493 research outputs found
Synthetic dimensions in ultracold molecules: quantum strings and membranes
Synthetic dimensions alter one of the most fundamental properties in nature,
the dimension of space. They allow, for example, a real three-dimensional
system to act as effectively four-dimensional. Driven by such possibilities,
synthetic dimensions have been engineered in ongoing experiments with ultracold
matter. We show that rotational states of ultracold molecules can be used as
synthetic dimensions extending to many - potentially hundreds of - synthetic
lattice sites. Microwaves coupling rotational states drive fully controllable
synthetic inter-site tunnelings, enabling, for example, topological band
structures. Interactions leads to even richer behavior: when molecules are
frozen in a real space lattice with uniform synthetic tunnelings, dipole
interactions cause the molecules to aggregate to a narrow strip in the
synthetic direction beyond a critical interaction strength, resulting in a
quantum string or a membrane, with an emergent condensate that lives on this
string or membrane. All these phases can be detected using measurements of
rotational state populations.Comment: 5-page article + 4 figures + references; 7 pages + 4 figures in
Supplemen
Correlations and enlarged superconducting phase of - chains of ultracold molecules on optical lattices
We compute physical properties across the phase diagram of the -
chain with long-range dipolar interactions, which describe ultracold polar
molecules on optical lattices. Our results obtained by the density-matrix
renormalization group (DMRG) indicate that superconductivity is enhanced when
the Ising component of the spin-spin interaction and the charge component
are tuned to zero, and even further by the long-range dipolar interactions.
At low densities, a substantially larger spin gap is obtained. We provide
evidence that long-range interactions lead to algebraically decaying
correlation functions despite the presence of a gap. Although this has recently
been observed in other long-range interacting spin and fermion models, the
correlations in our case have the peculiar property of having a small and
continuously varying exponent. We construct simple analytic models and
arguments to understand the most salient features.Comment: published version with minor modification
Accessing Rydberg-dressed interactions using many-body Ramsey dynamics
We demonstrate that Ramsey spectroscopy can be used to observe
Rydberg-dressed interactions. In contrast to many prior proposals, our scheme
operates comfortably within experimentally measured lifetimes, and accesses a
regime where quantum superpositions are crucial. The key idea is to build a
spin-1/2 from one level that is Rydberg-dressed and another that is not. These
levels may be hyperfine or long-lived electronic states. An Ising spin model
governs the Ramsey dynamics, for which we derive an exact solution. Due to the
structure of Rydberg interactions, the dynamics differs significantly from that
in other spin systems. As one example, spin echo can increase the rate at which
coherence decays. The results also apply to bare (undressed) Rydberg states as
a special case, for which we quantitatively reproduce recent ultrafast
experiments without fitting
Quantum magnetism with ultracold molecules
This article gives an introduction to the realization of effective quantum
magnetism with ultracold molecules in an optical lattice, reviews experimental
and theoretical progress, and highlights future opportunities opened up by
ongoing experiments. Ultracold molecules offer capabilities that are otherwise
difficult or impossible to achieve in other effective spin systems, such as
long-ranged spin-spin interactions with controllable degrees of spatial and
spin anisotropy and favorable energy scales. Realizing quantum magnetism with
ultracold molecules provides access to rich many-body behaviors, including many
exotic phases of matter and interesting excitations and dynamics.
Far-from-equilibrium dynamics plays a key role in our exposition, just as it
did in recent ultracold molecule experiments realizing effective quantum
magnetism. In particular, we show that dynamical probes allow the observation
of correlated many-body spin physics, even in polar molecule gases that are not
quantum degenerate. After describing how quantum magnetism arises in ultracold
molecules and discussing recent observations of quantum magnetism with polar
molecules, we survey prospects for the future, ranging from immediate goals to
long-term visions.Comment: 21 pages, 6 figures, 1 table. Review articl
A Model for Scattering with Proliferating Resonances: Many Coupled Square Wells
We present a multichannel model for elastic interactions, comprised of an
arbitrary number of coupled finite square-well potentials, and derive
semi-analytic solutions for its scattering behavior. Despite the model's
simplicity, it is flexible enough to include many coupled short-ranged
resonances in the vicinity of the collision threshold, as is necessary to
describe ongoing experiments in ultracold molecules and lanthanide atoms. We
also introduce a simple, but physically realistic, statistical ensemble for
parameters in this model. We compute the resulting probability distributions of
nearest-neighbor resonance spacings and analyze them by fitting to the Brody
distribution. We quantify the ability of alternative distribution functions,
for resonance spacing and resonance number variance, to describe the crossover
regime. The analysis demonstrates that the multichannel square-well model with
the chosen ensemble of parameters naturally captures the crossover from
integrable to chaotic scattering as a function of closed channel coupling
strength.Comment: 11 pages, 8 figure
Cooling Fermions in an Optical Lattice by Adiabatic Demagnetization
The Fermi-Hubbard model describes ultracold fermions in an optical lattice
and exhibits antiferromagnetic long-ranged order below the N\'{e}el
temperature. However, reaching this temperature in the lab has remained an
elusive goal. In other atomic systems, such as trapped ions, low temperatures
have been successfully obtained by adiabatic demagnetization, in which a strong
effective magnetic field is applied to a spin-polarized system, and the
magnetic field is adiabatically reduced to zero. Unfortunately, applying this
approach to the Fermi-Hubbard model encounters a fundamental obstacle: the
symmetry introduces many level crossings that prevent the system from
reaching the ground state, even in principle. However, by breaking the
symmetry with a spin-dependent tunneling, we show that adiabatic
demagnetization can achieve low temperature states. Using density matrix
renormalization group (DMRG) calculations in one dimension, we numerically find
that demagnetization protocols successfully reach low temperature states of a
spin-anisotropic Hubbard model, and we discuss how to optimize this protocol
for experimental viability. By subsequently ramping spin-dependent tunnelings
to spin-independent tunnelings, we expect that our protocol can be employed to
produce low-temperature states of the Fermi-Hubbard Model.Comment: References adde
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