What name is given to the quantum state of light containing exactly one photon?

A single-photon state is a quantum state of light containing exactly one photon in a specified optical mode.

In ideal notation it is often written as |1⟩, distinguishing it from the vacuum state |0⟩ and multiphoton number states. A photon is not a tiny classical pulse with a known route; its quantum state describes possible measurement outcomes such as detection time, polarization, frequency, and path.

Single photons are important in quantum optics and quantum information. Experiments can generate them using sources such as heralded spontaneous parametric down-conversion, quantum dots, trapped atoms, or color centers in diamond. Real devices are imperfect, so researchers measure properties such as brightness, purity, indistinguishability, and the probability of producing more than one photon.

A coherent laser state is different: it has an uncertain photon number and can contain zero, one, or many photons. A single-photon state is specifically a number state with photon number one.