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Jacob Roberts

Associate Professor
B.S., University of Notre Dame, 1993; Ph.D., University of Colorado, 2001.

Cold Atomic Gases

Atomic gas Bose-Einstein Condensates (BECs) provide excellent systems for studying superfluid behavior, many-body physics in dilute systems, and quantum coherence. These BECs are formed by cooling a trapped gas of atoms to temperatures well below a microkelvin so that a macroscopic number of atoms all occupy a single quantum state. The ability to image the BECs directly provides a clean diagnostic for measuring their properties and dynamics, making it possible to take pictures of an inherently quantum objects’ size, shape, motion and behavior. Not only are BECs interesting in their own right, but their cold temperatures and coherent properties are useful or even necessary as an initial step in experiments in atom interferometry, atom optics, and quantum information processing.

In almost every case, the first step in creating a BEC is to collect and cool atoms using laser cooling into a Magneto-Optic Trap. These traps can cool atoms down to sub-millikelvin temperatures, but limitations prevent the conditions necessary for BECs to be achieved in these traps. Usually, the atoms are transferred to a magnetic trap where evaporative cooling is used to cool the gas below the BEC transition temperature. Recently, it has been demonstrated that it is possible to create BECs in all-optical traps consisting of focused laser beams. Unlike magnetic traps, these all-optical traps can confine atoms in any spin state, enabling the study of spin dynamics in trapped atomic gas superfluids. The ability to alter the shape and structure of the optical trapping beams has the potential for allowing studies of BECs in novel geometries. Also, since many applications using BECs load them into optical lattices (which consist of interfering focused laser beams), creating the BECs in the optical trap in the first place leads to greater simplicity and more efficiency in the design of these experiments.

The thrust of the research program that I am developing is to investigate a novel cooling mechanism in these optical traps, called Collision-assisted Zeeman (CAZ) cooling. This cooling method does not use evaporative cooling and it concomitant loss of atoms from the gas, and so it has the promise of being more robust and efficient leading to easier exploitation of the properties of all-optical traps in the studies of BECs. The basic idea behind CAZ cooling is to use spin-changing collisions that can occur in a trapped gas of atoms. For particular arrangements of spin states and magnetic fields, these collisions will convert the kinetic energy of the colliding pair into Zeeman energy. Optical pumping using laser light can be used to return these atoms to their initial states, closing the cycle while at the same time removing energy from the gas. Repeating this cycle will thus cool the gas without the loss of atoms.

There are several outstanding questions about the necessary processes involved in this cooling, providing fertile ground for experiments beyond the evaluation of the cooling efficiency. For instance, optical pumping in cold dense samples is known to be inefficient, but its precise limitations have not yet been determined. How much will the ability to favorably alter the trap geometry improve the optical pumping efficiency? Can low levels of laser light be used to reduce inefficiencies due to reabsorption of the pumping photons? Will using a linewidth-broadened laser also disrupt this reabsorption-induced inefficiency? Can pumping on different atomic transitions produce more favorable results? What role will AC Stark shifts in the optical trap play in the improvement or degradation of the optical pumping efficiency? What is the role of atomic coherences that can result in the optical pumping process? The answers to these questions will not only have an impact on CAZ cooling but also on wider questions involving the efficient loading of other types of atoms traps, the ability to prepare cold atom samples for quantum information processing and precision spectroscopy, the viability of using certain schemes to create atom lasers, as well as possibly shedding light on the physics of optical pumping in other systems used in preparing gases for MRI imaging and spin-polarized nuclear targets. Besides optical pumping, measurement of the spin-changing collision rates, cold molecule formation rates, and details of the loading of the optical trap should also produce interesting results along the way to cooling these optically-trapped gases to create BECs.

Selected Publications

  • S. L. Rolston and J. L. Roberts, "ULTRACOLD NEUTRAL PLASMAS," XVIII ICAP 20002 Proceedings - MIT, Cambridge, MA July 27 - August 2, 2002 (online) http://cfa-www.harvard.edu/~hrs/icap2--2/proceedings/Rolston.pdf.

  • J. L. Roberts, N. R. Claussen, S. L. Cornish, E. A. Donley, E. A. Cornell, and C. E. Wieman, "CONTROLLED COLLAPSE OF A BOSE-EINSTEIN CONDENSATE," Phys. Rev. Lett. 86, 4211 (2001).

  • E. A. Donley, N. R. Claussen, S. L. Cornish, J. L. Roberts, E. A. Cornell, and C. E. Wieman, "DYNAMICS OF COLLAPSING AND EXPLODING BOSE-EINSTEIN CONDENSATES," Nature 412, 295-299 (2001).

  • J. L. Roberts, J. P. Burke, Jr., N. R. Claussen, S. L. Cornish, E. A. Donley, and C. E. Wieman, "IMPROVED CHARACTERIZATION OF ELASTIC SCATTERING NEAR A FESHBACH RESONANCE IN 85RB," Phys. Rev. A 64, 024702 (2001).

  • J. L. Roberts, N. R. Claussen, S. L. Cornish, and C. E. Wieman, "MAGNETIC FIELD DEPENDENCE OF ULTRACOLD INELASTIC COLLISIONS NEAR A FESHBACH RESONANCE," Phys. Rev. Lett., 85, 728-731 (2000).

  • N.R. Claussen, S.L. Cornish, J.L. Roberts, E.A. Cornell, and C.E. Wieman, "85RB BEC NEAR A FESHBACH RESONANCE," Atomic Physics 17, E. Arimondo, P. DeNatale, and M. Inguscio, Eds. (American Institute of Physics, New York, 2001), p. 325.

  • S. L. Cornish, N. R. Claussen, J. L. Roberts, E. A. Cornell, and C. E. Wieman, "STABLE 85RB BOSE-EINSTEIN CONDENSATES WITH WIDELY TUNABLE INTERACTIONS," Phys. Rev. Lett. 85, 1795-1798 (2000).

  • J. L. Roberts, N. R. Claussen, James P. Burke, Jr., Chris H. Greene, E. A. Cornell, and C. E. Wieman, "RESONANT MAGNETIC FIELD CONTROL OF ELASTIC SCATTERING IN COLD 85RB," Phys. Rev. Lett. 81, 5109-5112 (1998).

  • C. S. Wood, S. C. Bennett, J. L. Roberts, D. Cho, and C. E. Wieman, "PRECISION MEASUREMENT OF PARITY NONCONSERVATION IN CESIUM," Can. Journ. of Phys. 77, 7-75 (1999).

  • S. C. Bennett, J. L. Roberts and C. E. Wieman, "MEASUREMENT OF THE DC STARK SHIFT OF THE 6S -> 7S TRANSITION IN ATOMIC CESIUM," Phys. Rev. A 59, R16-18 (1999).

  • C. S. Wood, S. C. Bennett, D. Cho, B. P. Masterson, J. L. Roberts, C. E. Tanner, and C. E. Wieman, "MEASUREMENT OF PARITY NONCONSERVATION AND AN ANAPOLE MOMENT IN CESIUM," Science 275, 1759-1763 (1997).

  • D. Cho, C. S. Wood, S. C. Bennett, J. L. Roberts, and C. E. Wieman, "PRECISION MEASUREMENT OF THE RATIO OF SCALAR TO TENSOR TRANSITION POLARIZABILITIES FOR THE CESIUM 6S-7S TRANSITION," Phys. Rev. A 55, 1007-1011 (1997).

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Formation of 85Rb Bose-Einstein condensate
Formation of 85Rb Bose-Einstein condensate

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