Kochen-Specker theorems assure the breakdown of certain types of noncontextual hidden-variable theories through the nonexistence of global, holistic frame functions; however, they do not allow us to identify where this breakdown occurs, nor the extent of it. It was recently shown [Phys. Rev. A 86, 062109 (2012)] that this breakdown does not occur everywhere; here we show that it is maximal in that it occurs almost everywhere and thus prove that quantum indeterminacy, often referred to as contextuality or value indefiniteness, is a global property as is often assumed. In contrast to the Kochen-Specker theorem, we only assume the weaker noncontextuality condition that any potential value assignments that may exist are locally noncontextual. Under this assumption, we prove that once a single arbitrary observable is fixed to occur with certainty, almost (i.e., with Lebesgue measure one) all remaining observables are indeterminate.
Kochen-Specker theorems assure the breakdown of certain types of noncontextual hidden-variable theories through the nonexistence of global, holistic frame functions; however, they do not allow us to identify where this breakdown occurs, nor the extent of it. It was recently shown [Phys. Rev. A 86, 062109 (2012)] that this breakdown does not occur everywhere; here we show that it is maximal in that it occurs almost everywhere and thus prove that quantum indeterminacy, often referred to as contextuality or value indefiniteness, is a global property as is often assumed. In contrast to the Kochen-Specker theorem, we only assume the weaker noncontextuality condition that any potential value assignments that may exist are locally noncontextual. Under this assumption, we prove that once a single arbitrary observable is fixed to occur with certainty, almost (i.e., with Lebesgue measure one) all remaining observables are indeterminate.
The Kochen-Specker theorem proves the inability to assign, simultaneously, noncontextual definite values to all (of a finite set of) quantum mechanical observables in a consistent manner. If one assumes that any definite values behave noncontextually, one can nonetheless only conclude that some observables (in this set) are value indefinite. In this paper, we prove a variant of the Kochen-Specker theorem showing that, under the same assumption of noncontextuality, if a single one-dimensional projection observable is assigned the definite value 1, then no one-dimensional projection observable that is incompatible (i.e., non-commuting) with this one can be assigned consistently a definite value. Unlike standard proofs of the Kochen-Specker theorem, in order to localise and show the extent of value indefiniteness, this result requires a constructive method of reduction between Kochen-Specker sets. If a system is prepared in a pure state ψ, then it is reasonable to assume that any value assignment (i.e., hidden variable model) for this system assigns the value 1 to the observable projecting onto the one-dimensional linear subspace spanned by ψ, and the value 0 to those projecting onto linear subspaces orthogonal to it. Our result can be interpreted, under this assumption, as showing that the outcome of a measurement of any other incompatible one-dimensional projection observable cannot be determined in advance, thus formalising a notion of quantum randomness.
The Kochen-Specker theorem shows the impossibility for a hidden variable theory to consistently assign values to certain (finite) sets of observables in a way that is noncontextual and consistent with quantum mechanics. If we require noncontextuality, the consequence is that many observables must not have pre-existing definite values. However, the Kochen-Specker theorem does not allow one to determine which observables must be value indefinite. In this paper we present an improvement on the Kochen-Specker theorem which allows one to actually locate observables which are provably value indefinite. Various technical and subtle aspects relating to this formal proof and its connection to quantum mechanics are discussed. This result is then utilized for the proposal and certification of a dichotomic quantum random number generator operating in a three-dimensional Hilbert space.
The Kochen-Specker theorem demonstrates that it is not possible to reproduce the predictions of quantum theory in terms of a hidden variable model where the hidden variables assign a value to every projector deterministically and noncontextually. A noncontextual value assignment to a projector is one that does not depend on which other projectors-the context-are measured together with it. Using a generalization of the notion of noncontextuality that applies to both measurements and preparations, we propose a scheme for deriving inequalities that test whether a given set of experimental statistics is consistent with a noncontextual model. Unlike previous inequalities inspired by the Kochen-Specker theorem, we do not assume that the value assignments are deterministic and therefore in the face of a violation of our inequality, the possibility of salvaging noncontextuality by abandoning determinism is no longer an option. Our approach is operational in the sense that it does not presume quantum theory: a violation of our inequality implies the impossibility of a noncontextual model for any operational theory that can account for the experimental observations, including any successor to quantum theory.
