Session S32: Slow Molecular Beams and Quantum Optics

Slow beams of molecules with masses up to 6000 u

Slow molecules are desirable for various experiments, among them matter wave interferometry, precision metrology, collision studies and the improved control in the deposition of molecular nanopatterns. Here we report on effusive beams of intact per-fluorinated molecules with masses up to 6000 u and beyond. The molecules in these beams are observed to have a mean velocity of down to 30 m/s. And the mass selected signals of the post-ionized particles are so high that even molecules with a longitudinal velocity as low as 10 m/s and with a transverse velocities below 10 cm/s can still be detected. We discuss potential strategies and applications for further slowing, trapping and focusing of these molecules.   

Magnetic trapping of Stark decelerated OH

Ultracold, ground state polar molecules promise to revolutionarily impact AMO physics with the study of ultracold molecular collisions and quantum chemistry, implementation of quantum information processing, and the possibility of lattice-spin model simulations. Our research has focused on the use of a Stark decelerator to slow a supersonic expansion of OH. At a mean packet velocity of 20 m/s, we obtain a $\sim$10 mK sample at densities greater than $10^5$ cm$^{-3}$. The decelerator terminates at an anti-Helmholtz coil pair which we have used to demonstrate magnetic trapping of the polar molecule OH in the presence of tunable electric fields. We will present our latest results on trapping dynamics as well as discuss the feasibility of molecular cavity-assisted laser cooling, which may provide access to the ultracold regime.   

Semi-Classical Theory of Radiation Pressure Cooling of a Mechanical Oscillator by dynamical Backaction

Laser cooling of the thermal motion of mechanical oscillators has been predicted by Braginsky almost three decades ago, and has recently been demonstrated experimentally for the first time in a series of experiments. Cooling arises when the mechanical oscillator is coupled to an optical high finesse cavity, which causes the mechanical mode to be viscously damped on the red detuned optical sideband. The ultimate goal of these experiments is to reach the quantum ground state of the mechanical oscillator. Using a semiclassical theory for radiation pressure induced laser cooling, and taking into account the quantum back action of the radiation field on the mechanical oscillator noise spectrum, we derive the conditions for which ground state cooling is possible. In addition we elucidate the physical origin of the cooling and identify the similarities with atomic laser cooling.   

Cooling of a micro-mechanical oscillator using radiation pressure induced dynamical back-action

For more than three decades, dynamical backaction in the form of radiation pressure has been predicted to give rise to intricate coupled dynamics of the optical modes of a high-finesse cavity and the mechanical modes of its boundaries. In particular, if the mechanical oscillation period is comparable to the cavity's photon lifetime, and the cavity is pumped with a red-detuned laser, the Brownian motion of the mechanical mode can be reduced, corresponding to an effective temperature reduction or cooling. We have recently succeeded in an experimental demonstration of this phenomena, and exploited dynamical back-action to cool the radial breathing mode of a toroidal silica microcavity from room temperature to 11 K. Working with distinctively high mechanical frequencies (50 MHz) we can provide strong evidence for a virtually pure radiation-pressure effect. We further introduce a theoretical model to quantitatively predict the light-induced modifications in the mechanical oscillator's properties over a wide range of experimental parameters. These achievements constitute an important step towards ground-state cooling of a micromechanical oscillator.   

Stopping Single Photons in One-dimensional Circuit Quantum Electrodynamics Systems

We propose a mechanism to stop and time-reverse single photons in one-dimensional circuit quantum electrodynamics systems. As a concrete example, we exploit the large tunability of the superconducting charge quantum bit (charge qubit) to predict one-photon transport properties in multiple-qubit systems with dynamically controlled transition frequencies. In particular, two qubits coupled to a waveguide give rise to a single-photon transmission lineshape that is analogous to electromagnetically-induced transparency (EIT) in atomic systems. Furthermore, by cascading double-qubit structures to form an array and dynamically controlling the qubit transition frequencies, a single photon can be stopped, stored, and time-reversed. With a properly designed array, two photons can be stopped and stored in the system at the same time. Moreover, the unit cell of the array can be designed to be of deep sub-wavelength scale, miniaturizing the circuit.   

