# The cause of decoherence

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entropy1
Generally, what physical process causes loss of quantum coherence? Is it known?

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EPR
Big, "classical" objects.
This is a laden, blurry question that will not get a very clear and straighforward answer. For all practical purposes you can assume the above. Big things cause quantum systems to lose their quantum coherence via interaction.

entropy1
Big, "classical" objects.
This is a laden, blurry question that will not get a very clear and straighforward answer. For all practical purposes you can assume the above. Big things cause quantum systems to lose their quantum coherence via interaction.
I thought I asked a really smart question

Mentor
Generally, what physical process causes loss of quantum coherence? Is it known?
In many past threads (so many that people might suspect that I’m getting a cut of the sales - I’m not) I’ve recommended David Lindley’s book “Where does the weirdness go”. Read it.

But with that said... the answer to your question is: yes, it is known. Decoherence is caused by interaction with a complex system with an enormous number of degrees of freedom, informally “the environment“

mattt, Demystifier and atyy
Mentor
what physical process causes loss of quantum coherence?

As @Nugatory has said, it's caused by interaction with a system with a very large number of degrees of freedom. I would add that there will also be no way to keep track of the exact state of all of those degrees of freedom.

entropy1
Gold Member
Generally, what physical process causes loss of quantum coherence? Is it known?

To put a finer point on @Nugatory 's response: Consider a system partitioned into a system of interest ##s## and environment ##\epsilon## in an initial state $$|\Psi(t_0)\rangle = |\psi(t_0)\rangle_s\otimes|\phi(t_0)\rangle_\epsilon$$ We can construct a reduced density matrix $$\rho_s(t) = \mathrm{Tr}_\epsilon\left[|\Psi(t)\rangle\langle\Psi(t)|\right]$$ If ##\rho_s(t)## becomes diagonal at some time ##t > t_0##, then decoherence has occurred. Decoherence occurs especially quickly when the energy eigenstates of the environment are very close together, which happens when the environment is large.

As you can see, decoherence is somewhat a feature of our description of the system rather than the system itself, since it depends on our partitioning of the system into ##s## and ##\epsilon##. If we chose not to neglect any degrees of freedom, such that all degrees of freedom are considered a part of ##s##, and ##\rho_s(t) = |\Psi(t)\rangle\langle\Psi(t)|## then ##\rho_s(t)## would not become diagonal and no decoherence would occur.

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mattt, vanhees71 and entropy1
entropy1
If ##\rho_s(t)## becomes diagonal at some time ##t > t_0##, then decoherence has occurred.
This may be helpful as an introduction (in particular diagonalisation):
Decoherence - Sabine Hossenfelder

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Homework Helper
Gold Member
2022 Award
In many past threads (so many that people might suspect that I’m getting a cut of the sales - I’m not) I’ve recommended David Lindley’s book “Where does the weirdness go”. Read it.
I began to suspect you might actually be David Lindley!

jartsa
You don't actually need macroscopic systems to demonstrate some of the features of decoherence, very simple toy models can suffice. Have a look at the examples here https://www.damtp.cam.ac.uk/user/tong/aqm/aqmnine.pdf.

I could use some hint where in the paper those examples are.

Does this count as decoherence: A coherent wave-function of a photon meets a rough reflecting surface, and becomes a less coherent wave-function of a photon?

(I found the examples. Page 43)