Publications (19)34.56 Total impact

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ABSTRACT: We study the resonant control of two nonreactive polar molecules in an optical lattice site, focusing on the example of RbCs. Collisional control can be achieved by tuning bound states of the intermolecular dipolar potential by varying the applied electric field or trap frequency. We consider a wide range of electric fields and trapping geometries, showing that a threedimensional optical lattice allows significantly wider avoided crossings than free space or quasitwo dimensional geometries. Furthermore, we find that dipolar confinementinduced resonances can be created with reasonable trapping frequencies and electric fields, and have widths that will enable useful control in forthcoming experiments.Physical Review A 02/2012; 85(2). DOI:10.1103/PhysRevA.85.022703 · 2.99 Impact Factor 
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ABSTRACT: We present a Ramseytype atom interferometer operating with an optically trapped sample of 10^6 Bosecondensed Rb87 atoms. The optical trap allows us to couple the F =1, mF =0>\rightarrow F =2, mF =0> clock states using a single photon 6.8GHz microwave transition, while state selective readout is achieved with absorption imaging. Interference fringes with contrast approaching 100% are observed for short evolution times. We analyse the process of absorption imaging and show that it is possible to observe atom number variance directly, with a signaltonoise ratio ten times better than the atomic projection noise limit on 10^6 condensate atoms. We discuss the technical and fundamental noise sources that limit our current system, and outline the improvements that can be made. Our results indicate that, with further experimental refinements, it will be possible to produce and measure the output of a subshotnoise limited, large atom number BECbased interferometer.New Journal of Physics 11/2011; 13(11):119401. DOI:10.1088/13672630/13/11/119401 · 3.67 Impact Factor 
Article: Feshbach resonances in the 6 Li 40 K FermiFermi mixture: elastic versus inelastic interactions
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ABSTRACT: . We present a detailed theoretical and experimental study of Feshbach resonances in the 6Li40K mixture. Particular attention is given to the inelastic scattering properties, which have not been considered before. As an important example, we thoroughly investigate both elastic and inelastic scattering properties of a resonance that occurs near 155 G. Our theoretical predictions based on a coupled channels calculation are found in excellent agreement with the experimental results. We also present theoretical results on the molecular state that underlies the 155 G resonance, in particular concerning its lifetime against spontaneous dissociation. We then present a survey of resonances in the system, fully characterizing the corresponding elastic and inelastic scattering properties. This provides the essential information to identify optimum resonances for applications relying on interaction control in this FermiFermi mixture.The European Physical Journal D 11/2011; 65:111. DOI:10.1140/epjd/e2010105912 · 1.40 Impact Factor 
Conference Paper: Optically trapped atom interferometry using the clock transition of large Rubidium87 BoseEinstein condensates
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ABSTRACT: We present a Ramseytype atom interferometer operating with an optically trapped sample of 10^6 Bosecondensed 87 Rb atoms. We investigate this interferometer experimentally and theoretically with an eye to the construction of future high precision atomic sensors. Our results indicate that, with further experimental refinements, it will be possible to produce and measure the output of a subshotnoise limited, large atom number BECbased interferometer. The optical trap (shown in Figure 1 (c)) allows us to couple the F = 1, m F = 0> → F = 2, m F = 0> clock states using a single photon 6.8 GHz microwave transition, while state selective readout is achieved with absorption imaging. We analyse the process of absorption imaging and show that it is possible to observe atom number variance directly, with a signaltonoise ratio ten times better than the atomic projection noise limit on 10^6 condensate atoms. We discuss the technical and fundamental noise sources that limit our current system, and present theoretical and experimental results on interferometer contrast, dephasing and miscibility.IQEC/CLEO Pacific Rim 2011; 08/2011 
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ABSTRACT: Universal collision rate constants are calculated for ultracold collisions of two like bosonic or fermionic heteronuclear alkalimetal dimers involving the species Li, Na, K, Rb, or Cs. Universal collisions are those for which the short range probability of a reactive or quenching collision is unity such that a collision removes a pair of molecules from the sample. In this case, the collision rates are determined by universal quantum dynamics at very long range compared to the chemical bond length. We calculate the universal rate constants for reaction of the reactive dimers in their ground vibrational state v = 0 and for vibrational quenching of nonreactive dimers with v ≥ 1. Using the known dipole moments and estimated van der Waals coefficients of each species, we calculate electric field dependent loss rate constants for collisions of molecules tightly confined to quasitwodimensional geometry by a onedimensional optical lattice. A simple scaling relation of the quasitwodimensional loss rate constants with dipole strength, trap frequency and collision energy is given for like bosons or like fermions. It should be possible to stabilize ultracold dimers of any of these species against destructive collisions by confining them in a lattice and orienting them with an electric field of less than 20 kV cm(1).Physical Chemistry Chemical Physics 07/2011; 13(42):1911424. DOI:10.1039/c1cp21270b · 4.20 Impact Factor 
