Session X52: Atomic Physics: New Frontiers I

Comparison between two models of time-dependent absorption of matter waves

The interaction between an atom and a laser might give rise to transitions between two, or more, internal states of the atom. Such processes can be efficiently described within the framework of matter wave absorption, in which the laser beam is mimicked by an absorbing barrier. In this talk we present a quantitative comparison between two models describing the interaction between a non-relativistic quantum particle and a thin time-dependent absorbing barrier. The first model represents the barrier by time-dependent discontinuous matching conditions imposed on both the wave function of the particle and its spatial derivative. The second model treats the particle as a spinor submitted to a time-dependent off-diagonal $\delta$-potential. We show the two models to be in excellent agreement in a semiclassical regime. Reference: M. Barbier, M. Beau, A. Goussev, arXiv:1510.06996, \textit{Phys. Rev. A} (in press).   

Bell Experiment with Classical Optical Fields

We theoretically and experimentally explore the implications of entanglement in statistically classical optical fields \footnote{X.-F. Qian, Bethany Little, John Howell, and J. H. Eberly, Optica {\bf 2}, 611-615 (2015).}. The description of these fields in terms of polarization and amplitude degrees of freedom can take a non-separable form which employs a mathematical description of entanglement often associated with quantum phenomena. By subjecting these optical fields to a Bell analysis, we examine the role of entanglement in marking the quantum-classical boundary. We report a value of the Bell parameter greater than $\cal B$ $ = 2.54$, many standard deviations outside the limit $\cal B$ $ = 2$ established by the Clauser-Horne-Shimony-Holt Bell inequality \footnote{John F. Clauser, Michael A. Horne, Abner Shimony, and Richard Holt. Phys. Rev. Lett. {\bf 23}, 880-884 (1969)}. This suggests that Bell violation has less to do with quantum theory than previously thought, but everything to do with entanglement.   

Non-equilibrium Transport of Light

Non-equilibrium Transport of Light The thermalization of light under conditions of parametric coupling to a bath provides a robust chemical potential for light [1]. We study non-equilibrium transport of light using non-equilibrium Green's function approach under the parametric coupling scheme, and explore a potential photonic analogue to the Landauer transport equation. Our results provide understandings of many-body states of photonic matter with chemical potential imbalances. The transport theory of light paves the way for quantum simulation and even practical applications of diode-like circuits using quantum photonic sources in the microwave and optical domain. \\ $[1]$ M. Hafezi, P. Adhikari, J. M. Taylor, arXiv:1405.5821v2 (2014)   

Determination of Zak phase by reflection phase in 1D photonic crystals

For a one-dimensional (1D) periodic system with inherent mirror symmetry, the value of the geometric ``Zak'' phase in a bulk band is related to the sign of reflection phase for wavelengths inside the bandgaps sandwiching the bulk band. We designed an interference setup which allows us to measure the reflection phase of 1D phonic crystal fabricated for the optical range, and this in turn enabled us to determine the Zak phases of the bands. We then found interface states whose existence can be traced to the topological properties of the bandgaps and the geometric phases of the bulk bands. (accepted by optics letters)   

Experimental Apparatus for the Observation of the Topological Change Associated with Dynamical Monodromy

Monodromy means “once around a path,” therefore systems that have non-trivial monodromy are systems such that, when taken around a closed circuit in some space, the system has changed state in some way. Classical systems that exhibit non-trivial Hamiltonian monodromy have action and angle variables that are multivalued functions. A family, or loop, of trajectories of this system has a topological change upon traversing a monodromy circuit. We present an experimental apparatus for observing this topological change. A family of particles moving in a cylindrically symmetric champagne-bottle potential exhibits non-trivial Hamiltonian monodromy. At the center of this system is a classically forbidden region. By following a monodromy circuit, a loop of initial conditions on one side of the forbidden region can be made to evolve continuously into a loop that surrounds the forbidden region. We realize this system using a spherical pendulum, having at its end a permanent magnet. Magnetic fields generated by coils can then be used to create the champagne-bottle potential, as well as drive the pendulum through the monodromy circuit.   

