Elizabeth A. Donley

Dynamics of collapsing and exploding Bose–Einstein condensates

When atoms in a gas are cooled to extremely low temperatures, they will—under the appropriate conditions—condense into a single quantum-mechanical state known as a Bose–Einstein condensate. In such systems, quantum-mechanical behaviour is evident on a macroscopic scale. Here we explore the dynamics of how a Bose–Einstein condensate collapses and subsequently explodes when the balance of forces governing its size and shape is suddenly altered. A condensates equilibrium size and shape is strongly affected by the interatomic interactions. Our ability to induce a collapse by switching the interactions from repulsive to attractive by tuning an externally applied magnetic field yields detailed information on the violent collapse process. We observe anisotropic atom bursts that explode from the condensate, atoms leaving the condensate in undetected forms, spikes appearing in the condensate wavefunction and oscillating remnant condensates that survive the collapse. All these processes have curious dependences on time, on the strength of the interaction and on the number of condensate atoms. Although the system would seem to be simple and well characterized, our measurements reveal many phenomena that challenge theoretical models.

Atom-molecule coherence in a Bose-Einstein condensate

Recent advances in the precise control of ultracold atomic systems have led to the realisation of Bose–Einstein condensates (BECs) and degenerate Fermi gases. An important challenge is to extend this level of control to more complicated molecular systems. One route for producing ultracold molecules is to form them from the atoms in a BEC. For example, a two-photon stimulated Raman transition in a 87Rb BEC has been used to produce 87Rb2 molecules in a single rotational-vibrational state, and ultracold molecules have also been formed through photoassociation of a sodium BEC. Although the coherence properties of such systems have not hitherto been probed, the prospect of creating a superposition of atomic and molecular condensates has initiated much theoretical work. Here we make use of a time-varying magnetic field near a Feshbach resonance to produce coherent coupling between atoms and molecules in a 85Rb BEC. A mixture of atomic and molecular states is created and probed by sudden changes in the magnetic field, which lead to oscillations in the number of atoms that remain in the condensate. The oscillation frequency, measured over a large range of magnetic fields, is in excellent agreement with the theoretical molecular binding energy, indicating that we have created a quantum superposition of atoms and diatomic molecules—two chemically different species.

Controlled Collapse of a Bose-Einstein Condensate

The point of instability of a Bose-Einstein condensate (BEC) due to attractive interactions was studied. Stable 85Rb BECs were created and then caused to collapse by slowly changing the atom-atom interaction from repulsive to attractive using a Feshbach resonance. At a critical value, an abrupt transition was observed in which atoms were ejected from the condensate. By measuring the onset of this transition as a function of number and attractive interaction strength, we determined the stability condition to be N(absolute value of a) / a(ho) = 0.459+/-0.012+/-0.054, slightly lower than the predicted value of 0.574.

NIST-F1: recent improvements and accuracy evaluations

In the last several years we have made many improvements to NIST-F1 (a laser-cooled Cs fountain primary frequency standard at the National Institute of Standards and Technology (NIST) in Boulder, Colorado) resulting in a reduction by over a factor of 2 in the uncertainty of the realization of the SI second. The two most recent accuracy evaluations of NIST-F1 had combined standard fractional uncertainties of 0.61 × 10 −15 (June 2004) and 0.53 × 10 −15 (January 2005), which were submitted to the Bureau International des Poids et Mesures with total fractional uncertainties (including time-transfer contributions) of, respectively, 0.88 × 10 −15 and 0.97 × 10 −15 . Here we discuss the improvements and evaluation methods and present an updated uncertainty budget.

Double-pass acousto-optic modulator system

A practical problem that arises when using acousto-optic modulators (AOMs) to scan the laser frequency is the dependence of the beam diffraction angle on the modulation frequency. Alignment problems with AOM-modulated laser beams can be effectively eliminated by using the AOM in the double-pass configuration, which compensates for beam deflections. On a second pass through the AOM, the beam with its polarization rotated by 90° is deflected back such that it counterpropagates the incident laser beam and it can be separated from the input beam with a polarizing beam splitter. Here we present our design for a compact, stable, double-pass AOM with 75% double-pass diffraction efficiency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode fiber. The overall efficiency of the system (defined as the optical power out of the single-mode fiber divided by the optical power into the apparatus) is 60%.

Atomic Sensors – A Review

We discuss the basic physics and instrumentation issues related to high-performance physical and inertial sensors based on atomic spectroscopy. Recent work on atomic magnetometers, NMR gyroscopes, and atom interferometers is reviewed, with a focus on precision sensing of electromagnetic and gravitational fields and inertial motion. Atomic sensors have growing relevance to many facets of modern science and technology, from understanding the human brain to enabling precision navigation of moving platforms.

First accuracy evaluation of NIST-F2

We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (NIST) with a cryogenic (liquid nitrogen) microwave cavity and flight region. The 80?K atom interrogation environment reduces the uncertainty due to the blackbody radiation shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>50?000) at this low temperature, resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine in-house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems and the accuracy evaluation methods. The type B fractional uncertainty of NIST-F2 is shown to be 0.11???10?15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44???10?15.

Very-high-precision bound-state spectroscopy near a 85Rb Feshbach resonance

We precisely measured the binding energy ɛbind) of a molecular stale near the Feshbach resonance In a 85Rb Bose-Einstein condensate (BEC). Rapid magnetic-field pulses induced coherent atom-molecule oscillations in the BEC. We measured the oscillation frequency as a function B tield and fit the data TO a coupled-channel model. Our analysis constrained the Feshbach resonance position [155.041(18) Gl, width 10.71(2; G] and background scattering length [-443[3]a0] and yielded new values fur the Rb interaction parameters. These results improved our estimate lor the stability condition of an attractive BEC. We also found evidence for a mean-field shift to ɛbind.

Microscopic Dynamics in a Strongly Interacting Bose-Einstein Condensate

An initially stable 85Rb Bose-Einstein condensate (BEC) was subjected to a carefully controlled magnetic field pulse near a Feshbach resonance. This pulse probed the strongly interacting regime for the BEC, with the diluteness parameter (na(3)) ranging from 0.01 to 0.5. Condensate number loss resulted from the pulse, and for triangular pulses shorter than 1 ms, decreasing the pulse length actually increased the loss, until very short time scales (approximately 10 micros) were reached. The observed time dependence is very different from that expected in traditional inelastic loss processes, suggesting the presence of new microscopic BEC physics.

Operation of the NIST-F1 caesium fountain primary frequency standard with a maser ensemble, including the impact of frequency transfer noise

The operation of a caesium fountain primary frequency standard is greatly influenced by the characteristics of two other important facilities. The first is a stable frequency reference and the second is the frequency-transfer system. A stable frequency reference such as a hydrogen maser is a virtual necessity since essentially no fountain dead time can be tolerated without it. The frequency stability of this reference has a significant impact on the procedures for evaluating certain systematic biases in the fountain. State-of-the-art frequency transfer technology is also necessary if the fountain is intended to contribute to TAI or to be compared with other remotely located frequency standards without excessive degradation of stated uncertainties. We discuss the facilities available at the National Institute of Standards and Technology (NIST) and how they impact the operation of NIST-F1, the primary frequency standard at NIST.