The Kochen–Specker theorem establishes that noncontextual, globally consistent assignments of definite values to quantum observables are mathematically impossible. While this result places a fundamental constraint on pre-measurement value assignments, it does not by itself specify why observable values should exhibit contextuality or how the pre-measurement domain should be conceptually understood. This paper examines the Kochen–Specker theorem from the perspective of the Intrinsic Observability Framework (IOF), with particular emphasis on the relationship between contextuality and accessibility. In the IOF, observable values are not regarded as pre-existing entities awaiting discovery. Instead, the pre-measurement domain is characterized by potential observability, and measurement grants accessibility, thereby establishing observability and making observable values operationally meaningful. From this perspective, different measurement contexts generally participate in distinct accessibility conditions. Even when the same observable appears in distinct compatible sets, the conditions under which its observable value is operationally established are not necessarily identical. Contextuality can therefore be understood not merely as a restriction on global value assignment, but as a natural consequence of accessibility-dependent observability. The Kochen–Specker impossibility of constructing a single noncontextual global value map is thus interpreted as reflecting the attempt to combine observable values established under distinct accessibility conditions into a context-independent description. The IOF does not modify the mathematical content of the Kochen–Specker theorem or the standard quantum formalism. Rather, it provides an accessibility-based conceptual account of the contextual structure revealed by the theorem while explicitly characterizing the pre-measurement domain. Finally, the relationship between the IOF and representative interpretations of quantum mechanics is compared to clarify the distinct conceptual role of accessibility-dependent observability within quantum foundations.
The n-qubit Pauli group is an essential ingredient to most quantum applications, from computing and error correction to benchmarking and simulation. Despite comprising merely a discrete set of operators, it exhibits many quintessential features of quantum theory, including contextuality, which has been identified as a key resource to quantum advantage in a variety of different flavours. Here, we extend this analysis, introducing the notion of a `noncontextual property' whose nonexistence proves the Kochen-Specker theorem, similarly to and generalising common arguments based on the nonexistence of valuations. We relate this notion formally to the existence of Boolean-valued frame functions, and characterise all such frame functions in the case of the n-qubit Pauli group.
For two qubits, we show that the Pauli group admits noncontextual properties, despite admitting no valuations. We then establish this as the only nontrivial such case with n≥2, by proving that any Boolean-valued frame function on stabiliser states is constant for more than two qubits. We also perform a similar analysis for the symplectic theory underlying the n-qubit Pauli group, for which nonconstant Boolean-valued frame functions exist for all n, yet only in restricted form. By comparison, this shows that contextuality in the n-qubit Pauli group is not only a consequence of the projective nature of the Pauli group as a representation of its underlying symplectic vector space, but of the geometry of symplectic polar spaces itself. In geometric terms, our result determines all Cameron-Liebler sets of maximal totally isotropic flats in the binary affine-symplectic space.
The outcomes of quantum measurements are generally considered to be random, but despite the fact that this randomness is an important element in quantum information theory, its nature is not well understood. In this thesis, we study several issues relating to the origin and certification of quantum randomness and unpredictability. One of the key results in forming our understanding of quantum mechanics as an intrinsically indeterministic theory is the Kochen-Specker theorem, which shows the impossibility to consistently assign simultaneous noncontextual definite values to all quantum mechanical observables prior to measurement. However, the theorem, under the assumption that any definite values must be noncontextual, only strictly shows that some observables must be value indefinite. We strengthen this result, proving a stronger variant of the Kochen-Specker theorem showing that, under the same assumption, if a system is prepared in an arbitrary state j i, then every observable A is value indefinite unless j i is an eigenstate of A. The indeterministic nature of quantum measurements does little to explain how the quality of quantum randomness differs from classical randomness. We show that, subject to certain physical assumptions, a sequence of bits generated by the measurement of value indefinite observables is guaranteed, in the infinite limit, to be strongly incomputable. We further discuss how this can be used to build a quantum random number generator certified by value indefiniteness. Next, we study the notion of unpredictability, which is central to the concept of (quantum) randomness. In doing so, we propose a formal model of prediction that can be used to asses the predictability of arbitrary physical experiments. We investigate how the quantum features of value indefiniteness and complementarity can be used to certify different levels of unpredictability, and show that the outcome of a single measurement of a value indefinite quantum observable is formally unpredictable. Finally, we study the relation between this notion of unpredictability and the computability-theoretic certification of quantum randomness.