Strong-laser-induced quantum interference

Molecules are expected to be promising information devices(1-8). Theoretical proposals have been made for logic gates with a molecular wave packet modulated by a strong femtosecond laser pulse(9-15). However, it has not yet been observed how this changes the population of each eigenstate within the wave packet. Here we demonstrate direct observation of the population beating clearly as a function of the delay of the strong laser pulse. The period is close to the recurrence period of the wave packet, even though a single eigenstate should have no information on the wave-packet motion. This unusual beat arises from quantum interference among multiple eigenstates combined on a single eigenstate. This new concept, which we refer to as 'strong-laser-induced interference', is not specific to molecular eigenstates, but universal to the superposition of any eigenstates in a variety of quantum systems, being a new tool for quantum logic gates, and providing a new method to manipulate wave packets with femtosecond laser pulses in general applications of coherent control(16-20).