Pointer state

Eigenstates 3


In quantum Darwinism and similar theories, pointer states are quantum states that are less perturbed by decoherence than other states, and are the quantum equivalents of the classical states of the system after decoherence has occurred through interaction with the environment.

— Wikipedia on Pointer state


In calculation, if a quantum state is in a superposition, that superposition is a superposition of eigenstates.

However, real superposition does not just includes states that make macroscopic senses.


That is the major mistake of the many-worlds interpretation of quantum mechanics.

— Me@2017-12-30 10:24 AM

— Me@2018-07-03 07:24 PM



2018.07.03 Tuesday (c) All rights reserved by ACHK

Quantum Computing, 2

stcredzero 3 months ago

A note for the savvy: A quantum computer is not a magic bit-string that mysteriously flips to the correct answer. A n-qubit quantum computer is not like 2^n phantom computers running at the same time in some quantum superposition phantom-zone. That’s the popular misconception, but it’s effectively ignorant techno-woo.

Here’s what really happens. If you have a string of n-qubits, when you measure them, they might end up randomly in [one] of the 2^n possible configurations. However, if you apply some operations to your string of n-qubits using quantum gates, you can usefully bias their wave equations, such that the probabilities of certain configurations are much more likely to appear. (You can’t have too many of these operations, however, as that runs the risk of decoherence.) Hopefully, you can do this in such a way, that the biased configurations are the answer to a problem you want to solve.

So then, if you have a quantum computer in such a setup, you can run it a bunch of times, and if everything goes well after enough iterations, you will be able to notice a bias towards certain configurations of the string of bits. If you can do this often enough to get statistical significance, then you can be pretty confident you’ve found your answers.

— An Argument Against Quantum Computers

— Hacker News



2018.05.17 Thursday ACHK

Quantum observer 1.2

Single-world interpretation, 7.4

What if I have a microscopic measuring device, B, as a “quantum observer”?

If a particle A is in a superposition of eigenstates, another particle B, as a micro-observer, can also be in a superposition of eigenstates, before or after the observation.

An observation on A by B is an interaction between A and B.

If after the observation/interaction, B is in a superposition, what would B see? Would it see A as in a superposition? Or would it see A as in one of the eigenstates?

It depends on whether you regard individual eigenstates of the resulting B as individual particles “B1, B2, …” in multiple “worlds”, or you regard the superposition of all eigenstates of the resulting B as one single particle in this single universe. In other words, it depends on how you use the label “B”.

The identification of particle B as the superposition of all its eigenstates is more reasonable, because that is compatible with the meaning of the word “observer” in ordinary quantum mechanics. In ordinary quantum mechanics, an observer is a measuring device. A measuring device is a macroscopic object, following classical physical laws. If we have to express the classical laws in terms of quantum mechanics, we say that each classical state of that macroscopic object is a superposition of a lot of quantum states of a lot of the constituent particles.

Classical objects follow the Principle of Least Action, which is due to the superposition of a lot of microstates of the particles. If there is no quantum superposition, there is no Principle of Least Action. Classical mechanics does not work.

— Me@2013.01.14

2013.01.17 Thursday (c) All rights reserved by ACHK

Single-world interpretation, 7.2

Quantum Mechanics 3

Under the many-worlds interpretation, the Schrodinger equation, or relativistic analog, holds all the time everywhere. An observation or measurement of an object by an observer is modeled by applying the wave equation to the entire system comprising the observer and the object. One consequence is that every observation can be thought of as causing the combined observer-object’s wavefunction to change into a quantum superposition of two or more non-interacting branches, or split into many “worlds”. Since many observation-like events have happened, and are constantly happening, there are an enormous and growing number of simultaneously existing states.

If a system is composed of two or more subsystems, the system’s state will be a superposition of products of the subsystems’ states. Once the subsystems interact, their states are no longer independent. Each product of subsystem states in the overall superposition evolves over time independently of other products. The subsystems states have become correlated or entangled and it is no longer possible to consider them independent of one another. In Everett’s terminology each subsystem state was now correlated with its relative state, since each subsystem must now be considered relative to the other subsystems with which it has interacted.

— Wikipedia on Many-worlds interpretation

This is insightful, but incorrect. Please refer to my previous post “Single-world interpretation, 7” for details.

The main theme is that the macroscopic reality can never be an eigen-quantum-state. Instead, the macroscopic reality is the resultant effect of the superposition of eigen-quantum-states. For example, without quantum superposition, there would be no Principle of Least Action in classical mechanics.

— Me@2012-12-28 12:52:12 PM

In particular, Sidney explains that our world is a quantum world and any phenomena that look classical are approximate or derived. So it’s really nonsensical to ask for an “interpretation of quantum mechanics”. Instead, one should really discuss the “interpretation of classical physics” and its derivative appearance from the quantum framework.

Of course, Sidney was well aware of the fact – and made this fact explicit – that the people who have problems with these concepts have those problems simply because they believe that underneath quantum mechanics, there is still some classical physics operating.

— Sidney Coleman: Quantum mechanics in your face

— Lubos Motl

2012.12.28 Friday (c) All rights reserved by ACHK

Single-world interpretation, 6.2.2

Information lost, 5

In the Many-worlds interpretation (MWI), when we say that “a + b” collapses to “a”, there is a shift of definition of “you”.

MWI is in one sense correct: choice b version of you still exists. But the trick is that he is not in another universe. He is in the environment of this universe.

And perhaps in reverse, you are also part of the environment of him.

