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Matt Mackie

Publications and source records attributed to Matt Mackie.

At least 19 recordsLinked to original sources
arXiv2012

Bound states of two bosons in an optical lattice near an association resonance

We model two bosons in an optical lattice near a Feshbach or photoassociation resonance, focusing on the Bose-Hubbard model in one dimension. Whereas the usual atoms-only theory with a tunable scattering length yields one bound state for a molecular dimer for either attractive or repulsive atom-atom interaction, an atom-molecule theory gives two bound states that may represent attractively and repulsively bound dimers occurring simultaneously. Such unusual molecular physics should be observable for an atom-molecule coupling strength comparable to the width of the dissociation continuum of the lattice dimer, for example, using narrow Feshbach resonances in Na, $^{87}$Rb, and $^{133}$Cs or low-intensity photoassociation in $^{174}$Yb.

physics.atom-ph↗
arXiv2011

Feshbach-resonant Raman photoassociation in a Bose-Einstein condensate

We model the formation of stable heteronuclear molecules via pulsed Raman photoassociation of a two-component Bose-Einstein condensate near a strong Feshbach resonance, for both counterintuitive and intuitive pulse sequencing. Compared to lasers alone, weak Raman photoassociation is enhanced by as much as a factor of ten (five) for a counterintuitive (intuitive) pulse sequence, whereas strong Raman photoassociation is barely enhanced at all--regardless of pulse sequence. Stronger intra-atom, molecule, or atom-molecule collisions lead to an expected decrease in conversion efficiency, but stronger ambient inter-atom collisions lead to an unexpected increase in the efficiency of stable molecule production. Numerical results agree reasonably with an analytical approximation.

physics.atom-ph↗
arXiv2010

Creating a Quantum Degenerate Gas of Stable Molecules via Weak Photoassociation

Quantum degenerate molecules represent a new paradigm for fundamental studies and practical applications. Association of already quantum degenerate atoms into molecules provides a crucial shortcut around the difficulty of cooling molecules to ultracold temperatures. Whereas association can be induced with either laser or magnetic fields, photoassociation requires impractical laser intensity to overcome poor overlap between the atom pair and molecular wavefunctions, and experiments are currently restricted to magnetoassociation. Here we model realistic production of a quantum degenerate gas of stable molecules via two-photon photoassociation of Bose-condensed atoms. An adiabatic change of the laser frequency converts the initial atomic condensate almost entirely into stable molecular condensate, even for low-intensity lasers. Results for dipolar LiNa provide an upper bound on the necessary photoassociation laser intensity for alkali-metal atoms ~30 W/cm^2, indicating a feasible path to quantum degenerate molecules beyond magnetoassociation.

cond-mat.quant-gas↗
arXiv2010

Many-Body Rate Limit on Photoassociation of a Bose-Einstein Condensate

We briefly report on zero-temperature photoassociation of a Bose-Einstein condensate, focusing on the many-body rate limit for atom-molecule conversion. An upgraded model that explicitly includes spontaneous radiative decay leads to an unanticipated shift in the position of the photoassociation resonance, which affects whether the rate (constant) maximizes or saturates, as well as the limiting value itself. A simple analytical model agrees with numerical experiments, but only for high density. Finally, an explicit comparison with the two-body unitary limit, set by the size of the condensate, finds that the many-body rate limit is generally more strict.

cond-mat.quant-gas↗
arXiv2009

Theory of Combined Photoassociation and Feshbach Resonances in a Bose-Einstein Condensate

We model combined photoassociation and Feshbach resonances in a Bose-Einstein condensate, where the shared dissociation continuum allows for quantum interference in losses from the condensate, as well as a dispersive-like shift of resonance. A simple analytical model, based on the limit of weakly bound molecules, agrees well with numerical experiments that explicitly include dissociation to noncondensate modes. For a resonant laser and an off-resonant magnetic field, constructive interference enables saturation of the photoassociation rate at user-friendly intensities, at a value set by the interparticle distance. This rate limit is larger for smaller condensate densities and, near the Feshbach resonance, approaches the rate limit for magnetoassociation alone. Also, we find agreement with the unitary limit--set by the condensate size--only for a limited range of near-resonant magnetic fields. Finally, for a resonant magnetic field and an off-resonant laser, magnetoassociation displays similar quantum interference and a dispersive-like shift. Unlike photoassociation, interference and the fieldshift in resonant magnetoassociation is tunable with both laser intensity and detuning. Also, the dispersive-like shift of the Feshbach resonance depends on the size of the Feshbach molecule, and is a signature of non-universal physics in a strongly interacting system.

cond-mat.quant-gas↗
arXiv2009

All Optical Scheme for Strongly Enhanced Production of Dipolar Molecules in the Electro-Vibrational Ground State

