Pushing the Frontiers of Atomic Physics

The following sections are included:

• PREFACE

• Organizing Committees

• INVITED SPEAKERS

• CONTENTS

People who have a transforming influence on science come in many variations: there are of course the extraordinary researchers whose discoveries help open and define a new field of investigations; the great teachers who can inspire generations of students; and the visionary administrators who provide the financial and infrastructure support needed to carry out our work. Many physicists excel at one of these tasks, significantly less at two of them, and only very few at all three. Herbert Walther was one of these rare few…

The atomic and optical physics community lost one of its pioneers with the death of Willis E. Lamb, Jr. on May 15, 2008. Lamb was born on July 12, 1913, received the BS degree in Chemistry at Berkeley in 1934, and obtained his PhD under the tutelage of J. Robert Oppenheimer at Berkeley in 1938. He served on the faculties of Columbia University, Stanford University, Oxford University, Yale University, and the University of Arizona. Lamb received the Nobel prize in 1955 for his work on the fine structure of hydrogen and was awarded the President's National Medal for Science in 2000…

We report on measurements of the excitation spectrum of a strongly interacting Bose-Einstein condensate (BEC). A magnetic-field Feshbach resonance is used to tune atom-atom interactions in the condensate and to reach a regime where quantum depletion and beyond mean-field corrections to the condensate chemical potential are significant. We use two-photon Bragg spectroscopy to probe the condensate excitation spectrum; our results demonstrate the onset of beyond mean-field effects in a gaseous BEC.

A quantum emitted by any of a collection of identical atoms may be absorbed and re-emitted by other atoms many times before it eventually emerges. The radiation process is thus best described as collective or cooperative in nature. The atomic excitations are shown to attenuate as linear combinations of certain characteristic decay modes that lend a complex structure to the spectrum radiated. Instead of a single line, it becomes a closely-spaced multiplet of lines, the elements of which have a variety of lifetimes, line-shifts and line-widths. We calculate these quantities, first with an abstract two-state model for the atoms and then with an isotropic four-state model that accommodates the full polarization dependence of the radiation.

We describe our efforts to study the physics of the fractional quantum Hall effect using ultracold quantum gases in an optical lattice and to perform precision measurements using large-area atom interferometry.

A measurement reported in 2008 uses a one-electron quantum cyclotron to determine the electron magnetic moment in Bohr magnetons, g/2 = 1.001 159 652 180 73 (28) [0.28 ppt], with an uncertainty 2.7 and 15 times smaller than for previous measurements in 2006 and 1987. The electron is used as a magnetometer to allow lineshape statistics to accumulate, and its spontaneous emission rate determines the correction for its interaction with a cylindrical trap cavity. The new measurement and QED theory determine the fine structure constant, with α^-1 = 137.035 999 084(51) [0.37 ppb], and an uncertainty 20 times smaller than for any independent determination of α.

We use Bloch oscillations to coherently transfer many photon momenta to atoms. Then we can measure accurately the recoil velocity ħk/m and deduce the fine structure constant α. The velocity variation due to Bloch oscillations is measured using atom interferometry. This method yields a value of the fine structure constant α^-1 = 137.035 999 45 (62) with a relative uncertainty of about 4.5 × 10^-9.

We demonstrate an interferometric scattering technique that allows highly precise measurements of s-wave scattering phase shifts. We collide two clouds of cesium atoms in a juggling fountain clock. The atoms in one cloud are prepared in a coherent superposition of the two clock states and the atoms in the other cloud are prepared in one of the F,m_F ground states. When the two clouds collide, the clock states experience s-wave phase shifts as they scatter off of the atoms in the other cloud. We detect only the scattered part of the clock atom's wavefunction for which the relative phase of the clock coherence is shifted by the difference of the s-wave phase shifts. In this way, we unambiguously observe the differences of scattering phase shifts. These phase shifts are independent of the atomic density to lowest order, enabling measurements of scattering phase shifts with atomic clock accuracy. Recently, we have observed the changes in scattering phase shifts as a function of magnetic field over a range of values where Feshbach resonances may be expected and where inelastic scattering channels open and close. A number of these measurements will precisely test and tightly constrain our knowledge of cesium-cesium interactions. With such knowledge, future measurements may place stringent limits on the time variation of fundamental constants, such as the electron-proton mass ratio, by precisely probing phase shifts near a Feshbach resonance.

