Editing Dephasing

{{Short description|Mechanism recovering classical behavior from a quantum system}}
{{EngvarB|date = April 2019}}
{{Multiple issues|
{{In line citation|date=February 2020}}
{{Unfocused|date=February 2012}}
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[[File:Cavity Dephasing Simulation.png|thumb|266x266px|Cavity loses coherence due to dephasing.]]
In [[physics]], '''dephasing''' is a mechanism that recovers [[classical physics|classical]] behaviour from a [[quantum physics|quantum]] system. It refers to the ways in which [[coherence (physics)|coherence]] caused by perturbation decays over time, and the system returns to the state before perturbation. It is an important effect in molecular and atomic [[spectroscopy]], and in the [[condensed matter physics]] of [[mesoscopic]] devices.

The reason can be understood by describing the conduction in metals as a classical phenomenon with quantum effects all embedded into an [[Effective mass (solid-state physics)|effective mass]] that can be computed quantum mechanically, as also happens to [[Conductance quantum|resistance]] that can be seen as a [[Scattering theory|scattering]] effect of [[Valence and conduction bands|conduction electrons]]. When the temperature is lowered and the dimensions of the device are meaningfully reduced, this classical behaviour should disappear and the laws of quantum mechanics should govern the behavior of conducting electrons seen as waves that move [[Ballistic conduction|ballistically]] inside the conductor without any kind of dissipation. Most of the time this is what one observes. But it appeared as a surprise{{To whom?|date=February 2020}} to uncover that the so-called '''dephasing time''', that is the time it takes for the conducting electrons to lose their quantum behavior, becomes finite rather than infinite when the temperature approaches zero in mesoscopic devices violating the expectations of the theory of [[Boris Altshuler]], [[Arkady Aronov]] and David E. Khmelnitskii.<ref>{{Cite journal|last1=Altshuler|first1=B L|last2=Aronov|first2=A G|last3=Khmelnitsky|first3=D E|date=1982-12-30|title=Effects of electron-electron collisions with small energy transfers on quantum localisation|url=http://stacks.iop.org/0022-3719/15/i=36/a=018?key=crossref.0c3db443f7ce988f7111640a4057fb77|journal=Journal of Physics C: Solid State Physics|volume=15|issue=36|pages=7367–7386|doi=10.1088/0022-3719/15/36/018|bibcode=1982JPhC...15.7367A|issn=0022-3719|url-access=subscription}}</ref> This kind of saturation of the dephasing time at low temperatures is an [[open problem]] even as several proposals have been put forward.

The coherence of a sample is explained by the off-diagonal elements of a [[density state|density matrix]]. An external [[electric field|electric]] or [[magnetic field]] can create coherences between two [[quantum state]]s in a sample if the [[frequency]] corresponds to the energy gap between the two states. The coherence terms decay with the dephasing time or [[spin–spin relaxation]], ''T''<sub>2</sub>.

After coherence is created in a sample by light, the sample emits a [[Polarization (waves)|polarization wave]], the frequency of which is equal to and the [[phase (waves)|phase]] of which is inverted from the incident light. In addition, the sample is excited by the incident light and a population of molecules in the [[excited state]] is generated. The light passing through the sample is absorbed because of these two processes, and it is expressed by an [[absorption spectrum]]. The coherence decays with the [[time constant]], ''T''<sub>2</sub>, and the intensity of the polarization wave is reduced. The population of the excited state also decays with the time constant of the [[Spin–lattice relaxation|longitudinal relaxation]], ''T''<sub>1</sub>. The time constant ''T''<sub>2</sub> is usually much smaller than ''T''<sub>1</sub>, and the bandwidth of the absorption spectrum is related to these time constants by the [[Fourier transform]], so the time constant ''T''<sub>2</sub> is a main contributor to the bandwidth. The time constant ''T''<sub>2</sub> has been measured with ultrafast [[time-resolved spectroscopy]] directly, such as in [[Spin echo|photon echo]] experiments.

What is the dephasing rate of a particle that has an energy ''E'' if it is subject to a fluctuating environment that has a temperature ''T''? In particular what is the dephasing rate close to equilibrium (''E~T''), and what happens in the zero temperature limit? This question has fascinated the mesoscopic community during the last two decades (see references below).

==See also==
*[[Dephasing rate SP formula]]

==References==
<references />

=== Other ===

*{{cite book |last=Imry |first=Y. |year=1997 |title=Introduction to Mesoscopic Physics |publisher=[[Oxford University Press]]}} (And references therein.)
*{{cite journal |last1=Aleiner |first1=I. L. |last2=Altshuler |first2=B. L. |last3=Gershenson |first3=M. E. |year=1999 |title=Comment on "Quantum Decoherence in Disordered Mesoscopic Systems" |journal=[[Physical Review Letters]] |volume=82 |issue=15 |pages=3190 |arxiv=cond-mat/9808078 |bibcode=1999PhRvL..82.3190A |doi=10.1103/PhysRevLett.82.3190|s2cid=119348960 }}
*{{cite journal |last1=Cohen |first1=D. |last2=Imry |first2=Y. |year=1999 |title=Dephasing at low temperatures |journal=[[Physical Review B]] |volume=59 |issue=17 |pages=11143–11146 |bibcode=1999PhRvB..5911143C |doi=10.1103/PhysRevB.59.11143|arxiv=cond-mat/9807038 |s2cid=51856292 }}
*{{cite journal |last1=Golubev |first1=D. S. |last2=Schön |first2=G. |last3=Zaikin |first3=A. D. |year=2003 |title=Low-temperature dephasing and Renormalization in model systems |journal=[[Journal of the Physical Society of Japan]] |volume=72 |issue=Suppl. A |pages=30–35 |arxiv=cond-mat/0208548 |bibcode=2003JPSJ...72S..30S |doi=10.1143/JPSJS.72SA.30|s2cid=119036267 }}
*{{cite journal |last1=Saminadayar |first1=L. |last2=Mohanty |first2=P. |last3=Webb |first3=R. A. |last4=Degiovanni |first4=P. |last5=Bäuerle |first5=C. |year=2007 |title=Electron coherence at low temperatures: The role of magnetic impurities |journal=[[Physica E]] |volume=40 |issue=1 |pages=12–24 |arxiv=0709.4663 |bibcode=2007PhyE...40...12S |doi=10.1016/j.physe.2007.05.026|s2cid=13883162 }}
*{{Cite book |last=Mohanty |first=P. |year=2001 |chapter=Of decoherent electrons and disordered conductors|editor1-last=Skjeltorp |editor1-first=A. T. |editor2-last=Vicsek |editor2-first=T. |title=Complexity from Microscopic to Macroscopic Scales: Coherence and Large deviations |publisher=[[Kluwer]] |arxiv=cond-mat/0205274 |bibcode=2002cond.mat..5274M }}
*{{cite journal |last1=Frasca |first1=M. |year=2003 |title=Saturation of dephasing time in mesoscopic devices produced by a ferromagnetic state |journal=[[Physical Review B]] |volume=68 |issue=19 |pages=193413 |arxiv=cond-mat/0308377 |bibcode=2003PhRvB..68s3413F |doi=10.1103/PhysRevB.68.193413|s2cid=119498061 }}

[[Category:Wave mechanics]]
[[Category:Quantum optics]]
[[Category:Quantum information science]]
[[Category:Mesoscopic physics]]