracold atomic physics offers myriad possibilities to study strongly
correlated many-body systems in lower dimensions. Typically, only ground state
phases are accessible. Using a tunable quantum gas of bosonic cesium atoms, we
realize and control in one dimensional geometry a highly excited quantum phase
that is stabilized in the presence of attractive interactions by maintaining
and strengthening quantum correlations across a confinement-induced resonance.
We diagnose the crossover from repulsive to attractive interactions in terms of
the stiffness and the energy of the system. Our results open up the
experimental study of metastable excited many-body phases with strong
correlations and their dynamical properties
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