Claude Cohen-Tannoudji - Main Scientific Contributions#

Optical pumping
  • Derivation of the master equation describing the evolution of the atomic density matrix during the optical pumping cycle and considering for the first time the role of the off-diagonal elements of the density matrix.
  • Prediction from such a theory of new physical effects (Light-shifts, Conservation of coherence during the optical cycle...) and experimental demonstration of these effects.

Light shifts
  • First prediction that atomic energy levels are shifted by a quasiresonant light irradiation.
  • First experimental demonstration of the existence of light shifts.
  • Removal of the Zeeman degeneracy in zero magnetic field by light shifts and demonstration of the equivalence between light shifts and fictitious magnetic and electric fields.

Development of new optical detection methods
  • General theory of optical detection signals.
  • Demonstration of a new sensitive method using anomalous dispersion and transverse Faraday rotation.
  • Application to nuclear relaxation studies.

Ultra sensitive magnetometers using level crossing resonances
  • Prediction and demonstration of the existence of very narrow level crossing resonances in atomic ground states.
  • Detection of very weak magnetic fields, on the order of 3.10- 10 Gauss, by these resonances and application to the detection of the static magnetic field produced at a macroscopic distance by a gaseous sample of polarized nuclei.

Development of the dressed-atom approach
  • Quantizing the RF field used in magnetic resonance experiments allows one to consider the atom coupled to RF photons as a whole system described by a time independent hamiltonian, with true energy levels (dressed levels). Such an approach provides a simple interpretation of various physical effects (magnetic resonance, multiphoton transitions, parametric resonances, coherence resonances, frequency modulation...) in terms of level crossings and level anticrossings appearing in the dressed atom energy diagram.
  • Prediction and experimental observation of several new physical effects suggested by such a nonperturbative and global approach, in particular the fact that atomic magnetic moments can be modified and even cancelled by a non resonant RF irradiation.

Optical pumping with lasers
  • Theory of optical pumping with monochromatic coherent light sources.
  • Theory of the Hanle effect with monochromatic laser light. Description of the effect of phase fluctuations of the laser light on the fluorescence spectrum.
  • In connection with this work, development of a simple model for describing a discrete state coupled to a continuum with a finite width. Demonstration by simple graphic constructions of the possibility of a continuous transition between two extreme regimes, the Weisskopf-Wigner exponential decay and the Rabi oscillation.

Resonance fluorescence in intense resonant laser beams
  • Extension of the dressed atom approach to the optical domain and description of the effect of spontaneous emission.
  • Simple interpretation of the resonance fluorescence triplet in terms of spontaneous transitions between dressed states and extension to multilevel atoms.
  • Theoretical treatment of spontaneous Raman effect in intense laser fields.
  • Dressed atom description of collisional redistribution.

Doppler free spectroscopy
  • Prediction and demonstration of a new method of laser spectroscopy using velocity dependent light shifts for compensating the Doppler effect.
  • Demonstration of the possibility of observing optical Ramsey fringes with Doppler free two photon transitions.

Photon antibunching - Photon correlations - Quantum jumps
  • Prediction of photon antibunching in single atom resonance fluorescence. Interpretation of the effect in terms of quantum jumps.
  • Interpretation of photon correlations in terms of a radiative cascade of the dressed atom. Prediction by this approach of time correlations between the photons emitted in the two sidebands of the resonance fluorescence triplet and experimental demonstration of this effect.
  • Simple theoretical treatment of the intermittent fluorescence observable on a single trapped ion. Detection of quantum jumps by Dehmelt’s shelving method.
  • Interpretation,in terms of quantum jumps, of the physical mechanisms responsible for amplification without inversion.

Simple physical pictures for radiative processes
  • Derivation of an effective hamiltonian describing the effect of a high frequency irradiation on the slow motion of a weakly bound electron. Prediction of an energy shift of the Rydberg states.
  • By comparison with radiative corrections, identification of the respective contributions of vacuum fluctuations and radiation reaction to these corrections.
  • Interpretation of the positive sign of the spin anomaly g-2 as being due to the fact that the cyclotron motion of the electron charge is slowed down by radiation reaction more efficiently than the Larmor precession of the spin magnetic moment.
  • General description of dissipation and fluctuations in radiative processes using an Heisenberg equations approach and statistical functions such as symmetric correlation functions and linear response functions.

