Laser Spectroscopy

The following sections are included:

• PREFACE

• Organizing Committees

• CONTENTS

We have observed exponentially localized wave function of ultracold atoms released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. We present this result, and elaborate on the significance of 1D Anderson localization, and on the prospects of extending that type of study to quantum gases in higher dimensions (2D and 3D) and with controlled interactions. We will also point out its relevance to the rapidly evolving domain of quantum simulators to study difficult problems of Condensed Matter.

We summarize our recently obtained experimental results on the production of entangled - coherently spin squeezed - quantum states using an ensemble of ultracold atoms.¹ Coherent spin squeezing is obtained among two spatial modes of a Rubidium Bose-Einstein condensate in an optical multi well trap. We use an adiabatic cooling technique to realize these states even at finite temperature. By measuring the conjugate variables atom number difference and relative phase between two neighboring lattice sites, we detect coherent spin squeezing of -3.8dB. This type of entanglement allows for interferometry beyond the standard quantum limit, the bound attainable by classical means.^2,3

We study atomic Bose-Einstein condensates (BECs) with the magnetic dipole-dipole interaction. The long-range and anisotropic nature of the interaction causes anisotropic collapse in a spin-polarized BEC, ferrofluidity and supersolidity in a two-component BEC, and spontaneous circulation in a spinor BEC.

The nature of the vacuum state and its fluctuations constitutes one of the most fascinating aspects of modern physics. In particular, the parametric amplification of such fluctuations is crucial for phenomena ranging from optical parametric down-conversion¹ to stimulated positronium annihilation,² and boson creation in Universe inflation.³ Spinor Bose-Einstein condensates,^4–7 consisting of atoms with non-zero total spin, provide an optimal system for the investigation of the vacuum state.^8,9 Here we describe the amplification of vacuum fluctuations in gaseous spinor condensates in an unstable spin configuration. We observe strong instability resonances in the spinor condensate,¹⁰ induced by the confinement of the atomic ensemble. On these resonances we conclusively demonstrate that the system can act as a parametric amplifier for vacuum fluctuations,¹¹ providing a new microscope to investigate the vacuum state and a promising method for entanglement and squeezing production in matter waves.

One reviews the recently introduced field of matter-wave "meta-optics", i.e. the extension of optical negative-index media (NIM) to atom optics. After emphasizing the differences with light meta-optics and particularly the necessary transient character of NIM's in atom optics, we present the way of generating matter-wave NIM's and their general properties: negative refraction, atom meta-lenses. Finally their specific features are reviewed: longitudinal wave packet narrowing associated to a time-reversal effect, transient revivals of evanescent matter waves and atom reflection echoes at a potential barrier.

We report on our recent progress on the manipulation of single rubidium atoms trapped in optical tweezers and the generation of entanglement between two atoms, each individually trapped in neighboring tweezers. To create an entangled state of two atoms in their ground states, we make use of the Rydberg blockade mechanism. The degree of entanglement is measured using global rotations of the internal states of both atoms. Such internal state rotations on a single atom are demonstrated with a high fidelity.

We report on the storage and manipulation of hundreds of mesoscopic ensembles of ultracold ⁸⁷Rb atoms in a vast two-dimensional array of magnetic microtraps, defined lithographically in a permanently magnetized film. The atom numbers typically range from tens to hundreds of atoms per site. The traps are optically resolved using absorption imaging and individually addressed using a focused probe laser. We shift the entire array, without heating, along the surface by rotating an external bias field. We evaporatively cool the atoms to the critical temperature for quantum degeneracy. At the lowest temperatures, density dependent loss allows small and well defined numbers of atoms to be prepared in each microtrap. This microtrap array is a promising novel platform for quantum information processing, where hyperfine ground states act as qubit states, and Rydberg excitation may orchestrate interaction between neighbouring sites.

