Roman V. Krems

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

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Publications (124)347.46 Total impact

  • Timur V. Tscherbul, Roman V. Krems
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    ABSTRACT: We develop a theoretical method for solving the quantum mechanical reactive scattering problem in the presence of external fields based on a hyperspherical coordinate description of the reaction complex combined with the total angular momentum representation for collisions in external fields. The method allows us to obtain converged results for the chemical reaction LiF + H -> Li + HF in an electric field. Our calculations demonstrate that, by inducing couplings between states of different total angular momenta, electric fields with magnitudes <150 kV/cm give rise to resonant scattering and a significant modification of the total reaction probabilities, product state distributions and the branching ratios for reactive vs inelastic scattering.
    09/2014;
  • Zhiying Li, Roman V Krems, Eric J Heller
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    ABSTRACT: We explore the collision dynamics of complex hydrocarbon molecules (benzene, coronene, adamantane, and anthracene) containing carbon rings in a cold buffer gas of (3)He. For benzene, we present a comparative analysis of the fully classical and fully quantum calculations of elastic and inelastic scattering cross sections at collision energies between 1 and 10 cm(-1). The quantum calculations are performed using the time-independent coupled channel approach and the coupled-states approximation. We show that the coupled-states approximation is accurate at collision energies between 1 and 20 cm(-1). For the classical dynamics calculations, we develop an approach exploiting the rigidity of the carbon rings and including low-energy vibrational modes without holonomic constraints. Our results illustrate the effect of the molecular shape and the vibrational degrees of freedom on the formation of long-lived resonance states that lead to low-temperature clustering.
    The Journal of chemical physics. 09/2014; 141(10):104317.
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    Alisdair O. G. Wallis, Roman V. Krems
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    ABSTRACT: Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic 1Σ state. Unlike alkali metal atoms, 1Σ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for 1Σ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.
    02/2014; 89(3).
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    ABSTRACT: We show that the interaction of polar alkali-metal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkali-metal dimers (for all molecules except KRb) and into alkali-metal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkali-metal dimers in the a^{3}Σ^{+} state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimum-energy path for the chemical reactions of alkali-metal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkali-metal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkali-metal dimers in the a^{3}Σ^{+} electronic state.
    Physical Review A 11/2013; 88(5). · 3.04 Impact Factor
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    Jie Cui, Roman V. Krems
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    ABSTRACT: We calculate the cross sections for elastic scattering and Zeeman relaxation in binary collisions of molecules in the ro-vibrational ground state of a $^2\Sigma$ electronic state and the Zeeman state with the electron spin projection $M_S=1/2$ on the magnetic field axis. This is the lowest-energy state of $^2\Sigma$ molecules confined in a magnetic trap. The results are averaged over calculations with multiple molecule - molecule interaction potentials, which yields the expectation intervals for the cross sections and the elastic-to-inelastic cross section ratios. We find that the elastic-to-inelastic cross section ratios under conditions corresponding to trapped molecular ensembles at $T \sim 10^{-3}$ K exceed 100 for the majority of $^2\Sigma$ molecules. The range of $^2\Sigma$ molecules expected to be collisionally unstable in magnetic traps at $T < 10^{-3}$ K is limited to molecules with the spin-rotation interaction constant $\gamma_{\rm SR} > 0.5$ cm$^{-1}$ and the rotational constant $B_e < 4$ cm$^{-1}$.
    Physical Review A 08/2013; 88(4). · 3.04 Impact Factor
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    ABSTRACT: The goal of the present article is to review the major developments that have led to the current understanding of molecule-field interactions and experimental methods for manipulating molecules with electromagnetic fields. Molecule-field interactions are at the core of several, seemingly distinct, areas of molecular physics. This is reflected in the organization of this article, which includes sections on Field control of molecular beams, External field traps for cold molecules, Control of molecular orientation and molecular alignment, Manipulation of molecules by non-conservative forces, Ultracold molecules and ultracold chemistry, Controlled many-body phenomena, Entanglement of molecules and dipole arrays, and Stability of molecular systems in high-frequency super-intense laser fields. The article contains 853 references.
    Molecular Physics 06/2013; 111(12-13). · 1.67 Impact Factor
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    ABSTRACT: We determine the phase diagram of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for Su-Schrieffer-Heeger coupling) or no (for breathing-mode coupling) transition. We then show that ultracold molecules trapped in optical lattices can be used as a quantum simulator to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with external electric fields. The parameters of current experiments place them in the region where one of the transitions occurs. We also propose a scheme to measure the polaron dispersion using stimulated Raman spectroscopy.
    Physical Review Letters 05/2013; 110(22):223002. · 7.73 Impact Factor
