Critical velocity

The critical velocity of a superfluid mixture of the lowest two spin states of Lithium-6 was measured by moving a lattice at constant speed through the atom cloud. When the lattice velocity exceeds the critical velocity, dissipation kicks in and
an atom loss can be measured.

Fermionic superfluidity

Superfluidity of a fermionic gas in an optical lattice was shown for the first time. The diffraction peaks observed after
releasing the Fermi mixture from the lattice is characteristic for a superfluid. It was also shown that in a deeper lattice
superfluidity is lost in a reversible way.

Coherent molecular optics

Coherent molecular optics was performed using two-photon Bragg scattering. Molecules were produced by sweeping
an atomic Bose-Einstein condensate through a Feshbach resonance. The spectral width of the molecular Bragg resonance
corresponded to an instantaneous temperature of 20 nK, indicating that atomic coherence was transferred directly to the
molecules. An autocorrelating interference technique was used to observe the quadratic spatial dependence of the phase
of an expanding molecular cloud. Finally, atoms initially prepared in two momentum states were observed to cross pair
with one another, forming molecules in a third momentum state. This process is analogous to sum-frequency generation
in optics.

Formation of quantum-degenerate Sodium molecules

In 2003, several groups succeeded in converting ultracold atoms into ultracold molecules by magnetically tuning
a molecular level close to zero binding energy (Feshbach resonance). Atoms can then form molecules without release
of heat. Our experiment was the first to produce a molecular cloud clearly in the quantum-degerenate regime.
Ultracold sodium molecules were created from an atomic Bose-Einstein condensate by ramping an applied magnetic
field across a Feshbach resonance. More than 10^5 molecules were generated with a conversion efficiency of ~4%.
High phase-space density could only be achieved by rapidly removing residual atoms, before atom-molecule collisions
caused trap loss and heating. This was accomplished by a new technique for preparing pure molecular clouds, where
light resonant with an atomic transition selectively ”blasted” unpaired atoms from the trap. Time-of-flight analysis of
the pure molecular sample yielded an instantaneous phase-space density greater than 20.

Macroscopic pair-correlated atomic beams

By colliding two Bose-Einstein condensates we have observed strong bosonic stimulation of the elastic scattering process.
When a weak input beam (third wave) was applied as a seed, it was amplified by a factor of 20, and an initially unpopulated
conjugate wave was created. This large gain atomic four-wave mixing resulted in the generation of two macroscopically
occupied pair-correlated atomic beams. Since each collision process adds one atom each to the seed and conjugate
waves, fluctuations in the relative atom number are suppressed (squeezed). For the observed gain of twenty, the number
fluctuations should be below the shot noise by a factor of √40. We have also identified some limitations for using collisions
to create twin beams, including loss by subsequent collisions, and competition between other modes with similar gain.

Vortex arrays

Quantized vortices play a key role in superfluidity and superconductivity. We have observed the formation of highly
ordered vortex lattices in a rotating Bose-condensed gas. These triangular lattices contained over 100 vortices with
lifetimes of several seconds. Individual vortices persisted up to 40 seconds. The lattices could be generated over a
wide range of rotation frequencies and trap geometries, shedding light on the formation process. Our observation
of dislocations, irregular structure, and dynamics indicates that gaseous Bose-Einstein condensates may be a model
system for the study of vortex matter.

Publications

2004 & earlier  

• Dissociation and Decay of Ultracold Sodium Molecules

T. Mukaiyama, J. R. Abo-Shaeer, K. Xu, J. K. Chin, and W. Ketterle, Phys. Rev. Lett. 92, 180402 (2004)

• Formation of Quantum-Degenerate Sodium Molecules

      K. Xu, T. Mukaiyama, J.R. Abo-Shaeer, J.K. Chin, D. Miller, and W. Ketterle, Phys. Rev. Lett. 91, 210402 (2003)

• Amplification of Local Instabilities in a Bose-Einstein Condensate with Attractive Interactions

      J.K. Chin, J.M. Vogels, and W. Ketterle, Phys. Rev. Lett. 90, 160405 (2003)

• Coherent Collisions between Bose-Einstein Condensates

J.M. Vogels, J. K. Chin, and W. Ketterle, Phys. Rev. Lett. 90, 030403 (2003)

• Generation of Macroscopic Pair-Correlated Atomic Beams by Four-Wave Mixing in Bose-Einstein Condensates

      J.M. Vogels, K. Xu, and W. Ketterle, Phys. Rev. Lett. 89, 020401 (2002)

• Experimental Observation of the Bogoliubov Transformation for a Bose-Einstein Condensed Gas

      J.M. Vogels, K. Xu, C. Raman, J.R. Abo-Shaeer, and W. Ketterle, Phys. Rev. Lett. 88, 060402 (2002)

• Formation and Decay of Vortex Lattices in Bose-Einstein Condensates at Finite Temperatures

      J.R. Abo-Shaeer, C. Raman, and W. Ketterle, Phys. Rev. Lett. 88, 070409 (2002)

• Vortex Nucleation in a Stirred Bose-Einstein Condensate

C. Raman, J.R. Abo-Shaeer, J.M. Vogels, K. Xu, and W. Ketterle, Phys. Rev. Lett. 87, 210402 (2001)

• Realization of Bose-Einstein Condensates in Lower Dimensions

      A. Görlitz, J.M. Vogels, A.E. Leanhardt, C. Raman, T.L. Gustavson, J.R. Abo-Shaeer, A.P. Chikkatur, S. Gupta, S. Inouye, T. Rosenband, and W. Ketterle, Phys. Rev. Lett. 87, 130402 (2001)

• Observation of Vortex Lattices in Bose-Einstein Condensates

      J.R. Abo-Shaeer, C. Raman, J.M. Vogels, and W. Ketterle, Science 292, 476-479 (2001)

• Dissipationless flow and superfluidity in gaseous Bose-Einstein condensates

      C. Raman, R. Onofrio, J.M. Vogels, J.R. Abo-Shaeer, and W. Ketterle, J. Low Temp. Phys. 122, 99-116 (2001)

• Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

      R. Onofrio, C. Raman, J. M. Vogels, J.R. Abo-Shaeer, A.P. Chikkatur, and W. Ketterle, Phys. Rev. Lett. 85, 2228-2231 (2000)

• Surface Excitations of a Bose-Einstein Condensate

      R. Onofrio, D.S. Durfee, C. Raman, M. Köhl, C.E. Kuklewicz, and W. Ketterle, Phys. Rev. Lett. 84, 810-813 (2000)

• Evidence for a Critical Velocity in a Bose-Einstein Condensed Gas

      C. Raman, M. Köhl, R. Onofrio, D.S. Durfee, C.E. Kuklewicz, Z. Hadzibabic, and W. Ketterle, Phys. Rev. Lett. 83, 2502-2505 (1999)

Post-Doctoral Associates

• Yair Margalit
• Jeongwon Lee
• Ralf Gommers
• Yong-Il Shin, Professor at Seoul National University BEC group, South Korea
• Yingmei Liu, Professor at Oklahoma State University
• Takashi Mukaiyama, Professor at the University of
  Electro-Communications, Japan
• Johnny Vogels, Professor at the University of Utrecht, Netherlands
• Roberto Onofrio, Professor at Dartmouth College
• Chandra Raman, Professor at Georgia Tech

Undergraduate Students

• Sean Burchesky
• Widagdo Setiawan

Visitors

• Joseph Thywissen, (Jan-May 2009), Professor at the University
  of Toronto