arXiv subjects

I. B. Spielman

Publications and source records attributed to I. B. Spielman.

90 records · Page 5Linked to original sources

arXiv2010

Spin-charge-density wave in a squircle-like Fermi surface for ultracold atoms

We derive and discuss an experimentally realistic model describing ultracold atoms in an optical lattice including a commensurate, but staggered, Zeeman field. The resulting band structure is quite exotic; fermions in the third band have an unusual rounded picture-frame Fermi surface (essentially two concentric squircles), leading to imperfect nesting. We develop a generalized SO(3,1)xSO(3,1) theory describing the spin and charge degrees of freedom simultaneously, and show that the system can develop a coupled spin-charge-density wave order. This ordering is absent in studies of the Hubbard model that treat spin and charge density separately.

cond-mat.quant-gas↗

arXiv2010

Engineering Time-Reversal Invariant Topological Insulators With Ultra-Cold Atoms

Topological insulators are a broad class of unconventional materials that are insulating in the interior but conduct along the edges. This edge transport is topologically protected and dissipationless. Until recently, all existing topological insulators, known as quantum Hall states, violated time-reversal symmetry. However, the discovery of the quantum spin Hall effect demonstrated the existence of novel topological states not rooted in time-reversal violations. Here, we lay out an experiment to realize time-reversal topological insulators in ultra-cold atomic gases subjected to synthetic gauge fields in the near-field of an atom-chip. In particular, we introduce a feasible scheme to engineer sharp boundaries where the "edge states" are localized. Besides, this multi-band system has a large parameter space exhibiting a variety of quantum phase transitions between topological and normal insulating phases. Due to their unprecedented controllability, cold-atom systems are ideally suited to realize topological states of matter and drive the development of topological quantum computing.

cond-mat.quant-gas↗

arXiv2010

Phases of a 2D Bose Gas in an Optical Lattice

Ultra-cold atoms in optical lattices realize simple, fundamental models in condensed matter physics. Our 87Rb Bose-Einstein condensate is confined in a harmonic trapping potential to which we add an optical lattice potential. Here we realize the 2D Bose-Hubbard Hamiltonian and focus on the effects of the harmonic trap, not present in bulk condensed matter systems. By measuring condensate fraction we identify the transition from superfluid to Mott insulator as a function of atom density and lattice depth. Our results are in excellent agreement with the quantum Monte Carlo universal state diagram, suitable for trapped systems, introduced by Rigol et al. (Phys. Rev. A 79, 053605 (2009)).

cond-mat.quant-gas↗

arXiv2009

Raman processes and effective gauge potentials

A new technique is described by which light-induced gauge potentials allow systems of ultra-cold neutral atoms to behave like charged particles in a magnetic field. Here, atoms move in a uniform laser field with a spatially varying Zeeman shift and experience an effective magnetic field. This technique is applicable for atoms with two or more internal ground states. Finally, an explicit model of the system using a single-mode 2D Gross-Pitaevskii equation yields the expected vortex lattice.

physics.atom-ph↗

arXiv2009

Rapid production of $^{87}$Rb BECs in a combined magnetic and optical potential

We describe an apparatus for quickly and simply producing $\Rb87$ Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT) and then capture the atom in a magnetic quadrupole trap and force rf evaporation. We then transfer the resulting cold, dense cloud into a spatially mode-matched optical dipole trap by lowering the quadrupole field gradient to below gravity. This technique combines the efficient capture of atoms from a MOT into a magnetic trap with the rapid evaporation of optical dipole traps; the approach is insensitive to the peak quadrupole gradient and the precise trapping beam waist. Our system reliably produces a condensate with $N\approx2\times10^6$ atoms every $16\second$.

cond-mat.quant-gas↗

arXiv2009

Field-sensitive addressing and control of field-insensitive neutral-atom qubits

The establishment of a scalable scheme for quantum computing with addressable and long-lived qubits would be a scientific watershed, harnessing the laws of quantum physics to solve classically intractable problems. The design of many proposed quantum computational platforms is driven by competing needs: isolating the quantum system from the environment to prevent decoherence, and easily and accurately controlling the system with external fields. For example, neutral-atom optical-lattice architectures provide environmental isolation through the use of states that are robust against fluctuating external fields, yet external fields are essential for qubit addressing. Here we demonstrate the selection of individual qubits with external fields, despite the fact that the qubits are in field-insensitive superpositions. We use a spatially inhomogeneous external field to map selected qubits to a different field-insensitive superposition ("optical MRI"), minimally perturbing unselected qubits, despite the fact that the addressing field is not spatially localized. We show robust single-qubit rotations on neutral-atom qubits located at selected lattice sites. This precise coherent control is an important step forward for lattice-based neutral-atom quantum computation, and is quite generally applicable to state transfer and qubit isolation in other architectures using field-insensitive qubits.

quant-ph↗

arXiv2009

Quantum and Classical Dynamics of a BEC in a Large-Period Optical Lattice

We experimentally investigate diffraction of a Rb-87 Bose-Einstein condensate from a 1D optical lattice. We use a range of lattice periods and timescales, including those beyond the Raman-Nath limit. We compare the results to quantum mechanical and classical simulations, with quantitative and qualitative agreement, respectively. The classical simulation predicts that the envelope of the time-evolving diffraction pattern is shaped by caustics: singularities in the phase space density of classical trajectories. This behavior becomes increasingly clear as the lattice period grows.

