Since researchers created the first Bose-Einstein condensates (BECs) 3 years ago, they have been eagerly exploring this new state of matter and seeing what can be done with it—turning it into an atom laser, for example (see main text). But the magnetic traps used to confine the condensates put some kinds of studies off limits, because the magnetic fields freeze the spins of the trapped atoms in one orientation. Now Wolfgang Ketterle and his team at the Massachusetts Institute of Technology have removed the barrier with a new kind of trap.
Since researchers created the first Bose-Einstein condensates (BECs) 3 years ago, they have been eagerly exploring this new state of matter and seeing what can be done with it—turning it into an atom laser, for example (see main text). But the magnetic traps used to confine the condensates put some kinds of studies off limits, because the magnetic fields freeze the spins of the trapped atoms in one orientation. Now Wolfgang Ketterle and his team at the Massachusetts Institute of Technology have removed the barrier with a new kind of trap.
A year ago this month, physicists first created a Bose-Einstein condensate, a state of matter in which a cloud of trapped atoms is cooled so close to absolute zero that their quantum-mechanical waves merge. But with barely 1000 atoms in a condensate that lasted just a few seconds, the researchers could do no more than say they'd done it. Now the field is being transformed by a new atom trap that can create condensates of 5 million sodium atoms that survive for half a minute and a new technique for imaging condensates without destroying them. Several research groups are now probing the size, growth rate, optical properties, and even the viscosity of this new form of matter.
In 1925, Einstein predicted the condensation of bosons into the ground state of the system for low (but finite) temperatures. Several phenomena, including superfluidity and superconductivity have been associated with Bose-Einstein condensation, but these systems interact strongly with their environment and pure Bose-Einstein condensation could not be established. It took 70 years, in which time the laser was discovered, and laser cooling techniques to manipulate atoms in a dilute atomic gas, before Bose-Einstein condensation in dilute atomic gases could be demonstrated in 1995. In the first condensation experiments, BECs were created in a magnetic trap. Since in a magnetic trap not all mF states of the atom can be trapped simultaneously, thereby limiting the number of experiments that can be done, other ways of trapping and generating BECs were sought and found. In 2001, the first all-optical BEC was made, where the dipole force was used to trap atoms in the crossing of two far red detuned laser beams. In an optical dipole trap not only atoms in different internal states can be trapped, but also different atomic species simultaneously. In this thesis, the formation of an all-optical Bose-Einstein condensate with rubidium atoms is presented. Conventional all-optical BECs are usually created in high power CO2 laser dipole traps, or have complicated laser cooling schemes and complex dipole trap setups. In our simple and straightforward setup, we load rubidium atoms from a magneto-optical trap into a crossed optical dipole trap created by a single frequency Yb:YAG laser with a wavelength at 1030 nm. The small wavelength allows for a small diffraction limit, and permits us to use standard optical materials, thus making the experimental setup cost effective. Other attempts to achieve Bose-Einstein condensation in a multi-mode (frequency) fiber laser at 1064 nm failed, because the atom loss was quite high. It is assumed that the multi-mode character of the fiber laser induces Raman transitions in rubidium atoms, thereby heating them. We can trap about ∼ 5E7 atoms in a single beam dipole trap out of ∼ 5E9 atoms trapped in theMOT, and ∼ 350,000 atoms can be trapped in a crossed beam dipole trap due to the smaller trap volume. 70% of the atoms in the dipole trap is optically pumped into one mF state. Quantum degeneracy is reached by evaporatively cooling the atoms in the crossed dipole trap by ramping down the laser power with three linear ramps. We can independently change the power of each beam by an AOM. This allows us to use one beam as an atom waveguide for future experiments. After evaporation, we typically have about 10,000 atoms at a temperature below the critical temperature. We have proved Bose-Einstein condensation by using the anisotropic expansion of a quantum degenerate gas trapped in an anisotropic potential. The aspect ratio of our atom cloud changed during a time of flight from 0.7 to 1.2 in 10 ms, thus proving that we have reached quantum degeneracy. We have about 5,000 condensed atoms in our optical dipole trap at a temperature less than 100 nK. The remaining atoms are thermal. Bose-Einstein condensation is obtained within 8 s, and we can repeat the experiment every 30 s. It should be mentioned that the Bose-Einstein experiment was moved from the ”Leibniz Universität Hannover” to the ”Technische Universität Darmstadt”, and had to be completely rebuilt. All-optical Bose-Einstein condensation was reached within one year after the move. Our Bose-Einstein condensation setup presents an ideal starting point for using our condensates in combination with miniaturized atom optical setups based on our novel microfabricated optical elements. With our microlenses, we can create a number of possible dipole trap configurations, such as the dipole trap array or the cylindrical microlens array. Using microlenses in miniaturized atom optical setups opens a completely new field of coherent atom optics. Also because the tight confinement of the microtraps allows us to load a 3D BEC, a 1D BEC, or a Tonks-Girardeau gas in the micropotentials depending on the density.