Coherence Investigations of Erbium doped in Waveguide Structures for a Quantum Memory

Erbium doped waveguides are very promising candidates for the realization of a quantum memory based on reversible absorption in a controllably broadened absorption line (CRIB). First of all the wavelength of the ``storage transition'' matches well with the ``telecom wavelength'' most often used for long-distance quantum communications in the past. Secondly the interaction length between light and ions can be made very long within a waveguide. Thus high optical depth can be achieved as required for the proposal. We have measured the homogeneous linewidth of the I$_{15/2} \quad \to $I$_{13/2}$ transition in a Erbium-doped SiO$_{2 }$glass fiber and a LiNOb$_{3}$ Crystal with a waveguiding structure at a wavelength of $\lambda $=1530 nm. The homogeneous lifetime in the glass shows an abnormal magnetic field dependency and is in the order of several $\mu $s, which is an improvement of two orders of magnitude compared to existing data in similar material. Also we investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo.   

Dispersive, superfluid-like shock waves in optics

Dispersive shock waves arise from nonlinear wave breaking and mode dispersion, and are a fundamental type of fluid behavior in systems with none or near-zero viscosity, e.g. cold plasmas and superfluids. Here, we exploit the well-known (but underappreciated) relation between superfluids and nonlinear optics to study the photonic equivalent of dispersive, dissipationless shock waves. We experimentally demonstrate fundamental shock waves in one and two dimensions, examine their basic nonlinear properties, and observe collisions between two such shocks. We study spectral energy exchange during interactions, and find that energy and momentum transfer depend on details of the collision region. Results can be explained in terms of a nonlinear Huygens' principle, in which linear superposition of initial waves results in a nonlinear source of new shocks. In higher dimensions, wavefront geometry and expansion directions play a significant role. In addition to providing a versatile platform for new photonic physics, it is anticipated that the results reported here will lead to all-optical modeling of even richer (super)fluid-like phenomena in the near future.   

Coherent Quantum Engineering of Laser Cooling

Doppler laser cooling of two-level atoms is well understood, and has been utilized extensively for decreasing phase-space density of atomic gases. The temperature limit of Doppler cooling is on the order of the excited-state spectral linewidth, and cooling below this limit requires, for example, atomic sublevel degeneracy. Here we present a means of cooling that consists of three internal states of an atom and two lasers of distinct frequency. Employing sparse-matrix techniques, we find numerical solutions to the fully quantized master equation in steady state, allowing straightforward determination of laser-cooling temperatures. We develop a qualitative picture of the mechanism, related to the phenomenon of electromagnetically induced transparency, yielding a cooling scheme in which a dressing laser can be tuned to coherently engineer a two-level quantum system that has desirable Doppler-cooling properties. Effects of the induced asymmetric Fano-type lineshapes affect the detunings required for optimum cooling, as well as the predicted minimum temperatures which can be lower than the Doppler limit for either transition. This work was supported in part by the NSF.   

Three Level Systems for Quantum Memories in Erbium Doped Materials

Quantum memories for single photons could play an important role in quantum communication and optical quantum computing. We are working towards the realization of such a quantum memory based on the controlled reversible inhomogeneous broadening (CRIB) of a single absorption line in a rare earth ion. The implementation of the CRIB protocol for such a quantum memory requires a three level system such that the absorption over a broad bandwidth in a material can be greatly reduced via optical pumping to the auxiliary level. We report on the first experimental steps towards the realization of such a three level systems in Erbium doped materials with spectral hole burning techniques.   