quant-ph↗

arXiv2008

A Bose-Einstein Condensate in a Uniform Light-induced Vector Potential

We use a two-photon dressing field to create an effective vector gauge potential for Bose-condensed Rb atoms in the F=1 hyperfine ground state. The dressed states in this Raman field are spin and momentum superpositions, and we adiabatically load the atoms into the lowest energy dressed state. The effective Hamiltonian of these neutral atoms is like that of charged particles in a uniform magnetic vector potential, whose magnitude is set by the strength and detuning of Raman coupling. The spin and momentum decomposition of the dressed states reveals the strength of the effective vector potential, and our measurements agree quantitatively with a simple single-particle model. While the uniform effective vector potential described here corresponds to zero magnetic field, our technique can be extended to non-uniform vector potentials, giving non-zero effective magnetic fields.

cond-mat.other↗

arXiv2008

Condensate fraction in a 2D Bose gas measured across the Mott-insulator transition

We realize a single-band 2D Bose-Hubbard system with Rb atoms in an optical lattice and measure the condensate fraction as a function of lattice depth, crossing from the superfluid to the Mott-insulating phase. We quantitatively identify the location of the superfluid to normal transition by observing when the condensed fraction vanishes. Our measurement agrees with recent quantum Monte Carlo calculations for a finite-sized 2D system to within experimental uncertainty.

cond-mat.other↗

arXiv2008

Atoms in a radiofrequency-dressed optical lattice

We load cold atoms into an optical lattice dramatically reshaped by radiofrequency (rf) coupling of state-dependent lattice potentials. This rf dressing changes the unit cell of the lattice at a subwavelength scale, such that its curvature and topology departs strongly from that of a simple sinusoidal lattice potential. Radiofrequency dressing has previously been performed at length scales from mm to tens of microns, but not at the single-optical-wavelength scale. At this length scale significant coupling between adiabatic potentials leads to nonadiabatic transitions, which we measure as a function of lattice depth and dressing frequency and amplitude. We also investigate the dressing by measuring changes in the momentum distribution of the dressed states.

quant-ph↗

arXiv2007

The Mott insulator transition in two dimensions

Cold atoms confined in periodic potentials are remarkably versatile quantum systems for implementing simple models prevalent in condensed matter theory. In the current experiment, we realize the 2D Bose-Hubbard model by loading a Bose-Einstein condensate into an optical lattice, and we study the resulting Mott insulating state (a phase of matter in which atoms are localized on specific lattice sites). We measure momentum distributions which agree quantitatively with theory (no adjustable parameters). We also study correlations in atom shot nose and observe a pronounced dependence on the lattice depth, this dependence indicates geometric effects to first order and suggests deviations due to higher order corrections.

cond-mat.other↗

arXiv2005

Collisional de-excitation in a quasi-2D degenerate Bose gas

We separate a Bose-Einstein condensate into an array of 2D sheets using a 1D optical lattice, and then excite quantized vibrational motion in the direction normal to the sheets. Collisions between atoms induce vibrational de-excitation, transferring the large excitation energy into back-to-back outgoing atoms, imaged as rings in the 2D plane. The ring diameters correspond to vibrational energy level differences, and edge-on imaging allows identification of the final vibrational states. Time dependence of these data provides a nearly complete characterization of the decay process including the energies, populations, and lifetimes of the lowest two excited vibrational levels. The measured decay rates represent a suppression of collisional de-excitation due to the reduced dimensionality, a matter wave analog to inhibited spontaneous emission.

cond-mat.other↗

arXiv2004

Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems

Using a combination of heat pulse and nuclear magnetic resonance techniques we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at $ν_T = 1$ in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.

cond-mat.mes-hall↗

arXiv2004

Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance

Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer phase coherence in bilayer two-dimensional electron systems at total Landau level filling factor $ν_T = 1$. We find that the phase boundary between the interlayer phase coherent state and the weakly-coupled compressible phase moves to larger layer separations as the electron density distribution in the bilayer is imbalanced. The critical layer separation increases quadratically with layer density difference.

cond-mat.mes-hall↗

arXiv2002

Resistivity of dilute 2D electrons in an undoped GaAs heterostructure

We report resistivity measurements from 0.03 K to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, $0.16 \times 10^{10} {cm}^{-2}$ to $7.5 \times 10^{10} {cm}^{-2}$, are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This non-monotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.

cond-mat.mes-hall↗

arXiv2002

Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System

The frictional drag between parallel two-dimensional electron systems has been measured in a regime of strong interlayer correlations. When the bilayer system enters the excitonic quantized Hall state at total Landau level filling factor ν_T=1 the longitudinal component of the drag vanishes but a strong Hall component develops. The Hall drag resistance is observed to be accurately quantized at h/e^2.

cond-mat.mes-hall↗

arXiv2001

Observation of a Linearly Dispersing Collective Mode in a Quantum Hall Ferromagnet

Double layer two-dimensional electron systems can exhibit a fascinating collective phase believed to exhibit both quantum ferromagnetism and excitonic superfluidity. This unusual phase has recently been found to exhibit tunneling phenomena reminiscent of the Josephson effect. A key element of the theoretical understanding of this bizarre quantum fluid is the existence of linearly dispersing Goldstone collective modes. Using the method of tunneling spectroscopy, we have demonstrated the existence of these modes. We find the measured velocity to be in reasonable agreement with theoretical estimates.

cond-mat.mes-hall↗

arXiv2000

Evidence for a Goldstone Mode in a Double Layer Quantum Hall System

The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling $ν_T=1$ the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed.

cond-mat.mes-hall↗