Craig Savage: publications 1994-2003

We investigate a radial spin hedgehog, analogous to the Dirac monopole, in an optically trapped atomic spin-1 Bose-Einstein condensate. Full abstract.

We investigate a radial spin hedgehog, analogous to the Dirac monopole, in an optically trapped atomic spin-1 Bose-Einstein condensate. By joining together a monopole-antimonopole pair, we may form a vortex line with free ends. We numerically simulate the three-dimensional dynamics and imaginary time relaxation of these structures to nonsingular textures and show they can be observable for realistic experimental parameters.

We numerically show that a topologically nontrivial 3D Skyrmion can be energetically stable in a trapped two-component atomic Bose-Einstein condensate, for the parameters of Rb-87 condensate experiments. The separate conservation of the two atomic species can stabilize the Skyrmion against shrinking to zero size, while drift of the Skyrmion due to the trap-induced density gradient can be prevented by rotation or by a laser potential.

We solve the Gross-Pitaevskii equation numerically for the collapse induced by a switch from positive to negative scattering lengths. Full abstract.

We solve the Gross-Pitaevskii equation numerically for the collapse induced by a switch from positive to negative scattering lengths. We compare our results with experiments performed at JILA with Bose-Einstein condensates of Rb-85, in which the scattering length was controlled using a Feshbach resonance. Building on previous theoretical work we identify quantitative differences between the predictions of mean-field theory and the results of the experiments. Besides the previously reported difference between the predicted and observed critical atom number for collapse, we also find that the predicted collapse times systematically exceed those observed experimentally. Quantum field effects, such as fragmentation, that might account for these discrepancies are discussed.

We analyze the existence and stability of spatially extended (Bloch-type) and localized states of a Bose-Einstein condensate loaded into an optical lattice. Full abstract.

We analyze the existence and stability of spatially extended (Bloch-type) and localized states of a Bose-Einstein condensate loaded into an optical lattice. In the framework of the Gross-Pitaevskii equation with a periodic potential, we study the band-gap structure of the matter-wave spectrum in both the linear and nonlinear regimes. We demonstrate the existence of families of spatially localized matter-wave gap solitons, and analyze their stability in different band gaps, for both repulsive and attractive atomic interactions.

A multimode model of a continuously pumped atom laser is shown to be unstable below a critical value of the scattering length. Full abstract.

A multimode model of a continuously pumped atom laser is shown to be unstable below a critical value of the scattering length. Above the critical scattering length, the atom laser reaches a steady state, the stability of which increases with pumping. Below this limit the laser does not reach a steady state. This instability results from the competition between gain and loss for the excited states of the lasing mode. It will determine a fundamental limit for the linewidth of an atom laser beam.

We investigate the feasibility of a particular scheme for generating macroscopic quantum superposition states in two-species dilute gas Bose-Einstein condensates. Full abstract.

We investigate the feasibility of a particular scheme for generating macroscopic quantum superposition states in two-species dilute gas Bose-Einstein condensates. The scheme utilizes two-body interactions and Josephson coupling between the species. We report numerical studies that extend a previous two-mode model to include dissipation and extra modes.

An ideal atom laser would produce an atomic beam with highly stable flux and energy. Full abstract.

An ideal atom laser would produce an atomic beam with highly stable flux and energy. In practice, the stability is likely to be limited by technical noise and nonlinear dynamical effects. We investigate the dynamics of an atom laser using a comprehensive one-dimensional, mean-field numerical model. We fully model the output beam and experimentally important physics such as three-body recombination. We find that at high-pump rates, the latter plays a role in suppressing the high-frequency dynamics, which would otherwise limit the stability of the output beam.

We review the current theory of atom lasers. A tutorial treatment of second quantisation and the Gross-Pitaevskii equation is presented, and basic concepts of coherence are outlined. The generic types of atom laser models are surveyed and illustrated by specific examples. We conclude with detailed treatments of the mechanisms of gain and output coupling.

We present a fully quantum mechanical treatment of a single-mode atom laser including pumping and output coupling. Full abstract.

We present a fully quantum mechanical treatment of a single-mode atom laser including pumping and output coupling. By ignoring atom-atom interactions, we have solved this model without making the Born-Markov approximation. We find substantially less gain narrowing than is predicted under that approximation.

We present a model of a cw atom laser based on a system of coupled GP equations. The model incorporates continuous Raman outcoupling, pumping and three-body recombination. The outcoupled field has minimal atomic density fluctuations and is locally monochromatic.

