Laser Spectroscopy

Preface.

Programme Committee.

The 2005 Nobel Prize for Physics.

The convenient approximation of a real laser field by a Coherent State is again a relevant topic of interest, as laser spectroscopy scenarios are being developed in which remarkably long atomic lifetimes and extended interaction times (~100 s) can be enjoyed. Years ago, appropriate locking techniques were shown to allow precise locking of a laser field to a cavity, even in the milliHz domain, but lab vibrations modulated the cavity length and so the obtained optical frequency. Methods such as mechanical isolation (on a heroic scale) or active anti-vibration approaches are sufficiently productive such that, by now several groups have developed visible optical sources with ~Hz linewidths. Still, linewidths in the 100 milliHz domain have seemed very challenging — all the margins have been used up. We discuss mounting systems for an optical reference cavity, particularly an improved one based on implementing vertical symmetry, which provides dramatic reduction in the vibration sensitivity and can yield sub-Hz linewidths on an ordinary optical table in an ordinary lab. Interesting and commanding new issues — such as temporally-dependent spatial structure of the EO-modulated probe beam, and thermally-generated mechanical position noise — are found to dominate the laser phase errors in the sub-Hz linewidth domain. The theoretical scaling — and the spectral character — of this thermal noise motion of the cavity mirror surfaces have been studied and confirmed experimentally, showing an ~1 × 10^-16 m/√(Hz) thermal noise amplitude at 1 Hz, with a 1/√f amplitude spectral density, with f being the Fourier frequency of this noise process. For effective temperature stabilization, multi-point thermal control and dual thermal shells provide stable operation near the ULE thermally-stationary point. Spectral filtering in the optical and vacuum paths is critically important to prevent ambient thermal radiation from entering the inner shell. The observed frequency drift-rate of ~0.05 Hz/s is not yet ideally stable, but it appears possible to compensate drift accurately enough to allow 1 radian coherence times to approach ~100 s — if other problems such as the thermal noise can be adequately suppressed. Recent JILA spectra of lattice-trapped cold Sr atoms show an excellent prospect for ultrahigh resolution spectroscopy and highly stable optical atomic clocks and make us anxious to perfect improved phase-stable laser sources for the ¹S₀ – ³P₀ doubly-forbidden transition at 698 nm. These laser developments are aided by optical comb techniques, allowing useful phase comparison of several prototype stable laser sources, despite their various different wavelengths.

Optical spectroscopy and frequency metrology at the highest level of precision and resolution are being greatly facilitated by the use of ultracold atoms and phase stabilized light in the form of both cw and ultrashort pulses. It is now possible to pursue simultaneously coherent control of quantum dynamics in the time domain and high precision measurements of global atomic structure in the frequency domain. These coherent light-based precision measurement capabilities may be extended to the XUV spectral region, where new possibilities and challenges lie for precise tests of fundamental physical principles.

Experiments aimed at searching for gravitational waves from astrophysical sources have been under development for the last 40 years, but only now are sensitivities reaching the level where there is a real possibility of detections being made within the next few years by the long baseline detectors LIGO, VIRGO, GEO 600 and TAMA 300. There are a number of limitations to performance of these detectors and to their proposed upgrades, one of the most significant being thermal noise associated with mechanical losses in the mirror coatings.

The simple hydrogen atom has inspired many advances in quantum electrodynamics and experimental techniques. As the latest spin off, femtosecond optical frequency comb synthesizers have revolutionized the way optical frequencies are measured, and provide a reliable clock mechanism for optical atomic clocks. Precision spectroscopy of the hydrogen 1S-2S two-photon resonance has reached an accuracy of 1.4 parts in 10¹⁴, and considerable future improvements are envisioned. Such laboratory experiments are setting new limits for possible slow variations of the electromagnetic and strong interaction. The frequency comb technique has recently been extended into the extreme ultraviolet and may allow similar measurements on hydrogen like ions that can be held in a trap.

Quantum electrodynamic theory (QED) of the simplest bound-three-body system is now stringently tested by precise spectroscopic frequency measurements in Helium. Alternatively, comparison between He measurements and theory could be used for accurate determination of some fundamental quantities, as for example, the fine structure constant α, and the differences of nuclear charge radii between two ³He and ⁴He. In the following, we review our precise spectroscopic measurements on the 1083 nm Helium transition, connecting the triplet 2S and 2P states. Frequency differences among the 1083 nm measured frequencies give precise values of the fine structure (FS) and hyperfine structure (HFS) splittings of the 2³P level in ⁴He and ³He respectively, and of the isotope shift (IS) between these two isotopes. Implications of these results in the α determination as well as for nuclear charge radii and structure are discussed.

