Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with 'dead' time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise of the laser interrogating the atomic transition 1,2 . Despite recent advances in optical clock stability achieved by improving laser coherence, the Dick effect has continually limited optical clock performance. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock based on the interleaved interrogation of two cold-atom ensembles 3 . This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability of 6 10 17 / for an averaging time in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce 2 the standard quantum limit of clock stability, achieving a spectroscopy line quality factor Q 4 10 15 .An optical atomic clock operates by tuning the frequency of a laser (optical local oscillator, OLO) into resonance with a narrowband, electronic transition in an atomic
We observe interaction-induced broadening of the two-photon 5s-18s transition in 87 Rb atoms trapped in a 3D optical lattice. The measured linewidth increases by nearly two orders of magnitude with increasing atomic density and excitation strength, with corresponding suppression of resonant scattering and enhancement of off-resonant scattering. We attribute the increased linewidth to resonant dipole-dipole interactions of 18s atoms with blackbody induced population in nearby np states. Over a range of initial atomic densities and excitation strengths, the transition width is described by a single function of the steady-state density of Rydberg atoms, and the observed resonant excitation rate corresponds to that of a two-level system with the measured, rather than natural, linewidth. The broadening mechanism observed here is likely to have negative implications for many proposals with coherently interacting Rydberg atoms.
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