Topological Charge Screening in Disordered Aharonov-Bohm Wavefunctions

Free electrical charges are typically subject to screening relations. For example, in ionic fluids and Coulomb gases there is screening (both global and local) of the electrical charges, described by the first and second Stillinger-Lovett sum rules [1]. A topological analogy governs the statistical behaviour of the nodal points in Gaussian random superpositions of plane waves. These nodal points are integer topological charges, i.e. vortices and antivortices of the complex wavefunction, whose sign is that of the phase circulation. Such superpositions are known to model high energy eigenfunctions in the presence of wave chaos [2], and display topological charge screening in the bulk [3]. We investigate how these screening relations are affected by the introduction of a magnetic flux line [4], which may be fractional in strength. We find that the global screening relation is broken, with the average total topological charge of the vortices given by the flux strength, and that the local screening of the flux itself shows unexpected features. [1] F. H. Stillinger and R. Lovett, J. Chem. Phys. 49, 1991-94 (1968) [2] M. V. Berry, J. Phys. A 10, 2083-91 (1977) [3] M. V. Berry and M. R. Dennis, Proc. R. Soc. A 456, 2059-79 (2000) [4] Y. Aharonov and D. Bohm, Phys. Rev. 115, 485-91 (1959)   

Synthetic gauge flux and Weyl points in acoustic systems.

We consider acoustic systems comprising a honeycomb lattice in the xy plane and periodic along the z direction. As kz is a good quantum number here, for each fixed kz, this system can be treated as a reduced two-dimensional system. By engineering the interlayer coupling in the z-direction, we show that we can realize effective inversion symmetry breaking and synthetic staggered gauge flux in the reduced two-dimensional system. The realizations of chiral edge states for fixed values of kz are direct consequences of the staggered gauge flux. And we then show that the synthetic gauge flux is closely related to the Weyl points in the three-dimensional band structure.   

Atomic collisions, inelastic indeed.

At the turn of the twentieth century, a hot controversy raged about the ability of Boltzmann's framework to take care of irreversibility. The so-called Loschmidt's paradox progressively faded with time during the last hundred years, due to the predictive efficiency of statistical mechanics. However, one detail at the origin of the controversy -- the elasticity of atomic collisions -- was not completely challenged. A semi-classical treatment of two atoms interacting with the vacuum zero-point field permits to predict a friction force acting against the rotation of the pair of atoms [Bercegol H. {\&} Lehoucq R., \textit{Phys. Rev. Lett.} \textbf{115}, 090402 (2015)]. By its form and its level, the calculated torque is a candidate as a physical cause for diffusion of energy and angular momentum, and consequently for entropy growth. It opens the way to a revision of the standard vision of irreversibility. This presentation will focus on two points. First we will discuss the recent result in a broader context of electromagnetic interactions during microscopic collisions. The predicted friction phenomenon can be compared to and distinguished from Collision-Induced Emission and other types of inelastic collisions. Second we will investigate the consequences of the friction torque on calculated trajectories of colliding atoms, quantifying the generation of dimers linked by dispersion forces.   

Hydrogen Dissociation in Generalized Hartree-Fock Theory: Breaking the diatomic bond

Generalized Hartree Fock theory predicts molecular Hydrogen dissociation without correlation. A variational Gaussian-Sinc linear superposition is the basis of 50 calculations with 3-4 significant digits of quality. The spin singlet covalent bond spontaneously breaks into a pair of uncorrelated doublets at atomic separation of 1.22 Angstroms. Quantum spin numbers and energetic comparison with Configuration Interaction theory---correlation---point to a first order phase transition in the molecular Hydrogen bond without correlation.   

Stretching of Picosecond Laser Pulses with Uniform Reflecting Volume Bragg Gratings.

It is shown that a uniform reflecting volume Bragg grating (VBG) can be used as a compact monolithic stretcher of high-power picosecond laser pulses in cases when chirped Bragg gratings with an appropriate chirp rate are difficult to fabricate. A chirp-free reflected stretched pulse is generated of almost rectangular shape when incident short pulse propagates along a grating and experiences local Bragg diffraction. The increase in duration of the reflected pulse is approximately equal to twice the propagation times along the grating. We derived the analytic expression for diffraction efficiency, which incorporates incident pulse duration, grating thickness, and amplitude of refractive index modulation, enabling an optimum selection of the grating for pulse stretching. The typical expected theoretical value of diffraction efficiency is about 10{\%} after taking into account the spectral narrowing of the reflected emission. We believe that the relatively low energy efficiency of the proposed method is more than offset by a number of advantages, which are chirp-free spectrum of a stretched pulse, compactness, robustness, preservation of setup alignment and beam quality, and tolerance to high power. Obtained pulses of several tens of picoseconds can be amplified by standard methods which are not requiring special measures to avoid undesirable non-linear effects. We propose a simple and reliable method to control the temporal parameters of the high-power picosecond pulses using the same laser source and the VGB of variable thickness that can significantly simplify the experiments requiring different pulse durations.   

