76 research outputs found
Time-resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices
Quantum mechanical superexchange interactions form the basis of quantum
magnetism in strongly correlated electronic media. We report on the direct
measurement of superexchange interactions with ultracold atoms in optical
lattices. After preparing a spin-mixture of ultracold atoms in an
antiferromagnetically ordered state, we measure a coherent
superexchange-mediated spin dynamics with coupling energies from 5 Hz up to 1
kHz. By dynamically modifying the potential bias between neighboring lattice
sites, the magnitude and sign of the superexchange interaction can be
controlled, thus allowing the system to be switched between antiferromagnetic
or ferromagnetic spin interactions. We compare our findings to predictions of a
two-site Bose-Hubbard model and find very good agreement, but are also able to
identify corrections which can be explained by the inclusion of direct
nearest-neighbor interactions.Comment: 24 pages, 7 figure
Counting atoms using interaction blockade in an optical superlattice
We report on the observation of an interaction blockade effect for ultracold
atoms in optical lattices, analogous to Coulomb blockade observed in mesoscopic
solid state systems. When the lattice sites are converted into biased double
wells, we detect a discrete set of steps in the well population for increasing
bias potentials. These correspond to tunneling resonances where the atom number
on each side of the barrier changes one by one. This allows us to count and
control the number of atoms within a given well. By evaluating the amplitude of
the different plateaus, we can fully determine the number distribution of the
atoms in the lattice, which we demonstrate for the case of a superfluid and
Mott insulating regime of 87Rb.Comment: 4 pages, 4 figure
High-fidelity gates via RF-induced F\"{o}rster resonances
Registers of trapped neutral atoms, excited to Rydberg states to induce
strong long-distance interactions, are extensively studied for direct
applications in quantum computing. In this regard, new effective approaches to
the creation of multiqubit quantum gates arise high interest. Here, we present
a novel gate implementation technique based on RF-induced few-body F\"{o}rster
resonances. External radio frequency (RF) control field allows us to manipulate
the phase and population dynamics of many-atom system, thus enabling the
realization of universal quantum gates. We numerically
demonstrate RF-induced resonant interactions, as well as high-precision
three-qubit gates. The extreme controllability of interactions provided by RF
makes it possible to implement gates for a wide range of parameters of the
atomic system, and significantly facilitates their experimental implementation.
For the considered error sources, we achieve theoretical gate fidelities
compatible with error correction () using reasonable experimental
parameters.Comment: 6 pages, 3 figures, 1 tabl
The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison study
An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and Cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution.These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modifie
Phases and relativity in atomic gravimetry
The phase observable measured by an atomic gravimeter built up on stimulated
Raman transitions is discussed in a fully relativistic context. It is written
in terms of laser phases which are invariant under relativistic gauge
transformations. The dephasing is the sum of light and atomic contributions
which are connected to one another through their interplay with conservation
laws at the interaction vertices. In the case of a closed geometry, a compact
form of the dephasing is written in terms of a Legendre transform of the laser
phases. These general expressions are illustrated by discussing two techniques
used for compensating the Doppler shift, one corresponding to chirped
frequencies and the other one to ramped variations.Comment: 7 pages, 1 figur
Many-body Landau-Zener dynamics in coupled 1D Bose liquids
The Landau-Zener model of a quantum mechanical two-level system driven with a
linearly time dependent detuning has served over decades as a textbook paradigm
of quantum dynamics. In their seminal work [L. D. Landau, Physik. Z. Sowjet. 2,
46 (1932); C. Zener, Proc. Royal Soc. London 137, 696 (1932)], Landau and Zener
derived a non-perturbative prediction for the transition probability between
two states, which often serves as a reference point for the analysis of more
complex systems. A particularly intriguing question is whether that framework
can be extended to describe many-body quantum dynamics. Here we report an
experimental and theoretical study of a system of ultracold atoms, offering a
direct many-body generalization of the Landau-Zener problem. In a system of
pairwise tunnel-coupled 1D Bose liquids we show how tuning the correlations of
the 1D gases, the tunnel coupling between the tubes and the inter-tube
interactions strongly modify the original Landau-Zener picture. The results are
explained using a mean-field description of the inter-tube condensate
wave-function, coupled to the low-energy phonons of the 1D Bose liquid.Comment: 13 pages, 10 figures
Limits to the sensitivity of a low noise compact atomic gravimeter
A detailed analysis of the most relevant sources of phase noise in an atomic
interferometer is carried out, both theoretically and experimentally. Even a
short interrogation time of 100 ms allows our cold atom gravimeter to reach an
excellent short term sensitivity to acceleration of g at 1s.
This result relies on the combination of a low phase noise laser system,
efficient detection scheme and good shielding from vibrations. In particular,
we describe a simple and robust technique of vibration compensation, which is
based on correcting the interferometer signal by using the AC acceleration
signal measured by a low noise seismometer.Comment: 30 pages, 14 figure
Detecting inertial effects with airborne matter-wave interferometry
Inertial sensors relying on atom interferometry offer a breakthrough advance
in a variety of applications, such as inertial navigation, gravimetry or
ground- and space-based tests of fundamental physics. These instruments require
a quiet environment to reach their performance and using them outside the
laboratory remains a challenge. Here we report the first operation of an
airborne matter-wave accelerometer set up aboard a 0g plane and operating
during the standard gravity (1g) and microgravity (0g) phases of the flight. At
1g, the sensor can detect inertial effects more than 300 times weaker than the
typical acceleration fluctuations of the aircraft. We describe the improvement
of the interferometer sensitivity in 0g, which reaches 2 x 10-4 ms-2 / \surdHz
with our current setup. We finally discuss the extension of our method to
airborne and spaceborne tests of the Universality of free fall with matter
waves.Comment: 7 pages, 6 figures. The final version of this article is available in
OPEN access (free) from the editor website at
http://www.nature.com/ncomms/journal/v2/n9/full/ncomms1479.htm
Effect of Doublon-Holon Binding on Mott transition---Variational Monte Carlo Study of Two-Dimensional Bose Hubbard Models
To understand the mechanism of Mott transitions in case of no magnetic
influence, superfluid-insulator (Mott) transitions in the S=0 Bose Hubbard
model at unit filling are studied on the square and triangular lattices, using
a variational Monte Carlo method. In trial many-body wave functions, we
introduce various types of attractive correlation factors between a
doubly-occupied site (doublon, D) and an empty site (holon, H), which play a
central role for Mott transitions, in addition to the onsite repulsive
(Gutzwiller) factor. By optimizing distance-dependent parameters, we study
various properties of this type of wave functions. With a hint from the Mott
transition arising in a completely D-H bound state, we propose an improved
picture of Mott transitions, by introducing two characteristic length scales,
the D-H binding length and the minimum D-D exclusion length
. Generally, a Mott transition occurs when becomes
comparable to . In the conductive (superfluid) state, domains of
D-H pairs overlap with each other (); thereby D and
H can propagate independently as density carriers by successively exchanging
the partners. In contrast, intersite repulsive Jastrow (D-D and H-H) factors
have little importance for the Mott transition.Comment: 16 pages, 22 figures, submitted to J. Phys. Soc. Jp
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