- Research into Condensates Continues
Daniel S. Burgess
It has been nearly a decade since the first experiments were reported on Bose-Einstein condensates in ultracold atoms, and research into this and related phenomena continues to be fruitful. As the work advances, scientists worldwide are investigating new material systems, new cooling techniques and potential applications of other varieties of condensates.
Eighty years ago, Louis de Broglie recognized that, following the laws of quantum mechanics, the wavelength of a particle of matter is inversely proportional to its momentum. Satyendranath Bose and Albert Einstein further speculated that dense groups of particles moving extremely slowly would fall in unison to their lowest energy state. In de Broglie's terms, these Bose-Einstein condensates, or BECs, form when the particles move so slowly that their wavelengths are as large as the distance between them and overlap. Effectively, one wave function may then describe all the particles as if they were a single superparticle.
This condensed state requires exceptionally cold particles, and optical methods are central to the process of cooling matter to such an extreme. For example, in the "optical molasses" technique, pairs of counterpropagating laser beams are red-detuned relative to the transition between the material's ground and excited states. A particle moving in the opposite direction of one of the beams finds that the photons are blueshifted into resonance and can be absorbed, but that the photons from the other beams are Doppler-shifted even farther out of resonance and cannot. The particle re-emits the photon in a random direction and returns to its ground state, from which it can absorb another photon.
Each absorption event carries with it the demands of the conservation of momentum of the two-particle system, and the cumulative effect is a slowing of the particle. Because there are multiple pairs of counterpropagating beams, the direction of motion of the individual particles does not matter; each will encounter photons coming straight at it that it can absorb and that will slow it down.
Realization of this new state of matter finally came in 1995, earning colleagues Eric A. Cornell and Carl E. Wieman, along with Wolfgang Ketterle, the 2001 Nobel Prize in physics. Since then, groups around the world have produced bosonic (using particles whose spins are an integer) and fermionic (using particles whose spins are an odd multiple of 1/2 condensates; have demonstrated that they may display behaviors analogous to lasers but producing coherent matter waves; and have employed them to slow light and even to bring it to a halt. And the investigations into these material systems continue.
At Universität Innsbruck in Austria, a team of scientists has demonstrated that the principles behind the formation of Bose-Einstein condensates from atomic gases also apply to molecules made of fermionic atoms. In a report of the work, the group explained how the cooling of lithium atoms produces bosonic dimers that form a condensate. Beyond offering insight into this unique form of matter, the phenomenon promises to shed light on high-temperature superconductivity.
Rudolf Grimm, a professor at the university's Institut für Experimentalphysik, explained that this transition is similar to that displayed by electrons in a superconductor. To carry a supercurrent, fermionic electrons form bosonic Cooper pairs. By applying a magnetic field to the condensate, the Innsbruck researchers influence the combination process, yielding a Cooper-paired superfluid or a strongly interacting superfluid regime. "Our molecular BEC is a unique toy model for studying these phenomena," he said.
The setup incorporated an optical trap for evaporative cooling, a process in which the most energetic particles are allowed to escape so that the average energy per particle left in the trap, and hence the temperature, is reduced. The trap comprised a 1030-nm Gaussian beam from an Elektronik Laser System GmbH VersaDisk Yb:YAG laser that the researchers focused to a beam waist of 23 µm and loaded with lithium-6 atoms at a temperature of 80 µK from another trap (Figure 1). As they reduced the laser power from 10 W to a few milliwatts, the trap held up to 20 times as many atoms as it could if they were fermions, indicating that the particles had formed bosonic Li2 molecules. A calculated final trap temperature of 50 nK and subsequent experiments confirmed the formation of a molecular condensate, which exhibited a lifetime of approximately 20 seconds.
Figure 1. A team in Austria has demonstrated that fermionic lithium atoms may be induced to form molecular Bose-Einstein condensates. The setup incorporates an Yb:YAG optical trap for evaporative cooling of the sample. Courtesy of Rudolf Grimm.
Grimm said that the majority of experiments with condensates employ magnetic traps for evaporative cooling, but he noted that such an approach would not be suitable for the trapping of the lithium molecules. Advances in near-IR laser sources, however, are enabling the high powers required at the initial loading phase. "I expect a great future for optical trapping," he said.
