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Department of Chemistry

Prof. Jeremy M. Hutson, FRS

Personal web page

Professor in the Department of Chemistry
Telephone: +44 (0) 191 33 42147
Professor of Chemistry and Physics in the Department of Physics
Room number: CG103L

(email at j.m.hutson@durham.ac.uk)

Theory of Ultracold Molecules

At temperatures about a millionth of a degree above absolute zero, matter enters a new regime where all its motions are fully quantum-mechanical. Such quantum matter can be controlled very precisely, and new states with novel properties emerge. Perhaps the best-known of these is a Bose-Einstein Condensate (BEC), in which the wavefunctions for all the particles are identical and in phase with one another.  A BEC has the same relationship to ordinary matter as a laser has to ordinary light. BECs formed from ultracold atoms have transformed the field of atomic and optical physics, and many new quantum properties of matter have been observed, such as quantum vortices and optical lattices, where atoms are held in a regular array by forces creates with laser beams.

There is now much excitement over the preparation and properties of ultracold molecules. Molecules have important properties that atoms lack: they can vibrate and rotate, and have a much richer pattern of electron and nuclear spins. They can also have large electric dipole moments: because of this, the interactions between molecules are anisotropic (angle-dependent) and much longer-range than the interactions between atoms. Quantum gases of ultracold polar molecules will have important new properties that cannot occur for atoms.

Ultracold polar molecules have now been produced experimentally: in 2008, a group at the University of Colorado associated pairs of potassium and rubidium atoms to form so-called Feshbach molecules, in very high vibrational states, by tuning a carefully controlled magnetic field across a Feshbach resonance. The resulting KRb molecules were then transferred to the ground state using coherent laser pulses. The era of ultracold molecules has arrived!

Our group works on the theory of cold and ultracold molecules. We work closely with many of the world-leading experimental groups, including those in Colorado, Innsbruck and Berlin.

Examples of our recent work include:

  1. Production and properties of alkali metal dimers. Chemists usually neglect the tiny splittings in molecular energy levels due to nuclear spins. However, for ultracold molecules at temperatures below 1 microkelvin, these splittings are often all that remain. We have developed the theory needed to understand these splittings for molecules such as KRb and Cs2, and shown how they can be modified in the presence of electric and magnetic fields and used to manipulate the molecules [1,2]. This may be crucial for proposals to use ultracold molecules in quantum computing and quantum simulators. We have also recently worked with the Innsbruck experimental group to produce the first samples of ground-state ultracold molecules in an optical lattice [3].
  2. Sympathetic cooling. Formation of ultracold molecules by atom association is limited to atoms that can themselves be laser-cooled. In principle, it would be much more general to cool molecules directly from room temperature to the ultracold regime. However, although molecules can be cooled to 10 to 100 mK by various methods such as molecular beam deceleration, there is not yet a way to cool them the rest of the way. We believe that this can be achieved by sympathetic cooling, where molecules are cooled by collisions with ultracold atoms. However, very little is known about the collisions between polar molecules and laser-cooled atoms. In particular, inelastic collisions can prevent sympathetic cooling by ejecting atoms and molecules from the trap. We have developed ways to carry out quantum-mechanical calculations of molecular collisions in electric and magnetic fields, and have begun to look for systems where the inelastic collisions are weak enough for sympathetic cooling to work [4,5]. We have also shown how inelastic collision rates may be suppressed by tuning close to scattering resonances [6].
  3. Novel properties of ultracold molecules. If a molecule has both an electric dipole moment and unpaired electron spin, its energy can be tuned with both electric and magnetic fields. We have shown how such tuning can be used to produce a novel type of conical intersection, as a function of position in 3-d space instead of vibrational coordinates. Conical intersections famously produce a Berry Phase, such that the wavefunction must change sign along a path that encircles the intersection. This can produce half-integer quantization for rotation around the intersection. We have shown that this effect can produce vortices with half-integer quantum numbers in a Bose-Einstein condensate of ultracold molecules [7].

The field of ultracold molecules is very fast-moving. Five years ago, the production of quantum gases of polar molecules seemed a far-off dream. Now it is a reality. As new experiments and new systems are explored, new theoretical questions emerge all the time. Our theory group works closely with experimentalists to identify the most important questions, interpret the experiments, and propose new ones.

References

[1] J. Aldegunde, B. A. Rivington, P. S. Żuchowski and J. M. Hutson, "The hyperfine energy levels of alkali metal dimers: ground-state polar molecules in electric and magnetic fields", Phys. Rev. A 78, 033434 (2008).

[2] J. Aldegunde, H. Ran and J. M. Hutson, "Manipulating ultracold polar molecules with microwave radiation: the influence of hyperfine structure", Phys. Rev. A 80, 043410 (2009).

[3] J. G. Danzl, M. J. Mark, E. Haller, M. Gustavsson, R. Hart, J. Aldegunde, J. M. Hutson and H.-C. Nägerl, "An ultracold, high-density sample of rovibronic ground-state molecules in an optical lattice", Nature Physics 6, 265 (2010).

[4] P. S. Żuchowski and J. M. Hutson, "Low-energy collisions of NH3 and ND3 with ultracold Rb atoms", Phys. Rev. A 79, 062708 (2009).

[5] A. O. G. Wallis and J. M. Hutson, "Production of ultracold NH molecules by sympathetic cooling with Mg", Phys. Rev. Lett. 103, 183201 (2009).

[6] J. M. Hutson, M. Beyene and M. L. González-Martínez, "Dramatic reductions in inelastic cross sections for ultracold collisions near Feshbach resonances", Phys. Rev. Lett. 103, 163201 (2009).

[7] A. O. G. Wallis, S. A. Gardiner and J. M. Hutson, "Conical intersections in laboratory coordinates with ultracold molecules", Phys. Rev. Lett. 103, 083201 (2009).

Resources

Jeremy Hutson's full CV is available in pdf format,

Jeremy Hutson's publication list is available in both pdf format (formatted for printing) and in html format (with links to journal articles).