Quantum mechanics

What Quantum Measurements Measure

Robert Griffiths
Tuesday, December 6, 2016 - 12:00pm to 1:00pm

Discussions of the infamous measurement problem of quantum foundations tend to focus on how the output of a measurement, the pointer position, can be thought of in consistent quantum mechanical terms, while ignoring the equally important issue of what this outcome says about the earlier microscopic situation the apparatus was designed to measure. An experimental physicist is typically much more interested in the path followed by a particle before it triggered his detector than in what happened later, and if quantum mechanics cannot provide a clear explanation, how can...

Typical Worlds

Jeffrey Barrett
Friday, October 28, 2016 - 3:30pm to 4:30pm

Hugh Everett III presented pure wave mechanics, sometimes referred to as the many-worlds interpretation, as a solution to the quantum measurement problem. While pure wave mechanics is a deterministic physical theory with no probabilities, Everett sought to show how the theory might be understood as making the standard quantum statistical predictions as appearances to observers who were themselves described by the theory. We will consider his argument and how it depends on a particular notion of branch typicality. We will also consider the relationship between...

What Is "Orthodox" Quantum Mechanics?

David Wallace
Friday, November 18, 2016 - 3:30pm to 4:30pm

What is called "orthodox" quantum mechanics, as presented in standard foundational discussions, relies on two substantive assumptions --- the projection postulate and the eigenvalue-eigenvector link --- that do not in fact play any part in practical applications of quantum mechanics. I argue for this conclusion on a number of grounds, but primarily on the grounds that the projection postulate fails correctly to account for repeated, continuous and unsharp measurements (all of which are standard in contemporary physics) and that the eigenvalue-eigenvector link...

Department of Physics, Carnegie Mellon University
Ph.D., Physics, Stanford University, 1962

Quantum mechanics is hard to understand not only because it involves unfamiliar mathematics, but also because the usual discussion in  textbooks about how to relate the mathematics to the real world is incomplete. Supplying the missing link(s) and working out a fully consistent form of quantum theory is the goal of a research program which I initiated in 1984, and which, with major contributions by Roland Omnes, Murray Gell-Mann, and James Hartle, has resulted in what is now called the consistent (or decoherent) history approach to quantum theory. So far as is known at present, this approach is powerful enough to resolve the various quantum paradoxes (Schrodinger's cat, Einstein-Podolsky-Rosen, etc.) without any mysterious action-at-a-distance, and it makes good sense out of quantum  measurements.  I have written a book Consistent Quantum Theory (Cambridge University Press) which explains the essentials of this approach.

At present my research program is focused on applying consistent history methods and ideas to quantum information theory and quantum computation. Using the principle that quantum measurements, when properly interpreted, reveal a property of the measured system before the measurement took place, C.-S. Niu and I showed that one could greatly simplify the final step in Shor's algorithm for factoring long numbers. We made similar applications to eavesdropping in quantum cryptography. Understanding the significance of density matrices and entangled quantum states, and investigating the noise produced by quantum copying processes, are among the projects currently underway in my research group. We are also looking for (special) relativistic counterparts of some aspects of the consistent history interpretation which at present are best understood for nonrelativistic systems.  For further information about my research group see its web page.

My other interests include the problem of irreversibility in statistical mechanics, and various issues, such as determinism and free will, at the interface between science and Christian theology.

Selected Publications: 
Most Cited Publications
  1. "Ising model for the λ transition and phase separation in He 3-He 4 mixtures," Blume, M., V. J. Emery, and Robert B. GriffithsPhys. Rev. A. 4, 3 (1971)
  2. "Nonanalytic behavior above the critical point in a random Ising ferromagnet," Griffiths, Robert B., Phys. Rev. Let. 23, no. 1 (1969)
  3. "Critical points in multicomponent systems," Griffiths, Robert B., and John C. Wheeler., Phys. Rev. A. 2, no. 3 (1970)
  4. "Consistent histories and the interpretation of quantum mechanics," Griffiths, Robert B., Journal of Statistical Physics 36, no. 1-2 (1984)
  5. "Optimal eavesdropping in quantum cryptography. I. Information bound and optimal strategy," Fuchs, Christopher A., Nicolas Gisin, Robert B. Griffiths, Chi-Sheng Niu, and Asher Peres, Phys. Rev. A. 56, no. 2 (1997)
Recent Publications
  1. Reply to "comment on 'Particle path through a nested Mach-Zehnder interferometer'l, Griffiths, R.B." Physical Review A 97(2),026102 (2018)
  2. "Quantum Information: What is it all about?" Griffiths, R.B. Entropy 19(12), 645 (2017).
  3. "What quantum measurements measure." Griffiths, R.B. Physical Review A 96(3), 032110 (2017).
  4. "Degradable quantum channels using pure-state to product-of-pure-state isometries." Siddhu, V., Griffiths, R.B. Physical Review A 94(5), 052331 (2016).
  5. "Particle path through a nested Mach-Zehnder interferometer." Griffiths, R.B. Physical Review A 94(3), 032115 (2016).
Department of Physics and Astronomy, University of Pittsburgh
Ph.D., Theoretical Physics, University of California Berkeley, 1942

In the past Professor Gerjuoy published predominantly, though not entirely, in atomic physics collision theory. More recently, however, he has been devoting his research time solely to various theoretical problems related to quantum computing. During much of his career as a physicist he has worked on various American Physical Society (APS) and other professional society committees. In particular, he has been a member (and in some years Chair) of the following APS Committees: Panel on Public Affairs (1976-79, 1981 and 1994-96); Audit Committee (2002-4); Constitution and Bylaws Committee (1999-2002). In addition he has been a member (2000-03) of the APS Council, the APS governing body, as well as an officer (including Chair) of the APS Forum on Physics and Society (1994-7); membership (2005-07) on the APS Sakharov Prize Committee was his most recent APS committee service. In past years he also has been a member of: the Oak Ridge National Laboratory Health Physics Division Advisory Committee (1967-70, Chair 1971-74); the National Conference of Lawyers and Scientists, a joint Committee of the American Association for the Advancement of Science and the American Bar Association (1986-92); and the Pennsylvania Environmental Hearing Board (1981-86). These last two appointments reflect the fact that Professor Gerjuoy earned a law degree in 1977; indeed he has published numerous papers on issues arising at the interface of law and science.

Selected Publications: 
Most Cited Publications
  1. "Rotational Excitation by Slow Electron," E Gerjuoy and S. Stein, Phys. Rev. 97, 1671 (1955)
  2. "Cross Section for Energy Transfer between Two Moving Particles," E. Gerjuoy, Phys. Rev. 148, 54 (1966)
  3. "Applications of the Glauber approximation to atomic collisions," E. Gerjuoy, B. K. Thomas, Rep. Prog. Phys. 37, 1345 (1974)
  4. "Charge Transfer in Molecular Hydrogen," T. F. Tuan and E. Gerjuoy, Phys. Rev. 117, 756 (1960)
  5. "Glauber Theory of Atomic-Hydrogen Excitation by Electron Impac," H. Tai, R. H. Bassel, E. Gerjuoy, and Victor Franco Phys. Rev. A 1, 1819 (1970)