Jacob Roberts

Photo of Jacob Roberts

Professor and Chair of the Department of Physics

B.S., University of Notre Dame (1994)

Ph.D., University of Colorado (2001)

Research Interests

Ultracold Plasma Physics:  Ultracold plasmas are formed via the photoionization of gases of ultracold atoms.  The resulting plasmas that form have electron and ion temperatures (a few Kelvin) that can be much colder than other laboratory and natural plasma systems.  Yet, these plasmas have high ionization fractions.  As such, ultracold plasmas form an interesting system in which to conduct studies of fundamental plasma physics.

In our apparatus we have separated the laser cooling region where ultracold atoms are cooled and trapped from the region where the ultracold plasmas are formed.  Atoms are transported between the two regions in a magnetic trap formed by electromagnet coils that are mounted to a movable trap, similar to Bose-Einstein condensate apparatuses developed at JILA.  With this design we have built an apparatus that is capable of confining ultracold plasmas in a Penning trap similar to those used to confine antiprotons and positrons in anti-hydrogen experiments.  By trapping the ultracold plasmas we can extend their lifetimes and investigate longer timescale processes in these systems.

Current work, however, does not focus on trapping ultracold plasmas.  Rather, in our apparatus the density of the ultracold plasmas is much less than comparable experiments elsewhere.  We have found that at this lower density we can observe new plasma excitation responses to external driving fields, increased cooling due to electron evaporation, rapid initial expansion in response to applied external magnetic fields, colder electron temperatures due to reduced three-body recombination, and more experimentally accessible electron-electron and electron-ion collision times.

We have been investigating these properties of the ultracold plasmas formed in our system with an eye to achieving colder electron temperatures and thus more strongly-coupled plasmas and to understanding the physics necessary for effective loading of ultracold plasmas into a Penning trap.

Novel Ultracold Atom Cooling Techniques:  Evaporative cooling remains the most common experimental technique for cooling ultracold gases to the lowest achievable temperatures.  However, evaporative cooling suffers from a few notable disadvantages:  most of the initially cooled sample must be lost for effective cooling, favorable collision physics needs to be present, relatively long trap confinement times are required, and often the cooling rate can be slow on experimental timescales.  This leads to a motivation to find alternatives to evaporative cooling to cool ultracold gases — not only from a fundamental physics perspective but also for use in systems where evaporative cooling is difficult.

Extensions to laser cooling beyond Doppler and optical molasses cooling extend the range of achievable laser cooling to lower temperatures without the need for evaporative cooling.  We are investigating such an extension to laser cooling that we call SelecTive Optical Pumping cooling (STOP cooling).  This technique relies on the ability to selectively optically pump ultracold atoms that have relatively high energy from a dark state to a bright state while they are confined in a trap.  By waiting a predetermined amount of time and then using the ability to transfer momentum from light to atoms, the energy of these high-energy atoms can be reduced without disturbing the rest of the atoms that remain in the dark state.  The atoms can then be pumped from the bright back to the dark state, creating a closed cooling cycle that does not require atom loss.

The investigations in to this technique are in the early stages, although we have confirmed that the technique works as predicted for a limited number of cooling cycles in a gas of optically trapped 87Rb.  This technique should be widely applicable to any atoms or molecules that are confined in any type of trap as long as there is an accessible dark state.  Current research involves extending the measurement time to examine greater cooling in an 87Rb gas.  In the future, we plan to adapt this technique to cool trapped molecular gases.

Recent Publications

  • Wei-Ting Chen, Craig Witte, and Jacob L. Roberts, Damping of electron center-of-mass oscillation in ultracold plasmas, Phys. Plasmas 21, 052101 (2016)
  • John Guthrie and Jacob L. Roberts, A scalable theoretical mean-field model for the electron component of an ultracold neutral plasma, J. Phys. B 49, 045701 (2016)
  • Craig Witte and Jacob L. Roberts, Ultracold Plasma Expansion as a Function of Charge Neutrality, Phys. Plasmas 21, 103513 (2014)
  • Mathew S. Hamilton, Rebekah F. Wilson, and Jacob L. Roberts, Collision assisted Zeeman cooling with multiple types of atoms, Euro. J. Phys. D 68, 14 (2014)
  • Truman M. Wilson, Wei-Ting Chen, and Jacob L. Roberts, Density-dependent response of an ultracold plasma to few-cycle radio-frequency pulses, Phys. Rev. A 87, 013410 (2013).
  • Truman M. Wilson, Wei-Ting Chen, and Jacob L. Roberts, Influence of electron evaporative cooling on ultracold plasma expansion, Phys. Plasmas 20, 073503 (2013).
  • M. S. Hamilton, A. R. Gorges, and J. L. Roberts, Inter-isotope effects in optimal dual-isotope loading into a shallow optical trap, J. Phys. B 45, 095302 (2012).
  • Truman M. Wilson and Jacob L. Roberts, Enhanced light-assisted-collision rate via excitation to the long-lived 5S1/2-5D5/2 molecular potential in an 85-Rb magneto-optical trap, Phys. Rev. A 83, 033419 (2011)