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ABSTRACT: We discuss the longrange bound states of a pair of ground state polar molecules confined in a cylindrically symmetric optical lattice cell. We have solved the full twodimensional eigenvalue problem including van der Waals and anisotropic dipolar interactions. The dipoledipole interaction and lattice confinement are tunable, and with a large swave scattering length of the van der Waals potential it is possible to have coincidence of the three corresponding length scales. We study the bimolecular states, varying the z confinement from quasi2D to quasi1D geometry. In a quasi2D geometry, trap states are adiabatically converted to longrange bound states by increasing the electric field to more strongly align the dipoles along the axis of symmetry. In addition to confinement induced resonances, the electric field thereby provides opportunities for controlling collisional properties. Shallow bound states of the van der Waals potential are also strongly affected by the dipole moment and confinement. 
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ABSTRACT: We present a Ramseytype atom interferometer operating with an optically trapped sample of 10^6 Bosecondensed Rb87 atoms. The optical trap allows us to couple the F =1, mF =0>\rightarrow F =2, mF =0> clock states using a single photon 6.8GHz microwave transition, while state selective readout is achieved with absorption imaging. Interference fringes with contrast approaching 100% are observed for short evolution times. We analyse the process of absorption imaging and show that it is possible to observe atom number variance directly, with a signaltonoise ratio ten times better than the atomic projection noise limit on 10^6 condensate atoms. We discuss the technical and fundamental noise sources that limit our current system, and outline the improvements that can be made. Our results indicate that, with further experimental refinements, it will be possible to produce and measure the output of a subshotnoise limited, large atom number BECbased interferometer. In an addendum to the original paper, we attribute our inability to observe quantum projection noise to the stability of our microwave oscillator and background magnetic field. Numerical simulations of the GrossPitaevskii equations for our system show that dephasing due to spatial dynamics driven by interparticle interactions account for much of the observed decay in fringe visibility at long interrogation times. The simulations show good agreement with the experimental data when additional technical decoherence is accounted for, and suggest that the clock states are indeed immiscible. With smaller samples of 5 \times 10^4 atoms, we observe a coherence time of {\tau} = (1.0+0.50.3) s.New Journal of Physics 11/2010; 13(6):119401. DOI:10.1088/13672630/13/6/065020 · 3.67 Impact Factor 
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ABSTRACT: We present a simple technique for studying the collisions of ultracold atoms in the presence of a magnetic field and radiofrequency (rf) radiation. Resonant control of scattering properties can be achieved by using rf to couple a colliding pair of atoms to a bound state. We show, using the example of 6Li, that in some ranges of rf frequency and magnetic field this can be done without giving rise to losses. We also show that halo molecules of large spatial extent allow resonant control with much less rf power than deeply bound states. Another way to exert resonant control is with a set of rfcoupled bound states, linked to the colliding pair through the molecular interactions that give rise to magnetically tunable Feshbach resonances. This was recently demonstrated for 87Rb (Kaufman et al 2009 Phys. Rev. A 80 050701) [1]. We examine the underlying atomic and molecular physics that made this possible. Lastly, we consider the control that may be exerted over atomic collisions by placing atoms in superpositions of Zeeman states, and suggest that it could be useful where small changes in scattering length are required. We suggest other species for which rf and magnetic field control could together provide a useful tuning mechanism.New Journal of Physics 08/2010; 12(8):083031. DOI:10.1088/13672630/12/8/083031 · 3.67 Impact Factor 
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ABSTRACT: Scattering properties of ultracold atoms can be controlled using Feshbach resonances, which can be created using magnetic, optical or radiofrequency (rf) coupling of the colliding pair to a bound state. In the lossless case, the scattering length diverges at the resonance. However, in the presence of losses the scattering length has a range limited by the rate of decay. We study decaying resonances with both rigorous calculations and simple, intuitive models. We focus on rfinduced resonances for ^6Li, in which losses are created by rf coupling to several energetically lower channels. We show that creating a resonance by coupling to a broad, nearthreshold molecular state can provide useful control with low rf power and manageable losses. 
