Wednesday, September 29, 2010

A smart bird brain

Today Simon Benjamin (Oxford) gave a really nice talk, Will we succeed in creating entanglement in macroscopic quantum systems? Can it exist in living systems?

Do living systems already harness quantum mechanics in a "non-trivial" way?
How do we define "non-trivial"? A good criteria is whether biologists and chemists need quantum physicists to understand the phenomena of interest?

Specifically, Simon focused on recent theoretical work relating to the fascinating question of how migratory birds navigate (discussed in a previous post).
Specifically, they did a detailed analysis of decoherence and entanglement in the radical pair model for magnetoreception (where the small magnetic field of the earth causes a different decay rate for singlet and triplet channels). One thing I learnt is that a key to making this model work is that  the different spins experience a different spin anisotropy associated with the hyperfine interaction. (How can we justify this asymmetry?)

Recent experiments showed that a very weak oscillatory magnetic field resonant with the electron Larmor frequency will stop the compass working.

unknown rate = 10^4 per sec to disrupt compass

The Oxford group estimated the decoherence time must be shorter than 100 microsec.
This may seem long since  N @ C60  has a decoherence time of 80 microsec.

An open question is the biomolecular justification for the radical pair model and whether the spins need to be in a protected environment.
A candidate molecule cryptochrome have too many nuclear spins.

This is fascinating. Is it "non-trivial"? Well it is worth noting that 
1.  all the theoretical machinery used was more than 50 years old [i.e. preceded quantum information theory].
2. the entanglement involved involves two electrons and just two molecules and so is not "macroscopic"

A question was ask about whether the bird had evolved to develop this feature because it has a survival advantage. It is important to remember that not every function of an animal has been optimised by evolution, as discussed here.

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