Monday, August 24, 2009

Some questions about quantum control

I am struggling my way through trying to figure out what quantum control is really about and how quantum it is. On the plane to Toronto I read some of the book by Shapiro and Brumer. A few questions I have include:

When can quantum control be semi-classical?

Why is it possible to perform quantum control in condensed
phase systems at room temperature?

How much decoherence is needed to destroy it?

The basic idea of quantum control is to use multiple coherent laser pulses to create an initial state which is a coherent superposition of several quantum states. This state then evolves in a manner where the final products (branching ratios) depend on the relative phase and amplitude of the initial laser pulses.

One example is the Tannor-Rice scheme for the reaction
A+BC -> AB + C
which is shown in the figure below.

By the Franck-Condon principle absorption of a photon by the system in the electronic and vibrational ground state will produce a Gaussian vibrational wavepacket in the excited electronic state.
However, if one applies the relevant pulse sequence one can produce an excited state which has a vibrational wave packet with a net momemtum to the left or the right.

A similar issue arises with photo-isomerisation. Usually this occurs via a conical intersection between two potential energy surfaces.
One should be able to enhance the photo-isomerisation yield by producing an excited state in which the vibrational wave packet is a coherent state with a momemtum pointing towards the conical intersection.

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