OVERVIEW OF EXPERIMENT

The physical arrangement of the apparatus is shown in figure 1. The magneto-optical trap (MOT), used as a source of cold atoms, comprises three orthogonal pairs of laser beams of the requisite polarisations ( $\sigma^{+}\ \sigma^{-}$) which intersect in a region where a gradient of magnetic field is produced by a pair of anti-Helmholtz coils [6] [online] . Typically there are $10^{7}$ atoms in the trap after a 1 s loading time. These atoms are launched upwards to form a fountain of cold atoms using the following sequence: The magnetic field of the trap is switched off, to leave the atoms in optical molasses. The atoms are then launched upwards by introducing a frequency difference between the vertical beams so that the rest frame of the molasses is moving upwards at the required velocity ("moving molasses technique", [4]) and the cooling gives a narrow spread in velocities (the laser detuning below resonance is changed smoothly from -10 MHz to -65 MHz during this cooling period). The final stage of the launch sequence is to optically pump all the atoms into the lower hyperfine level (we discuss state selection in section 3). After launching, the atoms pass upwards though a 12 mm hole in a microwave cavity (a shorted section of X-band waveguide located 40 mm above the trap centre) and after a period of 0.5 s (for our highest launches) they fall back under gravity passing through the cavity a second time. Both the atoms which have undergone a transition and the total number of atoms are measured by monitoring the fluorescence from a 6 mm diameter probe beam (the photodiode and lens have a combined efficiency of 5%).

Figure 1: Experimental arrangement of the atomic fountain