We report a limit on the fractional temporal variation of the proton-to-electron mass ratio as , obtained by comparing the frequency of a rovibrational transition in SF₆ with the fundamental hyperfine transition in Cs. This result is direct and model-free. As far as we know, it is the most precise absolute frequency measurement of a molecular transition. It was performed using an optical link to transfer the primary standard frequency controlled with a Cs fountain from LNE-SYRTE to LPL. Last developments on that optical link show a resolution of a few 10^-16 at 1 s integration time and around 10^-19 at 1 day. This resolution is preserved when a non-dedicated fiber is used. In that case, the metrological signal is transferred together with the digital data signal from the Internet traffic using two different frequency channels in a dense wavelength division multiplexing scheme.

Here we present the first complete measurement of the radiative decay rate to the ground state of the lowest four triplet states of helium i.e. the metastable 2³S₁ state and the 2³P manifold. We employ laser cooling and trapping in an ultrahigh vacuum chamber to enable direct measurement of the trap loss rate for decay from the 2³P₁ state to the ground state. We then use this rate to calibrate the XUV emission decay to the ground state for all the remaining transitions. The 2³P₁ and 2³P₂ transition rates are measured for the first time, an upper bound is placed on the 2³P₀ decay rate, and the 2³S₁ metastable lifetime is determined for only the second time with a five-fold improvement in accuracy. These results are in excellent agreement with theoretical QED predictions, and anchor the helium-like isoelectronic sequences for these transitions at low Z.

We describe two possible configurations for optical lattice clocks; a one-dimensional (1D) lattice loaded with spin-polarized fermions and a three-dimensional (3D) lattice loaded with bosons. We realized these lattice clocks with fermionic ⁸⁷Sr and bosonic ⁸⁸Sr atoms and performed an optical frequency comparison between the two. The future prospects of a cryogenic Sr lattice clock and a blue-detuned lattice clock are also discussed.

Frequency standards based on narrow optical transitions in ²⁷Al⁺ and ¹⁹⁹Hg⁺ ions have been developed at NIST. Both standards have absolute reproducibilities of a few parts in 10¹⁷. This is about an order of magnitude better than the fractional uncertainty of the SI second, which is based on the ¹³³Cs hyperfine frequency. Use of femtosecond laser frequency combs makes it possible to compare the optical frequency standards to microwave frequency standards or to each other. The ratio of the Al⁺ and Hg⁺ frequencies can be measured more accurately than the reproducibility of the primary cesium frequency standards. Frequency measurements made over time can be used to set limits on the time variation of fundamental constants, such as the fine structure constant α or the quark masses.

We demonstrate a fiber-optical switch that operates with a few hundred photons per switching pulse. The light-light interaction is mediated by laser-cooled atoms. The required strong interaction between atoms and light is achieved by simultaneously confining photons and atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber.

Room-temperature atomic ensembles are useful and simple systems for experiments in quantum information science. Atomic ensembles have been entangled,¹ they have been used as quantum memories and states of light have been teleported into an ensemble.² They also find application in metrology, for instance in atomic magnetometry. Here we present recent progress of two experiments using almost the same setup consisting of two spin-polarized room-temperature atomic ensembles. In the first experiment, the two ensembles are used as a quantum memory for displaced states of light which are squeezed by 6.0 dB. In the second experiment, the ensembles are used as a sensor for radio-frequency magnetic fields. By using techniques from quantum information science, we propose a protocol for magnetic field measurements which is only limited by quantum spin-projection noise (PN) arising from the Heisenberg's uncertainty relation. We also present preliminary results of a magnetic field sensitivity of not too far from the projection noise limited sensitivity of our magnetometer.

We report recent progress in heralded pure-state single-photon sources based upon spontaneous nonlinear optical interactions – parametric down conversion in a bulk second-order nonlinear optical crystal and four-wave mixing in photonic-crystal fiber. Time-multiplexed photon-number resolving detection is discussed. A phase-sensitive photon-counting detector, with configurable positive operator-value measures, based upon time-multiplexed avalanche photodiodes is described. Characterization of quantum detectors using techniques similar to state and process tomography is reviewed. Experimental characterization of an avalanche photodiode is presented as an example.

