(1) Laser cooling and trapping in strong magnetic fields
By slowing and cooling atoms within a background field of several Tesla, we can perform precision measurements,
and study the effect of the large magnetic field upon the atomic level structure. Laser-cooling reduces motional
electric fields (vxB), which can otherwise cause significant blurring within spectroscopic data.
More importantly, it enables us to study dynamics of highly-excited atoms (Rydberg atoms) over a longer time scale.
For example, we were able to perform the first spectroscopic measurements on Landau levels in laser-excited Rydberg
atoms, and are currently studying how these excited states evolve over large time scales after the initial excitation
(t > ms).
Laser-excited hydrogen in B-fields is a well-known paradigm for quantum chaos, and has significant implications in the
classical/quantum physics correspondence. Rydberg atoms in fields of several Tesla are also analogous to ground-state
atoms in some astrophysical systems (e.g. neutron stars, with a field of >10^10 Gauss). Finally, novel hydrogenic
atoms which are dominated by these magnetic effects can be studied.
The atom trap produced within the large magnetic field also offers the possibility of studying density effects such as
collisional, or penning ionization, and atom-atom interactions. High angular momentum states, which behave more like
an electron within an atomic Penning trap than a typical atom in low fields, can be generated in such a collision-rich
Another important facet of this experiment is the ability to create two-component Penning traps. Using the electrode
package centered on the neutral atom trap, both electrons and positive ion cores (created through photo-ionization) can
be trapped by applying appropriate potentials. This confines the electrons and ions in the axial direction, while
tangential confinement is provided by the magnetron motion, produced by the large magnetic field. We can therefore create
two-component ultra-cold plasmas, and study effects such three-body recombination. Antihydrogen atoms recently generated
and trapped at CERN are the corresponding antimatter analog of these states. Studying the recombination process may
allow better control and understanding of these anti-matter experiments.
(2) Strong B-field??
Of course, the field should be strong from the point of view of atoms. In fact, a laboratory scale B-field is far from
being strong for ground state atoms. Atomic unit of field (=h/ (2 pi e a0^2)) is 2.35 x 10^5T, and a typical laboratory-size
B-field is of the order of several Telsa. This means that magnetic field effects for ground state atoms can be described very
well in a pertubative approach. However, for highly excited (Rydberg) atoms, this not longer holds. With Rydberg atoms, the
Coulomb field becomes weak at large principal quantum numbers (scaling as ~n^-2)), while magnetic effects scales up very
rapidly (~n^4*B^2). The Coulomb and diamagnetic terms in the Hamiltonian become comparable at B=3T and n=60. For larger n's
and/or B-fields, the magnetic field effect by far exceeds the Coulomb interaction, and leads to a qualitatively different
atomic structure. (Please note that n is not a good quantum number when B is nonzero, and is just used to describe the
effective energy level.)
The presence of a large B-field can significantly
affect plama physics as well. Collisional dynamics start to change when the cyclotron radius of charged particles becomes smaller
than the size of the plasma itself. In our experiments, we can generate a two-component neutral plasma by exciting laser-cooled
atoms slightly above the electron ionization energy threshold. Both the electrons and the ions have very small kinetic energies
compared other typical plasmas, and this rather new state of matter is called an ultracold plasma. At a field of ~3 Tesla, the
bound electron cyclotron radius in an ultracold plasma approaches the size of a Rydberg atom (10^3-10^4 a0), and the motion of
electrons is effectively pinned down to the field lines, leading to very different dynamics.