Measurement Contextuality and Planck's Constant

TL;DR

This paper links quantum contextuality to orders of Planck's constant within the Wigner-Weyl-Moyal formalism, explaining why qubits always show state-independent contextuality while odd-dimensional qudits can show state-dependent contextuality.

Contribution

It demonstrates that contextuality can be understood as a quantum correction in powers of Planck's constant within the WWM formalism, providing a new perspective on quantum-classical divide.

Findings

Qubits lack order   contributions, leading to universal contextuality.

Odd-dimensional qudits have non-zero order  terms, enabling state-dependent contextuality.

Contextuality correlates with higher-order  contributions in the WWM formalism.

Abstract

Contextuality is a necessary resource for universal quantum computation and non-contextual quantum mechanics can be simulated efficiently by classical computers in many cases. Orders of Planck's constant, $\hbar$, can also be used to characterize the classical-quantum divide by expanding quantities of interest in powers of $\hbar$---all orders higher than $\hbar^0$ can be interpreted as quantum corrections to the order $\hbar^0$ term. We show that contextual measurements in finite-dimensional systems have formulations within the Wigner-Weyl-Moyal (WWM) formalism that require higher than order $\hbar^0$ terms to be included in order to violate the classical bounds on their expectation values. As a result, we show that contextuality as a resource is equivalent to orders of $\hbar$ as a resource within the WWM formalism. This explains why qubits can only exhibit state-independent contextuality under Pauli observables as in the Peres-Mermin square while odd-dimensional qudits can also exhibit state-dependent contextuality. In particular, we find that qubit Pauli observables lack an order $\hbar^0$ contribution in their Weyl symbol and so exhibit contextuality regardless of the state being measured. On the other hand, odd-dimensional qudit observables generally possess non-zero order $\hbar^0$ terms, and higher, in their WWM formulation, and so exhibit contextuality depending on the state measured: odd-dimensional qudit states that exhibit measurement contextuality have an order $\hbar^1$ contribution that allows for the violation of classical bounds while states that do not exhibit measurement contextuality have insufficiently large order $\hbar^1$ contributions.