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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 2015

February 6, 2015 (Thurs, 2:30pm, YITP Common Room) Dongling Deng University of Michigan Probing knots and Hopf insulators with ultracold atoms (Host: Tzu-Chieh Wei)

Knots and links are fascinating and intricate topological objects that have played a prominent role in physical and life sciences. Their influence spans from DNA and molecular chemistry to vortices in superfluid helium, defects in liquid crystals and cosmic strings in the early universe. Here, we show that knotted structures also exist in a peculiar class of three dimensional topological insulators---the Hopf insulators. In particular, we demonstrate that the spin textures of Hopf insulators in momentum space are twisted in a nontrivial way, which implies various knot and link structures. We further illustrate that the knots and nontrivial spin textures can be probed via standard time-of-flight images in cold atoms as preimage contours of spin orientations in stereographic coordinates. The extracted Hopf invariants, knots, and links are validated to be robust to typical experimental imperfections. Our work establishes the existence of knotted structures in cold atoms and may have potential applications in spintronics and quantum information processing.

February 9, 2015 Dr. Alexander Turbiner National Autonomous University of Mexico, Mexico City Helium-like Coulomb system: two critical charges, 2nd excited state of He- ion, and all that (Host: Tom Bergeman)

Reduced Coulomb problem of two electrons in the field of charged fixed center $Z$ $(H^-, He, Li^+, \ldots)$ is discussed. Current situation with $1/Z$ expansion is reviewed, numerical deficiencies of previously firmly-established results are indicated. First nuclear critical charge (corresponding the zero ionization energy) is established with high accuracy (Drake et al, 2014; Olivares-Pilon et al, 2014)). Second nuclear critical charge predicted by Stillinger and Stillinger (1966, 1974) is calculated as well as its associated square-root branch point singularity with exponent 3/2. It implies (i) the level crossing $1S-2S$ and (ii) the existence of the bound state embedded to continuum. It seems it leads to the existence of the spin-singlet, second excited state of negative hydrogen ion of the same symmetry as the ground state situated very close to threshold, which looks as an experimental challenge to detect.

February 23, 2015 Dr. Ben Sparks The University of Melbourne, Australia Towards Single-Shot Ultra-fast Molecular Imaging with a Cold Atom Electron & Ion Source (Host: Eden Figueroa)

The holy grail of ultrafast electron microscopy is the production of single-shot diffraction images of non-crystalline structures, as they will allow for the study of structural dynamics of chemical and biological processes by creating ‘molecular movies’ [1], with spatial resolution of 0.1 nm and temporal resolution of 0.1 ps [2,3]. To achieve this goal we require a source that can produce a large number of electrons in a very small volume. One way of producing such bunches is by ionising clouds of cold atoms [4-5]. The electrons produced from such a source will be very cold (~10 K) and therefore will be much more focusable than standard hot electron sources and have a longer coherence length (~10 nm) allowing for imaging of structures such as proteins. The cost and bench-top nature of cold atom electron sources is also an advantage over the billion-dollar x-ray free electron lasers that are currently endeavouring to produce diffraction from single molecules. Here we present the latest experimental results from our rubidium cold atom electron and ion source [6,7], demonstrating its ability to shape charged bunches (to help overcome bunch expansion due to the self-repulsive Coulomb interactions [8]), as well as Rydberg-excitation (to overcome disorder-induced heating for creating sub—nm focused ion spots [9]), and single-shot electron diffraction patterns produced. References: [1] R. Dwyer et al., Phil. Trans. R. Soc. 364, 741 (2006) [2] B. J. Siwick et al., Science 302, 1382 (2003) [3] R. Srinivasan et al., Helv. Chim. Act. 86, 1763 (2008) [4] B. J. Claessens et al., Phys. Rev. Lett. 95, 164801 (2005) [5] B. J. Claessens et al., Phys. Plasmas 14, 093101 (2007) [6] A. J. McCulloch et al., Nature Phys. 7, 785 (2011) [7] A. J. McCulloch et al., Nature Commun. 4, 1692 (2013) [8] O. J. Luiten et al., Phys. Rev. Lett. 93, 094802 (2004) [9] e.g., see M. Robert-de-Saint-Vincent, C. S. Hofmann, H. Schempp, G. Günter, S. Whitlock, and M. Weidemüller, Phys. Rev. Lett. 110, 045004 (2013); G. Bannasch, T. C. Killian, and T. Pohl, Phys. Rev. Lett. 110, 253003 (2013).

