# Spring/Summer 2012 Atomic Molecular and Optical Physics Seminars at Stony Brook

All seminars are on Mondays at 4:00 PM in S-141 unless otherwise noted

 January 23, 2012

 Dr. Arman Cingoz, JILA, University of Colorado "Frequency combs and precision spectroscopy in the extreme ultraviolet" (Host: Tom Weinacht)

 Development of the optical frequency comb (a continuous train of stabilized ultrashort laser pulses) at beginning of the century has revolutionized optical metrology and precision spectroscopy due to its ability to provide a precise and direct link between microwave and optical frequencies. A novel application that aims to extend the precision and accuracy obtained in the visible and near-infrared part of the electromagnetic spectrum to the extreme ultraviolet (XUV) is the generation of XUV frequency combs via intracavity high harmonic generation (HHG). The main idea is to leverage both the ultrashort duration of each laser pulse and the exquisite phase coherence of the continuous pulse train by coupling a high power infrared frequency comb into a high finesse optical cavity. At the intracavity focus, the peak intensities reach ~1014 W/cm2, required to drive the extremely nonlinear HHG process, which produces radiation at the harmonics of the driving infrared laser frequency.   We will discuss the power scaling of this technique using a new 80-W average power Yb fiber frequency comb. Detailed understanding of plasma dynamics in the high-finesse cavity, coupled with technical improvements, have led to record level HHG yield of  >20 µW average power per harmonic reaching down to 50 nm. Recently, we have also demonstrated the comb structure of the high harmonics by resolving atomic argon and neon lines at 82 and 63 nm, respectively, via direct frequency comb spectroscopy. The argon transition linewidth of 10 MHz, limited by residual Doppler broadening, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. With these developments, ultrahigh precision spectroscopy in the XUV is within grasp and has a wide range of applications that include experimental tests of bound state and many body quantum electrodynamics in He+ and He, development of next-generation nuclear clocks, and searches for spatial and temporal variation of fundamental constants using the enhanced sensitivity of highly charged ions.

 January 30, 2012

 Dr. Nathaniel Brahms,  U. C. Berkeley "Quantum Optical Phenomena from Collective Motion and Spin" (Host: Dominik Schneble)

 One typically thinks of quantum physics as applying to the motion of atom-scale objects. Advances in atomic and device physics are now letting us access quantum phenomena in the collective motion of objects composed of thousands to billions of atoms. In this talk, I will discuss recent results from our experiment at Berkeley, in which we use the collective center-of-mass motion of a cloud of ultracold atoms as a many-atom mechanical oscillator, measured with light in a high-finesse optical cavity. We have observed new quantum phenomena: the generation of a nonclassical state of light arising from optical feedback with a massive moving object, and a direct measurement of the quantization of a many-atom mechanical oscillator.  I will also discuss how we use measurements of collective spin to perform microscopic imaging of atomic gases, and how these ideas might be applied to simulate many-body physics and magnetism.

 February 6, 2012

 Dr. Tom Allison, JILA, University of Colorado "High Brightness Extreme Ultraviolet Frequency Combs via Intracavity High-Order Harmonic" (Host: Tom Weinacht)

 We generate high order harmonics of a femtosecond frequency comb at the focus of a high-finesse optical cavity with 154 MHz repetition rate. The resulting tabletop high brightness extreme ultraviolet (XUV) light source has promising applications in XUV frequency metrology, strong field and molecular physics studies, and more traditional XUV applications currently served by synchrotron facilities. I will discuss our recent results of XUV yields greater than 200 microWatt per harmonic and the first direct frequency comb spectroscopy in the extreme ultraviolet, allowing MHz resolution of atomic lines at 82 nm and 63 nm with intrinsic absolute frequency calibration. With these developments, XUV spectroscopy has left the "light bulb" age.

 February 13, 2012

 Dr. Wesley Campbell, JQI, Univ. of Maryland "Coherent Manipulations with Ultrafast Pulses and Trapped Ions" (Host: Hal Metcalf)

 Quantum many-body systems with tens or hundreds of particles are often intractable to simulate on classical computers due to the exponential growth of the Hilbert space with the number of particles. We can nonetheless hope to simulate them efficiently by using another quantum system—one that also shows this exponential scaling but is initializable, well controlled, and easy to manipulate and probe. We use a collection of trapped atomic ions as a platform for the quantum simulation of lattice spin models. The use of mode-locked lasers to create the state-dependent potentials that simulate a variety of spin Hamiltonians allows operation in a spectral region that is relatively free of laser-induced decoherence. We also realize ultrafast gates where a single laser pulse can drive a high-fidelity single-qubit gate in ~50 ps. Single laser pulses may also be used for rapid deceleration of (neutral) molecular beams, which may provide us with a bridge over the "temperature gap" for direct cooling of molecules to ultracold temperatures.

