Bringing cold atoms to the classroom gives undergraduate students a hands-on way to learn about physics – and could inspire lifetime careers using photonics to manipulate atoms. Bringing cutting-edge physics into the undergraduate classroom is not always easy. “Cold atoms” (–7 torr), atom sources, ion pumps, magnetic fields, and optical and laser alignment is required. Figure 1. View of the miniMOT package showing the evacuated glass chamber located between the magnetic coils. Courtesy of ColdQuanta Inc. Within the field of experimental physics, researchers are increasingly aware of the importance of intensive experience with cold atoms. Now, a three-week project allows assistant professor Dr. Heather Lewandowski’s senior physics students at the University of Colorado at Boulder to get that experience through cooling and trapping clouds of rubidium atoms. Luckily, they don’t spend most of that time setting up or finely tuning their equipment; instead, they spend it on their experiments, thanks to a preassembled, prealigned setup called the miniMOT kit. Figure 2. Dr. Heather Lewandowski and a student work with the miniMOT at the University of Colorado at Boulder. MOT, which stands for magneto-optical trap, was coined by the physics research community. The miniMOT kit, which was developed by Boulder startup ColdQuanta Inc., allows undergrad students to jump over setup hurdles, right into the study of cold atoms. Rainer Kunz, ColdQuanta’s CEO, predicts that the device will become as indispensable to college physics laboratories as the oscilloscope is today. The central feature within the miniMOT package is an evacuated glass experimental chamber that allows laser-beam access along three mutually perpendicular axes. This geometry is critical for laser cooling of target atoms within the chamber. The chamber is sealed and shipped with the required vacuum, which means that users need no vacuum experience or equipment. The system uses rubidium atoms, which respond to general-purpose 780-nm lasers. An active rubidium dispenser resides within the evacuated chamber to make it easy to inject the material for experiments. The chamber also holds a nonevaporable getter and an ion pump to maintain the vacuum. All of the electronic circuits to power the ion pump and the rubidium source are integrated into the package, eliminating the need for external ion-pump controllers and current supplies for the vacuum system. The design includes the hardware needed to mount optics (mirrors and wave plates) along the vertical axis. The enclosure’s footprint is 2 (38 in2) and volume is ~3.1 liters (~190 in3). For the hardware included with the miniMOT kit, see “Just add lasers” on the right. Some instructors may be concerned that the convenience provided by prealigned kits could rob students of the learning experience of fine-tuning the experimental setups associated with research in atomic optics. That is a trade-off decision the instructor must make: Do I offer a class that covers many aspects of experimental physics in one semester or one that focuses on fewer experiments and provides time for preparatory alignments? For those who choose the latter, students can remove and restore the optics or use the nonkit version of the miniMOT. All kinds of experiments The following types of experiments with cold atoms are now accessible to a wide range of students and researchers through use of the miniMOT kit: • Demonstrating laser cooling. Students can go from classroom theories of how lasers can reduce atomic energies to actually accomplishing laser cooling in practice. They will see how a properly arranged laser configuration can create a “cloud,” faintly visible in the near-infrared, of cold atoms at the intersection of the beams. They also can experience how a secondary, parallel set of laser beams, detuned about 0.01 nm, can be used to re-pump errant atoms to maintain the trapped cloud. • Demonstrating magnetic trapping. Laser cooling reduces the velocity of the atoms, but the further confinement (trapping) of the atoms within a small region is effected by applying a tailored magnetic field. Students can energize two coils on the miniMOT’s supporting rods to establish an anti-Helmholtz magnetic configuration, which creates a small region of zero magnetic field within the glass chamber where a cloud of cold atoms will become trapped (see Figure 3). The zero-field region forms a stable location for the atoms in conjunction with their spin magnetic moments. Figure 3. View of cold-atom cloud (central bright spot) trapped in the zero-field region of the two coils. • Measuring atom number. Once an atomic cloud has been created and confined, students can experimentally estimate the number of atoms in the cloud. The brightness of the ensemble of photons, continuously re-radiated by the trapped rubidium atoms, is related to the number of atoms. By using a lens and a CCD sensor suitable for detecting the near-infrared radiation, a sector of the sphere of radiation can be imaged and its brightness measured – raw data needed to estimate the number of atoms. This experiment also trains students to account for the many error sources inherent in this type of research. • Measuring the temperature of the atom cloud, which is proportional to the average of the square of the atoms’ velocities. To deduce temperature, the average velocity can be revealed by disabling the magneto-optical trap and observing how quickly the atomic cloud expands. Data for cloud size versus time can be taken by sequential CCD images of the cloud’s shadow or by sequential images of the cloud fluorescing in response to brief laser pulses. • More advanced research. Besides the above examples, the miniMOT package permits advanced students and researchers to conduct more demanding experiments in atomic physics. These include applications to quantum entanglement, quantum optics, atomic clocks and electromagnetically induced transparency, or “slow light.” Dr. Andrew M.C. Dawes, assistant professor of physics at Pacific University in Forest Grove, Ore., has used the miniMOT to form a long, narrow (anisotropic) cloud of cold atoms to study the interaction of photons with the rubidium atoms. “By sending light along the long axis of the cloud, the light-matter interaction region is maximized,” he said. “These so-called anisotropic atom clouds combine the benefits of cold atomic media with long cloud geometry to achieve strong light-matter coupling. The anisotropic atom cloud is a promising medium for experimental tests of all-optical information processing.” He noted that the setup also enables experiments such as nonlinear wave mixing in cold atoms, tests of light-guiding in the anisotropic MOT and transverse optical pattern formation. With the use of additional hardware, future experiments incorporating the miniMOT will produce even colder atomic clouds (20 μK or colder) using sub-Doppler laser cooling. Lab guides to come To promote cold-atom research in the undergraduate physics curriculum, the University of Colorado’s Lewandowski is developing instructor and student lab guides for working with the miniMOT kit. These free guides, supported by a grant from the National Science Foundation, should be available in late 2011. Figure 4. View of technical team with the miniMOT. Importantly, the guides will help instructors evaluate how well the students achieve the educational goals. They are designed to confirm that the students have learned the appropriate laboratory skills and physics principles in the process of carrying out their experiments. In recent years, the photonics industry has searched for ways to inspire students to enter photonics-related careers, in part because of a shortage of qualified job applicants. Hands-on experience at the undergraduate level could help do just that. Meet the author John Cowley is a retired aerospace engineer and freelance writer living in Boulder, Colo. Just add lasers (and a few other things) The miniMOT is available preassembled and prealigned, with most of the additional hardware needed to produce cold-atom clouds in the undergraduate setting. Side view of the miniMOT kit The miniMOT kit includes: • Optics for manipulating the impinging laser beams (periscope, beamsplitting and polarization components). • Mirrors to facilitate alignment of the laser beams for atom cooling. • Coils to create the trapping magnetic field. • A small black-and-white CCD camera for imaging the trapped cold-atom cloud. • A 15 x 24-in. optics breadboard, upon which the supporting components are mounted and prealigned. The user need only provide the lasers, laser interface, video monitor and power supply for the magnetic coils.