What Is Quantum Optics?

Quantum optics is a branch of physics that studies the interaction between light and matter at a level of detail where the quantum nature of the electromagnetic field becomes essential. The field treats light not as a continuous classical wave but as a stream of discrete energy quanta called photons, each carrying energy proportional to its frequency as described by Planck's relation. Quantum optics extends classical electrodynamics by applying quantum field theory to optical frequencies, and its methods form the theoretical and experimental basis for lasers, photodetectors, entangled photon sources, and devices used in quantum information processing. The discipline draws from quantum mechanics, statistical physics, and photonics, with deep connections to the atomic physics of two-level and multi-level systems.

The field became a coherent discipline in the 1960s when Roy Glauber introduced the quantum theory of optical coherence, for which he later received the Nobel Prize in Physics. Glauber's coherence functions provided tools to distinguish classical from non-classical light, establishing that phenomena such as photon antibunching, squeezed light, and entangled photon pairs have no classical electromagnetic analog.

Nonclassical Light

Quantum optics distinguishes itself from classical photonics primarily through its treatment of nonclassical states of light. A coherent state, produced by an ideal laser, is the quantum state closest to a classical wave and obeys Poissonian photon statistics. Fock states, also called number states, contain an exactly defined number of photons and exhibit sub-Poissonian statistics; single-photon Fock states are essential resources for quantum cryptography and linear optical quantum computing. Squeezed light redistributes quantum noise between conjugate quadratures of the field, reducing noise in one quadrature below the vacuum level at the expense of increased noise in the other. These nonclassical properties are quantified using Glauber-Sudarshan P-functions and measured experimentally through techniques such as homodyne detection and photon correlation spectroscopy. Research on high-purity single-photon generation using cavity QED is described in arXiv literature on photonic Fock state generation.

Cavity Quantum Electrodynamics

Cavity quantum electrodynamics (cavity QED) studies the interaction between a single emitter, such as an atom or a quantum dot, and the quantized electromagnetic field confined in a high-finesse optical cavity. When the coupling strength between the atom and the cavity mode exceeds both the atomic decay rate and the cavity loss rate, the system enters the strong coupling regime, where energy is exchanged coherently between the atom and a single cavity photon. In this regime, the dressed-state energy levels split into the Jaynes-Cummings doublet, enabling photon-blockade effects that allow exactly one photon at a time to be transmitted through the cavity. Semiconductor quantum dots embedded in photonic crystal cavities extend these effects to solid-state platforms, enabling compact, chip-compatible nonclassical photon sources. The theory and experimental techniques of cavity QED with quantum dots in photonic crystals are reviewed in arXiv research on cavity QED and quantum dot systems.

Entangled Photon Pairs and Quantum Interference

Entangled photon pairs are generated through spontaneous parametric down-conversion (SPDC), in which a pump photon passing through a nonlinear crystal splits into two lower-frequency photons whose polarization, frequency, and momentum are correlated. The Hong-Ou-Mandel effect, in which two indistinguishable photons entering opposite ports of a beamsplitter always exit together, provides a sensitive test of photon indistinguishability and underpins photonic Bell state measurements used in quantum teleportation and entanglement swapping. Long-distance transmission of entangled photon pairs over optical fiber and free space, including experiments detailed in quantum communication survey literature, forms a physical layer for quantum networking.

Applications

Quantum optics has applications in a range of fields, including:

• Quantum key distribution using polarization-entangled or time-bin photon pairs
• Linear optical quantum computing using photons as qubits
• Quantum sensing and interferometry below the standard quantum limit
• Quantum imaging and ghost imaging using correlated photon pairs
• Optical atomic clocks and frequency standards