Groups | Seminars || Courses | Outreach

Seminars

Seminars will be held held at room S-141 in the Physics and Astronomy Department building on Mondays at 4:00 PM, unless noted otherwise.

Spring/Summer 2014

February 21, 2014 (Fri, 2:30pm) Prof. Thomas Baumert University of Kassel, Germany Charge oscillation controlled molecular excitation and chiral identification with fs-laser pulses (Host: Tom Weinacht)

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February 24, 2014 Prof. Ippei Danshita, Yukawa ITP, Kyoto University, Japan Superfluid-insulator transition of binary Bose mixtures: Quantum tricriticality and bright-like dark solitons (Host: Dominik Schneble)

Recent experiments on ultracold atoms in optical lattices have observed quantum criticality accompanying the second-order QPT between vacuum and SF, providing new opportunities for studying quantum criticality in optical-lattice systems [1]. Motivated by the experimental development, we study quantum criticality near a tricritical point (TCP) in the two-component Bose-Hubbard model on square lattices [2]. The existence of quantum TCP on a boundary of superfluid-insulator transition is confirmed by quantum Monte Carlo simulations. Moreover, we analytically derive the quantum tricritical scaling on the basis of an effective field theory. We find two significant features of the quantum tricriticality, namely, its characteristic chemical potential dependence of the superfluid transition temperature and a strong density fluctuation. We also analyze soliton solutions of a superfluid state and find that when the superfluid state changes from a ground state to a metastable one, a standard dark soliton turns into a bright-like dark soliton, which has a non-vanishing density dip and no pi phase kink even in the case of a standing soliton. We suggest that these features are directly observable in existing experimental setups of Bose-Bose mixtures in optical lattices. [1] X. Zhang, C.-L. Hung, S.-K. Tung, and C. Chin, Science 335, 1070 (2012). [2] Y. Kato, D. Yamamoto, and I. Danshita, arXiv:1311.2145 (2013), Phys. Rev. Lett. (in press)

March 24, 2014 Prof. Ignacio Franco, University of Rochester Electronic Decoherence in Molecules (Host: Tom Weinacht)

Electronic decoherence is a basic feature of the time evolution that accompanies photoexcitation, passage through conical intersections, electron transfer, or any other dynamical process that creates electronic superposition states. Understanding electronic decoherence is central to our description of fundamental processes such as photosynthesis and vision, and is also vital in the development of approximation schemes to the full vibronic evolution of molecules. In this talk, I will discuss three of our contributions toward the understanding of electronic decoherence in molecules: First, I will introduce a hierarchy of measures of decoherence for many-electron systems that is based on the purity and the hierarchy of reduced electronic density matrices [1]. While usual measures of decoherence are of limited applicability in molecules because they are based on the many-body electronic density matrix, the reduced purities can be used to characterize electronic decoherence in the common case when only reduced information about the electronic subsystem is available. Using the reduced purities and related measures, I will then discuss the decoherence dynamics of a model molecular system: the Su-Schrieffer-Heeger model of trans-polyacetylene [1,2]. The decoherence is modeled by following the coupled dynamics of electronic and vibrational degrees of freedom explicitly albeit approximately in an Ehrenfest mixed quantum-classical approximation. The simulations reveal the basic structure expected for the decoherence dynamics in molecules, provide insights into the main mechanisms for coherence loss, and illustrate how the decoherence dynamics changes with system size and initial state. Last, in this context, I will introduce a remarkable vibronic phenomenon that we call VIBRET [3] that looks like a long-lived coherent process while, in fact, it arises from incoherent vibronic dynamics. [1] I. Franco and H. Appel, J. Chem. Phys. 139, 094109 (2013). [2] I. Franco and P. Brumer, J. Chem. Phys. 136, 144501 (2012). [3] I. Franco, A. Rubio and P. Brumer, New J. Phys. 15, 043004 (2013)

March 31, 2014 Prof. Andrew Daley, University of Pittsburgh and University of Strathclyde, UK Dissipative dynamics, heating, and thermalisation of cold atoms in optical lattices (Host: Dominik Schneble)

