Randall G. Hulet was born on April 27, 1956. After his education at Stanford University (B.S. in Physics, 1978), he received his degree in Physics in 1984 at the Massachusetts Institute of Technology in the group of professor D. Kleppner. After that, he first spent another year in the same group and then went to Boulder, Colorado, where he was a National Research Council Fellow at the National Bureau of Standards in the group of professor D.J. Wineland. In 1987 he received a position at Rice University in Houston, Texas. Presently he occupies at Rice University the Fayez Sarofim chair in experimental physics. He received several awards, among which the I.I. Rabi Prize of the American Physical Society and the National Science Foundation Presidential Young Investigators Award. He is also a Fellow of the American Physical Society and a Fellow of the American Association for the Advancement of Science.
The scientific work of Randall G. Hulet is always of the highest quality. He is internationally well known for his many important contributions to atomic physics. In particular, he played a leading role in the development of laser cooling and laser trapping of atoms. He is even more famous for his influential experiments on Bose-Einstein condensation. His most important achievements in this respect are the first realisation of Bose-Einstein condensation in an atomic gas with attractive interactions, the first observation of the formation and collapse of a condensate, and most recently the creation of a degenerate Bose-Fermi mixture.
Randall G. Hulet has a long relation with Utrecht University, because the relevant theory for his experiments is developed to a large extent in collaboration with the group Quantum Fluids and Solids of the Institute for Theoretical Physics. His work is also closely related to the experiments that are being carried out in the group Atom Optics of the Debye Institute.
The two most successful theories in physics are statistical mechanics and quantum mechanics. Together these theories can explain everything we know about Nature so far, i.e., nobody has ever been able to make an observation that is not in agreement with statistical mechanics and quantum mechanics. A possible reason for the success of both theories is their intimate relationship. For instance in one mathematical formulation, quantum mechanics appears to be just a peculiar form of statistical mechanics. This has at various times during the last century lead to the speculation that quantum mechanics is not a fundamental theory and that, when all is said and done, only statistical mechanics will survive as the ultimate theory of physics.
Another, for our purposes more important, example of the intimate relationship between statistical mechanics and quantum mechanics, is the fact that Albert Einstein could predict purely on the basis of statistical arguments the existence of a phenomenon that we now know to be a manifestation of quantum mechanics. In fact, this phenomenon was discovered already in 1925, thus before the discovery of quantum mechanics. It is called Bose-Einstein condensation, because Einstein was inspired by the ideas of the Indian physicist Satyendranath Bose on the statistical properties of light.
The prediction is that, if a gas cloud of atoms is cooled to sufficiently low temperatures, a droplet of a much higher density is formed in the gas cloud. Although this resembles the well-known effect of condensation of a water vapour on a cold surface, the crucial difference is that in the case considered by Einstein, the condensation occurs due to the quantum-mechanical character of the atoms and not due to the fact that the atoms attract each other. As a result the temperatures at which Bose-Einstein condensation occurs are much lower than the temperature at which water liquefies and very close to absolute zero temperature.
More exciting for a physicist is that the high-density droplet, which is called the condensate, is a superfluid. This implies that if an object moves through the droplet, it experiences no friction. This stunning property of a superfluid can only be understood on the basis of quantum mechanics. Moreover, the droplet has a macroscopic size and can literally be seen with the naked eye. This is rather remarkable, because quantum mechanics was devised for understanding the behaviour of microscopically small particles, such as electrons, atoms and molecules and not of macroscopic objects, with which we are all familiar in our everyday lives.
The achievement of the required extremely low temperatures has obstructed the actual observation of Bose-Einstein condensation for a long time. However, in 1995, 70 years after the prediction by Albert Einstein, a group lead by Eric Cornell and Carl Wieman was finally able to confirm experimentally the existence of the quantum-mechanical condensation process by cooling an atomic gas to less than one ten-milliard of a degree above absolute zero. Shortly thereafter also two different groups lead by Randy Hulet, and Wolfgang Ketterle, respectively, achieved the same objective. These accomplishments are really extraordinary, since the temperatures obtained are so low that they do not exist anywhere else in the Universe. Moreover, reaching them requires the use of a combination of experimental techniques, which already took many years to develop separately. Combining these different techniques in a single experiment is, therefore, a major achievement in itself, and allowed the above-mentioned physicists to open up a completely new field of physics.