March 9, 2015 Prof. Alexander Gaeta Cornell University Chip-Based Optical Frequency Combs (Host: Tom Allison)

Optical frequency combs are having and will have enormous impact on many areas of science and technology, including time and frequency metrology, precision measurement, telecommunications, and astronomy.  I will describe our recent research on a novel type of frequency comb that is based on parametric nonlinear optical processes in silicon-based microresonators.  The dynamical behavior of how combs are generated in such a system is complex and include phase transitions, mode locking and synchronization, and femtosecond pulse generation.  Ultimately, such chip-based combs offer great promise for creating devices that are highly integrated and stable and can operate from the visible to mid-infrared regimes.

March 23, 2015 Prof. Lopez Vieyra National Autonomous University of Mexico, Mexico City Stable He^- can Exist in a Strong Magnetic Field (Host: Tom Bergeman)

The existence of bound states of the system (\alpha , e, e, e) in a magnetic field B is studied using the variational method. It is shown that for B > 0.13 a.u. (1 a.u. = 2.35 x 10^9 G) this system gets bound with total energy below the one of the (\alpha, e, e) system. It manifests the existence of the stable He- atomic ion.  Its ground state is a spin doublet ${}^2 (-1)^+$ at 0.74 a.u. > B > 0.13 a.u. and it becomes a spin quartet ${}^4 (-3)^+$  for larger magnetic fields. For 0.8 a.u. > B > 0.7 a.u. the He- ion has two (stable) bound states ${}^2 (-1)^+$ and ${}^4 (-3)^+$.

April 13, 2015 Prof. Jean-Pierre Wolf University of Geneva, Switzerland Towards Quantum Bioassays (Host: Tom Weinacht)

The identification and discrimination of molecules that exhibit almost identical structures and spectra using fluorescence spectroscopy is difficult. By addressing the molecular dynamics in “real time” and by making use of molecular quantum interference, quantum control already demonstrated its unique capability of selectively exciting or braking specific molecular bonds. In order to evaluate the capability of optimal control for discriminating between the optical emissions of nearly identical molecules, a specific strategy called “optimal dynamic discrimination (ODD) [1-3] was developed. The capability of ODD was demonstrated on the discrimination amongst riboflavin and flavin mononucleotide in aqueous solution, which are structurally and spectroscopically very similar. Closed-loop, adaptive pulse shaping discovers a set of 400 nm pulses that induce disparate nonlinear responses from the two flavins and allows for concomitant flavin discrimination of ~16\sigma. Additionally, attainment of ODD permits quantitative, analytical detection of the individual constituents in a flavin mixture. Most of the absorption bands of important biomolecules (DNA bases and proteins) lie, however, deeper in the UV. For this reason we developed a novel deep-UV all reflective pulse shaper based on MOEMS (Micro-opto-electromechanical systems)[4], which has laser pulse shaping capabilities down to 30 nm (XUV). We present recent experiments on the quantum control of the fluorescence of free aminoacids (Tryptophan and Tyrosin[5]), dipeptides (e.g.Trp-Leu, Trp-Ala, Trp-Gly) and large proteins (antibodies and albumin). In this latter case, we demonstrated that major serum proteins, namely immunoglobulins (IgG and IgM) and albumin can be discriminated and quantified, label free, in actual patients’ blood serum samples. These results pave the way to the future development of “quantum control based bio-assays”. (1) M. Roth, L. Guyon, J. Roslund, V. Boutou, F. Courvoisier, J.P. Wolf, H. Rabitz, Phys.Rev.Lett 102, 253001 (2009) (2) J. Petersen, R. Mitric, V. Bonacic-Koutecky, J.-P. Wolf, J. Roslund, H. Rabitz, Phys.Rev.Lett. 105, 073003 (2010) (3) J. Roslund, M. Roth, L. Guyon, V. Boutou, F. Courvoisier, J-P Wolf, H. Rabitz, J.Chem.Phys. 134, 034511 (2011) (4) J. Extermann, S. M.Weber, D. Kiselev, L. Bonacina, S. Lani, F. Jutzi,W. Noell, N. F. de Rooij, J-P Wolf, Opt Exp 19, 7580-7586 (2011) (5) A. Rondi,  L. Bonacina, A. Trisorio, C. Hauri, J.-P. Wolf, Phys.Chem.Chem.Phys. 14, 9317-9322 (2012) (6) S. Afonina et al, Appl. Phys. B, 111, 541-549 (2013)ba

