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| | "Why does my computer keep freezing up?" I was asked by a great deal of individuals the cause of their computer freeze problems. And I am fed up with spending much time in answering the query time and time again. This article is to tell we the real cause of the PC Freezes.<br><br>Document files let the consumer to input data, images, tables plus alternative elements to improve the presentation. The only issue with this formatting compared to different file kinds such as .pdf for illustration is its ability to be commonly editable. This signifies that anybody watching the file can change it by accident. Additionally, this file format can be opened by different programs but it refuses to guarantee that what we see inside the Microsoft Word application usually still be the same when we view it using another system. However, it really is still preferred by most computer users for its ease of utilize and features.<br><br>The 'registry' is a central database that shops info, settings plus options for a computer. It's really the most normal reason why XP runs slow plus in the event you fix this problem, you may make the computer run a lot faster. The issue is that the 'registry' shops a great deal of settings and details about your PC... plus considering Windows requirements to employ so many of these settings, any corrupted or damaged ones may straight affect the speed of your system.<br><br>Fixing tcpip.sys blue screen is simple to do with registry repair software.Trying to fix windows blue screen error on your may be tricky considering when you remove or damage the registry it can cause severe damage to your computer. The registry must be cleaned and all erroneous and incomplete information removed to stop blue screen errors from occurring.The benefit of registry repair software is not limited to just getting rid of the blue screen on startup.We could be amazed at the better and more improved speed plus performance of your computer system following registry cleaning is completed. Registry cleaning really can develop the computer's working abilities, particularly when you choose a certain registry repair software that is extremely efficient.<br><br>Use a [http://bestregistrycleanerfix.com/system-mechanic iolo system mechanic]. This may look the Windows registry for three kinds of keys which can definitely hurt PC performance. These are: duplicate, lost, plus corrupted.<br><br>Although I usually utilize the most recent variation of browser, occasionally different extensions and plugins become the cause of errors with my browser and the program. The same is the story with my browser which was crashing frequently potentially due to the Flash player error.<br><br>The System File Checker (SFC) could enable in resolving error 1721 because it, by its nature, scans the program files for corruption plus replaces them with their authentic versions. This requires you to have the Windows Installation DVD ROM for continuing.<br><br>If you need to have a computer with fast running speed, you'd better install a advantageous registry cleaner to wash the useless files for we. As long as we take care of your computer, it might keep inside good condition. |
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| In [[physics]], '''coherence''' is an ideal property of [[wave]]s that enables stationary (i.e. temporally and spatially constant) [[Interference (wave propagation)|interference]]. It contains several distinct concepts, which are limit cases that never occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. More generally, coherence describes all properties of the [[correlation function|correlation]] between [[physical quantities]] of a single wave, or between several waves or wave packets.
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| Interference is nothing more than the addition, in the mathematical sense, of wave functions. In quantum mechanics, 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 interference experiment|Young's slits experiment]]). 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.
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| 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 (waves)|phase]]. Two waves are said to be coherent if they have a constant relative phase. The [[degree of coherence]] is measured by the [[interference visibility]], a measure of how perfectly the waves can cancel due to destructive interference.
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| Spatial coherence describes the correlation between waves at different points in space. Temporal coherence describes the correlation or predictable relationship between waves observed at different moments in time. Both are observed in the [[Michelson–Morley experiment]] and [[Young's interference experiment]]. Once the fringes are obtained in the Michelson–Morley experiment, when one of the mirrors is moved away gradually, the time for the beam to travel increases and the infringes become dull and finally are lost, showing temporal coherence. Similarly, if in Young's double slit experiment the space between the two slits is increased, the coherence dies gradually and finally the infringes disappear, showing spatial coherence.
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| ==Introduction==
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| Coherence was originally conceived in connection with [[Thomas Young (scientist)|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]]. The property of coherence is the basis for commercial applications such as [[holography]], the [[Sagnac interferometer|Sagnac]] [[gyroscope]], [[radio]] [[Phased array|antenna array]]s, [[optical coherence tomography]] and telescope interferometers ([[Interferometry#Astronomical optical interferometry|astronomical optical interferometers]] and [[radio telescope]]s).