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ABSTRACT: Ultracold ground state dipolar 40K87Rb molecules have recently been produced in a loose harmonic trap by employing a magnetic field sweep across a Feshbach resonance followed by stimulated Raman adiabatic passage [K.K. Ni et al., Science 322, 231 (2008)]. The overall experimental efficiency for molecule formation was around 20%. We show that the efficiency can be increased to nearly 100% if one first loads the atomic gases into an optical lattice of the appropriate depth and tunes the interspecies attraction to have exactly one atom of each species at an occupied lattice site. Our proposed scheme provides a large enhancement to the dipolar molecule density even at relatively high temperatures, and avoids threebody recombination loss by preventing lattice sites from being triply occupied. Comment: (5 pages, 3 pages, submitted to Phys. Rev. Lett.)Physical Review A 01/2010; 81:011605. DOI:10.1103/PhysRevA.81.011605 · 2.99 Impact Factor 
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ABSTRACT: We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in Rb87 using radiofrequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rfcoupled bound states interacting with the collision threshold. Comment: 4+ pages, 3 figures, 1 table; revised introduction & references to reflect published versionPhysical Review A 11/2009; 80(5):050701(R). DOI:10.1103/PhysRevA.80.050701 · 2.99 Impact Factor 
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ABSTRACT: We will discuss recent computational work that employs both direct quantum Monte Carlo simulation and inhomogeneous dynamical meanfield theory to study the efficiency of preforming KRb pairs in an optical lattice. We will describe how to optimize the efficiency by adjusting the lattice depth and the interspecies interaction (via the Feshbach resonance) with parameters specific for fermionic ^40K and bosonic ^87Rb (since the groundstate dipolar molecule has already been formed from those atoms in free space). We work with a deep enough lattice that the K atoms are mobile, but the Rb atoms are localized, so the system is described by the spinless FalicovKimball model on a twodimensional lattice. We also calculate the entropy and estimate the temperature that one can achieve by cooling the atoms and adiabatically turning on the lattice. 
Article: Modeling a sodium spinor condensate
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ABSTRACT: An optically trapped condensate of F=1 sodium atoms shows coherent oscillations of the spin populations. These oscillations are well described by a theory based on the assumption that the three spin projections of the condensate share a single spatial mode [1]. Some time after the initial state preparation the oscillations damp and the system arrives in a state consistent with the ground state predictions of this theory. The transition from coherent oscillations to a ground state, however, is not understood. In this poster we describe a numerical model based on a three component coupled Gross Pitaevskii equation that simulates the behaviour of the gas. The model shows a coupling of spin interactions to motional degrees of freedom and shows behaviour similar to that seen in experiment. We also describe our efforts to produce an analytic model of this system. [1] W. Zhang et al, Phys. Rev. A, 72, 013602 (2005) 
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ABSTRACT: We have developed a model of Feshbach resonances in gases of ultracold alkali metal atoms using the ideas of multichannel quantum defect theory. Our model requires just three parameters describing the interactions  the singlet and triplet scattering lengths, and the long range van der Waals coefficient  in addition to known atomic properties. Without using any further details of the interactions, our approach can accurately predict the locations of resonances. It can also be used to find the singlet and triplet scattering lengths from measured resonance data. We apply our technique to $^{6}$Li$^{40}$K and $^{40}$K$^{87}$Rb scattering, obtaining good agreement with experimental results, and with the more computationally intensive coupled channels technique.Physical Review A 04/2009; 79(4). DOI:10.1103/PhysRevA.79.040701 · 2.99 Impact Factor 
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ABSTRACT: Multichannel quantum defect theory (MQDT) has a large number of applications in atomic physics, including the properties of collisions near threshold. The key concept is that the short range physics can be accounted for very simply and then matched to the asymptotic long range interaction. We have developed a model of Feshbach resonances based on the ideas of MQDT. This model allows calculation of the magnetic fields at which resonances occur, as well as properties such as the resonance width and background scattering length. Apart from known atomic properties, only three input parameters are required: the singlet and triplet scattering lengths, and the coefficient of the long range van der Waals potential. Analytic reference functions defined by the potential [1] are used to calculate the long range properties, which are linked to the short range physics through a frame transformation. We apply our theory to ^6Li^40K scattering, and obtain good agreement with experimental data and full coupled channels calculations, but with far less computational effort. This makes MQDT a useful tool for investigating collisions of new combinations of species. [1] B. Gao et al., Phys. Rev. A 72, 042719 (2005). 