We consider two-color heteronuclear photoassociation of atoms into dipolar molecules in the J=1 electro-vibrational ground state, where a free-ground laser couples atoms directly to the ground state and a free-bound laser couples the atoms to an electronically-excited state. This problem raises an interest because heteronuclear photoassociation from atoms to near-ground state molecules is limited by the small size of the target state. Nevertheless, the addition of the excited state creates a second pathway for creating ground state molecules, leading to quantum interference between direct photoassociation and photoassociation via the excited molecular state, as well as a dispersive-like shift of the free-ground resonance position. Using LiNa as an example, these results are shown to depend on the detuning and intensity of the free-bound laser, as well as the semi-classical size of both molecular states. Despite strong enhancement, coherent conversion to the LiNa electro-vibrational ground state is possible only in a limited regime near the free-bound resonance.

cond-mat.quant-gas↗
arXiv2008

Cross-Molecular Coupling in Combined Photoassociation and Feshbach Resonances

We model combined photoassociation and Feshbach resonances in a Bose-Einstein condensate. When the magnetic field is far-off resonance, cross coupling between the two target molecules--enabled by the shared dissociation continuum--leads to an anomalous dispersive shift in the position of laser resonance, as well as unprecedented elimination and enhancement of resonant photoassociation via quantum interference. For off-resonant lasers, a dispersive shift and quantum interference appear similarly in resonant three-body Feshbach losses, except that the Feshbach node is tunable with intensity.

physics.atom-ph↗
arXiv2006

Rogue decoherence in the formation of a macroscopic atom-molecule superposition

We theoretically examine two-color photoassociation of a Bose-Einstein condensate, focusing on the role of rogue decoherence in the formation of macroscopic atom-molecule superpositions. Rogue dissociation occurs when two zero-momentum condensate atoms are photoassociated into a molecule, which then dissociates into a pair of atoms of equal-and-opposite momentum, instead of dissociating back to the zero-momentum condensate. As a source of decoherence that may damp quantum correlations in the condensates, rogue dissociation is an obstacle to the formation of a macroscopic atom-molecule superposition. We study rogue decoherence in a setup which, without decoherence, yields a macroscopic atom-molecule superposition, and find that the most favorable conditions for said superposition are a density ~ 1e12 atoms per cc and temperature ~ 1e-10.

physics.atom-ph↗
arXiv2005

Simple Mean-Field Theory for a Zero-Temperature Fermi Gas at a Feshbach Resonance

We present a simple two-channel mean field theory for a zero-temperature two-component Fermi gas in the neighborhood of a Feshbach resonance. Our results agree with recent experiments on the bare-molecule fraction as a function of magnetic field [Partridge et al., cond-mat/0505353]. Even in this strongly-coupled gas of Li-6, the experimental results depend on the structure of the molecules formed in the Feshbach resonance and, therefore, are not universal.

cond-mat.supr-con↗
arXiv2005

Coherent Population Trapping in a Feshbach-Resonant Cesium Condensate

Recent experiments with Feshbach-resonant cesium Bose-Einstein condensates have led to unexplained molecule formation: a sudden switch of the magnetic field to its resonance value, followed by a finite hold time and another sudden switch to magnetic field values below threshold, converts about a third of the initial condensate atoms into molecules. Based on a model of coherent conversion between an atomic condensate, a molecular condensate, and magnetodissociated noncondensate atom pairs of equal and opposite momentum, we find that population trapping is strongly implicated as the physical mechanism responsible for molecule formation in switch experiments.

cond-mat.other↗
arXiv2005

Feshbach-Resonant Interactions in 40K and 6Li Degenerate Fermi Gases

We theoretically examine a system of Fermi degenerate atoms coupled to bosonic molecules by a Feshbach resonance, focusing on the superfluid transition to a molecular Bose-Einstein condensate dressed by Cooper pairs of atoms. This problem raises an interest because it is unclear at present whether bimodal density distributions observed recently in 40-K and 6-Li are due to a condensate of bosonic molecules or fermionic atom pairs. As opposed to 40-K, we find that any measurable fraction of above-threshold bosonic molecules are necessarily absent for the 6-Li system in question, which strongly implicates Cooper pairs as the culprit behind its bimodal distributions.

cond-mat.other↗
arXiv2005

Zero-Temperature Theory of Collisionless Rapid Adiabatic Passage from a Fermi Degenerate Gas of Atoms to a Bose-Einstein Condensate of Molecules