Bose–Einstein condensates (BECs) were formed five years ago from rubidium-87 and sodium atoms, whose interactions are repulsive. BECs were also formed from lithium-7 atoms, which attract one another. (See the article by Wolfgang Ketterle in PHYSICS TODAY, December 1999, page 30.) Now researchers in Boulder, Colorado, have created a single condensate of rubidium-85 atoms, which can be taken continuously from the repulsive to the attractive regimes. The experimenters can vary the strength of the interaction between the atoms in their condensate by a factor of 100 or more—and even flip its sign—by changing the external magnetic field. The ability to tune the interactions of a condensate opens the door to systematic explorations of atomic behavior in new regimes.
The experimental observation of Bose–Einstein condensation (BEC) in rubidium in 1995 demonstrated that a macroscopic number of bosons could be produced in a single quantum state of trapped atoms. The occupation of a single quantum state by a large number of identical bosons is the matter-wave analog of the storage of photons in a single mode of a laser cavity. In a conventional laser, one extracts a coherent beam of photons from a cavity by using a partially transmitting mirror as an output coupler. In 1997, Wolfgang Ketterle and his collaborators at MIT built a pulsed output coupler that extracted matter waves from a condensate, and they observed interference between atoms from two separate condensates, thereby demonstrating an atom laser for the first time (see PHYSICS TODAY, March 1997, page 17).
A Bose-Einstein condensate (BEC), the first of which was shown experimentally 22 years ago, isn’t your garden variety state of matter. It formed at a fraction above absolute zero and only in atoms that act like bosons, one of two types of fundamental particles. Bosons don’t follow the Pauli exclusion principle, which prohibits two particles from existing in the same quantum state. When bosonic atoms are cooled to form a condensate, they can lose their individuality. They behave like one big collective superatom, analogous to how photons become indistinguishable in a laser beam. But it’s even weirder than that.
This velocity-distribution data for a gas of rubidium atoms confirmed the discovery of the Bose–Einstein condensate in 1995. In these three snapshots in time, atoms—cooled to near absolute zero—condensed from less dense areas on the left (red, yellow, and green) to very dense areas at the center and the right (blue and white). Image courtesy of NIST/JILA/CU-Boulder.
“In a very good analogy, one can view a BEC as a bell, which begins to ring spontaneously when it is cooled below a certain temperature,” writes physicist Nick Proukakis at the Joint Quantum Centre Durham–Newcastle in the United Kingdom, in Universal Themes of Bose–Einstein Condensation , a forthcoming collection of research essays on progress in BECs (1). That behavior provides physicists with an extraordinary opportunity: to study bizarre quantum effects on a large scale, instead of having to probe individual particles.
For the last two decades, physicists have treated BECs something like Play-Doh. They poke it, smash it, tickle it with lasers, and trap it in magnetic fields. They mix condensates together to see what happens, and use it to slow down light (2). They have observed strange behaviors that would have been impossible to predict even two decades ago: solids that flow …
When Wolfgang Ketterle and his colleagues at MIT demonstrated an “atom laser” recently, the announcement excited the physics community and naturally grabbed the attention of the popular press. One essential property of a laser is, of course, its coherence, and the evidence provided by the MIT group is striking: Sharp interference fringes formed when two Bose–Einstein condensates from the same trap expanded and overlapped after being released from confinement. The fringes are reminiscent of the interference between coherent light waves emanating from a double slit. (See the figure below.)