Counting quantum pigeons with and without disturbing them

What is it about?

The laws of quantum mechanics allow for objects to exist in two states at once, such as Schrödinger's infamous cat, which occupies a superposition between life and death. This quantum superpower is essential for explaining the nanoscopic world, including the electrical properties of semi-conductors and the structure of atoms, yet it also leads to certain baffling conclusions. Once such conclusion is that three quantum pigeons can be crammed into only two pigeonholes without any of the pigeons being in the same pigeonhole. We experimentally confirm this prediction using photons in place of pigeons and uncover the role of measurement disturbance has to play in explaining this paradoxical phenomenon.

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Photo by Michael Dziedzic on Unsplash

Photo by Michael Dziedzic on Unsplash

Why is it important?

Quantum theory has proven wildly successful in predicting properties of systems whose past or future are specified. Applying the theory to systems with a definite past and future yields infamously counterintuitive predictions, e.g., three quantum pigeons can apparently occupy two pigeonholes without any pair occupying the same pigeonhole. Are such counterintuitive predictions merely an artifact of measurement disturbance? We answer this question empirically by measuring photonic “pigeons” with a variety of different measurement disturbances by implementing a scheme that achieves variable-strength measurements of nonlocal observables. We find that measurement disturbance can explain some, but not all, of the paradoxical phenomena of pre- and postselected systems, thus revealing the strange logic of quantum systems.