Antiprotonic helium atom is a metastable (τ ~ 3µs) neutral three-body Coulomb system consisting of an antiproton, a helium nucleus, and an electron. It was serendipitously discovered that about 3% of antiprotons stopped in a low-temperature helium gas target automatically form such metastable atoms. Precision laser spectroscopy of is possible by inducing a laser-resonant transition from a metastable antiproton orbit to a neighboring short-lived orbit and by detecting antiproton annihilation events. By comparing the experimental results with the state-of-the-art three-body QED calculations, antiproton to proton mass and charge comparison has been done to a precision of 10 parts per billion, which is currently the most precise test of the CPT symmetry (matter-antimatter symmetry) for baryons.

In this paper, we show that Bloch oscillations of ultracold atoms in an optical lattice is a strong tool to measure accurately some physical constants. In particular we describe two experimental approaches using a pure and an accelerated vertical standing wave to perform respectively a determination of the local acceleration of gravity g to ppm precision and of the fine structure constant α below 10 ppb.

We present the results of the discovery of optically pumped Astrophysical Lasers (APL) operating on the quantum transitions of the FeII ion and OI atom in the range 0.8-1.7 μm in gas condensations in the vicinity of Eta Carinae – most luminous and massive variable blue star of our Galaxy.

High-resolution metrology at wavelengths shorter than ultraviolet is in general hampered by a limited availability of appropriate laser sources. It is demonstrated that this limitation can be overcome by quantum-interference metrology with frequency up-converted ultrafast laser pulses. The required phase controlled pulses are obtained by amplifying the output of a frequency comb laser. Efficient harmonic up-conversion in crystals and gases is then possible because of the high peak power of the pulses, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft-x-ray spectral regions. A proof-of-principle experiment was performed in krypton on a two-photon transition that was excited with 2 × 212.55 nm light. The accuracy of the absolute transition frequency and isotope shifts were improved by more than an order of magnitude compared to previous experiments which used nanosecond timescale pulsed lasers.

We demonstrate the possibility to obtain an accurate value of the Boltzmann constant from measurements of the Doppler width of absorption lines of a molecular gas. An experiment using an ammonia line probed by a CO₂ laser spectrometer at 29 THz is described in detail.

A precise measurement of helium 2³P fine structure was carried out in a discharge cell using Doppler-free laser spectroscopy. It is the only known experiment to directly measure all three fine structure intervals at a 1 kHz level of accuracy. The 2³P₁ − 2³P₂ interval value agrees with other experiments but disagrees with theoretical predictions of two-electron QED. When this disagreement is resolved, the 2³P₀ − 2³P₁ interval measurement reported here will allow a determination of the fine structure constant to 14 parts in 10⁹, surpassing the precision of the well known QED-independent quantum Hall effect and Josephson effect determinations. The discharge cell is shown to be advantageous in the study and correction of systematic frequency shifts related to light pressure, and the use of the cell ensures that the possible systematic errors are substantially different from those reported in other experiments.

Recent research has led to the development of techniques that can produce large modifications in the velocity of propagation of light pulses. We have been especially interested in the development of techniques that can lead to ultraslow or ultrafast light propagation in room temperature solids, because such materials lend themselves to use in practical applications. Recent progress is reviewed. We also address the issue of the physical interpretation of negative group velocities.

We present our progress towards a new measurement of the electron electric dipole moment using a beam of YbF molecules. Data are currently being taken with a sensitivity of .

Bose-Einstein condensation was first created in a gas on June 5, 1995, thus it was almost exactly ten years old at the date of this conference. In fact, the first public appearance of BEC was made at the ICOLS meeting ten years ago when Eric Cornell presented our results. In this review, I will try to give a brief overview of what has been happening these last ten years. I will concentrate on a discussion of the experiments, but one reason for the rapid excitement and growth in BEC has been the close coupling between experiment and theory. A big reason for this coupling is because the interactions in the BEC have a uniquely simple form for a many body quantum system, and hence, are very tractable theoretically. The implications of this are that the comparison of theory and experiment provides a very rigorous test of theoretical ideas, and the theory has good predictive capabilities that can guide experiments in interesting new directions. This paper will be divided into two sections. The first will provide a very brief historical background on BEC and how we got there, and the second part will look at some examples of the most notable physics that has been done with BECs in the past ten years.