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    ABSTRACT: An elementary excitation in an aggregate of coupled particles generates a collective excited state. We show that the dynamics of these excitations can be controlled by applying a transient external potential which modifies the phase of the quantum states of the individual particles. The method is based on an interplay of adiabatic and sudden time scales in the quantum evolution of the many-body states. We show that specific phase transformations can be used to accelerate or decelerate quantum energy transfer and spatially focus delocalized excitations onto different parts of arrays of quantum particles. We consider possible experimental implementations of the proposed technique and study the effect of disorder due to the presence of impurities on its fidelity. We further show that the proposed technique can allow control of energy transfer in completely disordered systems.
    New Journal of Physics 09/2012; 15(6). · 4.06 Impact Factor
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    Jie Cui, Roman V. Krems
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    ABSTRACT: We present expressions demonstrating that collisional decoherence of ultracold atoms or molecules in a coherent superposition of nondegenerate quantum states is suppressed when both the real and imaginary parts of the scattering lengths for the states in the coherent superposition are equal. We show that the rate of collisional decoherence can be enhanced or suppressed by varying an external magnetic field near a Feshbach resonance. For some resonances, the suppression is very dramatic. We propose a method for measuring the scattering length of ultracold particles in excited quantum states exhibiting Feshbach resonances.
    Physical Review A 08/2012; 86(2). · 3.04 Impact Factor
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    Marina Litinskaya, Roman V. Krems
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    ABSTRACT: Ultracold polar molecules trapped on an optical lattice is a many-body system that, under appropriate conditions, may support collective excitations reminiscent of excitons in solid state crystals. Here, we discuss the rotational excitations of molecules on an optical lattice leading to rotational Frenkel excitons. Apart from solid hydrogen, there is no other natural system that exhibits rotational excitons. The rotational excitons have unique properties that can be exploited for tuning non-linear exciton interactions and exciton-impurity scattering by applying an external electric field. We show that this can be used to explore the competing role of the dynamical and kinematic exciton-exciton interactions in excitonic energy transfer and to study quantum localization in a dynamically tunable disordered potential. The rotational excitons can also be used as a basis for quantum simulation of condensed matter models that cannot be realized with ultracold atoms. As an example, we discuss the possibility of engineering the Holstein model with polar molecules on an optical lattice.
    08/2012;
  • Mikhail Lemeshko, Roman V. Krems, Hendrik Weimer
    Physical Review Letters 07/2012; 109(4). · 7.73 Impact Factor
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    ABSTRACT: We study the growth dynamics of ordered structures of strongly interacting polar molecules in optical lattices. Using a dipole blockade of microwave excitations, we map the system onto an interacting spin-1/2 model possessing ground states with crystalline order, and describe a way to prepare these states by nonadiabatically driving the transitions between molecular rotational levels. The proposed technique bypasses the need to cross a phase transition and allows for the creation of ordered domains of considerably larger size compared to approaches relying on adiabatic preparation.
    Physical Review Letters 07/2012; 109(3):035301. · 7.73 Impact Factor
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    ABSTRACT: We show that the cross sections for molecule-molecule collisions in the presence of an external field can be computed efficiently using a total angular momentum basis, defined either in the body-fixed frame or in the space-fixed coordinate system. This method allows for computations with much larger basis sets than previously possible. We present calculations for (15)NH-(15)NH collisions in a magnetic field. Our results support the conclusion of the previous study that the evaporative cooling of rotationally ground (15)NH molecules in a magnetic trap has a prospect of success.
    The Journal of Chemical Physics 07/2012; 137(2):024103. · 3.12 Impact Factor
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    ABSTRACT: Rotational excitation of ultracold polar molecules trapped on an optical lattice produces rotational Frenkel excitons (collective rotational excitations) [1]. We show that non-linear interactions between these excitons can be tuned by applying a dc electric field. We show that, at electric fields greater than a critical value, rotational Frenkel excitons form bound pairs -- biexcitons [2]. Frenkel biexcitons are strongly correlated states of two collective excitations in a molecular crystal, which are exceedingly hard to create and observe in solid-state crystals. We demonstrate that the binding energy of the rotational biexcitons can be controlled by tuning the angle between the applied field and the molecular array. Frenkel biexcitons can be used for many applications ranging from the controlled preparation of entanglement between quasi-particles to the study of bipolarons. [4pt] [1] ``Frenkel biexcitons in optical lattices with polar molecules,'' Ping Xiang, M. Litinskaya, R. V. Krems; cond-mat/1112.3942. [0pt] [2] ``Tunable disorder in a crystal of cold polar molecules,'' F. Herrera, M. Litinskaya, R. V. Krems, Phys. Rev. A 82, 033428 (2010).
    06/2012;
  • Felipe Herrera, Roman V. Krems
