arXiv subjects

David M. Weld

Publications and source records attributed to David M. Weld.

34 records · Page 2Linked to original sources

arXiv2020

Non-exponential decay in Floquet-Bloch bands

Exponential decay laws describe systems ranging from unstable nuclei to fluorescent molecules, in which the probability of jumping to a lower-energy state in any given time interval is static and history-independent. These decays, involving only a metastable state and fluctuations of the quantum vacuum, are the most fundamental nonequilibrium process, and provide a microscopic model for the origins of irreversibility. Despite the fact that the apparently universal exponential decay law has been precisely tested in a variety of physical systems, it is a surprising truth that quantum mechanics requires that spontaneous decay processes have non-exponential time dependence at both very short and very long times. Cold-atom experiments both classic and recent have proven to be powerful probes of fundamental decay processes; in this paper, we propose the use of Bose condensates in Floquet-Bloch bands as a probe of long-time non-exponential decay in single isolated emitters. We identify a range of parameters that should enable observation of long-time deviations, and experimentally demonstrate a key element of the scheme: tunable decay between quasienergy bands in a driven optical lattice.

cond-mat.quant-gas↗

arXiv2019

Phasonic Spectroscopy of a Quantum Gas in a Quasicrystalline Lattice

Phasonic degrees of freedom are unique to quasiperiodic structures, and play a central role in poorly-understood properties of quasicrystals from excitation spectra to wavefunction statistics to electronic transport. However, phasons are challenging to access dynamically in the solid state due to their complex long-range character and the effects of disorder and strain. We report phasonic spectroscopy of a quantum gas in a one-dimensional quasicrystalline optical lattice. We observe that strong phasonic driving produces a nonperturbative high-harmonic plateau strikingly different from the effects of standard dipolar driving. Tuning the potential from crystalline to quasicrystalline, we identify spectroscopic signatures of quasiperiodicity and interactions and map the emergence of a multifractal energy spectrum, opening a path to direct imaging of the Hofstadter butterfly.

cond-mat.quant-gas↗

arXiv2019

Quantifying and controlling prethermal nonergodicity in interacting Floquet matter

The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering.

quant-ph↗

arXiv2019

Transport in Floquet-Bloch bands

We report Floquet band engineering of long-range transport and direct imaging of Floquet-Bloch bands in an amplitude-modulated optical lattice. In one variety of Floquet-Bloch band we observe tunable rapid long-range high-fidelity transport of a Bose condensate across thousands of lattice sites. Quenching into an opposite-parity Floquet-hybridized band allows Wannier-Stark localization to be controllably turned on and off using modulation. A central result of this work is the use of transport dynamics to demonstrate direct imaging of a Floquet-Bloch band structure. These results demonstrate that transport in dynamical Floquet-Bloch bands can be mapped to transport in quasi-static effective bands, opening a path to cold atom quantum emulation of ultrafast multi-band electronic dynamics.

cond-mat.quant-gas↗

arXiv2018

Experimental Realization of a Relativistic Harmonic Oscillator

We report the experimental study of a harmonic oscillator in the relativistic regime. The oscillator is composed of Bose-condensed lithium atoms in the third band of an optical lattice, which have an energy-momentum relation nearly identical to that of a massive relativistic particle, with an effective mass reduced below the bare value and a greatly reduced effective speed of light. Imaging the shape of oscillator trajectories at velocities up to 98% of the effective speed of light reveals a crossover from sinusoidal to nearly photon-like propagation. The existence of a maximum velocity causes the measured period of oscillations to increase with energy; our measurements reveal beyond-leading-order contributions to this relativistic anharmonicity. We observe an intrinsic relativistic dephasing of oscillator ensembles, and a monopole oscillation with exactly the opposite phase of that predicted for non-relativistic harmonic motion. All observed dynamics are in quantitative agreement with longstanding but hitherto-untested relativistic predictions.

