On the wall: light and dark fringes. That’s what light looks like when it cancels itself out.
Way back in the 17th century, Christiaan Huygens suggested that light is a wave. But it was the experiments of Arago and Fresnel in the early 19th century that showed two beams produce such fringes only when their oscillations lie in the same plane. That discovery helped us understand polarization — the direction in which a wave oscillates.
Two centuries later, physicists returned to this experiment, but added a quantum twist, as envisioned by Roy Glauber. Instead of two ordinary beams, they used light of two different characters. On one side, a coherent beam, like a laser’s, where all photons march in step. On the other, a single photon, twisted left or right — like a tiny spinning top.
That ‘spinning top’ is the photon’s spin, its intrinsic rotation, linked to circular polarization (the very thing 3D glasses need). They mixed the beams and measured the resulting pattern. For that, they used photometry — measuring brightness and polarization. Unlike spectroscopy, which breaks light into a rainbow, here only polarization mattered.
The big surprise lay in a simple fact: the fringes depend on the photon’s spin. Left-handed spin gives one pattern; right-handed gives another.
So an ordinary optical mixer became a quantum-rotation detector. You can read the spin without fancy instruments — just look at the stripes. In the future, this will come in handy in quantum networks, where quickly determining the state of single photons is crucial.
Light travels at a constant speed of light — 300,000 km/s. Nothing’s faster. Yet in these experiments, it shows its wavy side.
And so an old experiment with fringes on a wall gets a new lease on life. Today it not only confirms light’s wave nature but also serves as a key to quantum secrets.
The word ‘spin’ comes from English, meaning ‘to rotate’. Although a photon doesn’t spin like a tiny ball, this property behaves like angular momentum.