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.

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Is my baby's gender an example of Schrodinger's cat?

At some time in the conception of a child, gender is determined by the X or Y chromosomes given by the male. This is, from my perception, a random selection, like the decaying atom. Like Schrodinger'...
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149 views

Is QED without measurement a local hidden variable theory?

An interesting thought experiment would be to create a large quantum wave-function describing both a pair of electrons with entangled spins and the equipment needed to measure the electrons' spins. In ...
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Average value of an observable

I got confused by the concept of the average value of an observable. I know that when we measure a physical quantity $A$ in a specific state described by $\psi_1$, we only get the eigenvalue $a$ of ...
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228 views

Delayed choice experiment clarification (concrete setup)

I know that clarifications about delayed choice experiment was asked million times, and I understand the idea, but I was not able to find the discussion of this particular situation anywhere, though I ...
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2answers
157 views

Probability of quantum transition

I have a question about a task: We have a particle, which is in a linear combination of the first two states of the harmonic oscillator, which we can parametrise as $|\psi\rangle=\cos(\frac{\...
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1answer
183 views

Uncertainty principle, statistical approach

I have a question about quantum uncertainty, let's say we have a system in the state $\psi=c_0\space|0\rangle \space+\space c_1|1\rangle\space+...$. I understand that if we measure the system, we will ...
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A spontaneous wavefunction collapse thought experiment

Suppose $^\dagger$ we have absolutely no knowledge of a hydrogen atom. It is well isolated. And we put a photon detector next to it (and pretty sure it has no interaction with the hydrogen atom.) ...
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Paradox of the wavefunction collapse into un unphysical state

"A measurement always causes the system to jump into an eigenstate of the dynamical variable that is being measured, the eigenvalue this eigenstate belongs to being equal to the result of the ...
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Momentum measurement of a particle in an infinite well [duplicate]

After measuring a quantum state for a particular measurable, the state collapses to one of the eigenstate of that measurable. If we measure for momentum in an infinite square well, the system will ...
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1answer
276 views

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 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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If “measurement” is just a quantum process, doesn't that imply an emergent multiverse?

Kurt Jacobs (Quantum Measurement Theory and its applications, 2014) writing about the Measurement Problem says (italics mine) ..there is no collapse. Any set of mutually orthogonal subspaces ...
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156 views

Measurement and change of quantum state

Suppose we have a quantum system that is in a certain initial state, together with a measuring device capable of limited precision measurement of some observable. After the measurement is complete, we ...
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248 views

What is the shape of the detector pattern in a Stern-Gerlach experiment with a beam source (instead of a fan)?

I've been delving into Bell test experiments and woefully most sources fall into either dense physics papers, or very low level simplifications with lots of handwavium. One thing in particular I'd ...
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Is the energy really infinitely large in a measurement of the energy immediately after the measurement of the position?

For instance, assume the particle rests on the ground state $\psi_0 (x)$ of a one-dimensional simply harmonic oscillator around the origin of axes $ x=0 $, and once we measure the position of the ...
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2answers
539 views

Commutator and Order of Measurement

I was going through Prof. Leonard Susskind's lectures on Quantum Field Theory (Lec 2). Professor said that the commutator of two observables $AB-BA$, has nothing to do with the 'measurement'- B ...
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Prepared states and quantum entanglement [duplicate]

Imagine two entangled quantum particles in the singlet state, one held by Alice and the other by Bob. Alice and Bob are both widely separated. Bob measures his spin in some axis and finds it to be ...
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Quantum mechanics stationary states

I am taking an intro course to quantum mechanics and I have some trouble understanding what exactly is going on with the particle in stationary states as opposed to their linear combination which is ...
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Does measuring operator $A$ on state $|\psi\rangle$ leave the system in $A |\psi\rangle$?

Given an observable $A$ and a system in the state represented by the vector $|\psi \rangle$, after the measurement is the system in the state $A |\psi \rangle$?
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226 views

Measurement and transition rates

We can use time dependant perturbation theory (specifically Fermi's Golden Rule) to calculate the transition rate (probability of transition per unit time) from one energy eigenstate to another. ...
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458 views

Measurement of position of particle

Say a particle was found to be at $C$. If I measure the position of particle immediately, its given in Griffith's book that it will still be at $C$. In the macroscopic world, isnt this true only if ...
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628 views

Does collapse of wave function to a momentum eigenstate violate speed of light restriction?