Ablation-cooled material removal with ultrafast bursts of pulses

Use of femtosecond pulses allows precise and thermal-damage-free material removal with broad applications. However, its potential is limited by low material removal speeds and complexity of the required lasers. The laser complexity arises from the high pulse energy threshold for ablation. Physics of the laser-material interaction precludes a straightforward scaling up of the removal rate by using more powerful lasers due to shielding and collateral damage from heat accumulation. Here, we exploit ablation cooling, a technique used in aerospace engineering since 1950’s, to circumvent this limitation. We apply rapid successions of pulses from specially developed lasers to ablate the target material before the residual heat deposited by previous pulses diffuse away from the interaction region. This constitutes a new physical regime of laser-material interactions, where heat removal due to ablation is comparable to conduction. Proof-of-principle experiments demonstrate reduction of required pulse energies by ~1000x, while simultaneously increasing efficiency and speed by ~10x.   

Formation of ultrashort pulses from monochromatic XUV radiation via interaction with a medium of IR-dressed He atoms.

Trains of high intensity ultrashort XUV pulses could find a lot of applications. Recently, a mechanism of high efficiency formation of a train of XUV pulses from quasi-monochromatic XUV field was suggested [Opt. Lett. 36, 2296 (2011)]. XUV field propagates through the medium of atoms, which are space-time modulated by IR field. The field scattered by modulated atoms contains sidebands of incident XUV field frequency and, for properly chosen parameters, train of ultrashort pulses is formed at the output of the medium. In this contribution, we study formation of ultrashort pulses in the medium of He atoms. Contrary to our recent work [Phys. Rev. A 91, 023830 (2015)], IR field is chosen to be weak enough, so that pulses are formed due to modulation of excited atomic levels, rather than tunnel ionization. The suggested method allows to form train of pulses with high efficiency and can be scaled to He-like ions in order to get even shorter pulses.   

Chiral Molecular Optical Response to Nano-Shaped Light

Chiral linear optical signals are an important spectroscopic tool for biomolecules and chemical sensing applications. Exact expressions are derived which express these signals as a convolution of a non-local linear susceptibility of matter with a non-local intrinsic property of the electric field. The chiral response can be enhanced and optimized using nano-optical fields with strong spatial variation. The approach is based on a gauge invariant calculation using the minimal coupling Hamiltonian. The multipolar expansion is avoided and all multipoles are naturally incorporated. We apply these expression to achiral (planar) and chiral (dihedral angle of $45^{\circ}$) bi-phenyl as a physically intuitive illustration.   

Study of Rydberg blockade mediated optical non-linearity in thermal vapor

We demonstrate Rydberg blockade by coupling to Rydberg state via two-photon excitation in rubidium thermal vapor. The probe beam coupling to the $D2$ transition was blue detuned by $1.3$ GHz and a coupling beam was scanned to excite the atoms to Rydberg state via two-photon transition (5s$_{1/2}\longrightarrow n$s$_{1/2}$). The dispersion of the probe beam is modified due to the 2-photon excitation and is measured using an optical heterodyne detection technique in the experiment. We have observed that the dispersion of the probe beam depends linearly on atomic vapor density while coupling to a Rydberg state with principal quantum number, $n=30$. However, density dependent suppression of the dispersion is observed while coupling to the Rydberg state with $n = 60$. Since the dispersion of the probe beam due to 2-photon excitation depends on the Rydberg population, the density dependent suppression is explained by introducing the concept of blockade. The blockade radius is measured to be about 2.2 $\mu$m which is consistent with the scaling due to Doppler width of the 2-photon resonance in thermal vapor. Our result promises the realization of single photon source and strong single photon non-linearity based on Rydberg blockade in thermal vapor.   

'Relativistic' corrections to the mass of a plucked guitar string

Quantum systems respond non-adiabaticity when parameters controlling them are ramped at a finite rate. If the parameters themselves are dynamical – for instance the position of a box that defines the boundary of a quantum field – the feedback of these excitations gives rise to effective Newtonian equations of motion for the parameter. For the age old problem of photons in a box, this correction gives rise to a mass proportional to the energy of the photons. We show that a similar correction arises for a classical guitar string plucked with energy $E$; moving clamps at the ends of the string requires inertial mass $m=2 E/c_s^2$, where $c_s$ is the speed of sound. This quasi-relativistic effect should be observable in freshman physics level experiments. We then comment on how these simple methods have been readily extended to treat problems such as ramps and quenches of strongly-interacting superconductors and dynamical trapping near a quantum critical point.