Quantum interference in microwave multiphoton transitions.

We observe interference in the resonant microwave multiphoton transition probability between the 21s state and the 19,3 Stark state of K in a static field as a function of the duration and intensity of a microwave pulse at 9 GHz. The 19,3 state adiabatically connects to the 19f state in zero field. The ac Stark effect shifts the 21s state through a multiphoton resonance with the 19,3 Stark state during both the rising and falling edges of the microwave pulse. The transitions that occur during each traversal of the resonance lead to an interference in the net transition probability as a function of the relative phase accumulated by each state during the pulse. We measure both the frequency of the interference oscillations for different detunings from resonance and the amplitude of the fringes for different degrees of pulse asymmetry. The latter results are in complete agreement with the predictions of a Landau-Zener model for the resonance transitions.