— Me@2011.11.20

This environment theory is not totally accurate. For example, in the photon double slit experiment, during the wave function collapse, 

sqrt(2) | left > + sqrt(2) | right >

–> | left >    ,

| right > as the unchosen choice, or the lost information, goes to the environment.      

However, the macroscopic reality of | photon goes left > requires not only the state of the photon but also the state of its environment, including the lost information | right >_micro. Just the lost information itself is not enough to form a macroscopic reality.

— Me@2012.04.03

2012.11.16 Friday (c) All rights reserved by ACHK

機會率哲學 1.6

這段改編自 2010 年 4 月 3 日的對話。

In fact, the spectrum of interpretations in quantum mechanics has a close analogue in probability theory. The “wave function is real” view is analogous to the “frequentist” view of probability theory where probabilities describe “random pheonomena” like rolling dice or radioactive decays and the “wave function represents what you know about the system” view is analogous to the Bayesian view where probability is just a consistent way of assigning [likelihoods] to propositions independent of whether they have anything to do with a “random process.”

— Bayesian Probability Theory and Quantum Mechanics

— John Baez






— Me@2012.11.03

致讀者:我於去年(2011)已經搞清了「機會率」的真正意義。如果你想知道,請參閱本網誌 quantum probability (量子機率)和 single-world interpretation(單重宇宙) 類的文章。你將會得到部分答案。


— Me@2012.11.03

2012.11.03 Saturday (c) All rights reserved by ACHK

Consistent histories, 2

Single-world interpretation, 8

The interpretation based on consistent histories is used in combination with the insights about quantum decoherence. Quantum decoherence implies that irreversible macroscopic phenomena (hence, all classical measurements) render histories automatically consistent, which allows one to recover classical reasoning and “common sense” when applied to the outcomes of these measurements.

— Wikipedia on Consistent histories

2012.04.14 Saturday ACHK

Single-world interpretation, 7

One consequence is that every observation can be thought of as causing the combined observer-object’s wavefunction to change into a quantum superposition of two or more non-interacting branches, or split into many “worlds”.

— Wikipedia on Many-worlds interpretation

That is incorrect.

Let’s consider the double-slit experiment. For simplicity, we regard the event “a person reads the device reading” as a classical event.

Before installing the measuring device, we do not know which slit a photon goes through. The photon state is in a superposition of eigenstates: 

| photon > = a | left > + b | right >

(According to the meaning of probability, |a|^2 + |b|^2 = 1.) In other words, if we send enough such kind of photons through the double-slit apparatus, we get the interference pattern. 

After installing the measuring device, we know which slit a photon goes through. According to the Copenhagen interpretation, when the photon passes through the double-slit apparatus, the photon-state “collapses” to one of the two eigenstates, such as | left >. However, a more accurate point of view is that, according to the quantum decoherence interpretation, the photon-and-device state becomes a superposition of a lot of eigenstates. Most of such eigenstates are corresponding to the macrostate of passing-through-the-left-slit, |left>_macro_state. 

The above many-worlds-interpretation statement assumes that there is a |right>_macro_state.

It is true in a sense that, since the photon-and-device involves a lot of particles, there are so many eigen-microstates. Some are certainly corresponding to the |right>_macro_state.

It is false in a sense that the weighting of the |right>_macro_state is so small that such macrostate is not meaningful in a macroscopic context, for example:

| photon-and-device > = 10^23 |left>_macro_state + 0.001 |right>_macro_state + other possible macrostates

— Me@2012-04-07 11:03:12 AM

2012.04.09 Monday (c) All rights reserved by ACHK

Everett’s relative-state formulation

Universal wave function, 13

Everett noticed that the unitary, deterministic dynamics alone decreed that after an observation is made each element of the quantum superposition of the combined subject-object wavefunction contains two “relative states”: a “collapsed” object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object have become correlated by the act of measurement or observation. The subsequent evolution of each pair of relative subject-object states proceeds with complete indifference as to the presence or absence of the other elements, as if wavefunction collapse has occurred, which has the consequence that later observations are always consistent with the earlier observations. Thus the appearance of the object’s wavefunction’s collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein’s early criticism of quantum theory, that the theory should define what is observed, not for the observables to define the theory). Since the wavefunction appears to have collapsed then, Everett reasoned, there was no need to actually assume that it had collapsed. And so, invoking Occam’s razor, he removed the postulate of wavefunction collapse from the theory.

— Wikipedia on Everett’s relative-state formulation

2012.02.26 Sunday ACHK

Single-world interpretation, 6.5

If this is the case, then wave function is deterministic. There is no free will. The free will is due to the ongoing superposition of eigenstates. Locally, we see superposition of “a and b” collapse to (such as) a. Globally, we also see b “goes to” the environment. Nothing is lost in a sense that no information is lost.

Then what happens when you make a choice by collapsing a wave function?

Free will, like wave function collapse, is a local illusion.

Since information cannot be lost, we always exist.

— Me@2011.11.20

2012.02.25 Saturday (c) All rights reserved by ACHK

Single-world interpretation, 6.2

In the Many-worlds interpretation (MWI), when we say that “a + b” collapses to “a”, there is a shift of definition of “you”.

MWI is in one sense correct: choice b version of you still exists. But the trick is that he is not in another universe. He is in the environment of this universe.

And perhaps in reverse, you are also part of the environment of him.

— Me@2011.11.20

2012.02.22 Wednesday (c) All rights reserved by ACHK