We have observed Bose-Einstein condensation of chromium atoms [1], whose large magnetic dipole moment is unique among all species that have been Bose-condensed so far. The arising magnetic forces are of anisotropic and long-range character and therefore introduce a novel type of interaction in the physics of ultra-cold quantum gases. In addition, it is expected that the character of the interaction present in a chromium BEC can be varied from mainly contact to purely dipolar utilizing one of the recently observed Feshbach resonances in ⁵²Cr-collisions.

We report our research on disordered complex systems using cold gases and trapped ions and address the possibility of using complex systems for quantum information processing. Two simple paradigmatic models of disordered complex systems are here revisited. The first one corresponds to a short range disordered Ising Hamiltonian (spin glasses) which can be implemented with a bose-fermi (bose-bose) mixture in a disordered optical lattice. The second model we address here is a long range disordered Hamiltonian characteristic of neural networks (Hopfield model) which can be implemented in a chain of trapped ions with appropriately designed interactions.

Experiments with ultracold atoms present an outstanding opportunity for implementing novel tests of theories of strongly correlated fermions. We present three new theoretical methods to treat these systems. Luttinger liquid theory and the exact one-dimensional solutions let us calculate collective mode frequencies at the metal-insulator transition in a lattice. A new Gaussian phase-space method for fermion systems can be used to simulate finite temperature atomic correlations. Finally, we introduce an approximate diagrammatic technique which correctly includes molecule-molecule interactions, and gives an accurate, quantitative theory of the BEC-BCS crossover regime.

Recently discovered light forces in high-finesse microcavities are ideal to capture single atoms, cool them to ultralow temperatures and trap them for long time intervals. Individual atoms at rest and strongly coupled to a cavity are interesting in quantum information science. Cavity cooling might also be useful to produce cold samples of particles like molecules which have no closed cycling transition for laser cooling.

We control and manipulate strings of neutral atoms, trapped inside a standing wave dipole trap. We show that such a string realizes a quantum register, where coherent information is encoded in the atomic hyperfine states using microwave transitions. Furthermore, using high resolution imaging optics, we measure the absolute and relative positions of the atoms with a sub-optical wavelength resolution. The overall position of the string is then actively controlled with an optical conveyor belt. Finally, by extracting and reinserting atoms at predetermined positions with a second, perpendicular dipole trap, we aim to control the interatomic distances, prepare equidistant strings, and rearrange their order.

We present a stand-alone interference method for the determination of the s- and d-wave scattering amplitudes in a quantum gas. Colliding two ultracold atomic clouds we observe the halo of scattered atoms in the rest frame of the collisional center of mass by absorption imaging. The clouds are accelerated up to energies at which the scattering pattern shows the interference between the s- (l = 0) and d- (l = 2) partial waves. With computerized tomography we transform the images to obtain the angular distribution, which is directly proportional to the differential cross section. This allows us to measure the asymptotic phase shifts of the s- and d-wave scattering channels. The method does not require knowledge of the atomic density. It allows us to infer accurate values for the s- and d-wave scattering amplitudes from the zero-energy limit up to the first Ramsauer-Townsend minimum using only the Van der Waals C₆ coefficient as theoretical input. For the ⁸⁷Rb triplet potential, the method yields an accuracy of 6%.

We propose a novel kind of electric field spectroscopy of ultracold, polar molecules. Scattering cross sections for these molecules will exhibit a quasi-regular series of resonance peaks as a function of applied electric field. In this contribution, we derive a simple approximate formula for F(n), the electric field F at which the nth resonance appears.

We present recent work resulting in the production of ultracold, polar RbCs molecules in their vibronic ground state. The production process consists of several steps: photoassociation of laser-cooled atoms; radiative stabilization of the resulting molecules; identification of the resulting population distribution; and laser-stimulated state transfer to the vibrational ground state. We discuss the properties of the resulting sample of X¹Σ⁺(ν = 0) molecules, with a view to the future directions of these experiments.

We study the properties of an optically-trapped, strongly-interacting Fermi gas of ⁶Li atoms, near the center of a broad Feshbach resonance. We observe a transition in the heat capacity, by precisely adding energy to the gas and measuring an empirical temperature that is based on the spatial profiles of the cloud. Recent theory, using a pseudogap formalism, enables the first temperature calibration, and interprets the transition as the onset of superfluidity in a strongly-attractive Fermi gas. We also measure the empirical temperature dependence of the radial breathing mode in the same regime. The frequency remains near the hydrodynamic value, while the damping rate reveals a clear transition in behavior near the predicted superfluid transition temperature. We consider quantum viscosity as a cause of damping and show that the predicted magnitude is consistent with observations, but the predicted scaling in the total atom number is not.

