Group of Vladan Vuletic

Experimental Atomic Physics

Department of Physics, Stanford University

 
 


Vladan Vuletic 
Assistant Professor, Department of Physics
Varian Building
382 Via Pueblo Mall
Stanford, CA 94305-4060
office Rm. 238, labs 243, 244
phone (650) 723-4233
fax   (650) 723-9173
vladan.vuletic@stanford.edu, vladan2@stanford.edu

 
People
 
Name
Position
office
   lab 
phone
(office)
phone
(lab)
phone
(private)
email address
Vuletic, Vladan
Assistant Professor
238
 
723-4233
 
843-0313
vladan.vuletic@stanford.edu
Black, Adam
Research Assistant
246
249/243
723-4238
 
776-7215
adam.black@stanford.edu
Chan, Hilton
Research Assistant
246
249/243
723-4238
 
 
chanhw@stanford.edu
Chin, Cheng Postdoc
240
243
723-2314
723-2314
575-6175
cchin@stanford.edu
Lin, Yu-ju Research Assistant
246
243
723-4238
723-2314
 
yjlin@stanford.edu
Teper, Igor
Research Assistant
246
243
723-4238
723-2314
 
teper@stanford.edu

 

Current research

We are currently building two experiments: The first is concerned with a new laser cooling method that can be applied to particles with an arbitrary internal level structure and may provide a way to cool certain molecules with large polarizability in the optical domain. The second experiment aims at realizing lower-dimensional ultracold atomic gases, prepared by optical cooling in very steep magnetic waveguides. The long term goal of both experiments is to develop new methods to manipulate particles in a regime where the quantum mechanical aspects dominate their behavior and their properties. On the one hand, this should lead to new tools that allow one to probe physical laws and to measure fundamental constants with increasing precision. On the other hand, the progress of experimental methods also drives the advances in our understanding of the ever mysterious, beautiful, accurate, yet deeply dissatisfying structure of quantum mechanics. This interplay between theoretical concepts and experimental realizations promises to be very fertile in fields such as quantum control, quantum feedback and its limits, many-particle quantum systems, and many-particle entanglement (quantum computing).
 
 
 

New laser cooling methods for atoms, ions or molecules: Cavity Doppler cooling and cavity sideband cooling by coherent scattering

Laser cooling of atoms has not only supplied the basis for the control and manipulation of matter at the quantum limit, e.g. in form of Bose-Einstein condensation, but has also resulted in a number of important applications and devices, many of which are tied to precision measurements and atomic clocks. However, laser cooling has so far been limited to atoms with a particular internal structure, and the cooling of molecules or even of atoms with a complicated level scheme has so far not been possible. If we could learn how to cool, trap and manipulate larger molecules in the same way as atoms, this would open the door for important developments in chemistry and possibly even in biology.
    Doppler cooling [1] is the dominant laser cooling mechanism at all but the lowest temperatures. In Doppler cooling counterpropagating laser beams are tuned to the red of a closed transition between an atomic ground and an atomic excited state. An atom that is moving towards a laser beam will experience photons that are blue-shifted into resonance with the atomic transition, while photons from a beam propagating in the same direction as the atom will be red-shifted further out of resonance. The momentum transfer associated with the preferred absorption of photons from the counterpropagating beam leads to slowing and cooling of the atoms, while the randomly emitted photons on average do not contribute to the force. The net effect is cooling to temperatures in the millikelvin to microkelvin range, corresponding to atomic velocities in the range of a few millimeters to a few centimeters per second.
    This principle behind laser cooling also entails its limitation. Since the momentum “kick” associated with each photon absorption event is much smaller than the momentum of a thermal atom, a larger number of absorption-emission events (on the order of thousand or more) is required to significantly change the atom’s velocity. Therefore laser cooling has only been demonstrated with atoms that can be optically cycled many times back to their initial ground state. However, most atoms (and all molecules) have multiple ground states to which the excited state can decay. Once the atom reaches a different ground state, the laser no longer has the correct detuning relative to the atomic transition, and the cooling stops. In particular, molecules have many vibrational and rotational levels, and consequently no laser cooling of molecules has been demonstrated.
    The novel proposed method [2,3] is based on coherent scattering, rather than on spontaneous emission from an excited state. Coherent scattering dominates when the laser is far detuned from atomic or molecular transitions and when its intensity is not too large. Coherent scattering is generic to all polarizable particles, independent of their internal structure, and describes the emission of radiation by an oscillating atomic dipole that is driven by an external (classical) electric field. The basic idea behind the proposed technique is that energy is conserved in the scattering process, and that therefore events where the scattered photon carries away a larger energy than the incident-photon energy are accompanied by a corresponding reduction of the atom’s energy. Such scattering events can be enhanced in an optical resonator that is tuned to be resonant with a frequency that is higher than that of the incident light. The new cooling mechanism depends only on the finesse (i.e. on the reflectivity of the cavity mirrors) and on the detuning of the photons relative to the cavity resonance, while it is independent of the detuning relative to atomic transitions. Therefore this new technique should be generic and be applicable to any sort of material. The target can be an atom in different ground states, a molecule in different rotational and vibrational states, or possibly even a scattering center (impurity) inside a solid. The only requirement is that at the given intensity and laser frequency the emission rate by the scatterer is large enough to produce efficient cooling. The cooling power is given by the product of the scattering rate and the energy difference between the incident and the scattered photon [2].
    We are currently preparing a proof-of-principle experiment where cesium atoms are cooled with the new technique using light that is far detuned from any atomic transitions. Since the new technique is closely related to Doppler cooling [2,4], the cooling limit will be given by the Doppler limit for the cavity linewidth, i.e. the temperature at which the thermal energy of the atoms equals the energy that corresponds to the linewidth of the resonator. Therefore temperatures near the recoil limit should be attainable with low loss mirror substrates, independent of the atomic structure or atomic linewidth.

[1]     T.W. Hänsch and A.L. Schawlow, Opt. Commun. 13, 68 (1975).
[2]     V. Vuletic and S. Chu, Phys. Rev. Lett. 84, 3787 (2000).
[3]     P. Horak, G. Hechenblaikner, K.M. Gheri, H. Stecher, and H. Ritsch, Phys. Rev. Lett. 79, 4974 (1997).
[4]     V. Vuletic, H. W. Chan, and A. T. Black, Phys. Rev. A 64, 033405 (2001).