Questions tagged [measurement-problem]

DO NOT USE THIS TAG just because your question involves measurements (either quantum or classical). The measurement problem asks how wave function collapse occurs during measurement in quantum mechanics, and how it can be reconciled with unitary evolution.

72 questions
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Experimental test of the non-statisticality theorem?

Context: The paper On the reality of the quantum state (Nature Physics 8, 475–478 (2012) or arXiv:1111.3328) shows under suitable assumptions that the quantum state cannot be interpreted as a ...
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Simultaneous measurements in Quantum Mechanics

In the notes I am using, it states that if two observables A and B are measured simultaneously, then the measurement of A does not affect the measurement of B, and vice versa. However, why does the ...
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How to think of matrices as observables?

I'm reading Nielsen and Chuang. In one of the early chapters, they introduce some matrices such as $$X = \begin{bmatrix} 0 & 1 \\ 1 & 0 \end{bmatrix}.$$ They interperet this as a gate that ...
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How does one compute the state of a quantum system following imperfect measurement?

Suppose I have a quantum system $S$ ("system") with Hamiltonian $H_S$ and initial density matrix $\rho_S(0)$. I allow $S$ to interact with another system $P$ ("probe"), which has Hamiltonian $H_P$ and ...
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Role of “hierarchic structure” in solving the Measurement Problem

Background Decoherence solves some aspects of the Measurement Problem by ensuring that at the end of a measurement process off diagonal terms in the density matrix are supressed. However decoherence ...
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Do restrictions on quantum mechanical measurement always just work out to avoid contradictions?

Classically it was said that measurement leads to a collapse of the wave function. However, if there wouldn't be any limit on the process on measurement itself, strange things can happen, e.g. a ...
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How can the pre-measurement be fulfilled?

In the decoherence program, the pre-measurement refers to the evolution in which the system and apparatus form a Schmidt state. In Maximilian Schlosshauer's review article(2005), I read "the linearity ...
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Measurement in the context of unitary evolution

My text describes how "projective measurements together with unitary dynamics are sufficient to implement a general measurement" and goes on (pp. 94-95) to demonstrate why this is so, but there are ...
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How are weak measurements taken into account in collapse models?

Collapse models have been introduced to solve the measurement problem in quantum mechanics. In collapse models the localization of wavefunction is not considered to be due to any kind of measurement ...
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QFT Superposition and the Higgs Ground State

In a quantum theory with zero spatial dimensions with Hamiltonian given by: $$H = \pi^\dagger \pi + \mu^2 \phi^\dagger \phi - \lambda (\phi^\dagger \phi)^2,$$ where the non hermitian field, $\phi$, is ...
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Does quantum mechanics require classical measurement apparatus?

I am trying to learn quantum mechanics and I have a question. Landau, in his quantum mechanics book says that it is in principle impossible to formulate basic concepts of quantum mechanics without ...
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How to explain Tsirelson's inequality using extended probabilities?

How to explain Tsirelson's inequality using extended probabilities? Some people have tried explaining the Bell inequalities using extended probabilities. For instance, a pair of entangled photons ...
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What is the simplest model of a quantum measurement on a 2 level system

What is the simplest physical system which can be used to model the quantum measurement of a 2 level system? For example, can the following, spin coupled to a harmonic bath, be used to model a ...
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Information Centric Interpretation of the Delayed Choice Experiment

The photon travels from the emmitter to the detector. Along the way it bounces off a few mirrors and off or through a beam splitter or two. This journey takes time, but only from OUR perspective/frame....
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Quantum Observation

Bear with me if I present a lack of knowledge - QM is not my field. There's a common notion in QM that until a particle is observed (measured), its properties are not definite, but rather are spread ...
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Reconstruct quantum state from probabilities

Given a quantum state, the Born rule lets us compute probabilities. Conversely, given probabilities, can we reconstruct the quantum state? I think the answer is almost trivially positive but how ...
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What does “contextuality” mean in the context of partial rather than complete quantum measurements?

The Kochen-Specker theorem is often described as ruling out "noncontextual" classical hidden variable theories. I understand the math behind the theorem, but I'm a little unclear on the exact, ...
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Are all kind of measurements of properties in the two slit-experiment destroying interference?

Usually when by measurement is found out through which slit the particle went then there is no interference. But suppose you don’t measure through which slit it went but for example his spin, will it ...
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Is an interaction-free measurement an irreversible process?

From what I've understood from reading different online sources including PSE*, measurements in quantum mechanics are generally argued to be irreversible (at least, when a macroscopic measuring device ...
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Weak measurement and weak value

The concept of weak measurements (and weak values) have become popular in Quantum information community, as I can see quite a few papers in arXiv. Since I am from Mathematical background (and the ...
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Uncertainty Principle - measuring momentum on one entangled particle, position on the other

If two entangled particles are sent far apart and then at exactly the same time the position of one, and the momentum of the other, is measured, won't this mean that, because the corresponding values ...
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Are universally valid possibilistic theories possible?

This is a spin-off of the following question: Are Thomas Breuer's subjective decoherence and Scott Aaronson's freebits with Knightian freedom the same things in essence? Given that Thomas ...
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What are you studying when you study a Harmonic Oscillator in QM?

This probably is a naive question - so please forgive a self-studier. In the text I am studying, one builds a HO by placing a particle in a potential that increases quadratically from the origin. The ...
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Quantum Eraser under Lorentz Boost

Suppose I am conducting the Quantum Eraser experiment. The results of this experiment are easy to understand with the traditional quantum mechanical interpretation of a pair of entangled photons. ...
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How does linearity of a measurement imply that the commutator of all measured observables are $c$-numbers?

I really don't understand with the linearity conditions I have where this comes from.
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finetuned quantum experiments by Murch lab, do any have dynamics outside of basic QM formalism/ axioms?

a series of very finetuned quantum experiments have been reported by the Murch lab eg in 2 articles in Nature & analysis there, some leading to dramatic accounts in the media. do ...
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Interesting (new to me) things in the exposition of Landau's book on QM

In section I.1 (The uncertainty principle), a principle I already know, the author suggests a "relaxing" picture (Unusual): "We have defined "apparatus" as a physical object which is governed, with ...