This chapter describes the National Bureau of Standards’ Observation of Atoms Laser-Cooled Below the Doppler Limit , relating the context in which the publication appeared, its impact on science, technology, and the general public, and brief details about the lives and work of the author. By carefully choosing the frequency of the laser, it appeared possible to cause the atoms to emit light at a slightly higher frequency (energy) than they absorbed, carrying away the thermal energy of the atom. This frequency difference derived from the Doppler shift due to the motion of the atoms. The sub-Doppler laser-cooling temperature for cesium can be 100 times lower than the Doppler limit. Using an idea from 1954, well ahead of its time but now made possible with sub-Doppler laser cooling, the cesium atoms form an atomic fountain, undergoing a one meter high parabolic trajectory in the earth's gravitational field with a resulting hundred-fold increase in the observation time.
This chapter describes the National Bureau of Standards’ Observation of Atoms Laser-Cooled Below the Doppler Limit , relating the context in which the publication appeared, its impact on science, technology, and the general public, and brief details about the lives and work of the author. By carefully choosing the frequency of the laser, it appeared possible to cause the atoms to emit light at a slightly higher frequency (energy) than they absorbed, carrying away the thermal energy of the atom. This frequency difference derived from the Doppler shift due to the motion of the atoms. The sub-Doppler laser-cooling temperature for cesium can be 100 times lower than the Doppler limit. Using an idea from 1954, well ahead of its time but now made possible with sub-Doppler laser cooling, the cesium atoms form an atomic fountain, undergoing a one meter high parabolic trajectory in the earth's gravitational field with a resulting hundred-fold increase in the observation time.
Experimental techniques of laser cooling and trapping, along with other cooling techniques, have produced gaseous samples of atoms so cold that they are, for many practical purposes, in the quantum ground state of their centre-of-mass motion. Such low velocities have virtually eliminated effects such as Doppler shifts, relativistic time dilation and observation-time broadening that previously limited the performance of atomic frequency standards. Today, the best laser-cooled, caesium atomic fountain, microwave frequency standards realize the International System of Units (SI) definition of the second to a relative accuracy of ≈3×10(-16). Optical frequency standards, which do not realize the SI second, have even better performance: cold neutral atoms trapped in optical lattices now yield relative systematic uncertainties of ≈1×10(-16), whereas cold-trapped ions have systematic uncertainties of 9×10(-18). We will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.
In sub-Doppler laser cooling of atoms, the lowest temperatures are known to occur at low (but not too low) excitation, and the temperature increases roughly linearly with laser intensity. However, under conditions for which the lowest temperatures are obtained, the small velocity capture range and low optical pumping rate limit the number of atoms that can be collected into the cold sample. In this study, we present measurements for laser cooling of Rb and metastable He atoms for two counterpropagating laser beams with orthogonal linear polarization (lin\ensuremath{\perp}lin) over a wide range of saturation parameters and laser detunings, together with results of semiclassical (Fokker-Planck) and quantum density matrix calculations. We find that at higher laser intensity, a larger number of atoms can be collected into a final velocity distribution that is significantly narrower than that given by a linear extrapolation vs the square root of laser intensity. Two cooling mechanisms are at work: the sub-Doppler Sisyphus mechanism and also Doppler cooling. Under certain conditions that we discuss, Doppler cooling aids Sisyphus cooling by collecting high-velocity atoms beyond the Sisyphus capture range. He∗ presents a rather unusual situation in that the minimum average kinetic energy (Eav) for purely Sisyphus cooling is comparable to the minimum value of Eav with Doppler cooling alone.
