BEC Interferometry

Overview

We have demonstrated a BEC interferometer in a waveguide. A double pulse of a standing light wave splits the condensate into two packets of equal size. The two packets propagate in opposite directions down the axis of the waveguide. A single pulse then reverses the momentum of the two wave packets simultaneously. A second double pulse reads out the phase shift, and is applied when the wave packets overlap. We can apply a magnetic gradient across the waveguide to induce a phase shift. The coherence of condensates in the guide is observed up to 10 ms with maximum separation of 120 mm.

The experimental apparatus consists of three vacuum chambers, as shown in the schematic at right:

• MOT chamber: Cools and traps ⁸⁷ Rb, |F = 1, m_F= -1>
• Glass cell: Further cools atoms to a few microkelvins using forced RF evaporative cooling
• Guiding chamber: Finish evaporation and then demonstrate interferometry, all in a microchip trap.

The three-chamber approach allows us to open and close the guiding chamber without affecting the vacuum integrity of the rest of the system.

Technical Synopsis

We begin with a cloud that has been cooled to a few microkelvins in the glass cell chamber. We then release confinement in the axial direction, allowing the atoms to travel towards the third chamber. While the atoms are traveling, we use several coils to apply fields which slow the atoms. The atoms are moving at about 10-20 cm/s as they are coupled into the magnetic guide on the chip. The guide is produced by the field from current running through microfabricated wires, added to a transverse bias field. During transfer, the cloud expands along the transfer/guide axis; it must be focused and collimated before evaporation towards BEC can begin again. To that end, we use a linear potential (produced by anti-Helmholtz coils that are wrapped around the chamber) to stop the cloud over the center of the chip. After the atoms reach the center of the chip and are slow enough, the current in a wire oriented perpendicular to the guiding wire is ramped up. This produces a dimple in the longitudinal bias field and creates tight axial confinement. 170,000 atoms are loaded into this magnetic T-trap, and after a short sweep of forced evaporation we get ~10,000 atoms in the condensate. The "Chip BEC" graphic at right illustrates the process; note that the time of flight image shows a larger cloud of about 70,000 atoms.