We observe coherent, purely collisionally driven spin dynamics of atom pairs confined at the sites of a deep optical lattice. Spin changing collisions induce coherent oscillations between two-particle Zeeman states with equal magnetization. This mechanism could be a way to create robust entangled atom pairs in an optical lattice with high efficiency. Moreover, measurement of the oscillation frequency allows for precise determination of the coupling parameters of the spin-changing collisional interaction, and thus constitute a stringent test of model potentials to describe the collision between two Rb atoms.

We observe the suppression of the 1D transport of an interacting elongated Bose-Einstein condensate in a random potential with a standard deviation small compared to the typical energy per atom, dominated by the interaction energy. Numerical solutions of the Gross-Pitaevskii equation reproduce well our observations. We propose a scenario for disorder-induced trapping of the condensate in agreement with our observations¹.

A continuous Raman output coupler for an atom laser has been the subject of many theoretical and numerical studies¹. However, prior to this work only a pulsed Raman output coupler has been demonstrated for magnetically trapped atoms². Here, we produce a continuous Raman atom laser and discuss, with reference to a simple model, how this system is superior to a radio frequency (RF) output coupler for future applications of the atom laser.

We review our recent experiments on interaction-induced effects in a gas of Rydberg atoms excited from a laser-cooled rubidium cloud. In density-dependent measurements we see a broadening of excitation lines and an on-resonance suppression of excitation, which can be explained by the van-der-Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii. Rapid ionization of the highly excited, initially stationary atoms is also observed. The initial ionization process is attributed to interaction-induced collisions of cold Rydberg atoms and subsequent Penning ionization. We present 3D degenerate Raman sideband cooling as a tool to reach ultralow temperatures and to provide structuring of the cold atom cloud for future studies on ultracold Rydberg gases.

We present a permanent magnetic film atom chip based on perpendicularly magnetized TbGdFeCo films. This chip routinely produces a Bose-Einstein condensate (BEC) of 10^{5 87}Rb atoms using the magnetic film potential. Fragmentation observed near the film surface provides unique opportunities to study BEC in a disordered potential. We show this potential can be used to simultaneously produce multiple spatially separated condensates. We exploit part of this potential to realize a time-dependent double well system for splitting a condensate.

We report on the realization of a quantum degenerate atomic Fermi gas in an optical lattice. Fermi surfaces of noninteracting fermions are studied in a three-dimensional lattice. Using a Feshbach resonance, we observe a coupling of the Bloch bands in the strongly interacting regime.

The Pauli exclusion principle inhibits s-wave collisions between fermionic ⁶Li atoms and renders thermalization and evaporative cooling at low temperatures inefficient. A way around this problem is sympathetic cooling by a different actively cooled atomic species. With this technique, using bosonic ⁸⁷Rb as cooling agent, we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for Bose-Einstein condensation and the Li cloud colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length .

We have observed two-particle correlations in a cloud of metastable helium atoms released from a magnetic trap. We use a microchannel plate detector to achieve single atom detection. For clouds above the Bose-Einstein condensation transition temperature, we observe the Hanbury Brown Twiss effect, an enhanced pair detection probability for separations smaller than the atomic coherence length. Below the BEC transition the enhancement vanishes. We observe the effect in three-dimensions and study its variation with the cloud size.

We report on a method of light–shift engineering where an additional laser is used to tune the energy of an excited atomic state. We show that the technique can be used to enhance the loading of a deep optical lattice or selectively load specific sites in a lattice.

The 5s²S_1/2 – 4d²D_5/2 transition at 674 nm in a single laser-cooled trapped ⁸⁸Sr⁺ ion serves as the reference for a highly stable and accurate optical frequency standard. A detailed study of the electric quadrupole shift of this reference transition has been carried out, and two techniques for nulling out the quadrupole shift have been experimentally demonstrated. The absolute frequency of the transition has been measured relative to the NPL caesium fountain standard, with an uncertainty of 1.5 Hz or 3.4 parts in 10¹⁵. Improvements to the experimental arrangement are underway, and are expected to lead to a ⁸⁸Sr⁺ optical frequency standard with stability and reproducibility exceeding that of the primary caesium standard.

