State-selective Rabi and Ramsey magnetic resonance line shapes

We carry out state-selective Rabi and Ramsey magnetic-resonance experiments on ground-state ¹³³Cs(F=4) atoms. Novel line shapes are obtained, which exhibit very sharp features with a width much smaller than the inverse duration of the magnetic-resonance pulse. The sensitivity of ordinary magnetic-resonance experiments with total spin F >1/2 is significantly less than the Heisenberg limit, which can be exactly realized only with maximally correlated spin states. We show that the state-selective resonances yield sensitivity very close to the Heisenberg limit, without any state preparation beyond ordinary optical pumping.