The detection of weak magnetic fields with high spatial resolution is an outstanding problem in diverse areas ranging from fundamental physics and material science to data storage and bio-imaging. Here we describe a new approach to magnetometry that takes advantage of recently developed techniques for coherent control of solid-state spin qubits. We experimentally demonstrate this novel magnetometer employing an individual electronic spin associated with a Nitrogen-Vacancy (NV) center in diamond. Using an ultra-pure diamond sample, we achieve shot-noise-limited detection of nanotesla magnetic fields at kHz frequencies after 100 seconds of averaging. In addition, we demonstrate sensitivity for a diamond nanocrystal with a volume of (30 nm)³. This magnetic sensor provides an unprecedented combination of high sensitivity and spatial resolution – potentially allowing for the detection of a single nuclear spin's precession within one second.

A key ingredient for a practical quantum repeater is a long memory coherence time. We describe a quantum memory using the magnetically-insensitive clock transition in atomic rubidium confined in a 1D optical lattice. We observe quantum lifetimes exceeding 6 milliseconds. We also demonstrate a dozen independent quantum memory elements within a single cold sample, and describe matter-light entanglement generation involving arbitrary pairs of these elements.

We describe Cavity QED experiments in which a beam of circular Rydberg atoms is used to manipulate and probe non-destructively microwave photons trapped in a very high-Q superconducting cavity. We realize an ideal quantum non-demolition (QND) measurement of light, observe the radiation quantum jumps due to cavity relaxation and prepare non-classical fields such as Fock and Schrödinger cat states. Combining QND photon counting with a homodyne mixing method, we reconstruct the Wigner functions of these non-classical states and, by taking snapshots of these functions at increasing times, obtain movies of the decoherence process in the cavity.

We generate input states with reduced quantum uncertainty (spin-squeezed states) for a hyperfine atomic clock by collectively coupling an ensemble of laser-cooled and trapped ⁸⁷Rb atoms to an optical resonator. A quantum non-demolition measurement of the population difference between the two clock states with far-detuned light produces an entangled state whose projection noise is reduced by as much as 9.4(8) dB below the standard quantum limit (SQL) for uncorrelated atoms. When the observed decoherence is taken into account, we attain 4.2(8) dB of spin squeezing, confirming entanglement, and 3.2(8) dB of improvement in clock precision over the SQL. The method holds promise for improving the performance of optical-frequency clocks.

In many experiments, isolated atoms and ions have been inserted into high-finesse optical resonators for fundamental studies of quantum optics and quantum information. Here, we introduce another application of such a system, as the realization of cavity optomechanics where the collective motion of an atomic ensemble serves the role of a moveable optical element in an optical resonator. Compared with other optomechanical systems, such as those incorporating nanofabricated cantilevers or the large cavity mirrors of gravitational observatories, our cold-atom realization offers direct access to the quantum regime. We describe experimental investigations of optomechanical effects, such as the bistability of collective atomic motion and the first quantification of measurement backaction for a macroscopic object, and discuss future directions for this nascent field.

Optomechanical devices in which a flexible SiN membrane is placed inside an optical cavity allow for very high finesse and mechanical quality factor in a single device. They also provide fundamentally new functionality: the cavity detuning can be a quadratic function of membrane position. This enables a measurement of "position squared" (x²) and in principle a QND phonon number readout of the membrane. However, the readout achieved using a single transverse cavity mode is not sensitive enough to observe quantum jumps between phonon Fock states.

Here we demonstrate an x²-sensitivity that is orders of magnitude stronger using two transverse cavity modes that are nearly degenerate. We derive a first-order perturbation theory to describe the interactions between nearly-degenerate cavity modes and achieve good agreement with our measurements using realistic parameters. We also demonstrate theoretically that the x²-coupling should be easily tunable over a wide range.