The Innsbruck group plans to continue the work by investigating the superfluid properties of ultracold Fermi gases created from the molecular condensate. The team will improve its optical detection methods, and it will introduce tools that will enable it to observe superfluidity.
Similarly, scientists at the University of Colorado and at the National Institute of Standards and Technology (NIST) in Boulder have produced Bose-Einstein condensates from fermionic potassium-40 atoms. Their approach included the application of a varying magnetic field to tune the interaction of the atoms.
The researchers loaded an optical trap formed by a Spectra-Physics 1064-nm Nd:YAG laser with atoms that they had cooled using optical molasses and evaporative cooling in a magnetic trap. Sweeping the ultracold sample in the optical trap with a magnetic field across a Feshbach resonance -- at which point the energy of a quasibound molecular state equals the energy of two free atoms -- converted a fraction of the fermionic atoms into bosonic molecules, which then formed a condensate (Figure 2). Time-of-flight and spin-selective absorption imaging offered information about the momenta of the atoms and the molecules.
Figure 2. Researchers in Colorado concurrently produced a molecular Bose-Einstein condensate from fermionic potassium atoms. The image in the background displays the cloud of molecules at slightly above the critical temperature; the image in the foreground displays a bimodal momentum distribution, indicating that the molecules have formed a condensate. Courtesy of Markus Greiner.
Markus Greiner of JILA, a collaboration of the university and NIST, said that such experiments might offer practical benefits. The ability to control the sample so that it moves from a fermionic gas to a condensate promises to enable the researchers to explore the transition from a superfluid of correlated pairs of atoms to a condensate. Specifically, the sample in the experiment passed through a crossover regime between the superfluid and condensate states, recently termed "resonance superfluidity."
"This regime is very interesting," Greiner explained, "since it cannot be studied with other fermionic systems, so far, but might be a key concept to better understand high-temperature superconductivity and other, related effects." Potential applications of the knowledge gained might also lie in quantum information, he suggested.
A collaboration of researchers from the US, Japan and France has investigated a condensate similar to a Bose-Einstein, but in a different sort of material system. In its report of the findings, the group discussed the potential application of polariton condensates as novel sources of coherent radiation.
Polaritons are quasiparticles made of photons and excitons, which are themselves quasiparticles comprising electron-hole pairs. Polaritons display bosonic properties, including the propensity to clump together in a single quantum state. The photon component, however, endows a polariton with unique properties. For example, the effective mass of a polariton is 100 million times lighter than that of an atom. As a result, it is theoretically possible to form a condensate of polaritons at temperatures millions to billions times higher than those necessary to create an atomic Bose-Einstein condensate (Figure 3).
Figure 3. Polaritons, half-matter/half-light quasiparticles in a semiconductor, also display bosonic behavior, including the formation of condensates. Researchers have exploited the decay of the photon component of polaritons to create a laser with a much lower threshold pump density and more stable beam characteristics than a photon laser. A plot of the polariton distribution in the energy-momentum plane reveals a sharp peak characteristic of a condensate. Courtesy of Hui Deng. Image by Gregor Weihs.
Hui Deng, a PhD candidate in the department of applied physics at Stanford University in California, explained that the researchers were able to exploit the decay of the photon component of the polaritons in the condensate to create a laser. They used 3-ps pulses from a Coherent Inc. mode-locked Ti:sapphire laser to excite a GaAs microcavity sandwiched between GaAl/AlAs distributed Bragg reflectors and GaAs quantum wells separated by AlAs layers. An Acton Research CCD camera captured the emissions from the sample.
To compare the performance of the polariton laser with a traditional laser, they also experimented on a different spot on the sample where polaritons would not form. They discovered that the threshold pump intensity of the polariton laser was up to two orders of magnitude lower than the photon laser. Moreover, the polariton laser maintained a uniform Gaussian beam profile even at high pump powers that forced the photon laser to produce multiple transverse modes.
Deng said that the next step in the work is to realize a polariton laser that operates at higher temperatures and that is pumped electrically. In this experiment, the sample was maintained at 4 K, but the operating temperature may be able to be pushed to 77 K in a GaAs system and to room temperature in ZnSe or GaN. The addition of dopants to the microcavity and proper electronics and device design may enable electrical pumping.
The researchers also plan to use the exciton-polariton system to investigate the transition between the superfluid and condensate state.
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