Over the past several decades, the field of atomic physics has seen outstanding developments, which have had an important impact on the development of atomic frequency standards. This is mainly due to improvements in laser technology, which has had a direct effect on the technique of optical pumping and made possible the cooling and trapping of ensembles of atoms to sub-Kelvin (sub-K) temperatures. The laser as such was developed in the 1960s. In early stages of its development, however, the use of the laser in the study of interactions between electromagnetic radiation and atoms was limited by the very restricted number of wavelengths available with the existing lasers and the rather narrow tunability of the instruments developed. The so-called dye laser extended the range of possible studies, but such an instrument is rather cumbersome and its size and complexity restricted its use to limited laboratory applications. The advent of room temperature solid-state diode lasers, which could be fabricated to emit wavelengths in resonance with many transitions in alkali atoms such as Cs and Rb, for example, as well as in ions such as Sr+ and Yb+, among many others, changed the whole picture. The power emitted by such lasers may be several tens of milliwatts. The spectrum of the radiation emitted is relatively narrow and the frequency can be adjusted over a relatively broad range by means of doping the substrate with selected elements at the time of fabrication. Fine frequency tuning of the device can also be done by means of the temperature of the substrate and its driving current. This property allows the exciting of transitions in selected atoms from one single ground-state hyperfine level to an excited state. This makes possible nearly complete inversion of population in ground states through optical pumping. Furthermore, as will be seen below, the exchange of momentum between a photon and an atom makes possible alteration of atomic velocities leading to so-called laser cooling of atoms. These two possibilities by themselves have totally altered the landscape of research in the field of atomic frequency standards leading to new realizations in both the microwave and the optical ranges. We will review selected subjects and recent advances in atomic physics that have had a specific impact on the development of atomic frequency standards. For a broader overview of such advances in atomic physics, the reader is referred to the excellent review on the subject by Cohen-Tannoudji and Guéry-Odelin (2011).
The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P
Over the past several decades, the field of atomic physics has seen outstanding developments, which have had an important impact on the development of atomic frequency standards. This is mainly due to improvements in laser technology, which has had a direct effect on the technique of optical pumping and made possible the cooling and trapping of ensembles of atoms to sub-Kelvin (sub-K) temperatures. The laser as such was developed in the 1960s. In early stages of its development, however, the use of the laser in the study of interactions between electromagnetic radiation and atoms was limited by the very restricted number of wavelengths available with the existing lasers and the rather narrow tunability of the instruments developed. The so-called dye laser extended the range of possible studies, but such an instrument is rather cumbersome and its size and complexity restricted its use to limited laboratory applications. The advent of room temperature solid-state diode lasers, which could be fabricated to emit wavelengths in resonance with many transitions in alkali atoms such as Cs and Rb, for example, as well as in ions such as Sr + and Yb + , among many others, changed the whole picture. The power emitted by such lasers may be several tens of milliwatts. The spectrum of the radiation emitted is relatively narrow and the frequency can be adjusted over a relatively broad range by means of doping the substrate with selected elements at the time of fabrication. Fine frequency tuning of the device can also be done by means of the temperature of the substrate and its driving current. This property allows the exciting of transitions in selected atoms from one single ground-state hyperfine level to an excited state. This makes possible nearly complete inversion of population in ground states through optical pumping. Furthermore, as will be seen below, the exchange of momentum between a photon and an atom makes possible alteration of atomic velocities leading to so-called laser cooling of atoms. These two possibilities by themselves have totally altered the landscape of research in the field of atomic frequency standards leading to new realizations in both the microwave and the optical ranges. We will review selected subjects and recent advances in atomic physics that have had a specific impact on the development of atomic frequency standards. For a broader overview of such advances in atomic physics, the reader is referred to the excellent review on the subject by Cohen-Tannoudji and Guéry-Odelin (2011) .
The high velocity of free atoms associated with the thermal motion, together with the velocity distribution of atoms has imposed the ultimate limitation on the precision of ultrahigh resolution spectroscopy. A sample consisting of low velocity atoms would provide a substantial improvement in spectroscopy resolution. To overcome the problem of thermal motion, atomic physicists have pursued two goals; first, the reduction of the thermal motion (cooling); and second, the confinement of the atoms by means of electromagnetic fields (trapping). Cooling carried sufficiently far, eliminates the motional problems, whereas trapping allows for long observation times. In this work the laser cooling and trapping of an argon atomic beam will be discussed. The experiments involve a time-of-flight spectroscopy on metastable argon atoms. Laser deceleration or cooling of atoms is achieved by counter propagating a photon against an atomic beam of metastable atoms. The solution to the Doppler shift problem is achieved using spatially varying magnetic field along the beam path to Zeeman shift the atomic resonance frequency so as to keep the atoms in resonance with a fixed frequency cooling laser. For trapping experiments a Magnetooptical trap (MOT) will be used. The MOT is formed by three pairs of counter-propagating laser beams with mutual opposite circular polarization and a frequency tuned slightly below the center of the atomic resonance and superimposed on a magnetic quadrupole field.