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| ==Mathematical definition==
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| {{Empty section|date=January 2014}}
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| ==Coherence and correlation==
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| The coherence of two waves follows from how well correlated the waves are as quantified by the [[cross-correlation]] function.<ref name="winter">
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| {{cite web
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| |author=Rolf G. Winter
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| |coauthors=Aephraim M. Steinberg
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| |year=2008
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| |title=Coherence
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| |url=http://accessscience.com/content/Coherence/146900
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| |work=AccessScience
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| |publisher=[[McGraw-Hill]]
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| }}</ref><ref name="BornWolf">
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| {{cite book
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| |author=M.Born
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| |coauthors=E. Wolf
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| |year=1999
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| |title=Principles of Optics
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| |edition=7th
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| |publisher=[[Cambridge University Press]]
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| |isbn= 978-0-521-64222-4
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| }}</ref><ref name="Loudon">
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| {{cite book
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| |author=Loudon, Rodney
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| |year=2000
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| |title=The Quantum Theory of Light
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| |publisher=[[Oxford University Press]]
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| |isbn=0-19-850177-3
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| }}</ref><ref name="mandel">
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| {{cite book
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| |author=Leonard Mandel
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| |coauthor=Emil Wolf
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| |year=1995
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| |title=Optical Coherence and Quantum Optics
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| |publisher=[[Cambridge University Press]]
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| |isbn=0-521-41711-2
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| }}</ref><ref name="mar">
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| {{cite book
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| |author=Arvind Marathay
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| |year=1982
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| |title=Elements of Optical Coherence Theory
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| |publisher=[[John Wiley & Sons]]
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| |isbn=0-471-56789-2
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| }}</ref> The cross-correlation quantifies the ability to predict the value of the second wave by knowing the value of the first. As an example, consider two waves perfectly correlated for all times. At any time, if the first wave changes, the second will change in the same way. If combined they can exhibit complete constructive interference/superposition at all times, then it follows that they are perfectly coherent. As will be discussed below, the second wave need not be a separate entity. It could be the first wave at a different time or position. In this case, the measure of correlation is the [[autocorrelation]] function (sometimes called '''self-coherence'''). Degree of correlation involves correlation functions.
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| ==Examples of wave-like states==
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| These states are unified by the fact that their behavior is described by a [[wave equation]] or some generalization thereof.
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| *Waves in a rope (up and down) or [[slinky]] (compression and expansion)
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| *[[Surface waves]] in a liquid
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| *Electric signals (fields) in transmission cables
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| *[[Sound]]
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| *[[Radio wave]]s and [[Microwaves]]
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| *[[Light wave]]s ([[optics]])
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| *[[Electron]]s, [[atom]]s and any other object (such as a baseball, as described by [[quantum physics]])
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| In most of these systems, one can measure the wave directly. Consequently, its correlation with another wave can simply be calculated. However, in optics one cannot measure the [[electric field]] directly as it oscillates much faster than any detector’s time resolution.<ref>
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| {{cite journal
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| |last1=Peng |first1=J.-L.
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| |last2=Liu |first2=T.-A.
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| |last3=Shu |first3=R.-H.
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| |year=2008
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| |title=Optical frequency counter based on two mode-locked fiber laser combs
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| |journal=[[Applied Physics B]]
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| |volume=92 |issue=4 |pages=513
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| |bibcode=2008ApPhB..92..513P
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| |doi=10.1007/s00340-008-3111-6
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| }}</ref> Instead, we measure the [[intensity (physics)|intensity]] of the light. Most of the concepts involving coherence which will be introduced below were developed in the field of optics and then used in other fields. Therefore, many of the standard measurements of coherence are indirect measurements, even in fields where the wave can be measured directly.
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| ==Temporal coherence==<!--Temporal coherence redirects here-->
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| [[File:single frequency.png|thumb|450px|right|Figure 1: The amplitude of a single frequency wave as a function of time ''t'' (red) and a copy of the same wave delayed by τ(green). The coherence time of the wave is infinite since it is perfectly correlated with itself for all delays τ.]]