Radiative forces - Laser cooling and trapping
  • Unified treatment for the deflection of an atomic beam by a quasiresonant laser standing wave, showing the continuous transition between the diffractive regime for short interaction times and the diffusive regime for long interaction times. Prediction of a new rainbow effect in the deflection profile.
  • Derivation of a Fokker-Planck equation describing atomic motion in laser light where, not only the diffusion coefficient, but also the friction coefficient are expressed in terms of correlation functions of the radiative force operator.
  • Proposal of stable optical traps for neutral atoms, with alternating cooling and trapping phases. All laser traps achieved so far use such a scheme.
  • Proposal of a new + - - laser configuration for laser cooling and trapping.
  • Dressed atom interpretation of the mean value and of the fluctuations of dipole forces in terms of dressed state energy gradients and spontaneous transitions between these states.
  • Proposal and experimental demonstration of a new type of laser cooling using dipole forces. This new scheme is called now ”Sisyphus cooling” because the atom is always climbing potential hills in the dressed atom energy diagram. Extension of this scheme to trapped ions.
  • Experimental demonstration of the possibility of channeling atoms in the nodes or in the antinodes of a laser standing wave.
  • First interpretation of the new cooling mechanisms allowing one to beat the Doppler limit of laser cooling in terms of optical pumping, light shifts and polarization gradients (56,58,59,60). Introdution of a new type of Sisyphus cooling due to correlations between the spatial modulations of light shifts and the spatial modulations of optical pumping rates. Applying this new cooling scheme to Cesium atoms, the Paris group has obtained very low kinetic 3-D temperatures (2 microkelvin).
  • Proposal and demonstration of a new cooling scheme using velocity selective coherent population trapping and allowing one to beat the limit associated with the recoil kinetic energy of an atom absorbing or emitting a single photon. Observation by this method of a subrecoil transverse temperature of 2 microkelvin on a beam of metastable Helium atoms. Starting from Helium atoms trapped in a magneto-optical trap, it has been possible recently to increase the interaction time by one order of magnitude and to measure, at one, two and three dimensions, temperatures of the order of 200 nanokelvin, about 20 times smaller than the recoil limit. Coherent manipulation of atomic wave packets by adiabatic transfer.
  • Mechanical detection of the Hanle effect.
  • Analogy between photon scattering and a quantum measurement process destroying atomic spatial coherences. Monte-Carlo simulation of the quantum evolution.
  • Demonstration of the existence of anomalous diffusion processes in subrecoil laser cooling. New statistical analysis of this cooling mechanism in terms of “Lévy flights”,providing precise analytical predictions in the long time limit, where the standards methods of quantum optics become inappropriate (70, 74). Application of such an approach to subrecoil Raman cooling and use of simpler sequences of pulses leading to 1D temperatures lower than 3nK for Cs atoms.
  • Development of a new method using the atomic spatial correlation function for measuring the momentum distribution of cooled atoms (75). Experimental evidence for non ergodic effects in subrecoil laser cooling.
  • First experimental demonstration of quantization of atomic motion in an optical potential associated with light shifts. Evidence for a long range spatial antiferromagnetic order of atoms in an optical molasses. This has been the starting point for the realization of “optical lattices” for neutral atoms, where atoms are trapped in a three-dimensional periodic array of potential wells.
  • Proposal of a gravitational cavity for neutral atoms and theoretical investigation of its quantum modes. Experimental observation of several bounces of Cesium atoms in such a cavity.

Bose-Einstein condensation

  • One of the most important applications of ultracold atoms is the possibility that they offer to observe quantum degeneracy effects in dilute bosonic or fermionic gases. Bose-Einstein condensation is one of these spectacular effects where a macroscopic mumber of bosonic atoms gather in the same quantum state, forming a mascroscopic matter wave. The coherence properties of theses matter waves can be described in termes of correlations functions similar to those used in quantum optics.
  • Realization, simultaneaously with another group in Orsay, of the first condensate where atoms are condensed, not in the electronic ground state, but in a long lived metastable state with a high internal energy. Bose-Einstein condensation of 4He atom in the 23S1 metastable state .
  • Production by one-photon photoassociation of giant dimers of ultracold metastable helium atoms. The 2 atoms are bound in a purely long range potential, where not only the attractive part, but also the repulsive part, are due to resonant dipole-dipole interactions. Experimental and theoretical study of these giant dimers.
  • The Scaterring length is a very important parameter for describing elastic collisions between ultracold atoms and for understanding the static and dynamic properties of gaseous Bose-Einstein condensates. The scattering length describing collisions beween metastable helium atoms has been determined very accurately by two different methods: study of the light shifts of one-photon photoassociation spectra; determination, by two photon photoassociation spectra of the binding energy of the least bound state of 2 metastable helium atoms in their interaction potential. Observation of atom-molecule "dark resonances".

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