We present a fully quantum-mechanical treatment of a single-mode atomic cavity with a pumping mechanism and an output coupling to a continuum of external modes. Full abstract.

We present a fully quantum-mechanical treatment of a single-mode atomic cavity with a pumping mechanism and an output coupling to a continuum of external modes. This system is a schematic description of an atom laser. In the dilute limit where atom-atom interactions are negligible, we have been able to solve this model without making the Born and Markov approximations. When coupling into free space, it is shown that for reasonable parameters there is a bound state which does not disperse, which means that there is no steady state. This bound state does not exist when gravity is included, and in that case the system reaches a steady state. We develop equations of motion for the two-time correlation in the presence of pumping and gravity in the output modes. We then calculate the energy spectrum of the steady-state output flux from the laser. We present a self-consistent Markov approximation which allows efficient calculation of this energy spectrum well above threshold.

We describe a graphics package we have developed for producing photo-realistic images of relativisticaly moving objects. The physics of relativistic images is outlined.

We use a simple two-mode model to investigate the quantum state dynamics of a two-species Bose-Einsten condensate, such as that produced in recent experiments, undergoing weak Josephson coupling. We find that in certain parameter regimes the quantum state of the system evolves into a macroscopic superposition of two states which differ in the atom number difference between the two species. The size of the macroscopic superposition created by such a method can be varied by adjusting the Josephson coupling coefficient, and is found to be near maximal for a certain critical value of this coefficient.

A two-level system is driven by two strong fields, one on resonance and one close to resonance. Full abstract.

A two-level system is driven by two strong fields, one on resonance and one close to resonance. There is a resonance-like response of the coupled atom-field system when the detuning of the second field is an integer fraction of the Rabi frequency of the resonant field. These subharmonic resonances are monitored by a transition to a third level and give a characteristic spectrum dominated by a series of doublets.

We discuss the use of the Born and Markov approximations in describing the dynamics of an atom laser. Full abstract.

We discuss the use of the Born and Markov approximations in describing the dynamics of an atom laser. In particular, we investigate the applicability of the quantum optical Born-Markov master equation for describing output coupling. We derive conditions based on the atomic reservoir and atom dispersion relations for when the Born-Markov approximations are valid and discuss parameter regimes where these approximations fail in our atom laser model. Differences between the standard optical laser model and the atom laser are due to a combination of factors, including the parameter regimes in which a typical atom laser would operate, the different reservoir state that is appropriate for atoms, and the different dispersion relations between atoms and photons. We present results based on an exact method in the regimes in which the Born-Markov approximation fails. The exact solutions in some experimentally relevant parameter regimes give a nonexponential loss of atoms from a cavity.

We numerically calculate the zero-temperature density profile and excitation spectrum of a two-species Bose-Einstein condensate for the parameters of recent experiments. Full abstract.

We numerically calculate the zero-temperature density profile and excitation spectrum of a two-species Bose-Einstein condensate for the parameters of recent experiments. We find that the ground-state density profile of this system becomes unstable in certain parameter regimes, which leads to a phase transition to a new stable state. This state displays spontaneously broken cylindrical symmetry. This behavior is reflected in the excitation spectrum: As we approach the phase transition point, the lowest excitation frequency goes to zero, indicating the onset of instability in the density profile. Following the phase transition, this frequency rises again.

We propose an optical method for increasing the number of atoms in a pair of dilute-gas Bose-Einstein condensates. Full abstract.

We propose an optical method for increasing the number of atoms in a pair of dilute-gas Bose-Einstein condensates. The method uses laser-driven Raman transitions which scatter atoms between the condensate and noncondensate atom fractions. For a range of condensate phase differences there is destructive quantum interference of the amplitudes for scattering atoms out of the condensates. Because the total atom scattering rate into the condensates is unaffected, the condensates grow.

We calculate the spectrum of a beam of atoms output from a single-mode atomic cavity. Full abstract.

We calculate the spectrum of a beam of atoms output from a single-mode atomic cavity. The output coupling uses an internal-state change to an untrapped state. We present an analytical solution for the output energy spectrum from a broadband coupler of this type. An example of such an output coupler, which we discuss in detail, uses a Raman transition to produce a nontrapped state.

We show that the phase difference of two overlapping ground-state Bose-Einstein condensates can affect the optical spontaneous emission rate of excited atoms. Full abstract.