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| [[File:phase drift.png|thumb|450px|right|Figure 2: The amplitude of a wave whose phase drifts significantly in time τ<sub>c</sub> as a function of time ''t'' (red) and a copy of the same wave delayed by 2τ<sub>c</sub>(green). At any particular time t the wave can interfere perfectly with its delayed copy. But, since half the time the red and green waves are in phase and half the time out of phase, when averaged over t any interference disappears at this delay.]]
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| Temporal coherence is the measure of the average correlation between the value of a wave and itself delayed by τ, at any pair of times. Temporal coherence tells us how monochromatic a source is. In other words, it characterizes how well a wave can interfere with itself at a different time. The delay over which the phase or amplitude wanders by a significant amount (and hence the correlation decreases by significant amount) is defined as the [[coherence time]] ''τ<sub>c</sub>''. At τ=0 the degree of coherence is perfect whereas it drops significantly by delay ''τ<sub>c</sub>''. The [[coherence length]] ''L<sub>c</sub>'' is defined as the distance the wave travels in time τ<sub>c</sub>.
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| One should be careful not to confuse the coherence time with the time duration of the signal, nor the coherence length with the coherence area (see below).
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| ===The relationship between coherence time and bandwidth===
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| It can be shown that the faster a wave decorrelates (and hence the smaller τ<sub>c</sub> is) the larger the range of frequencies Δf the wave contains. Thus there is a tradeoff:
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| :<math>\tau_c \Delta f \approx 1</math>.
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| Formally, this follows from the [[convolution theorem]] in mathematics, which relates the [[Fourier transform]] of the power spectrum (the intensity of each frequency) to its [[autocorrelation]].
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| ===Examples of temporal coherence===
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| We consider four examples of temporal coherence.
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| *A wave containing only a single frequency (monochromatic) is perfectly correlated at all times according to the above relation. (See Figure 1)
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| *Conversely, a wave whose phase drifts quickly will have a short coherence time. (See Figure 2)
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| *Similarly, pulses ([[wave packet]]s) of waves, which naturally have a broad range of frequencies, also have a short coherence time since the amplitude of the wave changes quickly. (See Figure 3)
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| *Finally, white light, which has a very broad range of frequencies, is a wave which varies quickly in both amplitude and phase. Since it consequently has a very short coherence time (just 10 periods or so), it is often called incoherent.
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| The most monochromatic sources are usually [[laser]]s; such high monochromaticity implies long coherence lengths (up to hundreds of meters). For example, a stabilized and monomode [[helium-neon laser]] can easily produce light with coherence lengths of 300 m.<ref name=saleh-teich>{{cite book|last=Saleh, Teich|title=Fundamentals of Photonics|publisher=Wiley}}</ref> Not all lasers are monochromatic, however (e.g. for a mode-locked [[Ti-sapphire laser]], Δλ ≈ 2 nm - 70 nm). LEDs are characterized by Δλ ≈ 50 nm, and tungsten filament lights exhibit Δλ ≈ 600 nm, so these sources have shorter coherence times than the most monochromatic lasers.
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| [[Holography]] requires light with a long coherence time. In contrast, [[Optical coherence tomography]] uses light with a short coherence time.
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| ===Measurement of temporal coherence===
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| [[File:wave packets.png|thumb|400px|right|Figure 3: The amplitude of a wavepacket whose amplitude changes significantly in time τ<sub>c</sub> (red) and a copy of the same wave delayed by 2τ<sub>c</sub>(green) plotted as a function of time ''t''. At any particular time the red and green waves are uncorrelated; one oscillates while the other is constant and so there will be no interference at this delay. Another way of looking at this is the wavepackets are not overlapped in time and so at any particular time there is only one nonzero field so no interference can occur.]]
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| [[File:interference finite coherence.png|thumb|390px|right|Figure 4: The time-averaged intensity (blue) detected at the output of an interferometer plotted as a function of delay τ for the example waves in Figures 2 and 3. As the delay is changed by half a period, the interference switches between constructive and destructive. The black lines indicate the interference envelope, which gives the [[degree of coherence]]. Although the waves in Figures 2 and 3 have different time durations, they have the same coherence time.]]
