Quantum Bayesian Networks

May 8, 2009

The QIS Workshop

Filed under: Uncategorized — rrtucci @ 2:40 am

Prof. Scott Aaronson (MIT) has just written a blog post about his experiences at the Quantum Information Science (QIS) Workshop (April 23-26 Vienna VA, 2009). (QIS research includes quantum computer research.) This is my comment about his blog post. (I also wrote an earlier blog post about the politics behind the QIS workshop.)

In 2002, a team of researchers published this paper:

Chi02: Andrew M. Childs, Richard Cleve, Enrico Deotto, Edward Farhi, Sam Gutmann, Daniel A. Spielman, “Exponential algorithmic speedup by quantum walk”. Abstract here

Chi02 uses a vector space V spanned by a set of orthogonal states, one state for each node of the following graph.

Fig.1- Chi02 graph

Fig.1- Chi02 graph

Chi02 considers a pure state from V. The state is a traveling wave moving from left to right of the above graph, evolving according to Schrodinger’s equation with a “quantum walk” Hamiltonian (proportional to the adjacency matrix of the graph). Chi02 finds that the quantum walk travels from entrance to exit exponentially faster than the classical random walk. (A sort of quantum tunneling effect)

More recently, Prof. Alán Aspuru-Guzik (Harvard) and Prof. Seth Loyd(MIT) have published this paper with their students:

Moh08:  Masoud Mohseni, Patrick Rebentrost, Seth Lloyd, and Alan Aspuru-Guzik “Environment-Assisted Quantum Walks in Photosynthetic Energy Transfer”. Abstract here

where they claim that a quantum walk (albeit a heavily damped one) can be used to describe photosynthesis. They also claim, without proof, that their model and photosynthesis exhibit the Chi02 effect. Of course, the Moh08 paper doesn’t use its model to predict any observed experimental data.

Aspuru gave a talk about his work at the QIS Workshop. In the words of Scott Aaronson

Fig. 2- Light Harvesting Molecule

Fig. 2- Light Harvesting Molecule

In a talk that was so good, you almost forgot it involved chemistry, Alán Aspuru-Guzik discussed applications of quantum complexity theory to understanding photosynthesis and the design of efficient solar cells (!). …Shown above is a light-harvesting molecule (image snagged from Alán’s slides), which apparently is efficient at concentrating light at its center for essentially the same reason the Childs et al. quantum walk reaches the target vertex exponentially faster than a classical walk:..

And according to Prof. Andrew Landhal (U. of New Mexico)

Just a quick comment on Alán Aspuru-Guzik’s talk, which was indeed fantastic and inspirational…

Well, let’s see. The Chi02 result is for a pure state with quantum coherence over all the nodes of Fig.1. There is no way that you are going to get quantum coherence over the thousands of atoms in Fig.2, for any significant amount of time, when these atoms are sitting inside a stew at room temperature or hotter. The quantum walk MohO8 is considering will degrade in this thermal environment to a classical random walk, so it can’t possibly exhibit Chi02’s purely quantum effect.

I don’t see how a paper that proposes a very speculative model of photosynthesis, makes dubious, amazing and unproven assertions about its connection to complexity theory, and predicts no observed data, can be so inspirational. But maybe I’m totally wrong. (would not be the first time 🙂 ) You be the judge. See Aspuru’s comment (#20) here. Extraordinary claims require extraordinary proof.



  1. Dear R. Tucci,

    Google Alerts (http://www.google.com/alerts) is great, as it told me about this post. I’d rather not spend too much time talking in blogs but rather working on research that ends in peer-reviewed publications. If you read our New Journal of Physics article (New Journal of Physics, 2009) you will see that we do indeed have results for the tree under decoherence conditions. I am talking about coherent evolution for 600 femtoseconds over the lengthscale of tens of nanometers. The physical chemistry community agrees this is going on. See for example, the paper by Collini and Scholes, Science (2009) on ultrafast experimental results on conjugated polymers at room temperature.

    I suggest that you spend some time reading the literature rather than bashing a field in which many are working on right now (including people like Martin Plenio and Birgitta Whaley) without having read the papers. It is too easy to make negative assertions without having a good basis. Happy to discuss POLITELTY if you have good arguments.

    PS. I enjoyed your Paulinesia paper.

    Comment by Alan Aspuru-Guzik — May 9, 2009 @ 10:55 pm

  2. Prof. Aspuru-Guzik. Thanks for your comment and for the extra information.
    Roberto Tucci Gómez

    Comment by rrtucci — May 10, 2009 @ 2:27 am

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