Category Archives: quantum decoherence

Universal wave function, 5

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Decoherence does not provide a mechanism for the actual wave function collapse; rather it provides a mechanism for the appearance of wavefunction collapse. The quantum nature of the system is simply “leaked” into the environment so that a total superposition of the wavefunction still exists, but exists — at least for all practical purposes — beyond the realm of measurement.

— Wikipedia on Quantum decoherence

There is no actual wave function collapse.

— Me@2011.12.26

2011.12.26 Monday (c) All rights reserved by ACHK

Universal wave function, 4

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Single-world interpretation, 4

That is, components of the wavefunction are decoupled from a coherent system, and acquiring phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue.

— Wikipedia on Quantum decoherence

The universal wave function is deterministic?

— Me@2011.11.20

2011.12.07 Wednesday ACHK

Hidden variable

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Universal wave function, 2

Assuming Bell’s theorem is correct, any hidden variable theory must be non-local.

Here is my guess:

The hidden variable is the wave function of the measuring device, aka the environment, which involves the rest of the universe.

In short, the hidden variable is the wave function of the whole universe.

— Me@2011.11.28

2011.11.28 Monday (c) All rights reserved by ACHK

Quantum decoherence 5.4

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Wave function collapse is a process of losing the superposition information to the environment.

The remaining problem is that we still cannot predict which part of the information will be lost.

— Me@2011.11.20

In quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior.

Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath), since every system is loosely coupled with the energetic state of its surroundings.

That is, components of the wavefunction are decoupled from a coherent system, and acquiring phases from their immediate surroundings.

— Wikipedia on Quantum decoherence

2011.11.23 Wednesday (c) All rights reserved by ACHK

Wheeler’s delayed choice experiment

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Quantum decoherence 5.3
 
 
In the delayed choice experiment, the wave function of the system (the photons and the environment) is also in a superposition of eigenstates, not just the wave functions of the individual photons are.

All the past is there, but our present measurement “chooses” which part to see. 

— Me@2011.10.21
 
 
The chosen part must be a consistent story, according the quantum mechanics.

The chosen part is what we called “an observer”.

— Me@2018-01-22 09:35:02 PM
 
 
 
2011.11.20 Sunday (c) All rights reserved by ACHK

Delayed choice quantum eraser

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Quantum decoherence 5.2 | Event Realism 5 | 事件實在論 5

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For a delayed choice quantum eraser, both interference patterns are there.

But since they overlap each other, you cannot see them individually.

— Me@2011.10.21

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One of the easiest ways of “making sense” of the delayed-choice paradox is to examine it using Bohmian mechanics. The surprising implications of the original delayed-choice experiment led Wheeler to the conclusion that “no phenomenon is a phenomenon until it is an observed phenomenon”, which is a very radical position. Wheeler famously said that the “past has no existence except as recorded in the present“, and that the Universe does not “exist, out there independent of all acts of observation”.

— Wikipedia on Wheeler’s delayed choice experiment

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What we do in the present does not change the past, but change we can see/say about the past.

— Wheeler on Delayed choice quantum eraser

— paraphrased

— Me@2018-02-04 03:40:27 PM

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2011.11.19 Saturday (c) All rights reserved by ACHK

EPR paradox

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Einstein–Podolsky–Rosen paradox | Quantum decoherence 5

How does quantum decoherence achieve EPR?

1. Everything is entangled, not just for those pairs of photons.

2. The measuring device is part of the system, the universe.

— Me@2011.10.21

2011.11.19 Saturday (c) All rights reserved by ACHK

Wave function 2

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Wavefunction collapse can be viewed as an epiphenomenon of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of its environment.

— Wikipedia on EPR paradox

Quantum decoherence: only the wave function that describes the whole universe is real.

A wave function that describes part of the universe is only partially real.

— Me@2011.10.21

2011.11.18 Friday (c) All rights reserved by ACHK

Single-world interpretation, 2

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The correct meaning of Hugh Everett’s thesis should not be “many-(different)-worlds interpretation”. Instead, it should refer to the fact that the whole universe is described by a single wavefunction, which is a superposition a lot of eigenstates. Also the wavefunction of the universe can never collapse since there is no “environment” for it to have decoherence with. 

