The Last of the Great Quantum Skeptics

Greg Gbur

H.J. Groenewold

Albert Einstein

Scott Aaronson

Last chapter, we talked about whether quantum states should be thought of as exponentially long vectors, and I brought up class BQP/qpoly and concepts like quantum advice. Actually, I’d say that the main reason why I care is something I didn't mention last time, which is that it relates to whether we should expect quantum computing to be fundamentally possible or not. There are people, like Leonid Levin and Oded Goldreich, who just take it as obvious that quantum computing must be impossible. Part of their argument is that it's extravagant to imagine a world where describing the state of 200 particles takes more bits then there are particles in the universe. To them, this is a clear indication something is going to break down. So part of the reason that I like to study the power of quantum proofs and quantum advice is that it helps us answer the question of whether we really should think of a quantum state as encoding an exponential amount of information. So, on to the Eleven Objections. Works on paper, not in practice. Violates Extended Church–Turing Thesis. Not enough “real physics.” Small amplitudes are unphysical. Exponentially large states are unphysical. Quantum computers are just souped-up analog computers. Quantum computers aren't like anything we've ever seen before. Quantum mechanics is just an approximation to some deeper theory. Decoherence will always be worse than the fault-tolerance threshold. We don't need fault-tolerance for classical computers. Errors aren't independent. What I did is to write out every skeptical argument against the possibility of quantum computing that I could think of. We'll just go through them, and make commentary along the way. Let me just start by saying that my point of view has always been rather simple: it's entirely conceivable that quantum computing is impossible for some fundamental reason. If so, then that's by far the most exciting thing that could happen for us. That would be much more interesting than if quantum computing were possible, because it changes our understanding of physics. To have a quantum computer capable of factoring 10000-digit integers is the relatively boring outcome – the outcome that we'd expect based on the theories we already have.

Peter Woit

Tarō Kashiwa, Yoshio Ohnuki, Masuo Suzuki

Abstract The last two decades have seen a consistent growth in the significance of path integrals. They are useful in quantum as well as statistical mechanics, providing more intuitive and easier methods of handling equations than more commonly-known treatments. However, the method is still greeted with skepticism. Where has the operator ordering problem gone? Do polar coordinates work in PI, despite the fact that they cannot exist in the canonical operator formalism? Is it possible to obtain a quantum mechanics on the sphere by inserting a delta function into the unconstrained formula? This book addresses and solves these fundamental issues, and covers more recent topics, such as the lattice fermion problem in quantum theory and the Monte Carlo method in statistical mechanics.