Assessment of a time-of-flight detection technique for measuring small velocities of cold atoms

Abstract

A low noise time-of-flight detection system for laser cooled atoms has been constructed and incrementally optimized. Here, a thorough description of the construction is presented along with an analysis of the capabilities of the system. The quality of the detection ͑the resolution, the reproducibility, the sensitivity, etc.͒ is crucial for, e.g., the ability to see details in the velocity distribution profile, which is of interest for fundamental studies of statistical physics and of the laser cooling processes, and for detection of small initial velocities of an atomic cloud, important, e.g., when studying small drifts induced by Brownian motors and ratchets. We estimate the signal-to-noise ratio of our signal to be better than 1000:1 for a typical single shot, and we discuss the effect of the initial atomic cloud size, the probe size, and the effects of the wave packet spread during the fall time on the measured quantities. We show that the shape of the velocity distribution is well conserved during the mapping done in the detection, i.e., in the convolution with the probe beam, and that velocities as small as a few percent of the single photon recoil velocity can be resolved.

Experimental measurement of efficiency and transport coherence of a cold-atom Brownian motor in optical lattices

“…To access the mean momentum spread (the kinetic temperature) δp 2 in Eq. ( 4), we use a time-of-flight technique that enables fast and accurate measurements of the distribution of the momentum, δp 2 z [28]. In the experiment we, adjust the potential depths by controlling the intensities in the lattice beams such that A 1 = A 2 .…”

Directed transport with real-time steering and drifts along predesigned paths using a Brownian motor

“…The drift velocities are constant for a fixed translation, and the change in direction when the potentials are translated appears to be very fast. To further investigate this, we have repeated the same set of relative potential translations, but on a shorter time scale, and measured the temperature using a (destructive) time-of-flight technique [31]. Figure 2c shows that the reaction time for the atoms to reach the new steady state is less than 1 ms, i.e., is not resolvable, given the time resolution and uncertainty of our control and detection systems.…”

Bimodal momentum distribution of laser-cooled atoms in optical lattices

“…The experimental set-up has been described in detail elsewhere (e.g., in Ref. [65]), and therefore the present description is kept brief.…”

“…The velocity distributions are observed by using the time-of-flight method [65], where atoms are released from the optical lattice and are allowed to expand under free fall. The expansion is then measured by a laser probe.…”