Wave-particle duality quantified for the first time

One of quantum physics’s most bizarre and fundamental concepts — that quantum objects behave both as particles and as waves — has a new and more quantitative foundation. Researchers have reimagined the archetypal double-slit experiment, using lasers and lithium niobate crystals to create two photons with a single quantum state. One photon’s wave-like nature was quantified using the interference pattern it created in an interferometer. The other photon’s particle-like qualities were measured by observing its trajectory. What’s more, scientists were able to tweak the lasers to test how the source influenced a single quantum particle’s wave–particle duality. This revealed how the whole system — photons, sources and detectors — is linked by quantum entanglement.