RESULTS

As an example of the possibilities of this of this new type of frequency standard, we show some Ramsey patterns obtained using this apparatus. The fraction of atoms detected in the level $\left\vert F = 4, M_{F} =0 \right>$ is plotted as a function of the microwave frequency detuning in Hz. Figure 4 corresponds to a launch of 110 mm above the cavity, corresponding to a width of 1.7 Hz, obtained using one-laser pumping (maximum of 14% of atoms in $M_{F}=0$). Each point is an average of 10 measurement cycles and the S/N is 30. Figure 5 corresponds to a launch of 57 mm obtained using the two-laser pumping scheme. To verify the mechanism the signal at the maximum of the central fringe was measured as a function of the angle of polarisation of the repumper relative to the bias B-field (5$\mu$T in the horizontal direction)(figure 6). This is a measurement of the number of atoms in the $\left\vert F = 3, M_{F} =0 \right>$ level and shows, that the relative number of atoms in this state can be increased by a factor of $\sim$2.5 without significant heating.

Figure 4: Central Ramsey fringe corresponding to a 11 cm launch above the cavity.

Figure 5: Ramsey pattern obtained with two laser pumping.

Figure 6: Fringe amplitude as a function of the angle of polarisation of the repumping light ($+$), and without repumping light ($\bullet$)

We also observed anomalies on some experimental patterns which cannot be explained using the standard model. We beleive that the central fringe amplitude may be affected by leakage of microwave from the waveguide. In order to illustrate this effect, Figure 7 shows a pattern obtained by numerical simulation. For a typical launch (T-0.5 s, $\tau$=2 ms, $b\tau=\pi$) and the velocity distribution is assumed gaussian: $\sigma$ = 10 ms) the atoms were submitted to an excitation of the amplitude 0.002 $\times b$ during the period $T$ (we assumed a constant amplitude and phase). We clearly see the collapse of the central fringe. This behavior is different to that due to incorrect power level as it affects mainly the central fringe.

Figure 7: Ramsey pattern showing the effect of a small, constant microwave excitation.

This result emphasizes the need for a well designed microwave cavity and further investigation of a possible frequency shift. [8] [online]