477 research outputs found
Jointly learning trajectory generation and hitting point prediction in robot table tennis
This paper proposes a combined learning framework for a table tennis robot. In a typical robot table tennis setup, a single striking point is predicted for the robot on the basis of the ball's initial state. Subsequently, the desired Cartesian racket state and the desired joint states at the striking time are determined. Finally, robot joint trajectories are generated. Instead of predicting a single striking point, we propose to construct a ball trajectory prediction map, which predicts the ball's entire rebound trajectory using the ball's initial state. We construct as well a robot trajectory generation map, which predicts the robot joint movement pattern and the movement duration using the Cartesian racket trajectories without the need of inverse kinematics, where a correlation function is used to adapt these joint movement parameters according to the ball flight trajectory. With joint movement parameters, we can directly generate joint trajectories. Additionally, we introduce a reinforcement learning approach to modify robot joint trajectories such that the robot can return balls well. We validate this new framework in both the simulated and the real robotic systems and illustrate that a seven degree-of-freedom Barrett WAM robot performs well
Finite temperature quantum simulation of stabilizer Hamiltonians
We present a scheme for robust finite temperature quantum simulation of
stabilizer Hamiltonians. The scheme is designed for realization in a physical
system consisting of a finite set of neutral atoms trapped in an addressable
optical lattice that are controllable via 1- and 2-body operations together
with dissipative 1-body operations such as optical pumping. We show that these
minimal physical constraints suffice for design of a quantum simulation scheme
for any stabilizer Hamiltonian at either finite or zero temperature. We
demonstrate the approach with application to the abelian and non-abelian toric
codes.Comment: 13 pages, 2 figure
A general T-matrix approach applied to two-body and three-body problems in cold atomic gases
We propose a systematic T-matrix approach to solve few-body problems with
s-wave contact interactions in ultracold atomic gases. The problem is generally
reduced to a matrix equation expanded by a set of orthogonal molecular states,
describing external center-of-mass motions of pairs of interacting particles;
while each matrix element is guaranteed to be finite by a proper
renormalization for internal relative motions. This approach is able to
incorporate various scattering problems and the calculations of related
physical quantities in a single framework, and also provides a physically
transparent way to understand the mechanism of resonance scattering. For
applications, we study two-body effective scattering in 2D-3D mixed dimensions,
where the resonance position and width are determined with high precision from
only a few number of matrix elements. We also study three fermions in a
(rotating) harmonic trap, where exotic scattering properties in terms of mass
ratios and angular momenta are uniquely identified in the framework of
T-matrix.Comment: 14 pages, 4 figure
Insulator-Superfluid transition of spin-1 bosons in an optical lattice in magnetic field
We study the insulator-superfluid transition of spin-1 bosons in an optical
lattice in a uniform magnetic field. Based on a mean-field approximation we
obtained a zero-temperature phase diagram. We found that depending on the
particle number the transition for bosons with antiferromagnetic interaction
may occur into different superfluid phases with spins aligned along or opposite
to the field direction. This is qualitatively different from the field-free
transition for which the mean-field theory predicts a unique (polar) superfluid
state for any particle number.Comment: 10 pages, 2 eps figure
Density Waves in Layered Systems with Fermionic Polar Molecules
A layered system of two-dimensional planes containing fermionic polar
molecules can potentially realize a number of exotic quantum many-body states.
Among the predictions, are density-wave instabilities driven by the anisotropic
part of the dipole-dipole interaction in a single layer. However, in typical
multilayer setups it is reasonable to expect that the onset and properties of a
density-wave are modified by adjacent layers. Here we show that this is indeed
the case. For multiple layers the critical strength for the density-wave
instability decreases with the number of layers. The effect depends on density
and is more pronounced in the low density regime. The lowest solution of the
instability corresponds to the density waves in the different layers being
in-phase, whereas higher solutions have one or several adjancet layers that are
out of phase. The parameter regime needed to explore this instability is within
reach of current experiments.Comment: 7 pages, 4 figures. Final version in EPJD, EuroQUAM special issue
"Cold Quantum Matter - Achievements and Prospects
Superfluidity versus Bloch oscillations in confined atomic gases
We study the superfluid properties of (quasi) one-dimensional bosonic atom
gases/liquids in traps with finite geometries in the presence of strong quantum
fluctuations. Driving the condensate with a moving defect we find the
nucleation rate for phase slips using instanton techniques. While phase slips
are quenched in a ring resulting in a superfluid response, they proliferate in
a tube geometry where we find Bloch oscillations in the chemical potential.
These Bloch oscillations describe the individual tunneling of atoms through the
defect and thus are a consequence of particle quantization.Comment: 4 pages, 1 figur
Experimental study of the transport of coherent interacting matter-waves in a 1D random potential induced by laser speckle
We present a detailed analysis of the 1D expansion of a coherent interacting
matterwave (a Bose-Einstein condensate) in the presence of disorder. A 1D
random potential is created via laser speckle patterns. It is carefully
calibrated and the self-averaging properties of our experimental system are
discussed. We observe the suppression of the transport of the BEC in the random
potential. We discuss the scenario of disorder-induced trapping taking into
account the radial extension in our experimental 3D BEC and we compare our
experimental results with the theoretical predictions
Theory of superfluidity and drag force in the one-dimensional Bose gas
The one-dimensional Bose gas is an unusual superfluid. In contrast to higher
spatial dimensions, the existence of non-classical rotational inertia is not
directly linked to the dissipationless motion of infinitesimal impurities.
Recently, experimental tests with ultracold atoms have begun and quantitative
predictions for the drag force experienced by moving obstacles have become
available. This topical review discusses the drag force obtained from linear
response theory in relation to Landau's criterion of superfluidity. Based upon
improved analytical and numerical understanding of the dynamical structure
factor, results for different obstacle potentials are obtained, including
single impurities, optical lattices and random potentials generated from
speckle patterns. The dynamical breakdown of superfluidity in random potentials
is discussed in relation to Anderson localization and the predicted
superfluid-insulator transition in these systems.Comment: 17 pages, 12 figures, mini-review prepared for the special issue of
Frontiers of Physics "Recent Progresses on Quantum Dynamics of Ultracold
Atoms and Future Quantum Technologies", edited by Profs. Lee, Ueda, and
Drummon
Thermodynamics of Dipolar Chain Systems
The thermodynamics of a quantum system of layers containing perpendicularly
oriented dipolar molecules is studied within an oscillator approximation for
both bosonic and fermionic species. The system is assumed to be built from
chains with one molecule in each layer. We consider the effects of the
intralayer repulsion and quantum statistical requirements in systems with more
than one chain. Specifically, we consider the case of two chains and solve the
problem analytically within the harmonic Hamiltonian approach which is accurate
for large dipole moments. The case of three chains is calculated numerically.
Our findings indicate that thermodynamic observables, such as the heat
capacity, can be used to probe the signatures of the intralayer interaction
between chains. This should be relevant for near future experiments on polar
molecules with strong dipole moments.Comment: 15 pages, 5 figures, final versio
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