Chris Fuchs is a research staff member at Bell Labs, Lucent Technologies,
an Adjunct Professor of Physics at the University of New Mexico, and an
associate editor for the journal Quantum Information and Computation.
His interests range from quantum channel theory to quantum cryptography
to quantum foundations. Perhaps the most outlandish thing he has done
is publish a collection of quantum emails; see
In the neo-Bayesian view of quantum mechanics that Appleby,
Caves, Pitowsky, Schack, the author and others (maybe D'Ariano?) are developing,
quantum states are taken to be compendia of partial beliefs about potential
measurement outcomes, rather than objective properties of quantum systems.
Different observers may validly have different quantum states for a single
system, and the ultimate origin of each individual state assignment is
taken to be unanalyzable within physical theory---its origin, instead,
ultimately comes from probability assignments made at stages of physical
investigation or laboratory practice previous to quantum theory. The objective
content of quantum mechanics (i.e., the part making no reference to observers)
thus resides somewhere else than in the quantum state, and various ideas
for where that "somewhere else" is are presently under debate---there
are adherents to the idea that it is purely in the "measurement clicks,"
there are adherents to the idea that it is in intrinsic, observer-independent
Hamiltonians, there are adherents to the idea that it is in the normative
rules quantum theory supplies for updating quantum states, and so on.
This part of the program is an active area of investigation; what is overwhelmingly
agreed upon is only the opening statement of this abstract---that quantum
states are compendia of beliefs. Still, quantum states are not simply
Bayesian probability assignments themselves, and different representations
of the theory (in terms of state vectors or Wigner functions or C*-algebras
and the like) can take one further from or closer to a Bayesian point
of view. It is thus worthwhile spending some time thinking about which
representation might be the most propitious for the point of view and
might, in turn, carry us the most quickly toward solutions of some of
the open problems. In this talk, I will explore various issues to do with
the above and explain why I prefer a representation of quantum mechanics
that makes crucial use of a single probability simplex.