| We have developed a prototype 87Rb fountain clock, and have used it to measure the frequency shift due to cold-collisions. We measure the shift by varying the atomic density on successive launches, looking for the relative shift of the simultaneously acquired Ramsey fringes. We find the shift to be 50 times smaller than for 133Cs, allowing a 87Rb clock to operate at a higher stability for the same accuracy. The atomic state detection has S/N = 500, but the instability of our quartz crystal local oscillator limits the short-term stability to deltanu/nu = 1.8 x 10-13 for 1 s of integration.; We have studied the cavity pulling frequency shift due to the interaction of atoms with their own radiated field in a microwave cavity. Furthermore, we have used this effect to cancel the collision shift. This technique has many advantages, including insuring immunity to long-term variations in the number of trapped atoms.; We have developed a novel method to measure the frequency response of a microwave cavity using the AC Zeeman shift due to a strong, detuned sideband. This technique seems to be widely applicable, as there are a number of different cavity designs that are difficult to probe electrically with accuracy.; We have developed a new method to directly measure the population difference between two clock states using FM absorption spectroscopy. This method could be used to achieve atom-shot-noise limited detection even for the high atomic densities possible (due to the small collision shift) in future Rb clocks.; We have made measurements of power-dependent loss-of-contrast for Ramsey and Rabi interactions in the clock microwave cavity, and have looked for possible effects due to microwave recoils.; One can achieve higher stabilities and eliminate dead time, thereby reducing the requirements for the local reference oscillator, by juggling---launching with a delay smaller than the Ramsey interrogation time. We have demonstrated a 2-ball juggling 87Rb clock that can be used to study the collisions between juggled balls of atoms.; Finally, we look to the future of cold-atom clocks, including microgravity clocks, and the role of microwave clocks in relation to the emerging field of all-optical clocks. |
