Quantum optics is a field of quantum physics that deals specifically with the interaction of photons with matter. The study of individual photons is crucial to understanding the behavior of electromagnetic waves as a whole.

To clarify exactly what this means, the word "quantum" refers to the smallest amount of any physical entity that can interact with another entity. Quantum physics, therefore, deals with the smallest particles; these are incredibly tiny sub-atomic particles which behave in unique ways.

The word "optics," in physics, refers to the study of light. Photons are the smallest particles of light (though it is important to know that photons can behave as both particles and waves).

### Development of Quantum Optics and the Photon Theory of Light

The theory that light moved in discrete bundles (i.e. photons) was presented in Max Planck's 1900 paper on the ultraviolet catastrophe in black body radiation. In 1905, Einstein expanded on these principles in his explanation of the photoelectric effect to define the photon theory of light.

Quantum physics developed through the first half of the twentieth century largely through work on our understanding of how photons and matter interact and inter-relate. This was viewed, however, as a study of the matter involved more than the light involved.

In 1953, the maser was developed (which emitted coherent microwaves) and in 1960 the laser (which emitted coherent light). As the property of the light involved in these devices became more important, quantum optics began being used as the term for this specialized field of study.

### Findings

Quantum optics (and quantum physics as a whole) views electromagnetic radiation as traveling in the form of both a wave and a particle at the same time. This phenomenon is called wave-particle duality.

The most common explanation of how this works is that the photons move in a stream of particles, but the overall behavior of those particles is determined by a *quantum wave function* that determines the probability of the particles being in a given location at a given time.

Taking findings from quantum electrodynamics (QED), it is also possible to interpret quantum optics in the form of the creation and annihilation of photons, described by field operators. This approach allows the use of certain statistical approaches that are useful in analyzing the behavior of light, although whether it represents what is physically taking place is a matter of some debate (although most people view it as just a useful mathematical model).

### Applications

Lasers (and masers) are the most obvious application of quantum optics. Light emitted from these devices is in a coherent state, which means the light closely resembles a classical sinusoidal wave. In this coherent state, the quantum mechanical wave function (and thus the quantum mechanical uncertainty) is distributed equally. The light emitted from a laser is, therefore, highly ordered, and generally limited to essentially the same energy state (and thus the same frequency & wavelength).