Classical optics relies on Maxwell’s equations, but it breaks down when very few photons are present or when single photons are counted. Glauber treated the electromagnetic field as a quantum operator and introduced correlation functions to evaluate photon statistics. He could thus explain the photon “bunching” observed in the Hanbury Brown–Twiss (HBT) experiment and showed that a laser’s perfectly correlated coherent state is qualitatively different from thermal light. His work founded the field of quantum optics and laid the groundwork for technologies such as quantum cryptography and precision quantum metrology.

Glauber’s theory uses canonical quantization of the electromagnetic field and employs normal ordering and time-ordering techniques to analyse photo-detection rigorously. While first-order coherence is accessible through conventional interferometry, he introduced higher-order coherence functions g^{(n)}(τ) to characterise full photon statistics. In particular, the criteria g^{(2)}(0)>1 for thermal light, g^{(2)}(0)=1 for an ideal laser, and g^{(2)}(0)<1 for non-classical light became experimental hallmarks for sub-Poissonian light and photon antibunching. Although embedded in QED, the framework effectively handles dissipative, non-perturbative systems, foreshadowing today’s open-quantum-systems approach.

Discarding an S-matrix viewpoint, Glauber developed a photo-detection model that clearly separates incident and scattered fields. By defining the photocurrent operator in a normal-ordered form of canonical conjugate fields, he generated phase-space distributions such as the P- and Q-functions. His calculation of multi-photon generation rates for a thermal distribution and the prediction of bunched statistics in a collinear HBT interferometer were seminal. Contemporary measurements of Mandel Q-parameters in cascade systems, Rabi-driven emitters, and design of heralded single-photon sources via parametric down-conversion all build directly on his correlation theory.