Written by: Stephen Hsu
Primary Source: Information Processing, 9/30/18.
My opening remarks:
On behalf of Michigan State University it is my pleasure to welcome all of you to this workshop on quantum information science.
In the fall of 1983 (my freshman year!) Feynman taught a graduate course at Caltech called Potentialities and Limitations of Computing Machines. Chapter 6 of the book developed from his lecture notes is entitled Quantum Mechanical Computers. In the prior years he had teamed with Professors Carver Mead and John Hopfield to teach a similar course. Carver Mead was the father of VLSI and coined the term “Moore’s Law”! John Hopfield, no slouch, was an early pioneer of neural nets, among other things.
It was in 1981, in a paper called Simulating Physics with Computers, that Feynman proposed the idea of a Universal Quantum Simulator. He was the first to discuss the simulation of quantum systems using a quantum computer, and to point out the difficulties of using classical computations to explore what could be exponentially large Hilbert spaces. Feynman analyzed reversible (unitary) computations using quantum elements, and wrote “… the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms, and quantum behavior holds dominant sway. I recount this little bit of history because we have finally, thanks to the sweat and ingenuity of many physicists, reached the era of noisy, but useful, quantum simulators. Personally I feel that universal quantum computers — of the type that could, for instance, implement Shor’s Algorithm — might still be far off. Nevertheless, quantum simulators are themselves an important step forward, and will likely become a very useful tool for physicists.
I can’t resist making a small prediction of my own here. Some of you might know that the foundations of quantum mechanics are still in disarray. As Steve Weinberg says: “… today there is no interpretation of quantum mechanics that does not have serious flaws.” Feynman himself said: “I think I can safely say that nobody understands quantum mechanics.”
Most physicists, even theorists, focus their efforts on practical matters and don’t worry about foundational questions. I believe that a side effect of work on quantum information and quantum computing will be a demystification of the process of measurement and of decoherence. By demystification I mean that many more physicists will develop a good understanding of something that was swept under the rug in von Neumann’s Projection or Collapse postulate, which we now teach in every QM course. Once we truly understand decoherence we realize that Schrodinger evolution of the wavefunction describing both observer and system can reproduce all the usual phenomenology of quantum mechanics — Collapse is not necessary. This was pointed out long ago by Everett, and well-appreciated by people like Feynman, Schwinger, Gell-Mann, Hawking, David Deutsch, and Steve Weinberg, although not widely understood in the broader physics community.
I apologize if these final comments are mysterious. Perhaps they will someday become clear… In the meantime, please enjoy the workshop :-)