The term *coherence* refers to many loosely related concepts in [[Classical Physics|classical]] and [[Quantum Mechanics|quantum physics]]. Usually coherence is a desirable property of physical systems, whereas its counterpart *decoherence* often refers to unwanted behavior. In physics, coherence is a fundamental property of [[Wave|waves]] that determines their ability to produce stable and predictable patterns of [[Interference|interference]]. When two [[Monochromatic Light|monochromatic]] waves interfere, they may cancel each other (*destructive* interference) or add up to create a wave with a larger amplitude (*constructive* interference). A familiar example of interfering waves can be seen on the surface of a lake, where ripples from two different sources meet -- sometimes amplifying each other into larger waves, sometimes canceling out into stillness. Two waves with the same [[Frequency|frequency]] interfere with each other, because their relative [[Phase - Wave|phase]] is constant. The phase relationship between waves is a defining characteristic of coherent waves. Wave coherence plays an important role in optics and [[Quantum Optics|quantum optics]], where [[Coherent Light|coherent light]] from [[Laser|laser]] sources is used for many technological and scientific applications. In quantum computing, quantum coherence is a desired property for a [[Qubit|qubit]]. The [[Coherence Time|coherence time]] of a qubit is a measure of how long it maintains its quantum state -- which is essentially the lifespan of a qubit. [[Decoherence|Decoherence]] of quantum systems is unavoidable and qubits are thus not perfect, but to judge the quality of a qubit, it is not sufficient to measure a qubits coherence time, but rather compare it against other important time scales such as the [[Quantum Gates|gate]] execution time. >[!read]- Further Reading >[!ref]- References