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At any particular time t the wave can interfere perfectly with its delayed copy. Temporal coherenceįigure 2: The amplitude of a wave whose phase drifts significantly in time τ c as a function of time t (red) and a copy of the same wave delayed by 2τ c(green). Therefore, many of the standard measurements of coherence are indirect measurements, even in fields where the wave can be measured directly. Most of the concepts involving coherence which will be introduced below were developed in the field of optics and then used in other fields. Instead, we measure the intensity of the light. However, in optics one cannot measure the electric field directly as it oscillates much faster than any detector’s time resolution. Consequently, its correlation with another wave can simply be calculated. In most of these systems, one can measure the wave directly.
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As an example, consider two waves perfectly correlated for all times. The cross-correlation quantifies the ability to predict the value of the second wave by knowing the value of the first. The coherence of two waves follows from how well correlated the waves are as quantified by the cross-correlation function. The property of coherence is the basis for commercial applications such as holography, the Sagnac gyroscope, radio antenna arrays, optical coherence tomography and telescope interferometers ( astronomical optical interferometers and radio telescopes). 4.1 The relationship between coherence time and bandwidthĬoherence was originally conceived in connection with Thomas Young's double-slit experiment in optics but is now used in any field that involves waves, such as acoustics, electrical engineering, neuroscience, and quantum mechanics.Similarly if in Young's double slit experiment if the space between the two slits is increased, the coherence dies gradually and finally the infringes disappear, showing Spatial Coherence. Once the infringes are obtained in Michelson–Morley experiment, if now one of the mirror is moved away gradually then the time for the beam to travel increases and the infringes become dull and finally are lost, showing Temporal Coherence. Temporal and Spatial Coherence can be exhibited by Michelson–Morley experiment and Young's slits experiment respectively. The degree of coherence is measured by the interference visibility, a measure of how perfectly the waves can cancel due to destructive interference. Two waves are said to be coherent if they have a constant relative phase. When interfering, two waves can add together to create a wave of greater amplitude than either one ( constructive interference) or subtract from each other to create a wave of lesser amplitude than either one ( destructive interference), depending on their relative phase. This implies that constructive or destructive interferences are limit cases, and that waves can always interfere, even if the result of the addition is complicated or not remarkable. In quantum physics, a single wave can interfere with itself, but this is due to its quantum behavior and is still an addition of two waves (see Young's slits experiment). One should note at this point that interference is nothing more than the addition, in a mathematical sense, of wave functions. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets. It contains in fact several distinct concepts, which are limit cases that never occur in reality but allows to understand the physics of waves and has become in particular a very important concept in quantum physics.
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temporally and spatially constant) interference. In physics, coherence is an ideal property of waves that enables stationary (i.e. Please improve this article by introducing more precise citations. This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations.