Photons Can Really Be in Two Places at Once, New Experiment Shows (Kind of)
Physicists at Hiroshima University say they’ve cleared up one of quantum physics’ biggest mysteries: Can a particle really be in two places at once? Or is the particle only ever in one place, and it’s just that the quantum maths that assigns it two places?
The researchers looked at this question with an interferometer, in which a photon (a quantum of light) gets split onto two paths which later recombine, according to the mathematics. The problem is that, while the maths does give a result that agrees with observations, there is usually no way to know that a single photon actually did take both paths, because the moment one measures which path the photon goes, the result collapses and no longer requires photons to go both paths.
What they did in the new experiment is to very slightly change the polarization on each path in opposite directions. Only then do they measure the probability that the photons still interfere. If they have a different polarization, they should not. And, importantly, the remaining probability that they do interfere depends on whether the photon only experienced one polarization shift on one arm, or on both arms.
The result is, unsurprisingly, that quantum mechanics is correct. They interpret this to say that, yes, the photon was really in both arms of the interferometer. And this is all well and good, except that according to their maths, photons can also have a negative presence in one arm of the interferometer, a fact that sheds strong doubts on their interpretation of the measurement results.
Image: Illustration of the experiment.
Figure: Fukuda et al, New J. Phys. 28 034515 (2026)