In very dim light, photons arrive one by one, like drops from a leaky faucet — never in pairs. This effect, predicted by Roy Glauber and confirmed by Leonard Mandel, directly proves that light is made of particles.
Now calculations show the same thing happens when ultraviolet flashes are born, as a powerful laser strikes a gas. Normally, a laser knocks out a whole shower of photons from atoms. But according to quantum laws (Werner Heisenberg), when infrared light is converted to ultraviolet, each particle forbids others from appearing nearby. The result is a stream of singles, not a crowd.
The discovery promises compact sources of single ultraviolet photons at room temperature — for quantum cryptography without bulky coolers, ultra-precise clocks, and microscopes that capture electron motion. And the biggest surprise: this effect has been hiding in plain sight for decades, lurking in the data of ordinary brightness measurements; no one simply thought to look for it.
Even the most stable laser always emits photons with a slight bunching. True “one-at-a-time” light is born only in this process.