Two thin planes of
cold atoms are held in an optical lattice by an array of laser beams.
Still another
laser beam, passed through a diffusing material, adds an element of disorder
to the atoms in the form of a speckle pattern. Courtesy
of Matthew Beeler.
(July 19,
2012) A new
experiment conducted at the Joint Quantum Institute (JQI)* examines the
relationship between quantum coherence, an important aspect of certain
materials kept at low temperature, and the imperfections in those materials.
These findings should be useful in forging a better understanding of disorder,
and in turn in developing better quantum-based devices, such as superconducting
magnets.
That’s where the JQI
experiment comes in. Specifically, Steve Rolston and his colleagues have set up
an optical lattice of rubidium atoms held at temperature close to absolute
zero. In such a lattice atoms in space are held in orderly proximity not by
natural inter-atomic forces but by the forces exerted by an array of laser
beams. These atoms, moreover, constitute a Bose Einstein condensate (BEC), a
special condition in which they all belong to a single quantum state.
Most things in nature are
imperfect at some level. Fortunately, imperfections---a departure, say, from an
orderly array of atoms in a crystalline solid---are often advantageous. For
example, copper wire, which carries so much of the world’s electricity,
conducts much better if at least some impurity atoms are present.
In other words, a pinch of
disorder is good. But there can be too much of this good thing. The issue of disorder
is so important in condensed matter physics, and so difficult to understand
directly, that some scientists have been trying for some years to simulate with
thin vapors of cold atoms the behavior of electrons flowing through solids
trillions of times more dense. With their ability to control the local forces
over these atoms, physicists hope to shed light on more complicated case of
solids.