Phase Control of Rotational Wave Packets and Quantum Information

Lasers can create rotational wave packets in gas-phase molecules which periodically revive as field-free, aligned distributions. We control the wave packet evolution with relatively weak laser pulses at fractional revivals which modify the phase between wave packet components. We demonstrate two phase control effects in oxygen: coherently switching revivals off and on, and doubling the revival frequency. When viewed as a quantum logic system, these effects correspond to a Hadamard and a T operation.

Figure 1

The upper panel shows the measured time evolution for an unmodified rotational wave packet in oxygen. The lower panels show second order alignment as seen by the modifying pulses. The dashed lines indicate the approximate location in time of the peak of the 50 fs FWHM pulses that performed the phase modifications.

Figure 2

The solid curve shows the coherent switching off and on of revivals in net alignment from $\pi /2$ phase changes applied to cross revivals at 4.23 and 10.1 ps after the aligning pulse as measured in oxygen. For comparison, the dotted line shows an unmodified wave packet, at arbitrary vertical scale, measured under similar conditions.

Figure 3

The solid curve shows the measurement of a doubled revival frequency in oxygen. Conventional revivals, shown by the dotted curve at arbitrary scale, were modified by the addition of $\pi /4$ of phase at the cross at 4.23 ps, and $\pi /2$ at the revival 5.7 ps after the aligning pulse.