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    ABSTRACT: We show that a strong off-resonant optical pulse can be used to create entanglement in an ensemble of polar molecules. The laser field modifies the rotational structure of molecules, enhancing the effect of the dipole-dipole interaction between molecules. This generates entanglement between molecules in different rotational states after the pulse is over. The degree of entanglement can be controlled by shaping the intensity and duration of the pulse. We show that a single nanosecond pulse can be used to produce an entangled state of molecules separated by several hundreds of nanometers, and that a sequence of pulses generate entanglement between molecules separated by tens of micrometers. We describe the possibility of using molecules trapped on an optical lattice to test Bell's inequalities by measuring orientation and alignment correlations. We also analyze the main sources of decoherence in the system and estimate the efficiency of two-qubit quantum gates for universal quantum computation with trapped polar molecules. Reference: F. Herrera, Ph.D. thesis, University of British Columbia, 2012
    06/2012;
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    ABSTRACT: We propose a method for sensitive parallel detection of low-frequency electromagnetic fields based on the fine structure interactions in paramagnetic polar molecules. Compared to the recently implemented scheme employing ultracold $^{87}$Rb atoms [B{\"o}hi \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 051101 (2010)], the technique based on molecules offers a 100-fold higher sensitivity, the possibility to measure both the electric and magnetic field components, and a probe of a wide range of frequencies from the dc limit to the THz regime.
    Physical Review A 02/2012; 86(1). · 3.04 Impact Factor
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    Ping Xiang, Marina Litinskaya, Roman V. Krems
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    ABSTRACT: Rotational excitation of polar molecules trapped in an optical lattice gives rise to rotational excitons. Here we show that non-linear interactions of such excitons can be controlled by an electric field. The exciton--exciton interactions can be tuned to induce exciton pairing, leading to the formation of biexcitons. Tunable non-linear interactions between excitons can be used for many applications ranging from the controlled preparation of entangled quasiparticles to the study of polaron interactions and the effects of non-linear interactions on quantum energy transport in molecular aggregates.
    Physical Review A 12/2011; 85(6). · 3.04 Impact Factor
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    Jie Cui, Roman V. Krems
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    ABSTRACT: We present expressions demonstrating that collisional decoherence of ultracold atoms or molecules in a coherent superposition of non-degenerate quantum states is suppressed when both the real and imaginary parts of the scattering lengths for the states in the coherent superposition are equal. We show that the rate of collisional decoherence can be enhanced or suppressed by varying an external magnetic field near a Feshbach resonance. For some resonances, the suppression is very dramatic. We propose a method for measuring the scattering length of ultracold particles in excited quantum states exhibiting Feshbach resonances.
    10/2011;
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    Alisdair O G Wallis, Roman V Krems
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    ABSTRACT: We study the rotational predissociation of atom-molecule complexes with very small binding energy. Such complexes can be produced by Feshbach resonance association of ultracold molecules with ultracold atoms. Numerical calculations of the predissociation lifetimes based on the computation of the energy dependence of the scattering matrix elements become inaccurate when the binding energy is smaller than the energy width of the predissociating state. We derive expressions that represent accurately the predissociation lifetimes in terms of the real and imaginary parts of the scattering length and effective range for molecules in an excited rotational state. Our results show that the predissociation lifetimes are the longest when the binding energy is positive, i.e., when the predissociating state is just above the excited state threshold.
    The Journal of Chemical Physics 09/2011; 135(12):124313. · 3.12 Impact Factor
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    Yury V Suleimanov, Roman V Krems
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    ABSTRACT: Collision properties of atoms and molecules in low temperature gases can be controlled by applying an external magnetic or electric field. The external field shifts the energy levels of the colliding particles, which gives rise to Feshbach resonances modifying the scattering cross sections. The resonances occur at particular magnitudes of the external field, where a bound state of the collision complex is degenerate with a scattering state. The positions of the resonances in the external field are usually identified by computing either the scattering cross sections or the bound states of the collision complex as functions of the external field magnitude. We propose a more efficient method for locating Feshbach resonances that requires neither of these computations. In particular, we show that the positions of Feshbach resonances can be identified by computing the log-derivative of the total wave function in a classically allowed region as a function of the external field strength. This procedure is particularly useful for locating narrow Feshbach resonances that may be hard to identify with the other methods.
    The Journal of Chemical Physics 01/2011; 134(1):014101. · 3.12 Impact Factor

Publication Stats

2k Citations
347.46 Total Impact Points

Institutions

  • 2006–2014
    • University of British Columbia - Vancouver
      • Department of Chemistry
      Vancouver, British Columbia, Canada
  • 2013
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 2012
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, MA, United States
  • 2011
    • University of Oxford
      • Physcial and Theoretical Chemistry Laboratory
      Oxford, ENG, United Kingdom
  • 2007–2010
    • Radboud University Nijmegen
      • Department of Theoretical Chemistry
      Nijmegen, Provincie Gelderland, Netherlands
  • 2003–2009
    • Harvard University
      • Department of Physics
      Cambridge, MA, United States
  • 2002–2008
    • Harvard-Smithsonian Center for Astrophysics
      • Institute for Theoretical Atomic, Molecular and Optical Physics
      Cambridge, Massachusetts, United States
  • 2000–2007
    • Moscow State Textile University
      Moskva, Moscow, Russia