Zombie cats on the quantum-classical frontier

What is it about?

Molecular properties fall on the frontier between the quantum and classical worlds---they are big enough to act like classical particles but small enough that manifestly quantum phenomena such as coherence and tunneling can sometimes be important. We explore the phenomenon of quantum coherence using both exact quantum and approximate semiclassical mechanics. We describe how the latter approach represents the superposition of coherent quantum states, such as the "live" and "dead" cats made famous by Schrodinger, with an approximate averaged description---a "zombie" state, neither alive or dead, which fails to capture important quantum behavior. We then go on to identify theoretical corrections that revive the correct live-dead superposition behavior in a classical framework.

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Photo by Lina Angelov on Unsplash

Photo by Lina Angelov on Unsplash

Why is it important?

This paper is fundamental theory, but its implications are experimentally observable. Quantum simulations are accurate but too expensive for all but the simplest problems. Semiclassical methods are a popular alternative, and allow the simulation of quantum molecular dynamics and spectroscopy with economical classical mechanical calculations. Our work identifies errors that can appear in those approaches and points the way to developing improved algorithms that are more accurate while remaining affordable.

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