The beam splitter is a fundamental and much used device in optics, and its counterpart for matter waves is highly sought after to facilitate experiments in atom interferometry. Giovanni Luca Gattobigio and co-workers from France and the United States have now demonstrated a photonics-assisted design that uses light to guide propagating matter waves into several different beam paths (Phys. Rev. Lett. 109, 030403; 2012). The device can be configured to operate as either a beam splitter or a beam switch.

Two laser beams are crossed with a 45° angle between them in an X configuration, creating four possible paths from the point of intersection. One of the laser beams is used as a guide for rubidium-87 atoms that have been out-coupled from a Bose–Einstein condensate. The atoms have a mean velocity of 13 ± 2 mm s−1, and the crossing takes place 700 μm downstream from the condensate trap.

Whether the system functions as a beam splitter or as a switch depends on the relative powers of the two crossed beams. When the crossing beam is much weaker than the laser beam guiding the atoms, no splitting occurs and the path of the atoms remains unchanged due to only weak coupling between the different modes in the crossing region. When the powers are roughly equal, the system enters the switching regime and the path of the atoms is completely switched to be guided by the crossing beam. Between these two regions exists the splitting regime, where all four beam paths become populated.

Credit: © APS 2012

To better understand the splitting regime, the researchers performed both quantum and classical numerical simulations. The split-step Fourier algorithm was used to compute the dynamics of the quantum wave packets, and a direct simulation Monte Carlo method allowed the atoms to be treated classically. Both quantum and classical simulations produced very similar results and the authors report that the splitting regime results from chaotic scattering dynamics. Within this chaotic region, a slight variation of the initial conditions changes the output path such that on average the guided atoms populate all possible paths.

The researchers say that many applications in guided atom optics, such as sensing and interferometry, could benefit from their device. For example, the smaller effective wavelength of matter waves could potentially provide interferometry that is several orders of magnitude more sensitive than optical interferometry.