 Time-of-flight atom detection

Time-of-flight atom detection measures particles after releasing a trapped atomic cloud and allowing it to expand for a known time. For ballistic motion, $\mathbf{r}(t)={\mathbf{r}}_0+{\mathbf{p}}_{0}t/m+\mathbf{g}{t}^2/2$, so after accounting for gravity the late position largely reflects the initial momentum when the initial cloud size is sufficiently small.

A position- and time-resolving detector can record individual arrivals and form spatial pair correlations. In fermionic atom antibunching, such correlations probe suppression of nearby detection pairs for the prepared state. Finite source size, interactions during early expansion, detector resolution and detection efficiency limit a literal initial-momentum interpretation; expansion is part of the experimental model, not merely a delay before measurement. A microchannel plate detector is one implementation for suitable metastable atoms.