Employing engineered electric fields, we demonstrate two coherence-preserving atom traps for precision measurements with neutral atoms. (1) A surface Stark trap has been developed to manipulate scalar atoms near solid surfaces. (2) An optical lattice clock has been realized for ⁸⁷Sr atoms and its absolute frequency was measured.

We describe recent research at NIST directed towards the development of microfabricated atomic clocks and magnetometers based on coherent population trapping spectroscopy. Clock physics packages based on the D₁ transition in ⁸⁷Rb achieve a fractional frequency instability below 4 × 10^-11/τ^1/2 [Knappe et al., Appl. Phys. Lett. 85, 1460 (2004)], while the sensitivity of the magnetic sensor reached 50 pT Hz ^-1/2 at 10 Hz [Schwindt et al. Appl. Phys, Lett. 85, 6509 (2004)]. With volumes around 10 mm³ and power consumptions below 150 mW they display an improvement over the previous state of the art by a factor of 100 in volume and a factor of 10 in power dissipation.

The change from a zero transition to the maximum amplitude of the electric field of visible light lasts shorter than one femtosecond (1 fs = 10^-15 s). By precisely controlling the hyperfast electric field oscillations in a short laser pulse we developed a measuring apparatus – the Atomic Transient Recorder - like an ultrafast stopwatch. This apparatus is capable of measuring the duration of atomic processes and electron dynamics with an accuracy of less than 100 attoseconds (1 as = 10^-18 s), which is the typical duration of electronic processes (transients) deep inside atoms. A 250-attosecond X-ray pulse initiates the atomic process to be measured and the attosecond stopwatch at the same time. For the first time it is now possible with this new measuring method to observe ultrafast processes in the electron shell of atoms.

Photons have a rich structure associated with their continuous degrees of freedom, the transverse wavevector and frequency. This modal structure can play an important role in quantum information processing based on photons. In particular, information may be coded into any of the degrees of freedom, and this means that photons may represent not only qubits, but also qudits, or qunats - where the continuous degrees of freedom are involved. Coding into quantum correlations in these degrees may be usefully employed to transmit more than one bit per photon in a secure communications link. However, it may also be detrimental: for example, it can hinder the preparation of pure states via conditional detection, and thus compromise the efficacy of quantum information processing schemes based on interference. We illustrate some general criteria that are useful for source design for QIP.

Circular Rydberg states are sensitive probes of a millimeter-wave field stored in a superconducting cavity. We show here that they can be used for a direct determination of two quasi-probability distributions in phase space, the Husimi-Q and the Wigner W functions, providing a detailed insight into the cavity mode quantum state. These experiments open the way to the study of non-local Schrödinger cat states, shared by two cavities.

Entanglement, its generation, manipulation, measurement and fundamental understanding is at the very heart of quantum mechanics. We here report on the creation and characterization of entangled states of up to 8 trapped ions, the investigation of long-lived two-ion Bell-states and on experiments towards entangling ions and photons.

We briefly discuss recent experiments on quantum information processing using trapped ions at NIST. A central theme of this work has been to increase our capabilities in terms of quantum computing protocols, but we have also applied the same concepts to improved metrology, particularly in the area of frequency standards and atomic clocks. Such work may eventually shed light on more fundamental issues, such as the quantum measurement problem.

Coherent transients result from the excitation of a two-level system by an ultrashort chirped pulse A sequence of two measurements provides a direct access to the excited state wave function from which the pulse electric field can be retrieved.

We present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a π pulse, we use this coherent control to make a high quality triggered source of single photons. We obtain an average single photon rate of 9600s^-1 at the detector. Measurements of the second-order temporal correlation function show almost perfect antibunching at zero delay. In addition, we present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sub-levels.

A large-scale ion trap quantum computer will require low-noise entanglement schemes and methods for networking ions between different regions. We report work on both fronts, with the entanglement of two trapped cadmium ions following a phase-insensitive Molmer-Sorensen quantum gate, the entanglement between a single ion and a single photon, and the development of advanced ion traps at the micrometer scale, including the first ion trap integrated on a semiconductor chip. We additionally report progress on the interaction of ultrafast resonant laser pulses with cold trapped ions. This includes fast Rabi oscillations on optical S-P transitions and broadband laser cooling, where the pulse laser bandwidth is much larger than the atomic linewidth. With these fast laser pulses, we also have developed a new method for precision measurement of excited state lifetimes.

Author Index.