Tuesday, July 2, 2013

Are quantum effects ever enhanced in condensed phases?

Previously, I asked the question: are there any condensed phase systems in chemistry where quantum effects [e.g. tunneling, interference, entanglement] are enhanced compared to the gas phase?

Let me clarify. Suppose we take a molecular system X and consider the magnitude of some quantum effect Y in the gas phase. We then put X in some condensed phase environment [e.g., solvent, protein, or glass] and measure or calculate Y.
It is quite possible that Y increases due to what I would call "physically trivial" effects, e.g. a change in the geometry of X which makes Y larger. For example, the polarity of a solvent can decrease the donor-acceptor distance for proton transfer in a molecule and thus increase quantum tunnelling.

To me a physically "non-trivial" effect is where the environment enhances the quantum effect for the same reference system X [e.g., one uses the same geometry of the molecule in the gas and condensed phases]. I am not sure this ever happens. Generally, environments decohere quantum systems.

But I stress that such environmental effects can be far from "trivial" to chemists and biologists. e.g., they can make an enzyme work!

Historically, this distinction between "trivial" and "non-trivial" environmental effects was important and confusing for the Caldeira-Leggett model for quantum tunneling in the presence of an environment. To clarify this issue Caldeira and Leggett added a "counter-term" to the Hamiltonian to subtract off the renormalisation of the potential barrier by the environment. This is discussed in detail in the book by Weiss [page 19 in the second edition]. Chemists call this "counter-term" the solvation energy.

Aside: in nuclear physics these renormalisation effects are observable and calculable, as described in this PRL.