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ABSTRACT: Recently ultracold vibrational ground state ^40K^87Rb polar molecules have been made using magnetoassociation of two cold atoms to a weakly bound Feshbach molecule, followed by a twocolor optical STIRAP process to transfer molecules to the molecular ground state [1]. We have used accurate potential energy curves for the singlet and triplet states of the KRb molecule [2] with coupled channels calculations to calculate all of the bound states of the ^40K^87Rb molecule as a function of magnetic field from the cold atom collision threshold to the v=0 ground state. We have also developed approximate models for understanding the changing properties of the molecular bound states as binding energy increases. Some overall conclusions from these calculations will be presented. [1] K.K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Peer, B. Neyenhuis, J. J. Zirbel, S. Kotochigova, P. S. Julienne, D. S. Jin, and J. Ye, Science, 2008, 322, 231235. [2] A. Pashov, O. Docenko, M. Tamanis, R. Ferber, H. Kn"ockel, and E. Tiemann, Phys. Rev. A, 2007, 76, 022511. 
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ABSTRACT: In recent experiments, Feshbach molecules have been associated using resonant modulation of a magnetic field close to a zeroenergy resonance [1, 2]. We analyse the dependence of this process upon the duration, amplitude and frequency of the modulation, as well as the temperature and density of the gas. A modulation of angular frequency phiL resonantly couples a pair of atoms with relative kinetic energy p^2/m = hphiL + Eb^av to the molecular state, where Eb^av is the molecular bound state energy. The presence of a continuum of modes around this energy has a strong influence on the final conversion efficiency. Shifts in the modulation frequency giving maximum conversion are created by the amplitude of the modulation and the temperature of the gas. We discuss the importance of meanfield effects at short times, and predict that resonant association can be effective for binding energies of order h x1 MHz. [1] S. T. Thompson, E. Hodby and C. E. Wieman, Phys. Rev. Lett. 95, 190404 (2005). [2] S. B. Papp and C. E. Wieman, Phys. Rev. Lett. 97, 180404 (2006). 
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ABSTRACT: We study the process of associating molecules from atomic gases using a magnetic field modulation that is resonant with the molecular binding energy. We show that maximal conversion is obtained by optimising the amplitude and frequency of the modulation for the particular temperature and density of the gas. For small modulation amplitudes, resonant coupling of an unbound atom pair to a molecule occurs at a modulation frequency corresponding to the sum of the molecular binding energy and the relative kinetic energy of the atom pair. An atom pair with an offresonant energy has a probability of association which oscillates with a frequency and timevarying amplitude which are primarily dependent on its detuning. Increasing the amplitude of the modulation tends to result in less energetic atom pairs being resonantly coupled to the molecular state, and also alters the dynamics of the transfer from continuum states with offresonant energies. This leads to maxima and minima in the total conversion from the gas as a function of the modulation amplitude. Increasing the temperature of the gas leads to an increase in the modulation frequency providing the best fit to the thermal distribution, and weakens the resonant frequency dependence of the conversion. Meanfield effects can alter the optimal modulation frequency and lead to the excitation of higher modes. Our simulations predict that resonant association can be effective for binding energies of order $h \times 1$ MHz.Physical Review A 10/2006; 75(1). DOI:10.1103/PhysRevA.75.013606 · 2.99 Impact Factor 
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ABSTRACT: We study the dissociation of Feshbach molecules by a magnetic field sweep across a zeroenergy resonance. In the limit of an instantaneous magnetic field change, the distribution of atomic kinetic energy can have a peak indicating dominance of the molecular closedchannel spin configuration over the entrance channel. The extent of this dominance influences physical properties such as stability with respect to collisions, and so the readily measurable presence or absence of the corresponding peak provides a practical method of classifying zeroenergy resonances. Currently achievable ramp speeds, e.g. those demonstrated by Duerr et al. [Phys. Rev. A 70, 031601 (2005)], are fast enough to provide magnetic field changes that may be interpreted as instantaneous. We study the transition from sudden magnetic field changes to asymptotically wide, linear ramps. In the latter limit, the predicted form of the atomic kinetic energy distribution is independent of the specific implementation of the twobody physics, provided that the nearresonant scattering properties are properly accounted for.Physical Review A 06/2006; 74(2). DOI:10.1103/PHYSREVA.74.023618 · 2.99 Impact Factor
Publication Stats
148  Citations  
34.56  Total Impact Points  
Top Journals
Institutions

2010–2012

National Institute of Standards and Technology
 Applied and Computational Mathematics Division
Maryland, United States


2009–2011

Loyola University Maryland
Baltimore, Maryland, United States


2006

University of Oxford
 Department of Physics
Oxford, ENG, United Kingdom