 February 20, 2012

 Dr. Xibin Zhou, MIT "Ultrafast Optics and Spectroscopy: from THz to Extreme Ultraviolet" (Host: Tom Weinacht)

 I will present work on recent developments of novel ultrafast light sources in two distant spectral regions from nonlinear frequency conversion of a near–IR ultrafast laser. The extreme ultraviolet (EUV) and soft X–rays are generated by high–order harmonic process in atomic gases and the high power ultrashort Terahertz (THz) light is generated by noncollinearly phase matched optical rectification in LiNbO3 crystal. Spectroscopy applications that take advantage of the ultrashort pulse durations or high electric field strength of these sources will also be discussed, including time resolved probing of non–Born–Oppenheimer dynamics in EUV excited molecular ion and more recent work towards ferroelectric polarization switching using intense THz light. The perspective on integration of these two techniques into a single experimental platform opens opportunities for probing and manipulating ultrafast dynamics in various physical systems.

 February 28, 2012 (Tuesday, 10am)

 Prof. Wolfgang Ketterle, CUA, MIT "Towards Quantum Magnetism with Ultracold Atoms" (Host: Dominik Schneble)

 Over the last 20 years, science with ultracold atoms has focused on motion:  slowing down motion, population of a single motional state (Bose-Einstein condensation, atom lasers), superfluid motion of bosons and fermion pairs.  In my talk, I will address the next challenge when motion is frozen out:  Spin ordering.  A two-component boson or fermion mixture can form magnetic phases such as ferromagnetic, antiferromagnetic ordering and a spin liquid.  The challenge is to reach the low temperature and entropy required to observe these phenomena.  I will describe our current efforts and progress towards this goal.  This includes the study of fermions with strong repulsive interactions where phase transition to itinerant ferromagnetism has been predicted, Bragg scattering of atoms in crystal of light (optical lattice), and a new adiabatic gradient demagnetization cooling scheme which has enabled us to realize spin temperatures of less than 50 picokelvin in optical lattices.  These are the lowest temperatures ever measured in any physical system.

 March 5, 2012

 Dr. Eden Figueroa, MPQ Garching "Optical Quantum Memory: From Atomic Ensembles to Single Atoms" (Host: Dominik Schneble)

 Quantum networks are at the forefront of modern research as they provide the technical frontier towards achieving quantum computation, communication and metrology. The realization of quantum networks composed of many nodes and channels requires new scientific capabilities; the most important is the development of quantum interconnects, featuring the coherent and reversible mapping of quantum information between light and matter. In this talk I will provide an overview of several exciting developments in this research direction. First, I will show you the engineered exchange of information between squeezed light and collective atomic excitations through the use of electromagnetically induced transparency (EIT). Later, we will explore the development of quantum interfaces between photons and single particles of matter taking advantage of a novel combination of EIT and Cavity Quantum Electrodynamics (QED). Lastly, I will show you our latest results combining the aforementioned building blocks to form an elementary quantum network in which entanglement is distributed and quantum states are transferred among two single-atom nodes located in completely independent laboratories.

 March 9, 2012 (Friday, 10am)

 Dr. Manas Kulkarni, University of Toronto "Finite temperature dynamical structure factor of the 1D Bose gas" (Host: Dominik Schneble)

 We study [1] the finite temperature dynamical structure factor of a 1D Bose gas using numerical simulations of the Gross--Pitaevskii equation appropriate to a weakly interacting system. The lineshape of the phonon peaks in $S(k,\omega)$ has a width $\propto |k|^{3/2}$ at low wavevectors. This anomalous width arises from resonant three-phonon interactions, and reveals a remarkable connection to the Kardar--Parisi--Zhang universality class of dynamical critical phenomena. [1] M. Kulkarni and A. Lamacraft,  arXiv:1201.6363

 March 9, 2012 (Friday, 1pm)

 Dr. Mikael Rechtsman, Technion (Haifa, Israel) "Disorder-enhanced transport in photonic quasicrystals and amorphous photonic lattices" (Host: Hal Metcalf)

