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1.
Phys Rev Lett ; 118(13): 130405, 2017 Mar 31.
Article in English | MEDLINE | ID: mdl-28409948

ABSTRACT

We measure the transport properties of two-dimensional ultracold Fermi gases during transverse demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to distinguish bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of a spin current around the local magnetization. When the two-dimensional scattering length is tuned to be comparable to the inverse Fermi wave vector k_{F}^{-1}, we find that the bare transverse spin diffusivity reaches a minimum of 1.7(6)ℏ/m, where m is the bare particle mass. The rate of demagnetization is also reflected in the growth rate of the s-wave contact, observed using time-resolved spectroscopy. The contact rises to 0.28(3)k_{F}^{2} per particle, which quantifies how scaling symmetry is broken by near-resonant interactions, unlike in unitary three-dimensional systems. Our observations support the conjecture that, in systems with strong scattering, the local relaxation rate is bounded from above by k_{B}T/ℏ.

2.
Phys Rev Lett ; 114(1): 015301, 2015 Jan 09.
Article in English | MEDLINE | ID: mdl-25615476

ABSTRACT

We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of (40)K precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter γ=1.08(9) and a bare transverse spin diffusivity D(0)(⊥)=2.3(4)ℏ/m for a normal-state gas initialized with full polarization and at one-fifth of the Fermi temperature, where m is the atomic mass. One might expect γ=0 at unitarity, where two-body scattering is purely dissipative. We observe γ→0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a nonzero γ. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0<(k(F)a)(-1)≲1.3, where k(F) is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that the sign change in γ indicates a switching of branch between a repulsive and an attractive Fermi gas.

3.
Science ; 344(6185): 722-4, 2014 May 16.
Article in English | MEDLINE | ID: mdl-24833387

ABSTRACT

Understanding the quantum dynamics of strongly interacting fermions is a problem relevant to diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasma. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions. Here, we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We observe the demagnetization of the gas, caused by diffusive spin transport. At low temperatures, the diffusion constant saturates to the conjectured quantum-mechanical lower bound ≃ h/m, where m is the particle mass. The development of pair correlations, indicating the transformation of the initially noninteracting gas toward a unitary spin mixture, is observed by measuring Tan's contact parameter.

4.
Phys Rev Lett ; 108(20): 205301, 2012 May 18.
Article in English | MEDLINE | ID: mdl-23003151

ABSTRACT

The concept of valence-bond resonance plays a fundamental role in the theory of the chemical bond and is believed to lie at the heart of many-body quantum physical phenomena. Here we show direct experimental evidence of a time-resolved valence-bond quantum resonance with ultracold bosonic atoms in an optical lattice. By means of a superlattice structure we create a three-dimensional array of independent four-site plaquettes, which we can fully control and manipulate in parallel. Moreover, we show how small-scale plaquette resonating valence-bond (RVB) states with s- and d-wave symmetry can be created and characterized. We anticipate our findings to open the path towards the creation and analysis of many-body RVB states in ultracold atomic gases.

5.
Phys Rev Lett ; 106(15): 155302, 2011 Apr 15.
Article in English | MEDLINE | ID: mdl-21568570

ABSTRACT

We simulate numerically the dynamics of strongly correlated bosons in a two-leg ladder subject to a time-dependent energy bias between the two chains. When all atoms are initially in the leg with higher energy, we find a drastic reduction of the interchain particle transfer for slow linear sweeps, in quantitative agreement with recent experiments. This effect is preceded by a rapid broadening of the quasimomentum distribution of atoms, signaling the presence of a bath of low-energy excitations in the chains. We further investigate the scenario of quantum quenches to fixed values of the energy bias. We find that for a large enough density the momentum distribution relaxes to that of an equilibrium thermal state with the same energy.

6.
Phys Rev Lett ; 107(25): 255301, 2011 Dec 16.
Article in English | MEDLINE | ID: mdl-22243087

ABSTRACT

We use Raman-assisted tunneling in an optical superlattice to generate large tunable effective magnetic fields for ultracold atoms. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of 1 flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for noninteracting particles. We provide a measurement of the local phase acquired from Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the magnetic field is directly revealed.