April 20, 2015 Dr. Johanan Odhner Temple University Filamentation: Pulse Dynamics and Applications in Spectroscopy (Host: Tom Weinacht)

Filamentation of high power femtosecond laser pulses is of growing interest for applications such as atmospheric sensing, control of plasma discharges (artificial lightning), laser plasma-induced droplet nucleation (artificial cloud and rain formation), and backward lasing. It is also widely used for continuum generation for spectroscopy and as a means to compression laser pulses to just a few optical cycles of the carrier wavelength. A necessity for developing a rigorous understanding the filamentation process is the means to study the filamentation process in situ without disturbing the filamentation process itself. This has proved to be particularly challenging for filaments generated in gaseous media, where pulse intensities can reach as high as 10^14 W/cm2, making traditional diagnostic techniques inapplicable. In this talk we explore two new methods for probing the pulse reshaping dynamics directly in gas phase filaments, as well as some of the spectroscopic tools and applications that have resulted from those studies.

June 16, 2015 (Tue, 11am) Prof. Giuseppe Vallone University of Padova, Italy From Bell inequalities to quantum information (Host: Eden Figueroa)

Quantum mechanics (QM) is one of the most amazing and revolutionary discoveries of the last century. Among its counterintuitive predictions we can recall the uncertainty principle, the wave-particle duality, the superposition of quantum states, the entanglement and the Bell inequalities. The predictions of QM have been verified in several experiments and now quantum mechanics is at the basis of Quantum Information (QI), that studies the ability to manipulate and transmit information in a totally innovative way with respect to the possibilities offered by classical physics. After a brief introduction to the fundamental concepts of quantum mechanics, we will present some QI applications such as quantum communication, Quantum Key Distribution, and random number generators.

June 17, 2015 (Wed, 2pm) Prof. Paolo Villoresi University of Padova, Italy Quantum  Communications for Distant Correspondents (Host: Eden Figueroa)

The paradigm shift that Quantum Communications represent vs. classical counterpart allows envisaging the application of global cryptographic key distribution as well as of other quantum technologies. Quantum Communications on planetary scale require complementary channels including ground and satellite links. As the former have progressed up to commercial stage using fiber-cables, it's crucial the study of links for space QC and eventually the demonstration of protocols such as quantum-key-distribution (QKD) and quantum teleportation along satellite-to-ground or intersatellite links. In recent announcements, the launch of dedicated mission was envisaged. Indeed, the sharing of quantum states among ground and orbiting terminal may be considered as feasible according to present optical technologies. However, the extension of the Quantum Communications and Technologies to long distances, on the surface of the Earth as well as from the Earth to an orbiting terminal in Space, is influenced by difficulties, among which the moving terminals, the large losses, the effects on the optical propagation of the turbulent medium. Indeed, the quantum state that is prepared and sent at the transmitter side experiences a transformation in the spatial spectrum in addition to the vacuum diffraction, that requires strategies to be compensated. We will address the experimental studies on the faithful transmission of qubits from Space to ground and from distant terminals on the Earth which demonstrates the means for realizing Quantum Communications on these channels.

July 13, 2015 Dr. Reihaneh Shahrokhshahi University of Virginia A cavity enhanced narrow-band multi-photon source for applications in quantum information (Host: Eden Figueroa)

Photons prepared in Fock states have been a subject of great interest due to the non-Gaussian (negative) nature of their Wigner function.  These quantum states are the essence of the quantum nature of light and have applications in quantum cryptography, quantum information processing and universal quantum computing. The generation of the Fock states has been most commonly achieved by using spontaneous parametric down-conversion (SPDC) in nonlinear crystals, but the fidelity and the success rate of Fock state generation was limited by the multimode nature of SPDC photons. In this talk, I will describe the investigation of cavity-enhanced SPDC modes for higher-fidelity photon pair generation. We have built an intrinsically stable OPO, whose well-defined cavity modes were used to herald photons, with up to 80% success rate in preliminary results. The heralding and measurements were performed by photon-number-resolving, high-quantum-efficiency, transition edge sensors, built at NIST by Sae Woo Nam's group.

