# Quantum Foundations

This series consists of talks in the area of Foundations of Quantum Theory. Seminar and group meetings will alternate.

## Seminar Series Events/Videos

Currently there are no upcoming talks in this series.

## Symmetric informationally complete measurements: Can we make big ones out of small ones?

Tuesday Dec 01, 2009
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For a quantum system with a d-dimensional Hilbert space, a symmetric informationally complete measurement (SIC) can be thought of as a set of d^2 pure states all having the same overlap. Constructions of SICs for composite systems usually do not make use of the composite structure but treat the system as a whole. Indeed for some cases, one can prove that a SIC cannot have the symmetry that one naturally associates with the composite structure.

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## What's Wrong with 'Measurement'?

Tuesday Nov 17, 2009
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In his brilliant article "Against 'Measurement'", John Bell famously
argued that the word has had such a damaging effect on the discussion,
that it should now be banned altogether in quantum mechanics. But in
the beginning was the word, and the word is still with us. Indeed,
David Mermin responded In Praise of Measurement that within the field
of quantum computer science the concept of measurement is precisely
defined, unproblematic, and forms the foundation of the entire
subject, a verdict reaffirmed by the development of measurement-based

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## Reduction and Emergence in Bose-Einstein Condensates

Thursday Oct 29, 2009
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A closer look at some proposed Gedanken-experiments on BECs promises to shed light on several aspects of reduction and emergence in physics. These include the relations between classical descriptions and different quantum treatments of macroscopic systems, and the emergence of new properties and even new objects as a result of spontaneous symmetry breaking.

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## The World According to De Finetti

Friday Oct 09, 2009

Bruno de Finetti is one of the founding fathers of the subjectivist school of probability, where probabilities are interpreted as rational degrees of belief. His work on the relation between the theorems of the probability calculus and rationality is among the corner stones of modern subjective probability theory. De Finetti maintained that rationality requires that an agent’s degrees of belief be coherent.

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## What would a consistent instrumentalism about quantum mechanics be? Or, why Wigner's friendly after all.

Friday Sep 25, 2009
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Instrumentalism about the quantum state is the view that this mathematical object does not serve to represent a component of (non-directly observable) reality, but is rather a device solely for making predictions about the results of experiments. One honest way to be such an instrumentalist is a) to take an ensemble view (= frequentism about quantum probabilities), whereby the state represents predictions for measurement results on ensembles of systems, but not individual systems and b) to assign some specific level for the quantum/classical cut.

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## Generating and detecting multi-qubit GHZ states in circuit QED

Tuesday Sep 22, 2009
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I will present recent work [1] on preparation by measurement of Greenberger–Horne–Zeilinger (GHZ) states in circuit quantum electrodynamics. In particular, for the 3-qubit case, when employing a nonlinear filter on the recorded homodyne signal the selected states are found to exhibit values of the Bell–Mermin operator exceeding 2 under realistic conditions. I will discuss the potential of the dispersive readout to demonstrate a violation of the Mermin bound, and present a measurement scheme avoiding the necessity for full detector tomography.

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## A First-Principles Implementation of Scale Invariance Using Best Matching

Tuesday Sep 15, 2009
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We present a first-principles implementation of {\em spatial} scale invariance as a local gauge symmetry in geometry dynamics using the method of best matching. In addition to the 3-metric, the proposed scale invariant theory also contains a 3-vector potential A_k as a dynamical variable. Although some of the mathematics is similar to Weyl's ingenious, but physically questionable, theory, the equations of motion of this new theory are second order in time-derivatives. It is tempting to try to interpret the vector potential A_k as the electromagnetic field.

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## Betting on Quantum Theory

Tuesday Sep 08, 2009

Betting (or gambling) is a useful tool for studying decision-making in the face of [classical] uncertainty. We would like to understand how a quantum "agent" would act when faced with uncertainty about its [quantum] environment. I will present a preliminary construction of a theory of quantum gambling, motivated by roulette and quantum optics. I'll begin by reviewing classical gambling and the Kelly Criterion for optimal betting. Then I'll demonstrate a quantum optical version of roulette, and discuss some of the challenges and pitfalls in designing such analogues.

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## Entanglement detection with bounded reference frames

Tuesday Aug 04, 2009
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Violation of local realism can be probed by theory–independent tests, such as Bell’s inequality experiments. There, a common assumption is the existence of perfect, classical, reference frames, which allow for the specification of measurement settings with arbitrary precision. However, if the reference frames are bounded'', only limited precision can be attained. We expect then that the finiteness of the reference frames limits the observability of genuine quantum features.

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## Classical fields as the natural ontic structure for quantum theory

Tuesday Jun 16, 2009
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Although most realistic approaches to quantum theory are based on classical particles, QFT reveals that classical fields are a much closer analog. And unlike quantum fields, classical fields can be extrapolated to curved spacetime without conceptual difficulty. These facts make it tempting to reconsider whether quantum theory might be reformulated on an underlying classical field structure.

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## RECENT PUBLIC LECTURE

### From the Mathematics of Supersymmetry to the Music of Arnold Schoenberg

Speaker: S. James Gates Jr.