Quantum nondemolition photon (QND) counting in a high Q cavity is performed by using circular Rydberg atoms. The atoms behave as clocks whose ticking rate is affected by light shifts induced by the cavity field. Measurning the atoms projects the field on non-classical states such as number states or Schrödinger cat states. We also use the QND measurement method for reconstructing the Wigner function of these states and to monitor their decoherence. These field manipulation methods can be applied to state preparation by quantum feedback and to demonstrate non locality with two fields located in separated cavities.

Frequency comb lasers in the infrared region of the spectrum have revolutionized many fields of physics. We demonstrate for the first time direct frequency comb spectroscopy at XUV wavelengths. Generation of an XUV comb is realized by amplification of two pulses from a frequency comb laser in a parametric amplifier, and subsequent high-harmonic generation to 51 nm (15^th harmonic). These XUV pulses, with a time separation between 5.4 and 10 ns, are then used to directly excite helium on the 1s^{2 1}S₀ - 1s5p ¹P₁ transition. The resulting Ramsey-like signal has up to 60% modulation contrast, indicating a high phase coherence of the generated XUV comb light.

We describe the generation of an ultrahigh-repetition-rate train of ultrashort pulses on the basis of an adiabatic Raman process. We also describe recent progress in studies toward the ultimate regime: realization of an ultrahigh-repetition-rate train of monocycle pulses with control of the absolute phase. We comment on the milestones expected in the near future in terms of the study of such novel light sources and the new field of optical science stimulated by their development.

Strong interactions between particles loaded into a periodic potential can lead to novel and surprising effects in the behaviour of a quantum many-body system. Here we discuss two examples: 1) Multi-orbital quantum phase diffusion for bosonic atoms loaded into a 3D optical lattice potential and 2) an anomalous expansion observed for a spin mixture of fermions as the attractive interactions between the particles are continuously increased.

Light scattering is used to extract information on the state of ultracold bosonic gases trapped in optical lattices. Different regimes of interactions are explored, ranging from weakly interacting 3D Bose-Einstein condensates to strongly interacting 1D gases in the crossover from superfluid to Mott-insulating states.

This article discusses two different approaches to study the physics of quantum gases. We load a two-component Fermi gas of potassium atoms into an optical lattice and realize the Fermi-Hubbard model. We probe the crossover from a metal to a Mott insulator by measuring the number of doubly occupied lattice sites. A Bose-Einstein condensate placed into an ultrahigh-finesse optical cavity provides a many-body system with global interactions. We investigate this system in a regime where the physics of cavity optomechanics is revealed.

Quantum degenerate ytterbium(Yb) gases in 3D optical lattices are studied for bosonic isotopes and mixtures of bosonic and fermionic isotopes. In 3D optical lattices, a quantum phase transition from a superfluid to a Mott insulating state is observed. In the deep Mott insulating regime, one-color photoassociation(PA) spectroscopy is performed to probe site occupancy. Bose-Fermi mixtures of Yb isotopes in 3D optical lattices are also studied using two different combinations of mixtures.

Recently, there has been important progress in the formation of ultracold polar molecules in the rovibrational ground state, thus opening intriguing perspectives for the investigation of strongly correlated quantum systems under the influence of long-range dipolar forces. After an brief introduction into the field of ultracold molecules, we will review our recent experiments on the formation of ultracold LiCs molecules in the absolute ground state X¹Σ⁺, v = 0, J = 0 via a single photo-association step starting from laser-cooled atoms.