July 16, 2015 (Thurs, 2:30pm, YITP Common Room)  Dr. Gyanyu Zhu Northwestern University Wonders in flat bands: from quantum liquid crystals to self-correcting quantum memory (Host: Tzu-Chieh Wei)

In this talk, I will discuss two cases where flat bands in frustrated lattice models lead to emergence of interesting physics. In the first part, I talk about a family of interacting boson models based on a kagome lattice with local synthetic gauge flux, which can be realized in optical lattices with ultra-cold atoms or circuit-QED lattices with interacting photons. Such models have a lowest flat band in the single-particle spectrum. The flat band is spanned by eigenstates forming localized loops on the lattice, with the maximally compact loop states typically breaking the discrete rotational symmetry of the lattice. When populated by locally-interacting particles, the close packing of such maximally compact loop states leads to a nematic loop crystal ground state. We predict that increasing the filling beyond the close packing fraction leads to the formation of quantum liquid crystals including a nematic supersolid and a nematic superfluid phase. We also show how the nematicity can be probed by time-of-flight experiments or phase imprinting techniques [1]. In the second part, I discuss how 4-body spin interactions can emerge in a 2D flat-band lattice with "Aharonov-Bohm cages", and in the presence of light-matter interactions.  Based on such an idea, one can realize the surface-code Hamiltonian in the ultra-strong coupling regime of a circuit-QED lattice, when the interaction strength is comparable to the microwave photon frequency.  Two types of 4-body stabilizer interactions are realized by utilizing the electro-magnetic duality in circuit-QED. In such case, the circuit-QED vacuum has topological degeneracies and can be used as a self-correcting quantum memory.   An alternative approach without ultra-strong coupling is to simulate the surface-code Hamiltonian in the rotating frame, with side-band driving through modulating the flux penetrating SQUID couplers. [1]  Guanyu Zhu, Jens Koch and Ivar Martin,  arXiv:1411.0043.

July 16, 2015 (Thurs, 4:00pm)  Dr. Aye Lu Win Old Dominion University Catalysis of Stark-tuned Interactions between Ultracold Rydberg Atoms (Host: Eden Figueroa)

The experimental investigations of the catalysis effect in the resonant energy transfer between ultracold 85Rb Rydberg atoms will be presented. We have investigated the energy transfer process of 34p + 34p → 34s + 35s, and observed Stark-tuned Förster resonances. When additional Rydberg atoms of 34d state are included in the interaction, an increase in the population of 34s states atoms was observed. Although the 34d state atoms do not directly participate in the resonant energy transfer that produces 34s state atoms, they add an additional interaction channel 34p + 34d → 34d + 34p that is resonant for all electric fields. We have also investigated the time dependence of the resonant interactions of 34p + 34p→ 34s + 35s, compared the experimental results with the numerical simulations of simple models, and found them to be in good agreement.

August 3, 2015 (2:45pm) time change Azure Hansen University of Rochester Topological spin textures in spinor Bose-Einstein condensates (Host: Tom Bergeman)

We generate spin textures in an 87-Rb Bose-Einstein condensate with diverse symmetry and topological properties, including spin monopoles, non-Abelian vortices, fractional vortices, and coreless vortices. Using a coherent optical Raman process we transfer spatially-dependent phase and polarization properties from complex singular optical modes to the condensate. This allows us to engineer the internal and external momenta, superfluid velocities, and spatial spin distribution of the condensate as well as control the complex relative phases of the magnetic spin components. This work will allow us to measure a new geometric phase in atom optics, study interactions between spin textures, and determine the ground state spin interactions in spin-2 87-Rb

August 7, 2015 (Friday, 3pm)  Prof. Ágnes Vibók University of Debrecen, Hungary Signatures of light-induced conical intersections on the dynamical properties of molecular systems (Host: Tom Weinacht)

Nonadiabatic effects are ubiquitous in physics, chemistry and biology. They are strongly amplified by conical intersections (CIs) which are degeneracies between electronic states of triatomic or larger molecules. A few years ago it has been revealed that CIs in molecular systems can be formed by laser light even in diatomics. The energetic and internuclear positions of these light-induced conical intersections (LICIs) are depend on the laser frequencies while the strength of their nonadiabatic couplings can be modified by the field intensities. Strong impact of these LICIs on different dynamical properties of the diatomics has been discussed in several papers. Recently our attention is focused on the description of the photodissociation dynamics of the D2+ molecule. Kinetic energy release (KER) and angular distribution of the photodissociation probabilities are calculated with and without LICIs for different values of laser parameters. By analyzing this dynamical process carefully, we found a robust effect in the angular distribution of the photofragments which serves as a direct signature of the LICI providing undoubted evidence for its existence.

last updated xx/xx/2015