Experimental test of non-macrorealistic cat-states in the cloud
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.
Huan-Yu Ku Neill Lambert Fong-Ruei Jhan Clive Emary Yueh-Nan Chen Franco Nori