One of the key challenges in current experiments with cold atoms in optical lattices is the realization of lower temperatures required for the preparation of many interesting quantum phases. In this context, it is very important to be able to characterise and control the competing heating processes. These include, for example, spontaneous emissions from incoherent scattering of the lattice light. Such heating processes give rise to decoherence of many-body states, and the resulting dynamics can be highly sensitive to the form of the state. Moreover, there is often a separation of timescales between some excitations that thermalize rapidly, and others that do not properly thermalize in the duration of an experimental run. This can strongly modify, and even reduce the overall effects of the heating processes. I will discuss some of our recent work in this direction, where we explore the relaxation of a system of bosons in an optical lattice in 1D after decoherence due to spontaneous emission events. For simple observables, we find regimes in which the system relaxes rapidly to values in agreement with a thermal distribution, and others where thermalization does not occur on typical experimental timescales. Because spontaneous emissions lead effectively to a local quantum quench, this behaviour is strongly dependent on the low-energy spectrum of the Hamiltonian, and undergoes a qualitative change at the Mott Insulator-superfluid transition point. These results have important implications for the understanding of thermalization after localized quenches in isolated quantum gases, as well as the characterization of heating in experiments. I will also briefly discuss the heating of two-species fermions in spin-ordered states due to spontaneous emission events.

April 2, 2014 (Wed, 4:30pm) Dr. Bryce Gadway, JILA Boulder Ultracold polar molecules: A many-body spin system with long-range interactions (Host: Dominik Schneble)

Nearly two decades ago, the field of atomic physics was revolutionized by the production of degenerate Bose and Fermi gases. These experimental systems, combined with a microscopic description of atomic interactions, have enriched our understanding of emergent collective phenomena, such as superfluidity and the BCS pairing of electrons. The recent production of ultracold gases of polar molecules has similarly opened up entirely new lines of research in quantum chemistry and the quantum simulation of many-body systems. I will describe our recent experiments that produce and study a system of ultracold ground-state molecules in an optical lattice. In particular, we have recently seen the first evidence for long-ranged, dipolar spin-exchange interactions between molecules in different rotational "pseudospin" states. By tuning the strength of dipole-dipole couplings, we have been able to confirm the microscopic description of dipolar interactions in our system. Furthermore, through comparisons to numerical simulations, we have found strong evidence that our system acts as an ideal "quantum simulator" of a specific spin-1/2 Heisenberg XY model. Lastly, I'll discuss future prospects for studies of ultracold polar molecules, including the coherent interplay between long-ranged interactions and the motion of molecules, relevant to the study of supersolidity and itinerant magnetism.

April 7, 2014 Prof. Hui Cao, Yale University Light transport through random media: physics and applications (Host: Hal Metcalf)

The first part of my talk is a fundamental study of light transport through disordered photonic waveguides. We recently observed position-dependent diffusion by probing light propagating inside a quasi-two-dimensional random system from the third dimension. The system size and shape are designed to enhance the interference of scattered waves so that the diffusion coefficient is modi�ed appreciably. We also use dissipation to control the effective system size, and tune the value of diffusion coefficient via the interplay of localization and dissipation. This work demonstrates the possibility of utilizing the geometry of a random system or the dissipation to manipulate wave diffusion. The second part is the application of random media to spectrograph. We have designed and fabricated an on-chip spectrometer based on random arrays of scatterers. The speckle patterns are recorded and used to reconstruct the input spectra after the spectral-spatial mapping is calibrated. Multiple scattering of light increases the effective optical path length, facilitating the spectral decorrelation of speckle. It allows us to dramatically reduce the device size without sacrificing spectral resolution. To reduce the insertion loss, we incorporate certain degree of order to the random spectrometer.

April 10, 2014 (Thurs, 4 pm) Dr. Mariana Assmann, Temple University Dynamics on radical cations after strong field ionization (Host: Tom Weinacht)

