Fachbereich Physik
Institut für Physik
Finding information in the noise: Tracking unusual quantum states of ultracold atoms
Researchers of Mainz University demonstrate the use of quantum statistics for
analyzing ultracold quantum gases
(Mainz, March 24, 2005) The concept of uncertainty is a fundamental component of
the laws of quantum mechanics: When releasing atoms which have originally been
confined separately, they should be found at random positions a short time later. In
fact, it is impossible to predict at which position an atom will appear. Nonetheless,
how the atoms organize among themselves is not entirely random. When atoms are
released from a lattice trap made of laser light, they tend to line up at specific distances from each other even after cloud of atoms has made an uncontrolled expansion. In the latest issue of the journal Nature, researches of the Johannes Gutenberg
University Mainz report on the observation of such correlations. They provide a
method for analyzing the behaviour of the atoms inside such traps, which serve as
models for effects in condensed matter systems such as magnetism or superconductivity.
In a spectacular experiment in 1956 Robert Hanbury Brown and Richard Twiss demonstrated that identical particles which are emitted randomly by independent sources
do not appear in a totally independent way when they are detected at a later time.
This happens even if there is no interaction or other “communication” between the
atoms. The reason for this is in the so-called quantum statistics: According to it’s internal angular momentum or “spin”, every object belongs to one of two classes of
particles called bosons and fermions. Bosons have a tendency to appear together at
the same location, even if there is no force “pulling” them together. In a recent experiment physicists at the Johannes Gutenberg University in Mainz have now dem-
onstrated that the effect can be exploited to obtain internal information about special
states of ultracold atom clouds. These clouds at temperatures just above absolute
zero (-273.15 degrees Celsius) are first converted to so-called Bose-Einstein Condensates and are then loaded in a trap made of laser beams and formed like a regular lattice. This drastically changes the nature of the cloud – in the BEC state the individual atoms are “delocalized”, each of them is occupying each site of the trap at
the same time. However, in a very strong lattice trap the cloud undergoes a transition
to a so-called “Mott insulator” state, where each atom in confined to exactly one lattice site.
After this preparation the atom cloud is released from the trap and expands in all directions. Since matter behaves like waves at these low temperatures and in the BEC
state the wave of each atom is extended over all lattice sites, interference effects occur: Atoms from the lattice can only fly in certain directions, similar to coloured light
which is scattered in different directions by the regular pattern of a compact disk. After the atoms have moved for a certain period of time, a photographic image of the
cloud is taken which shows how the atom cloud has expanded. In the case of a BEC,
the atoms can only fly to certain points in space which have a regular pattern, as a
result of the regular structure of the lattice trap.
In the Mott state, the picture is entirely different: Each atoms is launched independently from it’s own lattice site and moves in a random direction without exhibiting any
interference. The images of such an expanded atom cloud show a completely randomized distribution without any structure – whether the atoms originated from a lattice and what kind of lattice this was can no longer be determined. But, due to the
quantum statistics, the location of each atom with respect to the others is not entirely
random: Because the atoms are bosons, they have a tendency to appear at the
same locations in bunches. But in addition to this, the fact that the atoms originated
from a regular lattice trap leaves traces in the atom distribution: the atoms tend to
appear with certain, exactly defined distances separating them – the more closely
packed the lattice sites of the trap were, the larger these distances are now.
These properties of the distribution of atoms are obtained by analyzing the small random fluctuations in the images of atom clouds. This method of using what’s normally
considered as noise on the images was recently proposed by a group of theorists at
Harvard University and now demonstrated in the Mainz experiment. “For the first time
the regular structure of a Mott insulator in the lattice trap can be seen directly” says
Prof. Immanuel Bloch from Mainz University, “and we also have an elegant new demonstration of the Hanbury Brown-Twiss effect for ultracold atoms.” Atoms in lattice
traps are seen as model systems for the microscopic structure of crystals, superconductors, magnets and semiconductors. Therefore, the scientists hope to use the new
method for new insights into the behaviour of the atoms in such traps.
Figure 1: Image of an atom cloud after free expansion. Loading the cloud into an optical lattice of increasing depth first creates a regular interference pattern. For a very
deep lattice the pattern disappears, because the atoms are now confined to individual lattice sites and expand independently without interfering. In this case the regular
pattern disappears and only a featureless cloud remains.
Figure 2: The image on the left shows an atom cloud from a very deep lattice trap.
The interference pattern has disappeared, but small random fluctuations in the atom
distribution are still visible as “noise” on the image. These fluctuations are correlated
at certain distances, which can be uncovered by correlation analysis and represent a
regular pattern which is shown in the right image. This pattern reflects the regular
properties of the atom distribution in the original lattice trap.
Reference:
Simon Fölling, Fabrice Gerbier, Artur Widera, Olaf Mandel, Tatjana Gericke & Immanuel Bloch, Spatial quantum noise interferometry in expanding ultracold atom clouds
Nature 434, 481-484 (2005); doi:10.1038/nature03500
Kontakt und Informationen:
Univ.-Prof. Dr. rer. nat. Immanuel Bloch
Johannes Gutenberg-Universität Mainz
Institut für Physik
Tel. 06131 39-22279 und 39-20175
Fax 06131 39-25179
E-Mail: [email protected]
http://www.physik.uni-mainz.de/quantum/bec/