Of the three beautiful experiments just discussed, the experiment of Randy Hulet at Rice University is special, because it makes use of a gas of lithium atoms, which attract each other. This in contrast to the two other experiments, which use gases with repulsive interatomic interactions. In a gas of lithium atoms Bose-Einstein condensation therefore competes with the ordinary liquefication of the gas. As a result the condensate actually collapses if it becomes too big. This is analogous to the collapse of a white dwarf star, when it become too massive and the pressure of the electron gas in the star is no longer able to balance the effect of the gravitational attraction. In the case of a white dwarf the collapse results in a supernova. In a gas of lithium atoms we talk about a 'Bosenova'.
The occurrence of a Bosenova in the extremely cold gas cloud complicates the observation of the Bose-Einstein condensation enormously. The reason is that the number of lithium atoms in the high-density droplet is now limited to a rather small number. Under the experimental conditions of the Rice group, this number is only about 1250. If the condensate contains more atoms, it collapses and an explosion occurs, which almost completely destroys the condensate. Because of the small number of atoms involved, the creation of the superfluid droplet is very hard to observe. Indeed, it took Randy Hulet more than one year to develop the necessary detection techniques to accurately do so. It is interesting to mention here that the techniques used were actually inspired by the Dutch physicist Fritz Zernicke, who received in 1953 the Nobel Prize in Physics for this work.
Having overcome this experimental problem, Randy Hulet was not only able to observe the condensate, but he could also study in detail the actual growth and subsequent collapse of the condensate. He was the first to observe the upper limit on the number of atoms in the condensate. He also found that after the collapse a small remnant of the condensate remained in the gas, similar to the neutron star that exists in the centre of a supernova. The reason why such a small remnant remains in the gas is not known at present. As a result, more experimental and theoretical work is being carried out at this moment, to unravel the physical principles that are at work during the collapse of the condensate.
As mentioned previously, the pioneering Rice experiments on Bose-Einstein condensation in an atomic gas with attractive interactions were performed with a gas of lithium atoms. More precisely the experiments use a bosonic isotope of lithium. However, lithium has also a fermionic isotope, and a gas of such atoms will behave fundamentally different at low temperatures. In an abstract sense, they are the boys and girls of statistical physics. For example, a Bose-Einstein condensation cannot occur in this gas. Nevertheless, theoretical work in the group Quantum Fluids and Solids of the Institute for Theoretical Physics showed that under certain conditions the gas becomes also a superfluid. Interestingly, in the fermionic case it is precisely the attractive interaction between the lithium atoms that enables the formation of the superfluid. For a gas of lithium atoms the required conditions are indeed met, as was shown by the Rice group in high-resolution photoassociation experiments with cold lithium atoms. Such photoassociation experiments are presently also used in the group Atom Optics of the Debye Institute to perform detailed studies of cold collisions between various other atoms.
The experimental observation of this exciting phenomenon, which is related to the superconductivity of metal wires, is the main objective of Randy Hulet at the present time. As an important first step towards this goal, his group has recently been able to cool a gas of the fermionic isotope of lithium to record low temperatures. To obtain this record a gas consisting of both the bosonic and the fermionic isotopes of lithium is used, to make optimally use of the cooling techniques developed previously for a gas of the bosonic isotope alone. With this latest achievement he is now in a unique position to actually realise the necessary conditions for the superconducting phase of the gas.
Truly remarkable about the work of Randy Hulet is that he succeeds with only a relatively small group of collaborators to make very important contributions to the competitive field of atomic quantum gases, in which various large groups around the world are active. Crucial in this respect is his choice of the alkali-metal atom lithium, which offers many experimental challenges but is also a very rich in physical behaviour. Moreover, his research methods have a strongly innovative character and have inspired various experimental groups. Finally, he is very open to collaborations with theoretical groups, which gives him the opportunity to analyse and explain his results with the most sophisticated theoretical models existing at that time.
We are very honoured by the fact that Utrecht University recognizes the importance of the above contributions to the field of ultracold atomic gases and awards professor Randall G. Hulet today with an honorary degree. We do not only have the greatest respect for his scientific accomplishments, but also for his warm and outgoing personality. We hope that the fruitful and enjoyable collaborations between his group and the group Quantum Fluids and Solids of the Institute for Theoretical Physics and the group Atoms Optics of the Debye Institute will continue for many years to come.