Strong field ionization is the first step in high harmonic generation and it can be used to probe time-resolved molecular dynamics. The ionization of molecules to different cationic states can lead to either deactivation of the system to the lowest ionic state possibly followed by dissociation or direct dissociation into various fragments, depending on the nature of the ionic state. Furthermore, different delay times between pump and probe pulses entail different fragmentation patterns [1]. The different fragments can be experimentally detected with the help of time-of-flight mass spectrometry. In pump probe experiments the time decay of the fragments has been used as a signature of the dynamics on the neutral system [2,3]. The aim of the present work is to understand the dynamics of the cations produced upon ionization. Two systems are investigated: Cyclohexadiene (CHD) and the RNA base uracil. The reaction of CHD to Hexadiene (HT) is a prototypical system to describe photo isomerization [4]. The ionized uracil is an interesting subject of study because of its biological importance and also because it possesses conical intersections involving two and three electronic states [1,5]. Especially the latter are of interest due to the possibility of providing very efficient deactivation. We are investigating the dynamics in both systems starting from different ionic states with the help of ab initio molecular dynamics simulations. Initially, we consider the ionization to take place at the Franck-Condon point and focus on the investigation of the time scales of the processes that occur after ionization. [1] S.Matsika, M. Spanner, M. Kotur and T. C.Weinacht J. Phys. Chem. A 117, 12796 (2013). [2] M. Kotur, C. Zhou, S. Matsika, S. Patchkovskii, M. Spanner and T. C.Weinacht Phys. Rev. Lett. 109, 203007 (2012). [3] C. Zhou, S. Matsika, M. Kotur and T. C. Weinacht J. Phys. Chem. A 116, 9217 (2012). [4] M. Kotur, T.Weinacht, B. J. Pearson and S.Matsika The Journal of Chemical Physics 130, 134311 (2009). [5] S. Matsika Chem. Phys. 349, 356 (2008).

April 28, 2014 Dr. Bart McGuyer, Columbia University Precision measurements with ultracold Sr2 molecules (Host: Hal Metcalf)

We present ongoing studies of microkelvin Sr2 in an optical lattice.  The molecules are photoassociated from 88Sr atoms near a narrow intercombination line.  High-Q molecular spectra uncover peculiar physics, including multiply forbidden transitions and anomalously large linear and higher-order Zeeman shifts.  Measurements of linear Zeeman shifts yield nonadiabatic mixing angles of the molecular wave functions.  We strongly enable forbidden transitions using small magnetic fields and for the first time, quantitatively compare electric- and magnetic-dipole as well as electric-quadrupole transition strengths in molecules.  Current and future work made possible by this new type of long-lived molecule is discussed.

May 12, 2014 Dr. Daniel Stack, Army Research Laboratory, MD Quantum Communication Using Cold Atoms and Quantum Frequency Conversion (Host: Hal Metcalf)

The Quantum Sciences group at the Army Research Laboratory currently has three ongoing efforts: an atom chip for ultra-cold atoms, a MOT (magneto-optical trap) based quantum memory, and PPLN (periodically-poled lithium niobate) based frequency conversion. I will give a brief overview of the atom chip experiment.This experiment is focused on exploring the capabilities of ultra-cold/degenerate ensembles in a compact physics package. We use microwave and RF fields for coherent spin control of magnetically trapped atoms.  The bulk of my talk will focus on the other two experiments: a MOT based quantum memory, and PPLN based frequency conversion.Our quantum memory utilizes off-axis, spontaneously emitted single photons generated by the interaction of a 795 nm write-laser beam with a cold ^{87}Rb ensemble. To minimize losses during transport through optical fiber links, the single photons are frequency-converted to the telecomm band by difference frequency generation in a PPLN crystal. Quantum memories and single photon transport are key ingredients for a scalable quantum network that could enable high fidelity transmission of quantum information over long distances.

June 12, 2014 (Thurs,  1:00PM) Prof. Arvinder Sandhu, Univ. of Arizona, Tucson Probing non-adiabatic, coupled dynamics in molecules and materials (Host: Tom Weinacht)

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June 13, 2014 (Fri,  2:00PM) Loic Henriet, Ecole Polytechnique / CNRS, France Spin dynamics and light-matter interaction (Host: Dominik Schneble)

Coupling spins to a many-body bosonic environment leads to complex non-Markovian dynamics, and entanglement stems from the coherent evolution of spin and bosons. In this talk we introduce and apply an exact stochastic method that allows to compute the real-time dynamics of spin variables in contact with a bosonic environment and subject to time-dependent driving terms. The restriction to one bosonic mode leads to the Rabi Hamiltonian, which describes the fundamental interaction between light and matter. Energy is periodically transfered from the spin to the light giving rise to well-known Rabi oscillations. Recent on-chip experiments, by using articial two-level systems made of superconducting qubits, allow a high control on the coupling between the system and the light eld and have triggered renewed interest in the exact description of the spin dynamics. Eective photon-photon interactions and photon blockade effects may be engineered, and such systems are of importance for applications in quantum computing. We will show the applicability of the stochastic method in various conditions. Dissipation and decoherence eects on the two-level system can be modelled by the spin-boson model where the spin is coupled to a bath of harmonic oscillators responsible for ohmic dissipation. This dissipative impurity system displays a delocalization-localization quantum phase transition for sufficiently strong coupling. Additionally there exists an exact mapping to the anisotropic Kondo model and the one-dimensional Ising model with 1/r^2 interactions, so that the study of its dynamics exhibits several interesting physical phenomena. This spin-boson model may be engineered in cold atomic setups, where a single two-level system is embedded in a one-dimensional Bose-Einstein condensate. These experiments would also eventually lead to the implementation of the dissipative quantum Ising model in a transverse field.