cond-mat.quant-gas↗

arXiv2018

Quantum Simulation of Ultrafast Dynamics Using Trapped Ultracold Atoms

Ultrafast electronic dynamics are typically studied using pulsed lasers. We demonstrate a complementary experimental approach: quantum simulation of ultrafast dynamics using trapped ultracold atoms. Counter-intuitively, this technique emulates some of the fastest processes in atomic physics with some of the slowest, leading to a temporal magnification factor of up to twelve orders of magnitude. In these experiments, time-varying forces on neutral atoms in the ground state of a tunable optical trap emulate the electric fields of a pulsed laser acting on bound charged particles. We demonstrate the correspondence with ultrafast science by a sequence of experiments: nonlinear spectroscopy of a many-body bound state, control of the excitation spectrum by potential shaping, observation of sub-cycle unbinding dynamics during strong few-cycle pulses, and direct measurement of carrier-envelope phase dependence of the response to an ultrafast-equivalent pulse. These results establish cold atom quantum simulation as a complementary tool for studying ultrafast dynamics.

physics.atom-ph↗

arXiv2018

Observation and uses of position-space Bloch oscillations in an ultracold gas

We report the direct observation and characterization of position-space Bloch oscillations using an ultracold gas in a tilted optical lattice. While Bloch oscillations in momentum space are a common feature of optical lattice experiments, the real-space center-of-mass dynamics are typically too small to resolve. Tuning into the regime of rapid tunneling and weak force, we observe real-space Bloch oscillation amplitudes of hundreds of lattice sites, in both ground and excited bands. We demonstrate two unique capabilities enabled by tracking of Bloch dynamics in position space: measurement of the full position-momentum phase-space evolution during a Bloch cycle, and direct imaging of the lattice band structure. These techniques, along with the ability to exert long-distance coherent control of quantum gases without modulation, may open up new possibilities for quantum control and metrology.

cond-mat.quant-gas↗

arXiv2017

Quantum Emulation of Extreme Non-equilibrium Phenomena with Trapped Atoms

Ultracold atomic physics experiments offer a nearly ideal context for the investigation of quantum systems far from equilibrium. We describe three related emerging directions of research into extreme non-equilibrium phenomena in atom traps: quantum emulation of ultrafast atom-light interactions, coherent phasonic spectroscopy in tunable quasicrystals, and realization of Floquet matter in strongly-driven lattice systems. We show that all three should enable quantum emulation in parameter regimes inaccessible in solid-state experiments, facilitating a complementary approach to open problems in non-equilibrium condensed matter.

cond-mat.quant-gas↗

arXiv2016

Fibonacci optical lattices for tunable quantum quasicrystals

We describe a quasiperiodic optical lattice, created by a physical realization of the abstract cut-and-project construction underlying all quasicrystals. The resulting potential is a generalization of the Fibonacci tiling. Calculation of the energies and wavefunctions of ultracold atoms loaded into such a lattice demonstrate a multifractal energy spectrum, a singular continuous momentum-space structure, and the existence of controllable edge states. These results open the door to cold atom quantum simulation experiments in tunable or dynamic quasicrystalline potentials, including topological pumping of edge states and phasonic spectroscopy.

cond-mat.quant-gas↗

arXiv2015

Adiabatic cooling of bosons in lattices to magnetic ordering

We suggest and analyze a new scheme to adiabatically cool bosonic atoms to picokelvin temperatures which should allow the observation of magnetic ordering via superexchange in optical lattices. The starting point is a gapped phase called the spin Mott phase where each site is occupied by one spin-up and one spin-down atom. An adiabatic ramp leads to an xy-ferromagnetic phase. We show that the combination of time-dependent density matrix renormalization group methods with quantum trajectories can be used to fully address possible experimental limitations due to decoherence, and demonstrate that the magnetic correlations are robust for experimentally realizable ramp speeds. Using a microscopic master equation treatment of light scattering in the many-particle system, we test the robustness of adiabatic state preparation against decoherence. Due to different ground-state symmetries, we also find a metastable state with xy-ferromagnetic order if the ramp crosses to regimes where the ground state is a z-ferromagnet. The bosonic spin Mott phase as the initial gapped state for adiabatic cooling has many features in common with a fermionic band insulator, but the use of bosons should enable experiments with substantially lower initial entropies.

