rf-Induced Quantum Interference between Low-Field Stark Levels in Rydberg States of Sodium

Quantum phase interference is shown to occur in sodium Rydberg states ($n=26-31\mathrm{}$) when they are exposed to weak rf electric fields during the first few volts per centimeter of an increasing electric-field ramp. The effect is observed as an oscillation of intensity in a component of the selective-field-ionization signal as a function of applied frequency in the range 250-990 MHz. The phenomenon is explained in terms of dressed states, and the frequencies of the peaks are modeled in good agreement with experiment.