Noncontextuality, Finite Precision Measurement and the Kochen-Specker Theorem 0309017

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  • 7/27/2019 Noncontextuality, Finite Precision Measurement and the Kochen-Specker Theorem 0309017





    Non-Contextuality, Finite Precision Measurement

    and the Kochen-Specker Theorem

    Jonathan BarrettPhysique Theorique CP 225,

    Universite Libre de Bruxelles,Bvd. du Triomphe,

    1050 Bruxelles, Belgiumand

    Theorie de lInformation et des Communications CP 165/59,Universite Libre de Bruxelles,

    Av. F. D. Roosevelt 50,1050 Bruxelles, Belgium

    Adrian KentCentre for Quantum Computation,

    Department of Applied Mathematics and Theoretical Physics,University of Cambridge,

    Wilberforce Road, Cambridge CB3 0WAUnited Kingdom

    August 2003, revised November 2003


    Meyer originally raised the question of whether non-contextual hiddenvariable models can, despite the Kochen-Specker theorem, simulate thepredictions of quantum mechanics to within any fixed finite experimentalprecision (Meyer, D. 1999. Phys. Rev. Lett., 83, 3751-3754). Meyersresult was extended by Kent (Kent, A. 1999. Phys. Rev. Lett., 83, 3755-3757). Clifton and Kent later presented constructions of non-contextualhidden variable theories which, they argued, indeed simulate quantummechanics in this way (Clifton, R and Kent, A. 2000. Proc. Roy. Soc.Lond. A, 456, 2101-2114).

    These arguments have evoked some controversy. Among other things,it has been suggested that the Clifton-Kent models do not in fact repro-duce correctly the predictions of quantum mechanics, even when finiteprecision is taken into account. It has also been suggested that care-ful analysis of the notion of contextuality in the context of finite preci-sion measurement motivates definitions which imply that the Clifton-Kent

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    models are in fact contextual. Several critics have also argued that theissue can be definitively resolved by experimental tests of the Kochen-

    Specker theorem or experimental demonstrations of the contextuality ofNature.

    One aim of this paper is to respond to and rebut criticisms of theMeyer-Clifton-Kent papers. We thus elaborate in a little more detailhow the Clifton-Kent models can reproduce the predictions of quantummechanics to arbitrary precision. We analyse in more detail the relation-ship between classicality, finite precision measurement and contextuality,and defend the claims that the Clifton-Kent models are both essentiallyclassical and non-contextual. We also examine in more detail the sensesin which a theory can be said to be contextual or non-contextual, and inwhich an experiment can be said to provide evidence on the point. In par-ticular, we criticise the suggestion that a decisive experimental verificationof contextuality is possible, arguing that the idea rests on a conceptualconfusion.

    Keywords: Kochen-Specker, contextual, finite precision, experiment,loophole

    1 Introduction

    1.1 The Kochen-Specker theorem

    Consider a set K of Hermitian operators that act on an n-dimensional Hilbertspace. Suppose that V is a map that takes a Hermitian operator in K to a realnumber in its spectrum. We call such a map a colouring ofK. If the followingconditions are satisfied

    V(A + B) = V(A) + V(B)

    V(AB) = V(A)V(B)

    A, B K such that [A, B] = 0, (1)

    then the map is a KS-colouring ofK. We call these conditions the KS criteria.Kochen and Speckers celebrated theorem (Specker, 1960; Kochen & Specker,1967) states that if n > 2 there are KS-uncolourable sets, i.e., sets K for whichno KS-colouring exists. It follows trivially that the set of all Hermitian operatorsacting on a Hilbert space of dimension > 2 is KS-uncolourable.

    The fact that the set of all Hermitian operators in dimension > 2 is KS-uncolourable is a corollary of Gleasons theorem (Gleason, 1957). This was firstpointed out in Bell (1966), where an independent proof was also given. Kochen

    and Specker constructed the first finite KS-uncolourable set. Many proofs alongthe lines of Kochen and Speckers have since been produced by constructingdemonstrably KS-uncolourable sets (see, e.g., Peres, 1995; Zimba & Penrose,1993; Conway & Kochen; Cabello et al., 1996). The most common type of proofdescribes a set of 1-dimensional projection operators in n dimensions that is KS-uncolourable. If we represent 1-dimensional projections by vectors onto which


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    they project, and colour the corresponding set of vectors with a 1 or a 0, theKS criteria would imply that for each orthogonal n-tuple of vectors, exactly one

    must be coloured 1, and all the rest 0. The Kochen-Specker theorem can thenbe proved by showing that the colouring condition cannot be satisfied. In theiroriginal proof, Kochen and Specker describe a set of 117 vectors in 3 dimensionsthat is KS-uncolourable.1

    Of course, the well-known proofs of the Kochen-Specker theorem referred toabove are logically correct. Moreover, the Kochen-Specker theorem undeniablysays something very important and interesting about fundamental physics: itshows that the predictions of quantum theory for the outcomes of measurementsof Hermitian operators belonging to a KS-uncolourable set cannot be preciselyreproduced by any hidden variable theory that assigns real values to these op-erators in a way that respects the KS criteria, since no such hidden variabletheory exists. However, debate continues over the extent to which Kochen andSpecker succeeded in their stated aim, to give a proof of the nonexistence ofhidden variables(Kochen & Specker, 1967, p.59), even when this is qualified (asit must be) by restricting attention to non-contextual hidden variables. Beforesummarising and continuing this debate, we review why one might be interestedin hidden variable theories in the first place.

    Consider a system in a state | and a set of observables A , B , C , . . . suchthat | is not an eigenstate of A, B, C , . . .; here we use capital letters withhats to denote Hermitian operators and capital letters without hats to denotethe corresponding observables. Orthodox quantum mechanics leads us to saysomething like this: if we measure A, we will obtain the result a with probability

    pa, if we measure B, we will obtain the result b with probability pb, and so on.With an ease born of familiarity, the well trained quantum mechanic will notbat an eyelid at such statements. But, one might well ask: why are they so

    oddly phrased? Could this just be a rather awkward way of saying that withprobability pa, the value of A is a, or with probability pb, the value of B is b,and so on?

    Suppose that the set A , B , C , . . . corresponds to a KS-uncolourable set ofoperators A, B, C , . . .. The suggestion is that at a given time, each observablein the set has some definite value associated with it, defined by some hiddenvariables of the system. The significance of the KS criteria is that if the Her-mitian operators associated with two observables commute, then according toquantum mechanics, the observables can be simultaneously measured, and thevalues obtained will satisfy the KS criteria (and in general will satisfy any func-tional relationships that hold between the operators themselves). We are notlogically compelled to assume that any hidden variable theory shares these prop-erties. However, the standard motivation for considering hidden variables is to

    examine the possibility that quantum theory, while not incorrect, is incomplete,Thus motivated, it seems natural to assume that the colouring defined by thehidden variables must also satisfy the KS criteria. But given this assumption,

    1How small can a KS-uncolourable set of vectors be? The current records stand at 31vectors in 3 dimensions (Conway & Kochen) and 18 in 4 dimensions (Cabello et al., 1996).


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    since there is no such colouring, the original supposition that the observableshave definite values must be wrong.

    The contradiction obtained in the Kochen-Specker theorem is avoided if, in-stead of defining a map V, we assign values to Hermitian operators in such a waythat the value assigned to a particular Hermitian operator depends on whichcommuting set we are considering that operator to be part of. Such a valueassignment