July 11, 2014 (Fri, 10:00AM) Dr. Yev Lushtak SAES Getters USA Sorption Mechanisms and Pumping Characteristics of Non-Evaporable Getter (NEG) Pumps for AMO Physics (Host: Dominik Schneble)

In the ultrahigh vacuum (UHV) regime, the main source of pressure is outgassing of physisorbed gas molecules from walls of vacuum chambers. NEGs are comprised of sintered porous reactive alloys that efficiently trap active atmospheric gases and hydrogen. SAES Getters produces a number of pumps based on NEG technology. These units are compact, possess large pumping speeds for active gases, produce no vibrations, and are amagnetic. These characteristics allow for simpler vacuum systems with reduced magnetic interference and a better level of vacuum than can be achieved with conventional means. In addition to the active gases and hydrogen, the atmosphere contains non-getterable gases like argon and methane. Therefore, NEG pumps must be combined with other pumping systems such as ion pumps to obtain low UHV and XHV pressures. The NEXTorr pump represents a perfect compromise in the combination of NEG and ion pump technology. The sorption mechanisms of NEG materials and examples of NEG and NEXTorr pumps used in cold trap systems will be presented.

July 29, 2014 (Tue, 4:00PM) Prof. Jin-Tae Kim Chosun University, South Korea Spectroscopic Analysis on the Hetero-nuclear KRb Molecule Using Both Molecular Beam and Ultracold Molecule Experiments (Host: Tom Bergeman)

We have been combining the results of molecular beam spectroscopic experiments and ultracold molecule spectroscopic experiments. In particular, the combination allows definitive assignments of nearly all lines in both spectra if the energy regions of the excited states involved are the same. Moreover, it can be used to find the optimum pathways for stimulated Raman population transfer from translationally ultracold molecules [typically formed in high rovibrational levels near the ground state dissociation limit] to fully [translationally and rovibrationally] ultracold molecules in the lowest rovibrational level of the ground electronic state. This topic has been of particular interest to the ultracold community because such fully ultracold molecules cannot undergo inelastic collisions, and therefore are promising candidates for degenerate Bose and Fermi gases.

July 31, 2014 (Thurs, 4:00 PM) Michael Keller University of Vienna, Austria Towards experiments with momentum entangled He* atom pairs (Host: Hal Metcalf)

The high internal energy of metastable helium atoms can be exploited to extend the field of cold atom physics to a single particle level. I will present our work towards experiments with momentum entangled helium atoms, starting from the source to create a bright beam of He* atoms, followed by various slowing and cooling steps until we obtain a Bose-Einstein condensate as a coherent source of ultracold metastable atoms. I will present the single-atom detector that allows for a full 3-dimensional reconstruction of momentum space, talk about ways to create momentum entangled atom pairs by s-wave scattering and discuss the feasibility to directly demonstrate the presence of entanglement between individual atoms

August 14, 2014 (Thurs, 4:00 PM) Prof. Peter van der Straten University of Utrecht, Netherlands Spin drag in a Bose gas (Host: Hal Metcalf)

The field of spintronics, where the spin of the electron is manipulated rather than its charge, has recently also led to interest in spin currents. Contrary to charge currents, these spin currents can be subject to strong relaxation due to collisions between different spin species, a phenomenon known as spin drag.  Here,  we perform a transport experiment using ultra-cold bosonic atoms and observe spin drag for bosons for the first time. By lowering the temperature we find that spin drag for bosons is enhanced in the quantum regime due to Bose stimulation, which is in agreement with recent theoretical predictions. Our work on bosonic transport shows that this field may be as rich as transport in solid-state physics and may lead to the development of advanced devices in atomtronics.

last updated 07/31/2014