Lithium-7 exhibits a broad Feshbach resonance that we exploit to tune the interactions in a Bose-Einstein condensate (BEC). We find that the rate of photoassociation can be enhanced by several orders of magnitude by tuning close to the resonance, and use this effect to observe saturation in the rate of association of a BEC for the first time. We have also used a lithium BEC to explore the effects of disorder on the transport and coherence properties of the condensate. We also show that the scattering length goes through a shallow zero-crossing far from the resonance, where it may be made positive or negative with a magnitude of less than 0.1 a_o, and have made preliminary transport measurements in the regime of weak repulsive and attractive interactions.

We report on experiments exploring the physics of dipolar quantum gases using a ⁵²Cr Bose-Einstein condensate (BEC). By means of a Feshbach resonance, it is possible to reduce the effects of short range interactions and reach a regime where the physics is governed by the long-range, anisotropic dipole-dipole interaction between the large (6 µ_B) magnetic moments of Chromium atoms. Several dramatic effects of the dipolar interaction are observed: the usual inversion of ellipticity of the condensate during time-of flight is inhibited, the stability of the dipolar gas depends strongly on the trap geometry, and the explosion following the collapse of an unstable dipolar condensate displays d-wave like features.

This article summarizes recent work on the exciton-polariton BEC at Stanford, which was presented at ICAP 2008. The covered topics include cooperative cooling of exciton-polariton spin mixtures, quantum degeneracy at thermal equilibrium condition, Bogoliubov excitation spectrum, first and second order coherence, and dynamical condensation at excited Bloch bands in a one-dimensional lattice.

In 1958, P.W. Anderson predicted the exponential localization¹ of electronic wave functions in disordered crystals and the resulting absence of diffusion. It was realized later that Anderson localization (AL) is ubiquitous in wave physics² as it originates from the interference between multiple scattering paths, and this has prompted an intense activity. Experimentally, localization has been reported in light waves³ microwaves,⁴ sound waves,⁵ and electron gases⁶ but to our knowledge there was no direct observation of exponential spatial localization of matter-waves (electrons or others). We present in this proceeding the experiment that lead to the observation of Anderson Localization (AL)⁷ of a Bose-Einstein Condensate (BEC) released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. Direct imaging allows for unambiguous observation of an exponential decay of the wavefunction when the conditions for AL are fulfilled. The disorder is created with a one-dimensional speckle potential whose noise spectrum has a high spatial frequency cut-off, hindering the observation of exponential localization if the expanding BEC contains atomic de Broglie wavelengths that are smaller than an effective mobility edge corresponding to that cut-off. In this case, we observe the density profiles that decay algebraically.⁹

One of the most intriguing phenomena in physics is the localization of waves in disordered media. This phenomenon was originally predicted by P. W. Anderson, fifty years ago, in the context of transport of electrons in crystals, but it has never been directly observed for matter waves. Ultracold atoms open a new scenario for the study of disorder-induced localization, due to the high degree of control of most of the system parameters, including interactions. For the first time we have employed a noninteracting Bose-Einstein condensate to study Anderson localization. The experiment is performed in a 1D lattice with quasi-periodic disorder, a system which features a crossover between extended and exponentially localized states as in the case of purely random disorder in higher dimensions. We clearly demonstrate localization by investigating transport properties, spatial and momentum distributions. Since the interaction in the condensate can be controlled, this system represents a novel tool to solve fundamental questions on the interplay of disorder and interactions and to explore exotic quantum phases.

Fermi gases with magnetically tunable interactions provide a clean and controllable laboratory system for modelling interparticle interactions between fermions. Near a Feshbach resonance, the s-wave scattering length diverges and Fermi gases are strongly interacting, enabling tests of nonperturbative many-body theories in a variety of disciplines, from high temperature superconductors to neutron matter and quark-gluon plasmas. We measure the entropy and energy of this model system, enabling model-independent comparison with thermodynamic predictions. Our experiments on the expansion dynamics of rotating strongly interacting Fermi gases reveal extremely low viscosity hydrodynamics. Combining the thermodynamic and hydrodynamic measurements enables an estimate of the ratio of the shear viscosity to the entropy density. A strongly interacting Fermi gas in the normal fluid regime is found to be a nearly perfect fluid, where the ratio of the viscosity to the entropy density is close to a universal minimum that has been conjectured by string theory methods. In the weakly interacting regime near a zero crossing in the s-wave scattering length, we observe coherently prepared Fermi gases that slowly evolve into long-lived spin-segregated states that are far from equilibrium and weakly damped.

