Coherent Control of Molecular Motion

Pulse Shaper

The use of ultrafast pulse shapers to coherently control atomic and molecular quantum states is being intensely pursued by many groups. In our lab, we use a liquid-crystal spatial light modulator (LC-SLM) in a Martinez-stretcher configuration to shape both pulse trains from a Ti:Sapphire oscillator, as well as high energy pulses from an amplifier.

Genetic Algorithms

The range of pulse shapes that can be created with the pulse shaper is enormous. Since the specific pulse required to effect a particular control on a quantum system is rarely known a priori, we use a genetic algorithm (GA) to scour the search space for optimal solutions to this problem.

The GA uses an experimentally-measured fitness funcition to find the appropriate phase mask for the LC-SLM. In one example, the shaper was configured to find the optimal compression phase for a pulse stretched by a dispersive material. The second-harmonic signal of the pulse was used as the feedback.

In contrast to traditional genetic algorithms, we introduced a covariance-matrix adaptation of the evolutionary strategy (CMA-ES). This encourages the search algorithm to manage its search step sizes individually for each search dimension. In both simulation and measurements, we found that the CMA-ES outperformed standard a GA for high-dimensional searches in noisy laboratory environments, converging on a better result faster.

Molecular Motion

We used a short, intense pump pulse, focused into a hollow-core capillary to excite vibrational modes of SF₆ gas. The Raman shifting can be observed in the spectrum of a narrow-band probe pulse that interrogates the molecules shortly after the pump. The probe yields information about the molecular motion and can act as a feedback to the pulse shaper, which shapes the temporal characteristics of the pump.

References

• The pulse shaper used is available from CRi, Inc.