 The theory of Anderson localization states that disorder acts to inhibit wave transport in periodic systems because it causes extended Bloch states to be replaced with localized states (this applies equally to electrons in solids, light in photonic crystals, and matter waves, among others). In the first part of my talk, I will present our experimental demonstration of a counterintuitive effect: disorder-enhanced transport in quasicrystals (directly opposite to the inhibition of transport in periodic crystals). Quasicrystals are structures that have long-range order but are not periodic. Specifically, they contain rotational order with symmetries that are forbidden to periodic systems (for example, 5-fold rotational symmetry). Photonic quasicrystals have shown promise as low-index-contrast band gap materials due to the near-isotropy of their quasi-Brillouin zone. Our observation provides definitive proof for what occurs in solid-state quasicrystals (as found in metal alloys): when these materials are more disordered (i.e., with more structural defects), their conductivity increases. A holy grail of soft-matter physics has been to find a self-assembling (possibly disordered) system that has a photonic band gap. In the second part of my talk, I will present our observation of photonic band gaps in amorphous photonic lattices, in which waveguides are arranged much like atoms in a two-dimensional liquid. The very presence of a band gap is not obvious at all: band gaps are commonly thought to open only as a result of Bragg scattering (which means long-range order), and liquids have only short-range order.

 March 19, 2012

 Prof. Chandra Raman, Georgia Tech "Quantum Magnetism with Spinor Bose-Einstein condensates" (Host: Dominik Schneble)

 Bose-Einstein condensates (BECs) have revolutionized atomic physics, a revolution which, seventeen years after their discovery, shows little sign of stopping.  In this talk I will present experimental work from our laboratory that investigates the spinor nature of a BEC of atomic sodium, an example of a quantum antiferromagnet.  The interplay between the quadratic Zeeman effect and spin-spin interactions has allowed us to realize a rich phase diagram of possibilities, especially those associated with quantum phase transitions at (nearly) zero temperature.  While the traditional view of such phase transitions has focused on scaling laws associated with the many-body ground state, our work highlights the wealth of non-equilibrium phenomena that can be explored, particularly the spatial nature of quantum quenches.

 March 26, 2012

 Dr. Adam Kirrander, ITAMP "Theoretical dynamics of heavy Rydberg states" (Host: Tom Weinacht)

 [talk cancelled]

 April 16, 2012

 Prof. Mukund Vengalattore, Cornell University "Hybrid Quantum Systems for Nanoscale Matrology" (Host: Dominik Schneble)

 Ultracold atomic gases have long held promise as a quantum many-bodyresource for field and force metrology. A powerful route to extend such quantum-limited sensors to sub-micron length scales is tointerface the atomic gas to a 'solid-state' device such as amechanical oscillator combining, in principle, the tunability and coherence of the atomic gas with the bandwidth and spatial resolution of the microfabricated device. I will describe our progress toward the realization of a hybrid quantum system composed of a Bose condensate magnetically coupled to a micromechanical oscillator. This is a rich system both for studies of macroscopic quantum physics as well as precision magnetic microscopy at sub-micron length scales with applications ranging from electronic spin detection to studies of correlated electronic materials. I will also describe ongoing efforts in our group to create and study novel non-equilibrium states of an ultracold magnetic quantum fluid.

 Thursday, May 3, 2012 (4:00 PM)

 Dr. Ippei Danshita RIKEN (Japan) "Superflow decay induced by quantum phase slips in one-dimensional Bose gases" (Host: Dominik Schneble)

 In recent years, experiments with ultracold atoms [1,2,3,4] have  investigated transport properties of one-dimensional (1D) Bose gases in optical lattices and shown that the transport in 1D is drastically suppressed even in the superfluid state compared to that in higher dimensions. Motivated by the experiments, we study superfluid transport of 1D Bose gases. In 1D, superflow at zero temperature can decay via quantum nucleation of phase slips even when the flow velocity is much smaller than the critical velocity predicted by mean-field theories. Using instanton techniques, we calculate the nucleation rate \Gamma_{prd} of a quantum phase slip for a 1D superfluid in a periodic potential and show that it increases in a power-law with the flow momentum p, as \Gamma_{prd} ~ p^{2K-2}, when p is much smaller than the critical momentum [5]. Here, L and K denote the system size and the Luttinger parameter. To make a connection with the experiments, we simulate the dipole oscillations of 1D Bose gases in a trapped system with use of the quasi-exact numerical method of time-evolving block decimation. From the simulations, we relate the nucleation rate with the damping rate of dipole oscillations, which is a typical experimental observable [1,3], and show that the damping rate indeed obeys the power-law, meaning that the suppression of the transport in 1D is due to quantum phase slips.  We also suggest a way to identify the superfluid-insulator transition point from the dipole oscillations. References: [1] C. D. Fertig et al., Phys. Rev. Lett. 94, 120403 (2005). [2] J. Mun et al., Phys. Rev. Lett. 99, 150604 (2007). [3] E. Haller et al., Nature 466, 597 (2010). [4] B. Gadway et al., Phys. Rev. Lett. 107, 145306 (2011). [5] I. Danshita and A. Polkovnikov, Phys. Rev. A 85, 023638 (2012).