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| In optics, temporal coherence is measured in an interferometer such as the [[Michelson interferometer]] or [[Mach–Zehnder interferometer]]. In these devices, a wave is combined with a copy of itself that is delayed by time τ. A detector measures the time-averaged [[intensity (physics)|intensity]] of the light exiting the interferometer. The resulting interference visibility (e.g. see Figure 4) gives the temporal coherence at delay τ. Since for most natural light sources, the coherence time is much shorter than the time resolution of any detector, the detector itself does the time averaging. Consider the example shown in Figure 3. At a fixed delay, here 2τ<sub>c</sub>, an infinitely fast detector would measure an intensity that fluctuates significantly over a time ''t'' equal to τ<sub>c</sub>. In this case, to find the temporal coherence at 2τ<sub>c</sub>, one would manually time-average the intensity.
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| <br style="clear:both" />
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| ==Spatial coherence==<!--Spatial coherence redirects here-->
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| In some systems, such as water waves or optics, wave-like states can extend over one or two dimensions. Spatial coherence describes the ability for two points in space, ''x<sub>1</sub>'' and ''x<sub>2</sub>'', in the extent of a wave to interfere, when averaged over time. More precisely, the spatial coherence is the [[cross-correlation]] between two points in a wave for all times. If a wave has only 1 value of amplitude over an infinite length, it is perfectly spatially coherent. The range of separation between the two points over which there is significant interference is called the coherence area, ''A<sub>c</sub>''. This is the relevant type of coherence for the Young’s double-slit interferometer. It is also used in optical imaging systems and particularly in various types of astronomy telescopes. Sometimes people also use “spatial coherence” to refer to the visibility when a wave-like state is combined with a spatially shifted copy of itself.
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| ===Examples of spatial coherence===
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| <gallery caption="Spatial coherence" perrow=5>
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| Image:spatial_coherence_infinite_ex1.png|<small>Figure 5: A plane wave with an infinite [[coherence length]].</small>
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| Image:spatial_coherence_infinite_ex2.png|<small>Figure 6: A wave with a varying profile (wavefront) and infinite coherence length.</small>
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| Image:spatial_coherence_finite.png|<small>Figure 7: A wave with a varying profile (wavefront) and finite coherence length.</small>
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| Image:spatial_coherence_pinhole.png|<small>Figure 8: A wave with finite coherence area is incident on a pinhole (small aperture). The wave will [[diffraction|diffract]] out of the pinhole. Far from the pinhole the emerging spherical wavefronts are approximately flat. The coherence area is now infinite while the coherence length is unchanged.</small>
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| Image:spatial_coherence_detector.png|<small>Figure 9: A wave with infinite coherence area is combined with a spatially shifted copy of itself. Some sections in the wave interfere constructively and some will interfere destructively. Averaging over these sections, a detector with length D will measure reduced [[interference visibility]]. For example a misaligned [[Mach–Zehnder interferometer]] will do this.</small>
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| </gallery>
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| Consider a tungsten light-bulb filament. Different points in the filament emit light independently and have no fixed phase-relationship. In detail, at any point in time the profile of the emitted light is going to be distorted. The profile will change randomly over the coherence time <math>\tau_c</math>. Since for a white-light source such as a light-bulb <math>\tau_c</math> is small, the filament is considered a spatially incoherent source. In contrast, a radio [[Phased array|antenna array]], has large spatial coherence because antennas at opposite ends of the array emit with a fixed phase-relationship. Light waves produced by a laser often have high temporal and spatial coherence (though the degree of coherence depends strongly on the exact properties of the laser). Spatial coherence of laser beams also manifests itself as speckle patterns and diffraction fringes seen at the edges of shadow.
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| Holography requires temporally and spatially coherent light. Its inventor, [[Dennis Gabor]], produced successful holograms more than ten years before lasers were invented. To produce coherent light he passed the monochromatic light from an emission line of a [[mercury-vapor lamp]] through a pinhole spatial filter.