The advent of a quantum degenerate gas of ground-state polar molecules opens the door to a wide range of scientific explorations. Novel molecular interactions, quantum-controlled chemical reactions, exotic phase transitions and strongly correlated states of matter are among a few prominent examples to be studied. We discuss recent experimental progresses at JILA, including the production of a high-density gas of ultracold KRb polar molecules in their absolute rovibrational ground state, coherent manipulations of the nuclear spin degrees of freedom, observations of barrier-less chemical reactions with the associated rates controlled by pure long-range, quantum mechanical effects, and control of inelastic and elastic collision cross sections via the tuning of the molecular dipole moment.

Recent years have seen tremendous progress in the field of cold and ultracold molecules. A central goal in the field is currently the realization of stable rovibronic ground-state molecular samples in the regime of quantum degeneracy, e.g. in the form of molecular Bose-Einstein condensates, molecular degenerate Fermi gases, or, when an optical lattice is present, molecular Mott-insulator phases. However, molecular samples are not readily cooled to the extremely low temperatures at which quantum degeneracy occurs. In particular, laser cooling, the 'workhorse' for the field of atomic quantum gases, is generally not applicable to molecular samples. Here we take an important step beyond previous work¹ and provide details on the realization of an ultracold quantum gas of ground-state dimer molecules trapped in an optical lattice as recently reported in Ref. 2. We demonstrate full control over all internal and external quantum degrees of freedom for the ground-state molecules by deterministically preparing the molecules in a single quantum state, i.e. in a specific hyperfine sublevel of the rovibronic ground state, while the molecules are trapped in the motional ground state of the individual lattice wells. We circumvent the problem of cooling by associating weakly-bound molecules out of a zero-temperature atomic Mott-insulator state and by transferring these to the absolute ground state in a four-photon STIRAP process. Our preparation procedure directly leads to a long-lived, lattice-trapped molecular many-body state, which we expect to form the platform for many of the envisioned future experiments with molecular quantum gases, e.g. on precision molecular spectroscopy, quantum information science, and dipolar quantum systems.

X-ray parametric down-conversion, one of the most fundamental nonlinear optical processes, is investigated quantitatively based on precision measurements. It is found that the nonlinear process is observed indirectly though a quantum mechanical interference known as the Fano effect. The second order nonlinear susceptibility is, for the first time, estimated for wide idler energies between vacuum ultraviolet and soft x-rays. The experimental results presented here give a firm basis to explore the frontier of x-ray nonlinear optics, which will be fully accessible with the forthcoming x-ray free-electron lasers.

An overview of the new field of Gas in Scattering Media Absorption Spectroscopy (GASMAS) is presented. The GASMAS technique combines narrow-band diode-laser spectroscopy with optical propagation in diffuse media. Whereas solids and liquids have broad absorption features, free gas in pores and cavities in the material is characterized by sharp spectral signatures. These are typically 10,000 times sharper than those of the host material. Many applications in materials science, food packaging, pharmaceutics and medicine have been demonstrated. Molecular oxygen and water vapor have been studied around 760 and 935 nm, respectively. Liquid water, an important constituent in many natural materials, such as tissue, has a low absorption at such wavelengths, allowing propagation. Polystyrene foam, wood, fruits, food-stuffs, pharmaceutical tablets, and human sinus cavities have been studied, demonstrating new possibilities for characterization and diagnostics. Transport of gas in porous media can readily be studied by first immersing the material in, e.g., pure nitrogen gas, and then observing the rate at which normal air, containing oxygen, reinvades the material. The conductance of the human sinus connective passages can be measured in this way by flushing the nasal cavity with nitrogen, while breathing normally through the mouth. A clinical study comprising 40 patients has been concluded.

Laser spectroscopy on high quality ion beams allows to explore physical properties of atoms and molecules and to test fundamental theories. An experiment to measure time dilation shows the potential.

I will first review our works in achieving planar random microcavity laser, then report our recent works on generating single frequency lasing from coupled microcavities, I will also show the possibility of using the single frequency laser source to achieve ultrasensitive optical bio-sensing.

The following sections are included:

• AUTHOR INDEX