# Experimental test of non-macrorealistic cat-states in the cloud

## TL;DR

This study uses cloud-based quantum computing to experimentally test macrorealism by observing violations of the Leggett-Garg inequality in multi-qubit cat states, revealing size-dependent quantum behavior.

## Contribution

It demonstrates the first experimental violation of the Leggett-Garg inequality in multi-qubit cat states using cloud quantum devices, exploring the limits of macrorealism.

## Key findings

- Two-qubit and four-qubit cat states violate the inequality.
- Six-qubit cat state does not violate beyond the measurement bound.
- Un-entangled states show low disconnectivity and no violation.

## Abstract

The Leggett-Garg inequality attempts to classify experimental outcomes as arising from one of two possible classes of physical theories: those described by macrorealism (which obey our intuition about how the macroscopic classical world behaves), and those that are not (e.g., quantum theory). The development of cloud-based quantum computing devices enables us to explore the limits of macrorealism in new regimes. In particular, here we take advantage of the properties of the programmable nature of the IBM quantum experience to observe the violation of the Leggett-Garg inequality (in the form of a ``quantum witness") as a function of the number of constituent systems (qubits), while simultaneously maximizing the `disconnectivity', a potential measure of macroscopicity, between constituents. Our results show that two-qubit and four-qubit ``cat states" (which have large disconnectivity) are seen to violate the inequality, and hence can be classified as nonmacrorealistic. In contrast, a six-qubit cat state does not violate the ``quantum-witness" beyond a so-called clumsy invasive-measurement bound, and thus is compatible with ``clumsy macrorealism". As a comparison, we also consider un-entangled product states with n = 2, 3, 4, and 6 qubits, in which the disconnectivity is low.