A Kapitza-Dirac Talbot-Lau interferometer for molecules

We present a novel matter-wave interferometer setup which is designed for particles with wavelengths down to 0.5 pm. Such a short wavelength corresponds for instance to a mass of 7000 atomic mass units (amu) at a velocity of 100m/s. Such an advance in mass and complexity can only be accomplished by introducing a standing light wave [1,2,3] to replace the central material grating used in a standard Talbot-Lau interferometer [4]. Light gratings combine high transmission with the absence of the perturbing van der Waals forces otherwise encountered at material gratings. This is particularly desirable for the investigation of the wave-particle duality of large molecules with high polarizabilities. We show the first successful application of this interferometer with C$_{70}$-Fullerenes. Preliminary studies with sources and detection schemes for molecules of up to 7000 amu are very promising for interference experiments with such large and heavy objects in the immediate future. [1] P. Gould et al., Phys. Rev. Lett. 56, 827 (1986) [2] D. Freimund et al., Nature 413, 142 (2001) [3] O. Nairz et al., Phys. Rev. Lett. 87, 160401 (2001) [4] B. Brezger et al., J. Opt. B 5, 82 (2003)   

Quantum phase transitions for light and XY spin models in coupled cavity arrays

The realization of insulator to superfluid transitions in optical lattices have opened great possibilities for simulating many body systems. It is thus interesting to explore which other systems permit such phases and simulations, especially if the problem of accessibility of the individual sites is not present. Particularly arresting will be to find such phases in a system of photons which, by being non-interacting, are unlikely candidates for the studies of many-body phenomena. Here we show that a Mott phase can arise in an array of coupled high Q electromagnetic cavities between which photons can hop, when each cavity is coupled to a {\em single} two level system (atom/quantum dot/superconducting qubit). In this phase each atom-cavity system has the same integral number of net (atomic plus photonic) excitations. It occurs for resonant photonic and atomic frequencies when the {\em photon blockade} effect provides an {\em effective repulsion} between the excitations in each atom-cavity system. Detuning the atomic and photonic frequencies suppresses this repulsion and induces a transition from the Mott phase to a photonic superfluid. We show that for zero detuning, the system can simulate the dynamics of an XY spin chain with arbitrary number of excitations.   

Moments Formulation of Optical-Pulse Propagation in Insulators

We have developed general expressions for the group velocity and its dispersion in insulators in terms of moments of the material's IR (ionic) and UV (electronic) absorptions. The formulation, which is based on Kramers-Kronig dispersion theory, is independent of material models, and involves only independently measurable quantities. The carrier frequency at which a signal propagates with minimum distortion is determined by the ratio of the first moment of the ionic absorption to the inverse-third moment of the electronic absorption*. This represents a balance between ionic and electronic effects and depends only on their respective contributions to dispersion in the index, not on the magnitude of the refractive index. Physically, minimum distortion corresponds to propagation of a compound ionic-electronic polaron at a frequency for which the ionic and electronic components remain in phase. Applications to silicate-glass fibers will be considered. *This is a generalization of a result given by S. H. Wemple, Appl. Opt. \textbf{18}, 31 (1979).   

Controlled Spontaneous Emission

The problem of spontaneous emission has been studied by numerical simulations. The dynamics of the combined system atom + radiation field, involving up to 15 k field oscillators, has been calculated by direct diagonalization of the Hamiltonian. Optimization of the discrete model's parameters was made by comparing results with the exact solution for the model with equidistant frequencies of the oscillators and equal coupling constants. A numerical approach made it possible to address problems too complex for analytical treatment, which involve interaction with external fields and emission by multi-atom systems. Our major findings are the following. 1) Irradiation by a periodic sequence of laser pulses may shift the frequency in a continuous way by attenuating the power of the pulses. 2) In a two-atom system, the linewidth of the emitted spectrum can be made arbitrary small, and can be regulated by changing a difference between the transition frequencies of the atoms. Therefore, both the frequency and linewidth of spontaneous emission can be controlled.