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| In February 2011, Dr [[Andrew Truscott]], leader of a research team at the ARC Centre of Excellence for Quantum-Atom Optics at [[Australian National University]] in [[Canberra, Australian Capital Territory]], showed that [[helium]] atoms cooled to near [[absolute zero]] / [[Bose-Einstein condensate]] state, can be made to flow and behave as a coherent beam as occurs in a laser.<ref>
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| {{cite journal
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| |last1=Hodgman |first1=S. S.
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| |last2=Dall |first2=R. G.
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| |last3=Manning |first3=A. G.
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| |last4=Baldwin |first4=K. G. H.
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| |last5=Truscott |first5=A. G.
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| |year=2011
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| |title=Direct Measurement of Long-Range Third-Order Coherence in Bose-Einstein Condensates
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| |journal=[[Science (journal)|Science]]
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| |volume=331 |issue=6020 |pages=1046–1049
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| |bibcode=2011Sci...331.1046H
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| |doi=10.1126/science.1198481
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| |pmid=21350171
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| }}</ref><ref>
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| {{cite web
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| |last=Pincock |first=S.
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| |date=25 February 2011
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| |title=Cool laser makes atoms march in time
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| |url=http://www.abc.net.au/science/articles/2011/02/25/3149175.htm
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| |work=[[ABC Science]]
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| |publisher=[[ABC News Online]]
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| |accessdate=2011-03-02
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| }}</ref>
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| ==Spectral coherence==
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| [[File:spectral coherence pulse.png|thumb|right|350px|Figure 10: Waves of different frequencies (i.e. colors) interfere to form a pulse if they are coherent.]]
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| [[File:spectral coherence continuous.png|thumb|right|350px|Figure 11: Spectrally incoherent light interferes to form continuous light with a randomly varying phase and amplitude]]
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| Waves of different frequencies (in light these are different colours) can interfere to form a pulse if they have a fixed relative phase-relationship (see [[Fourier transform]]). Conversely, if waves of different frequencies are not coherent, then, when combined, they create a wave that is continuous in time (e.g. white light or [[white noise]]). The temporal duration of the pulse <math>\Delta t</math> is limited by the spectral bandwidth of the light <math>\Delta f</math> according to:
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| :<math>\Delta f\Delta t \ge 1</math>,
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| which follows from the properties of the Fourier transform and results in [[Küpfmüller's uncertainty principle]] (for quantum particles it also results in the [[Heisenberg uncertainty principle]]).
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| If the phase depends linearly on the frequency (i.e. <math>\theta (f) \propto f</math>) then the pulse will have the minimum time duration for its bandwidth (a ''transform-limited'' pulse), otherwise it is chirped (see [[Dispersion (optics)|dispersion]]).
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| ===Measurement of spectral coherence===
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| Measurement of the spectral coherence of light requires a [[nonlinear optics|nonlinear]] optical interferometer, such as an intensity [[optical autocorrelation|optical correlator]], [[frequency-resolved optical gating]] (FROG), or [[spectral phase interferometry for direct electric-field reconstruction]] (SPIDER).
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| <br style="clear:both" />
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| ==Polarization coherence==
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| Light also has a [[polarization (waves)|polarization]], which is the direction in which the electric field oscillates. Unpolarized light is composed of incoherent light waves with random polarization angles. The electric field of the unpolarized light wanders in every direction and changes in phase over the coherence time of the two light waves. An absorbing [[polarizer]] rotated to any angle will always transmit half the incident intensity when averaged over time.
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| If the electric field wanders by a smaller amount the light will be partially polarized so that at some angle, the polarizer will transmit more than half the intensity. If a wave is combined with an orthogonally polarized copy of itself delayed by less than the coherence time, partially polarized light is created.
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| The polarization of a light beam is represented by a vector in the [[Polarization (waves)#Parameterization|Poincaré sphere]]. For polarized light the end of the vector lies on the surface of the sphere, whereas the vector has zero length for unpolarized light. The vector for partially polarized light lies within the sphere
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| ==Applications==
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| ===Holography===
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| Coherent superpositions of ''optical wave fields'' include [[holography]]. Holographic objects are used frequently in daily life in bank notes and credit cards.
