Kohärente

Kohärente is an adjective used in German to describe phenomena that preserve a fixed phase relationship or show a consistent phase across different parts of a system. In scientific contexts it often appears in phrases such as kohärente Lichtquelle (coherent light source) or kohärente Zustände (coherent states). The noun Kohärenz stands for the property of coherence, while kohärent describes objects or states that possess it.

In optics, coherence is typically separated into temporal and spatial categories. Temporal coherence refers to a stable phase over time, related to the spectral bandwidth and to coherence length. Spatial coherence concerns uniform phase across a wavefront, enabling clear interference fringes. Sources with high coherence include lasers; incoherent sources such as ordinary white light lack stable relative phase and produce blurred fringes.

In quantum mechanics and quantum optics, kohärente Zustände—coherent states of the quantum harmonic oscillator—are special quantum states that closely resemble classical oscillations. They minimize the Heisenberg uncertainty relation and are eigenstates of the annihilation operator. In quantum optics, laser light is often modeled as a coherent state, providing a useful bridge between wave and particle pictures.

Additionally, coherence is measured using correlation functions, such as the first-order coherence g1 for optical fields and higher-order coherences for more complex light. In signal processing, coherence describes linear correlations between signals in the frequency domain, quantifying how well two signals maintain a fixed phase relation across frequencies. While related, coherence is distinct from entanglement and from mere statistical correlation.