SCHRÖDINGER’S cat, both dead and alive at once, was always meant to be a thought experiment, but will ordinary objects or your favourite feline ever enter the quantum world? No one knows, but now there is a way to measure how close physicists are to realising the thought experiment.

The laws of quantum mechanics should apply to everything, from the tiniest particle to the entire universe. However, their effects are usually too small to see on everyday scales. Physicists are continually testing quantum effects at increasing scales, to probe the divide between the classical and quantum realms. But without a ranking system it wasn’t clear how far they had got.

Klaus Hornberger of the University of Duisburg-Essen in Germany and his colleague Stefan Nimmrichter have come up with such a system. If a quantum-classical divide exists, quantum laws will have to change. But successful quantum experiments at larger and larger scales rule out some of the possible modifications. The more changes an experiment rules out, the higher it scores.

An experiment in which a cat sits in two places at once would score very highly on the scale


This makes it possible to compare theoretical and real experiments, and creates a list of chart-toppers. Attempts at neutron superposition in the 1960s score around 5 or 6, while modern experiments involving nearly 500 atoms hit 12. It is a logarithmic scale, so this is roughly a million-fold improvement, but it pales next to Schrödinger’s cat. Implementing this thought experiment with existing quantum technology would max out at 24. A version in which an actual cat simultaneously sits in two spots that are 10 centimetres apart, would score 57 (Physical Review Letters, doi.org/mbm1).

That’s not to say it could never happen, says Hornberger, as 50 years ago people would have thought today’s experiments impossible. “I don’t think there is a fundamental limit.”