We theoretically examine a zero-temperature system of Fermi degenerate atoms coupled to bosonic molecules via collisionless rapid adiabatic passage across a Feshbach resonance, focusing on saturation of the molecular conversion efficiency at the slowest magnetic-field sweep rates. Borrowing a novel MANY-FERMION Fock-state theory, we find that a proper model of the magnetic-field sweep can systematically remove saturation. We also debunk the common misconception that many-body effects are responsible for molecules existing above the two-body threshold.

physics.atom-ph↗
arXiv2004

Collective molecule formation in a degenerate Fermi gas via a Feshbach resonance

We model collisionless collective conversion of a degenerate Fermi gas into bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a classical field and seeding the pairing amplitudes with random phases. A dynamical instability of the Fermi sea against association into molecules initiates the conversion. The model qualitatively reproduces several experimental observations {[Regal et al., Nature {\bf 424}, 47 (2003)]}. We predict that the initial temperature of the Fermi gas sets the limit for the efficiency of atom-molecule conversion.

physics.atom-ph↗
arXiv2004

New Directions in Degenerate Dipolar Molecules via Collective Association

We survey results on the creation of heteronuclear Fermi molecules by tuning a degenerate Bose-Fermi mixture into the neighborhood of an association resonance, either photoassociation or Feshbach, as well as the subsequent prospects for Cooper-like pairing between atoms and molecules. In the simplest case of only one molecular state, corresponding to either a Feshbach resonance or one-color photoassociation, the system displays Rabi oscillations and rapid adiabatic passage between a Bose-Fermi mixture of atoms and fermionic molecules. For two-color photoassociation, the system admits stimulated Raman adiabatic passage (STIRAP) from a Bose-Fermi mixture of atoms to stable Fermi molecules, even in the presence of particle-particle interactions. By tailoring the STIRAP sequence it is possible to deliberately convert only a fraction of the initial atoms, leaving a finite fraction of bosons behind to induce atom-molecule Cooper pairing via density fluctuations; unfortunately, this enhancement is insufficient to achieve a superfluid transition with present ultracold technology. We therefore propose the use of an association resonance that converts atoms and diatomic molecules (dimers) into triatomic molecules (trimers), which leads to a crossover from a Bose-Einstein condensate of trimers to atom-dimer Cooper pairs. Because heteronuclear dimers may possess a permanent electric dipole moment, this overall system presents an opportunity to investigate novel microscopic physics.

physics.atom-ph↗
arXiv2004

Improved efficiency of stimulated Raman adiabatic passage in photoassociation of a Bose-Einstein condensate

We theoretically examine Raman photoassociation of a Bose-Einstein condensate, revisiting stimulated Raman adiabatic passage (STIRAP). Due to collisional mean-field shifts, efficient molecular conversion requires strong coupling and low density, either of which can bring about rogue photodissociation to noncondensate modes. We demonstrate explicitly that rogue transitions are negligible for low excited-state fractions and photodissociation that is slower than the STIRAP timescale. Moreover, we derive a reduced-parameter model of collisions, and thereby find that a gain in the molecular conversion efficiency can be obtained by adjusting the atom-atom scattering length with off-resonant magnetoassociation. This gain saturates when the atom-atom scattering length is tuned to a specific fraction of either the molecule-molecule or atom-molecule scattering length. We conclude that a fully-optimized STIRAP scheme may offer the best chance for achieving coherent conversion from an atomic to a molecular condensate with photoassociation.

physics.atom-ph↗
arXiv2004

Raman Photoassociation of Bose-Fermi Mixtures and the Subsequent Prospects for Atom-Molecule Cooper Pairing

We theoretically investigate Raman photoassociation of a degenerate Bose-Fermi mixture of atoms and the subsequent prospect for anomalous (Cooper) pairing between atoms and molecules. Stable fermionic molecules are created via free-bound-bound stimulated Raman adiabatic passage which, in contrast to purely bosonic systems, can occur in spite of collisions. With the leftover atomic condensate to enhance intrafermion interactions, the superfluid transition to atom-molecule Cooper pairs occurs at a temperature that is roughly an order of magnitude below what is currently feasible.

physics.atom-ph↗
arXiv2004

Comment on "Stimulated Raman adiabatic passage from an atomic to a molecular Bose-Einstein condensate"

Collective two-color photoassociation of a freely-interacting 87Rb Bose-Einstein condensate is theoretically examined, focusing on stimulated Raman adiabatic passage (STIRAP) from an atomic to a stable molecular condensate. In particular, Drummond et al. [Phys. Rev. A 65, 063619 (2002); cond-mat/0110578] have predicted that particle-particle interactions can limit the efficiency of collective atom-molecule STIRAP, and that optimizing the laser parameters can partially overcome this limitation. We suggest that the molecular conversion efficiency can be further improved by treating the initial condensate density as an optimization parameter.

physics.atom-ph↗