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| ===Non-optical wave fields===
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| Further applications concern the coherent superposition of ''non-optical wave fields''. In quantum mechanics for example one considers a probability field, which is related to the wave function <math>\psi (\mathbf r)</math> (interpretation: density of the probability amplitude). Here the applications concern, among others, the future technologies of [[quantum computing]] and the already available technology of [[quantum cryptography]]. Additionally the problems of the following subchapter are treated.
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| ==Quantum coherence==<!-- This section is linked from [[quantum entanglement]] -->
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| {{merge|Quantum decoherence|target=Quantum coherence and decoherence|section=y|date=September 2013}}
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| In [[quantum mechanics]], all objects have wave-like properties (see [[Matter wave|de Broglie wave]]s). For instance, in Young's [[double-slit experiment]] electrons can be used in the place of light waves. Each electron's wave-function goes through both slits, and hence has two separate split-beams that contribute to the intensity pattern on a screen. According to standard wave theory (Fresnel, Huygens) these two contributions give rise to an intensity pattern of bright bands due to constructive interference, interlaced with dark bands due to destructive interference, on a downstream screen. (Each split-beam, by itself, generates a diffraction pattern with less noticeable, more widely spaced dark and light bands.) This ability to interfere and diffract is related to coherence (classical or quantum) of the wave. The association of an electron with a wave is unique to quantum theory.
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| When the incident beam is represented by a quantum [[pure state]], the split beams downstream of the two slits are represented as a [[Quantum superposition|superposition]] of the pure states representing each split beam. (This has nothing to do with two particles or [[Bell's inequalities]] relevant to an entangled state: a 2-body state, a kind of coherence between two 1-body states.) The quantum description of imperfectly coherent paths is called a [[Mixed state (physics)|mixed state]]. A perfectly coherent state has a [[density matrix]] (also called the "statistical operator") that is a projection onto the pure coherent state, while a mixed state is described by a classical probability distribution for the pure states that make up the mixture.
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| Large-scale ([[macroscopic]]) quantum coherence leads to novel phenomena, the so-called [[macroscopic quantum phenomena]]. For instance, the [[laser]], [[superconductivity]] and [[superfluidity]] are examples of highly coherent quantum systems whose effects are evident at the macroscopic scale. The macroscopic quantum coherence (Off-Diagonal Long-Range Order, ODLRO) [Penrose & Onsager (1957), [[Chen Ning Yang|C. N. Yang]] (1962)] for laser light, and superfluidity, is related to first-order (1-body) coherence/ODLRO, while superconductivity is related to second-order coherence/ODLRO. (For fermions, such as electrons, only even orders of coherence/ODLRO are possible.) Superfluidity in liquid He4 is related to a partial [[Bose–Einstein condensate]]. Here, the condensate portion is described by a multiply-occupied single-particle state. [e.g., Cummings & Johnston (1966)]
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| Regarding the occurrence of quantum coherence at a macroscopic level, it is interesting to note that the classical electromagnetic field exhibits macroscopic quantum coherence. The most obvious example is the carrier signal for radio and TV. They satisfy [[Roy J. Glauber|Glauber]]'s quantum description of coherence.
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| ==See also==
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| {{Commons category|Coherence}}
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| * [[Atomic coherence]]
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| * [[Coherence length]]
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| * [[Coherent state]]
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| * [[Laser linewidth]]
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| * [[Measurement in quantum mechanics]]
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| * [[Measurement problem]]
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| * [[Optical heterodyne detection]]
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| * [[Quantum decoherence]]
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| * [[Quantum Zeno effect]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *{{cite web
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| |author="Dr. SkySkull"
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| |title=Optics basics: Coherence
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| |url=http://skullsinthestars.com/2008/09/03/optics-basics-coherence/
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| |work=Skulls in the Stars
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| }}
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| {{DEFAULTSORT:Coherence (Physics)}}
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| [[Category:Concepts in physics]]
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| [[Category:Wave mechanics]]
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| [[Category:Quantum mechanics]]
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| [[Category:Radar signal processing]]
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