New forms of matter near absolute
zero temperature
Wolfgang Ketterle
Massachusetts Institute of Technology, Cambridge, USA
Abstract. First, I want to make some remarks about my career. In
1990, I made a major change in my life. I went from my native Germany to the US. I changed my research field from hot flames to ultracold atoms. And I went from applied physics to fundamental physics. I
had enjoyed applied research in Germany. In combustion diagnostics,
it is easy to explain what motivates you (clean environment, efficient
combustion) and you receive a lot of acknowledgement from your
family and friends. However, I realized that my talents were better
matched to fundamental questions, where goals are sometimes fuzzy,
research is exploratory, and real applications are decades away. It is
much harder for me to explain now why a little puff of gas fascinates
me, a tiny amount of matter, almost nothing, suspended in a stainless
steel vacuum chamber. But I know that my current work is even more
relevant in the long run than what I did earlier.
In the US, at the Massachusetts Institute of Technology, I experienced
an American elite university. As an assistant professor, I enjoyed the
independence at an early stage of my career. In contrast to the
“Juniorprofessur” in Germany, these positions are tenure track.
This means that if the candidate is successful, he or she will be promoted to a permanent position at the same institution. I think it is
important that young researchers know when they accept a junior
position, that the number of permanent positions is matched to the
8
Wolfgang Ketterle
number of people hired on junior positions. Young researchers are
usually highly motivated and accept major sacrifices for their passion. They don’t expect to be hired immediately on a permanent faculty position, but they deserve to know that if they are successful,
their academic career will continue. This perspective is provided by
the tenure track system, but not by the old German system of “Habilitation” or by the new system of “Juniorprofessur”.
In the scientific part of my talk, I explained how we reach extremely
low temperatures below one nanokelvin, less than a billionth degree
above absolute zero temperature. Atoms and molecules at room
temperature zip around at the speed of jet airplanes (300 m/s),
whereas at one nanokelvin, their velocity is less than 1 mm/s. However, the special and remarkable fact is that at such low temperatures, the atoms stop moving randomly, but rather march in lockstep. This is the phase transition called Bose-Einstein condensation,
which was predicted in 1925 but observed only 70 years later. This
discovery of a new form of matter has led to a flurry of worldwide
activities and was recognized with the Nobel Prize in 2001.
1
Ultracold gases
Ultracold gases have now become a new approach to discover and
explore new forms of matter. When engineers design new airplanes,
they build small models and test them in a wind tunnel. When scientists design new forms of matter, they are facing the problem that at
ordinary densities atoms are pressed against each other and the
forces between atoms are complicated and not fully understood.
What would be nice is to build a magnified model where the atoms
are far apart and interact only with well known electrical forces.
However, such an enlarged model is matter at much lower density
and therefore much lower temperatures are required to study the
phenomena of interest. This vision has become a reality with ultracold gases. We are studying matter which is 100,000 times thinner
than air, 100 million times less dense than ordinary solids. We need
100 million times lower temperature to observe phase transitions and
New forms of matter near absolute zero temperature
9
other interesting physics, but those temperatures can now be realized
in many laboratories.
An important example of this line of research has been the realization of a new form of high-temperature superfluidity in atomic
gases. Superfluidity requires particles to march in lockstep. This is
only possible for bosons, particles which consist of an even number
of electrons, protons and neutrons. Atoms with an odd number have
to form atom pairs before they can become superfluid. This is similar to electrons in metals, which have to pair up to become superconducting. Superconductivity is superfluidity for charged particles.
A major goal in condensed matter physics has been the achievement
of superconductivity at higher temperatures. However, the ultimate
goal of room temperature superconductors has not been achieved,
and it is not clear theoretically if it will ever be possible or not.
Recently, the MIT group demonstrated superfluidity for pairs of
fermionic atoms. Superfluidity was observed at low density and
nanokelvin temperatures. However, when scaled up to the density of
electrons in a metal, this new form of superfluidity occurs above
room temperature and therefore represents the highest-temperature
superfluidity demonstrated so far. Will this research lead to roomtemperature superconductors? Nobody knows, but our knowledge
about the nature of superfluidity and superconductivity is rapidly
advancing.
I mentioned key factors for my success.
• Technical infrastructure;
• Excellent collaborators;
• International exchange;
• Tradition and mentors;
• Physical endurance.
Our experiments are at the limit of feasibility and require high-quality
labs and state-of-the-art equipment. At MIT, we can select our stu-
10
Wolfgang Ketterle
Fig. 1. Family tree of atomic physicists. People with name in italics are
Nobel laureates. Many excellent researchers were educated by Dan Kleppner and Dave Pritchard.
dents from the best applicants from all over the world and therefore
have excellent graduate students. Half of our graduate students are
foreign.
2
Perspectives on excellence
At MIT, atomic physics has a long-standing tradition of excellence.
The two senior people, Profs. Daniel Kleppner and David Pritchard,
have trained a school of excellent physicists. The figure shows that
several generations of Nobel laureates are connected in a family tree.
This is remarkable since Nobel prizes are given in atomic physics
only every five to ten years.
Last but not least, physical endurance is an important factor (see
figure). Our experiments are complicated prototypes, often the first
New forms of matter near absolute zero temperature
11
Fig. 2. A histogram shows the number of images saved for later analysis and
the hour at which they were taken. The histogram is based on a one-year
period (2/1998 – 1/1999) during the thesis work of Dan Stamper-Kurn.
of their kind, built with improvisation, and they need a lot of tweaking
and alignments. If it is only possible to get an experiment ready late in
the evening, then the data taking continues through the night. Of
course, what took all day and night a few years ago, can now be accomplished before lunch, but the current experiments have more
complexity added, so again, the data taking is often a good night’s
work.
Given the recent dynamics in the field, I expect many more years of
exciting discoveries.
http://www.springer.com/978-3-540-72620-3