We show that the phase difference of two overlapping ground-state Bose-Einstein condensates can affect the optical spontaneous emission rate of excited atoms. Depending on the phase difference the atom-stimulated spontaneous emission rate can vary between zero and the rate corresponding to all the ground-state atoms in a single condensate. Besides giving control over spontaneous emission this provides an optical method for detecting the condensate phase difference. It differs from previous methods in that no light fields are applied. Instead the light is spontaneously emitted when excited atoms make a transition into either condensate.

In this paper we present an atom laser scheme using a Raman transition for the output coupling of atoms. Full abstract.

In this paper we present an atom laser scheme using a Raman transition for the output coupling of atoms. A beam of thermal atoms (bosons) in a metastable atomic state is pumped into a multimode atomic cavity. This cavity is coupled through spontaneous emission to another cavity for the atomic ground state. Above a certain threshold pumping rate a large number of atoms build up in the lowest energy state of the second cavity, while the higher energy states remain unpopulated. Atoms are then coupled to the outside of the cavity with a Raman transition. This changes the internal level of the atom and imparts a momentum kick, allowing the atoms to leave the system. We propose an implementation of our scheme using hollow optical-fiber atom waveguides.

Nuclear processes can be stimulated by the presence of a macroscopic number of bosons in one of the final states. Full abstract.

Nuclear processes can be stimulated by the presence of a macroscopic number of bosons in one of the final states. We describe the conditions necessary to observe the atom-stimulation of a beta decay process. The stimulation may be observable if it becomes possible to produce a Bose-Einstein condensate with the order of 10¹⁴ atoms in a trap.

This paper examines the feasibility of constructing an experiment that detects the atomic stimulation of a photon-emission process. Full abstract.

This paper examines the feasibility of constructing an experiment that detects the atomic stimulation of a photon-emission process. A beam of atoms (bosons) in an excited state is passed through an atomic trap that traps the atoms when they are in their internal ground state. When the trap contains a Bose-Einstein condensate, the cross section for absorption of the atomic beam is increased. We examine a particular model in which this atom stimulation is observable and is also characterized by the emission of photons in a narrow cone in the direction of the atomic beam.

We investigate the mechanical potential of an atom due to a Raman transition. This is a generalization of the usual two-level dipole potential. Full abstract.

We investigate the mechanical potential of an atom due to a Raman transition. This is a generalization of the usual two-level dipole potential. With sufficiently large one- and two-photon detunings the forms of the potential and excitation probability are the same as for the two-level case, but with a scaled detuning. This allows greater effective blue detunings to be accessible with the Raman scheme. This means that with the same available laser power, atoms can move coherently in the light field for many orders of magnitude longer without spontaneously emitting. Furthermore the sign of the potential may be controlled independently of the one-photon detuning.

Diffraction of multi-level atoms by an evanescent wave reflective diffraction grating is modeled by numerically solving the time-dependent Schrödinger equation. Full abstract.

Diffraction of multi-level atoms by an evanescent wave reflective diffraction grating is modeled by numerically solving the time-dependent Schrödinger equation. We are able to explain the diffraction observed in experiments with metastable neon. This is not possible using a two-level atom model. The multi-level model predicts sensitive dependence of diffraction on the laser polarization. For pure p-polarization of the lasers a dark state forms. Associated with this is high order diffraction.

The energy spectrum for a system of atoms in a periodic potential can exhibit a gap in the band structure. Full abstract.

The energy spectrum for a system of atoms in a periodic potential can exhibit a gap in the band structure. We describe a system in which a laser is used to produce a mechanical potential for the atoms, and a standing-wave light field is used to shift the atomic levels using the Autler-Townes effect, which produces a periodic potential. The band structure for atoms guided by a hollow optical fiber waveguide is calculated in three dimensions with quantized external motion. The size of the band gap is controlled by the light guided by the fiber. This variable band structure may allow the construction of devices that can cool atoms. The major limitation on this device would be the spontaneous emission losses.

This paper introduces and reviews light forces, atom cooling and atom trapping. Full abstract.

This paper introduces and reviews light forces, atom cooling and atom trapping. The emphasis is on the physics of the basic processes. In discussing conservative forces the semi-classical dressed states are used rather than the usual quantized field dressed states.

We numerically solve the time-dependent Schrödinger equation for a two-level atom interacting with an evanescent light field. Full abstract.