We perform momentum-resolved rf spectroscopy on a Fermi gas of ⁴⁰K atoms in the region of the BCS-BEC crossover. This measurement is analogous to photoemission spectroscopy, which has proven to be a powerful probe of excitation gaps in superconductors. We measure the single-particle spectral function, which is a fundamental property of a strongly interacting system and is directly predicted by many-body theories. For a strongly interacting Fermi gas near the transition temperature for the superfluid state, we find evidence for a large pairing gap.

Experiments on ultra-cold atomic gases at nano-Kelvin temperatures are revolutionizing many areas of physics. Their exceptional adaptability and simplicity allows tests of many-body theory in areas long thought to be inaccessible, due to strong interactions. Ultra-cold Fermi gases are now providing new insight into the foundations of quantum theory. They are expected to exhibit a universal thermodynamic behaviour in the strongly interacting limit, independent of any microscopic details of the underlying interactions. Here, we present a systematic theoretical study of strong interacting fermions, using different field-theoretic methods and comparisons with quantum Monte Carlo simulations. Pioneering measurements have dramatically confirmed our theoretical predictions, giving the first known evidence for universal fermion thermodynamics.

We describe recent experimental studies of a spin-polarized Fermi gas with strong interactions. Tomographically resolving the spatial structure of an inhomogeneous trapped sample, we have mapped out the superfluid phases in the parameter space of temperature, spin polarization, and interaction strength. Phase separation between the superfluid and the normal component occurs at low temperatures, showing spatial discontinuities in the spin polarization. The critical polarization of the normal gas increases with stronger coupling. Beyond a critical interaction strength all minority atoms pair with majority atoms, and the system can be effectively described as a boson-fermion mixture. Pairing correlations have been studied by rf spectroscopy, determining the fermion pair size and the pairing gap energy in a resonantly interacting superfluid.

A universal characterization of interactions in few- and many-body quantum systems is often possible without detailed description of the interaction potential, and has become a defacto assumption for cold atom research. Universality in this context is defined as the validity to fully characterize the system in terms of two-body scattering length. We discuss universality in the following three contexts: closed-channel dominated Feshbach resonance, Efimov physics near Feshbach resonances, and corrections to the mean field energy of Bose-Einstein condensates with large scattering lengths. Novel experimental tools and strategies are discussed to study universality in ultracold atomic gases: dynamic control of interactions, run-away evaporative cooling in optical traps, and preparation of few-body systems in optical lattices.

Bose-Einstein condensates have long been considered the most appropriate source for interferometry with matter waves, due to their maximal coherence properties. However, the realization of practical interferometers with condensates has been so far hindered by the presence of the natural atom-atom interaction, which dramatically affects their performance. We describe here the realization of a lattice-based interferometer based on a Bose-Einstein condensate where the contact interaction can be tuned by means of a Feshbach resonance, and eventually reduced towards zero. We observe a strong increase of the coherence time of the interferometer with vanishing scattering length, and see evidence of the effect of the weak magnetic dipole-dipole interaction. Our observations indicate that high-sensitivity atom interferometry with Bose-Einstein condensates is feasible, via a precise control of the interactions.

Topological matter is an unconventional form of matter: it exhibits a global hidden order which is not associated with the spontaneous breaking of any symmetry. The defects of this exotic type of order are anyons, quasiparticles with fractional statistics. Moreover, when living on a surface with non-trivial topology, like a plane with a hole or a torus, this type of matter develops a number of degenerate states which are locally indistinguishable and could be used to build a quantum memory naturally resistant to errors. Except for the fractional quantum Hall effect there is no experimental evidence as to the existence of topologically ordered phases, and it remains a huge challenge to develop theoretical techniques to look for them in realistic models and find them in the laboratory. Here we show how to use ultracold atoms in optical lattices to create and detect different instances of topological order in the minimum non-trivial system: four spins in a plaquette. By combining different techniques we show how to prepare these spins in mimimum versions of topical topological liquids like resonant valence bond or Laughlin states, probe their fractional quasiparticle excitations, and exploit them to build a mini-topological quantum memory.