7.
Phys Rev Lett ; 103(3): 033003, 2009 Jul 17.
Article in English | MEDLINE | ID: mdl-19659274

ABSTRACT

We experimentally demonstrate electromagnetically induced transparency and light storage with ultracold 87Rb atoms in a Mott insulating state in a three-dimensional optical lattice. We have observed light storage times of approximately 240 ms, to our knowledge the longest ever achieved in ultracold atomic samples. Using the differential light shift caused by a spatially inhomogeneous far detuned light field we imprint a "phase gradient" across the atomic sample, resulting in controlled angular redirection of the retrieved light pulse.

8.
Phys Rev Lett ; 101(15): 155303, 2008 Oct 10.
Article in English | MEDLINE | ID: mdl-18999609

ABSTRACT

We analyze the interference pattern produced by ultracold atoms released from an optical lattice, commonly interpreted as the momentum distributions of the trapped quantum gas. We show that for finite times of flight the resulting density distribution can, however, be significantly altered, similar to a near-field diffraction regime in optics. We illustrate our findings with a simple model and realistic quantum Monte Carlo simulations for bosonic atoms and compare the latter to experiments.

9.
Phys Rev Lett ; 101(9): 090404, 2008 Aug 29.
Article in English | MEDLINE | ID: mdl-18851591

ABSTRACT

We report on the observation of an interaction blockade effect for ultracold atoms in optical lattices, analogous to the 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.

10.
Science ; 319(5861): 295-9, 2008 Jan 18.
Article in English | MEDLINE | ID: mdl-18096767

ABSTRACT

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 measured coherent superexchange-mediated spin dynamics with coupling energies from 5 hertz up to 1 kilohertz. 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 and 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 that can be explained by the inclusion of direct nearest-neighbor interactions.

11.
Nature ; 448(7157): 1029-32, 2007 Aug 30.
Article in English | MEDLINE | ID: mdl-17728753

ABSTRACT

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism.

12.
J Emerg Med ; 16(3): 395-401, 1998.
Article in English | MEDLINE | ID: mdl-9610966

ABSTRACT

Attempts at modification of test-ordering practices among housestaff demonstrate reversion to prior behavior after relaxation of surveillance. To test the hypothesis that use of pre-specified criteria for obtaining emergency department (ED) lumbosacral spine (LSS) films would reduce LSS utilization, we designed a cross-sectional observational study, with 1-year follow-up. The primary endpoint was estimate of efficacy, expressed as a proportionate change in LSS films from 1982 to 1992, adjusted for ED volume. The secondary endpoint was estimate of safety of this protocol. We found a 28% proportionate decrease in LSS films from 1982 to 1992. Among 520 patients with back complaints not meeting criteria for films, 4 had serious back pathology on follow-up that may have been detected on the index ED visit if LSS films had been ordered. All four were HIV(+), three of whom initially denied, then subsequently admitted, to active intravenous drug use. We conclude that use of a simple requisition form is associated with a quantitatively and statistically significant sustained reduction in utilization of ED LSS radiography. This protocol appears to provide a safe, cost-efficient means of limiting LSS film ordering by housestaff, with the possible exception of HIV(+) individuals. Because the association between HIV and radiographic abnormalities may be confounded by intravenous drug use, we cannot determine, on the basis of our data, whether HIV(+) status should be considered an independent criterion for obtaining LSS films.


Subject(s)
Emergency Service, Hospital/statistics & numerical data , Guideline Adherence , Low Back Pain/diagnostic imaging , Radiography/statistics & numerical data , Adult , Aged , Clinical Protocols , Cross-Sectional Studies , Diagnostic Errors/statistics & numerical data , Emergency Service, Hospital/standards , Female , Follow-Up Studies , Forms and Records Control , Humans , Internship and Residency , Lumbosacral Region/diagnostic imaging , Male , Middle Aged , Patient Selection , United States , Utilization Review/statistics & numerical data
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