All light is made of photons — tiny particles. A simple detector merely notes whether they hit or not: it's like a light meter that says 'yes' or 'no'. For a long time, it was thought such a device couldn't distinguish a laser beam from quantum light — their signals seemed identical.

The solution: place an adjustable attenuator in front of the detector — like a dimmer. At different levels of dimming, classical and quantum light behave differently. Classical light is predictable: the stronger the attenuation, the fewer the flashes. Quantum light, however, brought a surprise: when heavily dimmed, the detector starts firing noticeably more often than expected. It's as if the particles push through the barrier together. This anomaly is a hallmark of non-classical behavior. Thus, a dimmer turned a blunt indicator into a keen spectroscopic probe.