Interacting bosons, fermions and Bose-Fermi mixtures in optical lattices form novel model systems for the investigation of fundamental quantum many body effects. This article summarizes some of our recent work on interacting bosonic and fermionic quantum gases in optical lattices. We show how the compressibility of a fermionic quantum gas mixture can be evaluated by measuring its size vs trap confinement. The results are compared to ab-initio Dynamical Mean Field Theory (DMFT) calculations, for which we find very good agreement with the experiment. Furthermore, quantum phase diffusion is introduced as a powerful method for the measurement of the renormalized Hubbard parameters underlying most lattice models.

The cold-molecules field is very active trying to transfer laser cooling techniques from atoms to molecules. Photoassociation of cold atoms, followed by spontaneous emission of the electronically excited molecules, produces translationally cold molecules, but in several vibrational levels v of the ground state. We have recently shown that vibrational cooling can be obtained by optical pumping with a shaped broadband femtosecond laser. The broadband laser electronically excites the molecules, leading via a few absorption - spontaneous emission cycles to a redistribution of the vibrational population in the ground state. By removing the laser frequencies corresponding to the excitation of the v = 0 level, a dark state is produced by the so-shaped laser, yielding with successive laser pulses an accumulation of the molecules in the v = 0 level.

Strongly correlated states in many-body systems are traditionally created using elastic interparticle interactions. Here we show that inelastic interactions between particles can also drive a system into the strongly correlated regime. This is shown by an experimental realization of a specific strongly correlated system, namely a one-dimensional molecular Tonks-Girardeau gas.

When ultracold (T < 1 mK) molecules are formed by photoassociation (PA) followed by spontaneous emission, they commonly are formed in high vibrational levels, typically within 30 cm^-1 of the dissociation limit.^1,2 Such levels are difficult to produce in other ways, and, because their bands include only a few rotational quantum numbers (typically < 5 for KRb at 0.2 mK), their electronic spectra are readily assignable, especially when accurate ab initio calculations are also available. Research at the University of Connecticut has demonstrated how the spectroscopy of KRb ultracold molecules formed by PA^3,4 can be studied using multiple resonance spectroscopy.^1,5–7

Single molecular ions can be sympathetically cooled to a temperature in the mK-range and become spatially localized within a few µm³ through the Coulomb interaction with laser-cooled atomic ions, and hence be an excellent starting point for a variety of single molecule studies. By applying a rather simple, non-destructive technique for the identification of the individual molecular ions relying on an in situ mass measurement of the molecules, studies of the photofragmentation of singly-charged aniline ions (C₆H₇N⁺) as well as investigations of isotope effects in reactions of Mg⁺ ions with HD molecules have been carried out.

The genesis of light pulses with attosecond (10^-18 seconds) durations signifies a new frontier in time-resolved physics. The scientific importance is obvious: the time-scale necessary for probing the motion of an electron(s) in the ground state is attoseconds (atomic unit of time = 24 as). The availability of attosecond pulses would allow, for the first time, the study of the time-dependent dynamics of correlated electron systems by freezing the motion, in essence exploring the structure with ultra-fast snapshots, then following the subsequent evolution using pump-probe techniques. This paper examines the fundamental principles of attosecond formation by Fourier synthesis of a high harmonic comb and phase measurements using two-color techniques. Quantum control of the spectral phase, critical to attosecond formation, has its origin in the fundamental response of an atom to an intense electromagnetic field. We will interpret the laser-atom interaction using a semi-classical trajectory model.

Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular physics has begun. Not only is it possible to control outer-shell electron dynamics with intense optical lasers, but now control of ultrafast inner-shell processes has become possible by combining strong optical laser fields with tunable sources of X-ray radiation. This marriage of strong-field laser and X-ray physics has led to the discovery of methods to control reversibly resonant X-ray absorption in atoms and molecules on ultrafast timescales. Here we describe three scenarios for control of resonant X-ray absorption: ultrafast field ionization, electromagnetically induced transparency in atoms and strong-field molecular alignment.

The following sections are included:

• AUTHOR INDEX