We numerically solve the time-dependent Schrödinger equation for a two-level atom interacting with an evanescent light field. The atom may be reflected or diffracted. Using the experimental parameter values we quantitatively model the evanescent field dopplerons (velocity-tuned resonances) observed by Stenlake et al. [Phys. Rev. A 49, 16 (1994)]. Besides successfully modeling the experiment, our approach provides complementary insights to the usual solution of the time-independent Schrödinger equation. We neglect spontaneous emission.

We analyze a type of cavity, or trap, for atoms: a hollow optical fiber bent into a vertical U shape. Full abstract.

We analyze a type of cavity, or trap, for atoms: a hollow optical fiber bent into a vertical U shape. The atoms are confined by gravity and by light forces due to the evanescent wave on the fibers interior surface. A unique feature of this cavity is its mechanical flexibility, which allows tailoring of the gravitational potential experienced by the atoms. In particular a cycloid shape gives simple harmonic motion along the fiber. It can achieve confinement times similar to the parabolic reflector type of gravitational cavity. Quantized motion and an intracavity cooling scheme are considered.

We numerically investigate the classical limit of quantum trajectories in optical second-harmonic generation. Full abstract.

We numerically investigate the classical limit of quantum trajectories in optical second-harmonic generation. This is a dissipative system of two nonlinearly coupled harmonic oscillators, corresponding to the fundamental and second-harmonic optical cavity modes. Classically it is described by a system of nonlinear differential equations which have a range of attractors: fixed points, limit cycles, and chaotic attractors. We consider the field-amplitude mean values corresponding to the quantum trajectory wave functions. We find that in the classical limit of large photon number the trajectories of these mean values approach the corresponding classical attractors. This is because the mean values of operator products factorize into products of mean values in the classical limit.

We consider the theoretical problem of finding the bound eigenstates of an infinite nonuniform two-dimensional waveguide with Dirichlet boundary conditions. Full abstract.

We consider the theoretical problem of finding the bound eigenstates of an infinite nonuniform two-dimensional waveguide with Dirichlet boundary conditions. Using a coordinate transformation we show that this is equivalent to finding the eigenstates of a uniform waveguide with a potential proportional to the eigenvalue. Hence there is a sense in which the bound states occurring in nonuniform waveguides are analogous to bound states due to potentials in uniform waveguides.

We present a theoretical analysis of coherent atomic motion through a straight atomic waveguide constructed from a hollow optical fiber. Full abstract.

We present a theoretical analysis of coherent atomic motion through a straight atomic waveguide constructed from a hollow optical fiber. Atoms are guided by the evanescent light field at the fibers interior glass-vacuum interface. The atoms internal structure is modeled by a J_g=0 to J_e=1 transition. The atomic wave functions are determined and the loss rates due to spontaneous emission, tunneling to the wall, and nonadiabatic transitions are estimated. The influence of Casimir-Polder forces is considered. We conclude with a discussion of the feasibility of the proposed waveguides.

We consider a two-level atom driven by modulated light and find that complete population inversion can be induced by light without any resonant frequency component. Full abstract.

We consider a two-level atom driven by modulated light and find that complete population inversion can be induced by light without any resonant frequency component. This is in contrast to the familiar case of monochromatic driving in which complete population inversion is only possible with resonant light. Our results concern experimentally realizable systems, and hence the effect of spontaneous emission is considered. We relate our results to recent work on double quantum wells and hence extend the parameter regime for low-frequency radiation generation

We present a quantum-mechanical analysis of an above-threshold laser amplifier made phase sensitive by an injected field. Full abstract.

We present a quantum-mechanical analysis of an above-threshold laser amplifier made phase sensitive by an injected field. The gain and noise are calculated in a high-Q cavity with four-level atoms as the lasing medium. We find that for coherent-state inputs this amplifier can preserve the signal-to-noise ratio thus achieving the theoretical best noise performance.

Various configurations of dual recycling laser interferometers are tested, numerically, for their tolerance to wave-front distortions induced by mirror tilts and curvature mismatch. Full abstract.

Various configurations of dual recycling laser interferometers are tested, numerically, for their tolerance to wave-front distortions induced by mirror tilts and curvature mismatch. We verify that, in the presence of such geometric imperfections, dual recycling designs experience significantly less power loss than straight power recycling devices. Importantly, we show that the power loss reduction from the detection port is accompanied by maintenance of circulating power in the instrument and hence maintenance of signal response. We confirm predictions that when a cavity is placed at the output port of the interferometer, tolerance to geometric imperfections is further improved by typically an order of magnitude.

Publications 1994-2003 mainly cold atoms

Publications 1994-2003
mainly cold atoms