 May 14, 2012

 Dr. Daniel Pertot, University of Cambridge (UK) "Ultracold Fermions in Two Dimensions" (Host: Dominik Schneble)

 Quantum gases of fermionic atoms are ideal systems to experimentally study the behavior of interacting fermions under conditions and in regimes that have so far not been accessible in traditional condensed matter and solid state systems. Recently, we have performed experiments with two-component Fermi gases of K-40 atoms confined to two-dimensional planes by an optical standing wave. Examining the collective modes, we find that the trapped 2D gas exhibits a dynamical SO(2,1) scaling symmetry and we have studied its shear viscosity. Further, we have measured the single-particle spectral function using momentum-resolved rf spectroscopy. For a balanced two-component mixture in the strongly interacting regime, we observe a many-body pairing gap, or pseudo gap, above the BKT transition temperature. For a strongly imbalanced system, we have measured the effective mass and energy of two-dimensional Fermi polarons. A sudden divergence of the effective mass extracted from the spectral function indicates a transition from the polaronic to the molecular regime. In addition, for repulsive interactions, we have seen signatures of the metastable repulsive polaron and have determined its lifetime.

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 May 21, 2012

 Francis Paraan, YITP, Stony Brook University "Tonks-Girardeau Gases of Bosons, Anyons, and Fermions" (Host: Tom Bergeman)

 In this talk I describe the boson-anyon-fermion correspondence in one-dimensional Bose gases with short range interactions. This mapping is very useful at and near the Tonks-Girardeau limit (impenetrable bosons) where the gas shares properties with weakly interacting Fermi gases. The presentation has two parts. First, I use the boson-fermion relation to obtain the ground state energy of a confined strongly interacting Bose gas near the Tonks-Girardeau limit. This approach gives finite-number corrections to the Thomas-Fermi result. Second, I present a boson-anyon mapping that allows one to study the effects of exchange statistics in interacting anyon gases. I end with a brief description of recent proposals to realize anyonic Hubbard models in optical lattices.

 June 11, 2012

 Dr. Zsolt Kis, KFKI Institute (Budapest, Hungary) "Resonant Optical Sideband Generation" (Host: Tom Weinacht)

 Optical  sideband   generation  is   an  important  method   to  obtain frequencies which  are not available from  a laser source,  or if many evenly spaced frequency components  are needed. There are some methods available today to get sidebands  of a seed laser, e.g. application of acousto   optic   modulators,    spontaneous   or   stimulated   Raman scattering. In  my talk  I show two  alternative proposals:  one which works in two-level  systems and it is able  to generate sidebands from kHz to  several tens, maybe hundreds of MHz,  the other works  in harmonically trapped two-level systems and the frequency  spacing of the sidebands is equal to the oscillation frequency in  the harmonic potential. In the second process there  is some  flexibility to tailor  the pulse shape  of the generated sidebands.

 Tues, Wed, Thurs, June 12-14, 2012 (2:00 PM)

 Prof. Bruce W. Shore, Lawrence Livermore National Laboratory "Visualizing Quantum-State Changes" (Host: Hal Metcalf)

 [Tutorial on the present basic theory of quantum-state manipulation and ways to picture the changes, with examples]

 July 9, 2012 (2:00PM)

 Dr. Andrew MacRae, University of Calgary "Title" (Host: Hal Metcalf)

 abstract

 Wed, July 18, 2012 (4:00pm)

 Dr. Scott Beattie, University of Cambridge (UK) "Spinor superflow in an annular BEC" (Host: Dominik Schneble)

 We have studied the persistent flow of a spinor condensate in an all-optical ring trap.  After initially rotating the atoms, we have observed high-charge vortices persist in pure spin states for long times (>100s) and decay stochastically in a quantized fashion.  These decay events correspond to the occurrence of 2p phase slips in the condensate.  By adding a second spin component, we are able to study how the addition of a spin degree of freedom affects the stability of the superflow.  I will discuss these studies, in which we observe that the existence a second spin component drastically reduces the lifetime of the persistent flow.