cond-mat.quant-gas↗

arXiv2014

Effusive Atomic Oven Nozzle Design Using an Aligned Microcapillary Array

We present a simple and inexpensive design for a multichannel effusive oven nozzle which provides improved atomic beam collimation and thus extended oven lifetimes. Using this design we demonstrate an atomic lithium source suitable for trapped-atom experiments. At a nozzle temperature of 525$^{\circ}$C the collimated atomic beam flux directly after the nozzle is $1.2 \times 10^{14}$ atoms per second with a peak beam intensity greater than $5.0 \times 10^{16}$ atoms per second per steradian. This suggests an oven lifetime of several decades of continuous operation.

physics.atom-ph↗

arXiv2011

Bragg Scattering as a Probe of Atomic Wavefunctions and Quantum Phase Transitions in Optical Lattices

We have observed Bragg scattering of photons from quantum degenerate $^{87}$Rb atoms in a three-dimensional optical lattice. Bragg scattered light directly probes the microscopic crystal structure and atomic wavefunction whose position and momentum width is Heisenberg-limited. The spatial coherence of the wavefunction leads to revivals in the Bragg scattered light due to the atomic Talbot effect. The decay of revivals across the superfluid to Mott insulator transition indicates the loss of superfluid coherence.

cond-mat.quant-gas↗

arXiv2011

Spin gradient demagnetization cooling of ultracold atoms

A major goal of ultracold atomic physics is quantum simulation of spin Hamiltonians in optical lattices. Progress towards this goal requires the attainment of extremely low temperatures. Here we demonstrate a new cooling method which consists of applying a time-varying magnetic field gradient to a spin mixture of ultracold atoms. We have used this method to prepare isolated spin distributions at positive and negative spin temperatures of +/-50 picokelvin. The spin system can also be used to cool other degrees of freedom, and we have used this coupling to reduce the temperature of an apparently equilibrated sample of rubidium atoms in a Mott insulating state to 350 picokelvin. These are the lowest temperatures ever measured in any system.

cond-mat.quant-gas↗

arXiv2010

Thermometry and Refrigeration in a Two-Component Mott Insulator of Ultracold Atoms

Interesting spin Hamiltonians can be realized with ultracold atoms in a two-component Mott insulator (2CMI). It was recently demonstrated that the application of a magnetic field gradient to the 2CMI enables new techniques of thermometry and adiabatic cooling. Here we present a theoretical description which provides quantitative analysis of these two new techniques. We show that adiabatic reduction of the field gradient is capable of cooling below the Curie or Néel temperature of certain spin ordered phases.

cond-mat.quant-gas↗

arXiv2010

Spin gradient thermometry for ultracold atoms in optical lattices

We demonstrate spin gradient thermometry, a new general method of measuring the temperature of ultracold atoms in optical lattices. We realize a mixture of spins separated by a magnetic field gradient. Measurement of the width of the transition layer between the two spin domains serves as a new method of thermometry which is observed to work over a broad range of lattice depths and temperatures, including in the Mott insulator regime. We demonstrate the thermometry in a system of ultracold rubidium atoms, and suggest that interesting spin physics can be realized in this system. The lowest measured temperature is 1 nK, indicating that the system has reached the quantum regime, where insulating shells are separated by superfluid layers.

cond-mat.quant-gas↗

arXiv2008

Improved constraints on non-Newtonian forces at 10 microns

Several recent theories suggest that light moduli or particles in "large" extra dimensions could mediate macroscopic forces exceeding gravitational strength at length scales below a millimeter. Such new forces can be parameterized as a Yukawa-type correction to the Newtonian potential of strength $α$ relative to gravity and range $λ$. To extend the search for such new physics we have improved our apparatus utilizing cryogenic micro-cantilevers capable of measuring attonewton forces, which now includes a switchable magnetic force for calibration. Our most recent experimental constraints on Yukawa-type deviations from Newtonian gravity are more than three times as stringent as our previously published results, and represent the best bound in the range of 5 - 15 microns, with a 95 percent confidence exclusion of forces with $|α| > 14,000$ at $λ$ = 10 microns.

hep-ex↗