Two pebbles drop into water at a distance. The waves from them run toward each other and at certain spots they cancel out — the water becomes smooth, even though each stone alone would create ripples. Much the same happens with a pair of photons (light particles) born in a single crystal and flying off in different directions.
In the new work, physicists made such a pair create a 'dip' not across the beam but along it — in the direction the photons are traveling. To do this, they used a ring-shaped pump beam (a light ring, not a solid spot) and passed it through a crystal that generates photon pairs. One photon of the pair travels a slightly longer path to the detector than the other. Because of this path difference, they accumulate a different 'twist' of the wave — the so-called phase (the shift of wave crests relative to each other). When the phases become opposite, the waves cancel each other completely, and at that point there is no light.
The authors found that the 'vortex' at such a dead point has a strict order: the phase around it twists by an integer number of turns. The magnitude of this number is set by the shape of the pump ring. If the ring is simple, the twist is single; if double, the twist is double. But the most interesting part: the trick works only when the photons are truly intertwined (entangled). If their states can be separated, like socks in a drawer, there will be no shared dead point.
Practical use is still in the future: such points could serve as ultra-sensitive markers for measuring distances or for encoding information in quantum communication lines.
A similar effect can be seen in noise-canceling headphones: a microphone picks up external noise, and a speaker plays its inverted copy, and silence falls in your ear. Here, too, two waves cancel each other, but they work not with noise but with light.