author={Lucien Hardy},
  journal={International Journal of Modern Physics B},
  • L. Hardy
  • Published 7 May 2012
  • Physics, Mathematics, Computer Science
  • International Journal of Modern Physics B
In this paper we consider theories in which reality is described by some underlying variables, λ. Each value these variables can take represents an ontic state (a particular state of reality). The preparation of a quantum state corresponds to a distribution over the ontic states, λ. If we make three basic assumptions, we can show that the distributions over ontic states corresponding to distinct pure states are nonoverlapping. This means that we can deduce the quantum state from a knowledge of… 
No ψ-epistemic model can fully explain the indistinguishability of quantum states.
This work considers models that are defined for a single quantum system of dimension d, such that the independence condition does not arise, and derives an upper bound on the extent to which the probability distributions can overlap.
Conclusive exclusion of quantum states and aspects of thermo-majorization
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On Modern Approaches to the Einsteinian View of Quantum States
Quantum mechanics notoriously faces the measurement problem, the problem that if read thoroughly, it implies the nonexistence of definite outcomes in measurement procedures. A plausible reaction to
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A century after the development of quantum theory, the interpretation of a quantum state is still discussed. If a physicist claims to have produced a system with a particular quantum state vector,
Is the Quantum State Real in the Hilbert Space Formulation?
It is shown that an unambiguous proof of reality of the quantum states gleaned from thereality of quantum fields can also provide an explicit substantiation of the reality of quantum states in Hilbert space.
Is a time symmetric interpretation of quantum theory possible without retrocausality?
  • M. Leifer, Matthew F Pusey
  • Physics, Mathematics
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
  • 2017
It is shown that the assumption that a time symmetric ontology for quantum theory must necessarily be retrocausal can be replaced by a different assumption, called λ-mediation, that plausibly holds independently of the status of the quantum state, and that this assumption implies a timelike analogue of Bell's local causality criterion.
Reality, causality, and quantum theory
Quantum theory describes our universe incredibly successfully. To our classically-inclined brains, however, it is a bizarre description that requires a reimagining of what fundamental reality, or
No-go theorems for ψ-epistemic models based on a continuity assumption.
This argument is different from the recent proof of Pusey, Barrett, and Rudolph and it already yields a nontrivial constraint on ψ-epistemic models using a single copy of the system in question.
ψ-Epistemic Models, Einsteinian Intuitions, and No-Gos. A Critical Study of Recent Developments on the Quantum State
Quantum mechanics notoriously faces the measurement problem, the problem that if read thoroughly, it implies the nonexistence of definite outcomes in measurement procedures. A plausible reaction to


Einstein, Incompleteness, and the Epistemic View of Quantum States
It is shown that for models wherein the quantum state has the status of something real, the failure of locality can be established through an argument considerably more straightforward than Bell’s theorem.
Evidence for the epistemic view of quantum states: A toy theory
We present a toy theory that is based on a simple principle: the number of questions about the physical state of a system that are answered must always be equal to the number that are unanswered in a
Phase Groups and the Origin of Non-locality for Qubits
A general framework in which two theories: quantum mechanics restricted to qubit stabiliser states and operations, and Spekkens's toy theory are compared, finding that viewed within this framework these theories are very similar, but differ in one key aspect - a four element group which emerges naturally within the authors' framework.
Alternative Experimental Protocol for a PBR-Like Result
Pusey, Barrett and Rudolph (PBR) [1] have proven a new theorem which imposes a significant constraint on the interpretation of quantum mechanics [2]. The theorem shows that if one assumes that
Reconstruction of Gaussian quantum mechanics from Liouville mechanics with an epistemic restriction
How would the world appear to us if its ontology was that of classical mechanics but every agent faced a restriction on how much they could come to know about the classical state? We show that in
In this paper, we shall show how the theory of measurements is to be understood from the point of view of a physical interpretation of the quantum theory in terms of hidden variables developed in a
Distinct quantum states can be compatible with a single state of reality.
This work considers models for quantum systems with measurement outcomes determined by an underlying physical state of the system but where several quantum states are consistent with a single underlying state-i.e., probability distributions for distinct quantum states overlap.
The Problem of Hidden Variables in Quantum Mechanics
Forty years after the advent of quantum mechanics the problem of hidden variables, that is, the possibility of imbedding quantum theory into a classical theory, remains a controversial and obscure
Conditions for compatibility of quantum-state assignments
Suppose N parties describe the state of a quantum system by N possibly different density operators. These N state assignments represent the beliefs of the parties about the system. We examine
QBism, the Perimeter of Quantum Bayesianism
This article summarizes the Quantum Bayesian point of view of quantum mechanics, with special emphasis on the view's outer edges---dubbed QBism. QBism has its roots in personalist Bayesian