A no-go theorem for theories that decohere to quantum mechanics

TL;DR

This paper proves that no post-quantum theory can exist that reduces to quantum mechanics via decoherence while satisfying causality and purification principles, reinforcing the fundamental nature of quantum theory.

Contribution

It establishes a no-go theorem showing that theories extending quantum mechanics cannot satisfy both causality and purification principles simultaneously.

Findings

No post-quantum theory reduces to quantum via decoherence under causality and purification.

Quantum theory's fundamental status is reinforced by the impossibility of such extensions.

Any viable post-quantum theory must violate causality, purification, or both.

Abstract

To date, there has been no experimental evidence that invalidates quantum theory. Yet it may only be an effective description of the world, in the same way that classical physics is an effective description of the quantum world. We ask whether there exists an operationally-defined theory superseding quantum theory, but which reduces to it via a decoherence-like mechanism. We prove that no such post-quantum theory exists if it is demanded that it satisfy two natural physical principles: causality and purification. Causality formalises the statement that information propagates from present to future, and purification that each state of incomplete information arises in an essentially unique way due to lack of information about an environment. Hence, our result can either be viewed as evidence that the fundamental theory of Nature is quantum, or as showing in a rigorous manner that any post-quantum theory must abandon causality, purification, or both.