While the success of quantum mechanics was beyond question, the notion of a theory based on stationary wave functions instead of moving particles was unsettling, and to show the connection between the classical and quantum theories Schrodinger constructed wave packets, coherent superpositions of harmonic oscillator wave functions in which the probability density moved just as a classical particle would. The case Schrodinger chose, the harmonic oscillator, is unique in having evenly spaced levels, so that once created, the wave packet lasts indefinitely. In reality most physical systems are not composed of evenly spaced levels, and wave packets constructed in such systems disperse in time.

Wave packets remained theoretical constructs until the advent of mode-locked lasers, which have allowed their formation and detection in atomic and molecular systems. Recent technical developments of short pulse laser and pulse shaping have offered fascinating possibilities of quantum control of these systems and even quantum information processing with wave packets. “Normal” wave packets, however, which are coherently created by short optical pulses in general, show dispersions and are immediately destroyed due to decoherence.

Alternatively, nondispersing wave packets (Fig. 1) survive for an extremely long time without showing any dispersion, as was observed by us very recently (Fig. 2). Unlike the normal wave packets, they are immune to decoherence. In terms of classical dynamics, the nondispersing wave packets are constructed based on the nonlinear resonance effects, in which nonlinear phase-locking of the wave packet by a resonant driving field plays crucial role in the robustness of the wave packets against decoherence. Furthermore, it allows to control the wave packet dynamics, as have been demonstrated in our experiments where the wave packet has been sped up or slowed down by exploiting the phase locking. Thus, a use of the nondispersing wave packets should be quite promising for decoherence-free quantum control in multi-level quantum ladder systems.

In the present project we aim to fully understand still unknown properties of the nondispersing wave packets and demonstrate their applicabilities to decoherence-free quantum control of atoms and molecules.

Figure caption

Fig. 1 Nondispersiong wave packet

Fig. 2 Ionization signal of the nondispersing wave packet for 100-ns and 3100-ns coarse delays.