Amazing stuff!
"... To see whether any system is behaving classically, scientists use a mathematical test called the Leggett-Garg inequality (LGI). Classical systems always obey the LGI limit while quantum systems violate it, proving they are non-classical.
Breaking a quantum limit
But even in quantum systems, this violation has a limit called the temporal Tsirelson's bound (TTB). In this research, scientists wanted to see if they could break the TTB limit and find even more extreme forms of quantum weirdness.
... theorized that a new kind of quantum motion, in which a particle follows two distinct sets of movement instructions simultaneously, could be powerful enough to break the TTB limit. They called this superposition of unitaries.
The team tested their idea in an NMR (nuclear magnetic resonance) machine, which let them control a qubit (the basic building block of quantum information). In this experiment, the qubit was a carbon nucleus within a molecule. The researchers designed a precise quantum circuit using a helper particle (an ancillary qubit) to make the qubit follow two sets of instructions at the same time. Specifically, they combined two different kinds of magnetic rotation on the qubit.
Results and applications
When the system evolved under this new combined motion, the changes were immediate and dramatic. The researchers measured the LGI violation and found it had smashed the TTB limit, confirming a new level of quantum weirdness. ..."
From the abstract:
"The violation of Leggett-Garg inequality (LGI) indicates general temporal correlations in quantum systems that cannot be explained classically.
Under unitary dynamics and projective measurements, the violation of LGI is restricted up to the temporal Tsirelson’s bound (TTB).
Here, we consider superposition of unitary time evolutions and find them to produce an enhancement in the violation of LGI beyond the TTB, growing monotonically with increasing superposition.
We experimentally realize superposition of unitaries in NMR systems and demonstrate this enhanced violation.
In the presence of noise, such superposition of unitaries remarkably extend the time of LGI violation, showcasing improved robustness against decoherence.
This opens up possibilities of employing such nontrivial dynamical maps for robust quantum control, along with provoking research on characterizing correlations in general sequential measurement scenarios."
Extreme Violations of Leggett-Garg Inequalities for a System Evolving under Superposition of Unitaries (no public access)
Enhanced non-macrorealism: Extreme violations of Leggett-Garg inequalities for a system evolving under superposition of unitaries (preprint, open access)
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