Use of the relative phase in a bichromatic field pulse to control a quasienergy gap.

We show experimentally and theoretically how the relative phase in a short, linearly polarized microwave pulse consisting of two phase-locked frequencies is effective for strong-field quantum control of He Rydberg atoms when the frequency ratio p:q is 3:1. It is also effective when p:q = 2:1 but less effective for 3:2. Parameters in our experiment are similar to those investigating quantum control of tightly bound atoms or molecules with ultrashort pulsed lasers.