Wherein I give some evidence that, despite this webpage, I am a practicing physicist.
In this paper, I try to cause some good-natured trouble. The issue is, when will we ever stop burdening the taxpayer with conferences devoted to the quantum foundations? The suspicion is expressed that no end will be in sight until a means is found to reduce quantum theory to two or three statements of crisp physical (rather than abstract, axiomatic) significance. In this regard, no tool appears better calibrated for a direct assault than quantum information theory. Far from a strained application of the latest fad to a time-honored problem, this method holds promise precisely because a large part---but not all---of the structure of quantum theory has always concerned information. It is just that the physics community needs reminding.
This is predominantly an abridged, more tree-friendly version of "Quantum Mechanics as Quantum Information (and only a little more)." There is, however, a significant number of changes of phrase throughout, along with a small amount of new material in Sections 3 and 5.
The world we live in is well-described by quantum mechanics. What should we make of that? In a way, the answer to this question was once less positive than it is today. For although quantum theory is a tool of unprecedented accuracy in predicting and controlling the phenomena about us---and by way of that is the basis of our technological society---the intellectual lesson we have come to derive from it has been one largely of limitations. The best place to see this attitude at work is in a standard presentation of the Heisenberg uncertainty relations. It is almost as if the world were holding something back that we really had every right to possess: the task of physics, or so it was believed, is simply to sober up to this fact and make the best of it.
In contrast to this textbook lesson, the last five years have seen the start of a significantly more positive, almost intoxicating, attitude about the basic role of quantum mechanics. This is evidenced no more clearly than within the small, but growing, community of workers in Quantum Information Theory and Quantum Computing. The point of departure in both these disciplines is not to ask what limits quantum mechanics places upon us, but instead what novel, productive things we can do in the quantum world that we could not have done otherwise. In what ways can we say that the quantum world is fantastically better than the classical world?
Quantum States: What the Hell Are They?
This collection of letters traces my transition from taking the quantum state's essence as information or knowledge, to being instead freely-chosen Bayesian degrees of belief, and finally to being nothing more than the pragmatic gambling commitments one endorses in any given instance. As I (almost) put it to one of the correspondents in this collection: "I think there is no doubt I've gone through a phase transition. For all my Bayesian rhetoric in the last few years, I simply had not realized the immense implications of holding fast to the view that "probabilities are subjective degrees of belief." Of course, one way to look at this revelation is that it is a reductio ad absurdum for the whole point of view---and that will be the first thing the critics pick up on. But I view it as a godsend. For with this simple train of logic, one can immediately stamp out the potential reality/objectivity of any of a number of terms in quantum theory that might have clouded our vision. With so much dead weight removed, the little part left behind may finally have the strength to support an ontology."
In this note, I try to accomplish two things. First, I fulfill Andrei Khrennikov's request that I comment on his "Växjö Interpretation of Quantum Mechanics," contrasting it with my own present view of the subject matter. Second, I try to paint an image of the hopeful vistas an information-based conception of quantum mechanics indicates.
This is the third and final installment of three in the Cerro Grande Fire Series. It reflects the following thought. What has always struck me as most wonderful in quantum mechanics is its indication of how our world may be more malleable than was thought in classical times. With our experimental interventions into nature, we---in the capacity of physical systems and nothing more---may have the opportunity to shape the world in unforeseen and perhaps significant ways. This document catalogs and annotates various materials exploring this idea, from the potentially deeply profound to the just-plain silly. Personally I suspect many of the works cited herein lean toward the profound, but that is an issue for science to decide. The 522 citations below are meant predominantly as historical tabulation and as motivation for such a future science.
I say no interpretation of quantum mechanics is worth its salt unless it raises as many technical questions as it answers philosophical ones. In this talk, I hope to convey the essence of a salty, if not downright briny, point of view about quantum theory: The deepest truth of quantum information and computing is that our world is a world wildly sensitive to the touch. When we irritate it in the right way, the result is a pearl. The speculation is that this sensitivity alone gives rise to the whole show, with the quantum calculus portraying the best shot we can take at making predictions in such a world. True to form, I ask more questions than I know how to answer. However, along the way, I give a variant of Gleason's theorem that works even for rational and two-dimensional Hilbert spaces, give another variant of Gleason's theorem that gives rise to the tensor-product rule for combining quantum systems, and finally derive a new form for expressing how quantum states change upon the action of a measurement.
This is a talk dedicated to Anton Zeilinger on the occasion of his 60th birthday. In it, I strive to make quantum mechanics look as much like Bayesian probability theory as possible. The shape of the gap where the two structures differ---it is speculated---gives the clearest indication yet of what the "reality" underlying quantum theory may actually look like.