Macroscopically, a lot of, if not all, the eigenstates are corresponding to one single classical reality, e.g.

1 + 1 + 2 = 4

1 + 3 + 0 = 4

So there are no multiple (different) universes. Even if the universe is a superposition of several distinct macrostates, the wavefunction cannot collapse to a single macrostate, because there is no “environment” for it to lose information to.

— Me@2011.10.06

— Me@2011.10.18

2011.10.18 Tuesday (c) All rights reserved by ACHK

Single-world interpretation

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Everett’s thesis introduction reads:

    Since the universal validity of the state function description is asserted, one can regard the state functions themselves as the fundamental entities, and one can even consider the state function of the entire universe. In this sense this theory can be called the theory of the “universal wave function,” since all of physics is presumed to follow from this function alone.

The universal wave function is the wavefunction or quantum state of the totality of existence, regarded as the “basic physical entity” or “the fundamental entity, obeying at all times a deterministic wave equation”.

Criticism

Ray Streater writes:

    The idea of the wave-function of the universe is meaningless; we do not even know what variables it is supposed to be a function of. […] We find the laws of Nature by reproducible experiments. The theory needs a cut, between the observer and the system, and the details of the apparatus should not appear in the theory of the system.

Hugh Everett’s response

    If we try to limit the applicability so as to exclude the measuring apparatus, or in general systems of macroscopic size, we are faced with the difficulty of sharply defining the region of validity. For what n might a group of n particles be construed as forming a measuring device so that the quantum description fails? And to draw the line at human or animal observers, i.e., to assume that all mechanical aparata obey the usual laws, but that they are not valid for living observers, does violence to the so-called principle of psycho-physical parallelism.

— Wikipedia on Universal wavefunction

2011.10.16 Sunday ACHK

Complementarity

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Quantum coherence, 1

Berthold-Georg Englert, Marlan O. Scully & Herbert Walther, Quantum Optical Tests of Complementarity, Nature, Vol 351, pp 111–116 (9 May 1991) and (same authors) The Duality in Matter and Light Scientific American, pg 56–61, (December 1994). Demonstrates that complementarity is enforced, and quantum interference effects destroyed, by decoherence (irreversible object-apparatus correlations), and not, as was previously popularly believed, by Heisenberg’s uncertainty principle itself.

— Wikipedia on Complementarity (physics)

2011.09.28 Wednesday ACHK

T-symmetry 8

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The quantum arrow of time

According to the Copenhagen interpretation of quantum mechanics, quantum evolution is governed by the Schrodinger equation, which is time-symmetric, and by wave function collapse, which is time irreversible. As the mechanism of wave function collapse is philosophically obscure, it is not completely clear how this arrow links to the others. Despite the post-measurement state being entirely stochastic in formulations of quantum mechanics, a link to the thermodynamic arrow has been proposed, noting that the second law of thermodynamics amounts to an observation that nature shows a bias for collapsing wave functions into higher entropy states versus lower ones, and the claim that this is merely due to more possible states being high entropy runs afoul of Loschmidt’s paradox. According to the modern physical view of wave function collapse, the theory of quantum decoherence, the quantum arrow of time is a consequence of the thermodynamic arrow of time.

— Wikipedia on Arrow of time

2011.08.07 Sunday ACHK

The Sixth Sense

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Cole: I see dead people.
Malcolm: In your dreams? (Cole shakes his head no)
Malcolm: While you’re awake? (Cole nods)
Malcolm: Dead people like, in graves? In coffins?
Cole: Walking around like regular people. They don’t see each other. They only see what they want to see. They don’t know they’re dead.
Malcolm: How often do you see them?
Cole: ALL THE TIME.

— The Sixth Sense

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Cole: I see dumb people.
Malcolm: In your dreams? (Cole shakes his head no)
Malcolm: While you’re awake? (Cole nods)
Malcolm: Dumb people like, in graves? In coffins?
Cole: Walking around like regular people. They don’t see each other. They only see what they want to see. They don’t know they’re dumb.
Malcolm: How often do you see them?
Cole: ALL THE TIME.

— Me@2010.09.15

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2010.09.16 Thursday ACHK