Let's consider a free system where the Hamiltonian is $\hat{p}^2/2m$. At time $t=0$, we start with a state at position $x$. An instantaneous time $\delta t$ later, where $\delta t\rightarrow 0$, we ...
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Is classical logic inconsistent with some facts in QM?

Quantum logic is a set of rules for reasoning about propositions that takes the principles of quantum theory into account. This research area and its name originated in a 1936 paper1 by Garrett ...
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Collapse of wave function

Suppose a quantum system is initially at a state $\psi_0$ and that a measurement of an observable $f$ is performed. Immediately after the measurement, the system will be in a state that is an ...
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188 views

When will a quantum measurement occur?

I am confused about the nature of quantum measurements. In short: if a particle is coupled with a field, when will the wave collapse. A field might be coupled to a particle for a long time, 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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Measuring momentum and energy of a free particle

Regarding the free particle in QM, we are given that the general wave function is: $$\Psi(x,t) = \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^{\infty}\phi(k)e^{i(kx-\frac{\hbar k^2}{2m}t)}dk.$$ The ...
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Does the Copenhagen interpretation say that a tree only has definite mass after I look at it?

Assume that the Copenhagen interpretation holds. Imagine a Universe, a nearly identical copy of our Universe, that contains only our Earth and me (i.e., I'm the lone conscious being and the lone self-...
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single Observer iterpretation, experiment design

I am thinking of an interpretation of QM in which there is a single Observer (O). That is, its mind's history is marking a special path on the many-world graph, which we identify with reality. This ...
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3answers
766 views

How can a spin totally aligned along the $x$ axis have uncertain $z$ component?

Let's work with the eigenvectors $$|x+ \rangle = [\sqrt{1/2}, \sqrt{1/2}] \text{ and } |z+\rangle = [1, 0].$$ Suppose I measure $S_x$ and find our state vector is $|x+\rangle$. If spin is angular ...
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3answers
200 views

Has any quantum mechanics experiment ever suggested the past or future could be changed?

This question has already been asked in many guises but unfortunately all the responses run the gamut of possible answers with no one who appears to be in agreement therefore I will ask in a slightly ...
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136 views

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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397 views

How does an electron jump from one lobe to another of the same hourglass-shaped $2p_x$-orbital?

Imagine an electron in the hourglass-shaped $2p_x$-orbital. It has two lobes. The probability of it being in the centre of the orbital is zero. This is the point that connects the two lobes of the ...
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133 views

Quantum states after real world measurements

Regarding measurements of an observable in a quantum system. My understanding, from the postulates of quantum mechanics, is that when we measure an observable quantity, the state of the system ...
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On a measurement level, is quantum mechanics a deterministic theory or a probability theory?

Quantum mechanics is a non-commutative probability theory. As such, it fundamentally behaves differently from classical probability theories. This manifests itself most pronouncedly in the uncertainty ...
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Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But ...
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1answer
121 views

Quantum state after change of magnetic field

I have the following conditions: $\lvert\psi(0)\rangle=\lvert+\rangle_x=\frac{1}{\sqrt{2}}\lvert+\rangle+\frac{1}{\sqrt{2}}\lvert-\rangle$. So the state at $t=T$ is $\lvert\psi(t)\rangle=\frac{1}{\...
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Want to measure entanglement of the state [closed]

Good day, I want to measure the state with concurrence and negativity. I do local unitary transformation with represented by $U\in SU(4)$ (Lie group). After the transformation (rotation of angle) ...
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1answer
124 views

Probabilities with the Density Matrix

The density matrix of the system is given by: $$ [\rho_{S}(t)]_{mn} = [\rho_{S}(0)]_{mn} e^{-i\omega_{0}(m - n)t} e^{-i \delta(t)(m^2 - n^2) - \gamma(t)(m - n)^2}, ...
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An example of a nonlinear but deterministic physical transformation in Hilbert space

Supposedly all physically realisable transformations are either linear or non-deterministic (measurements are not linear transformations, but they are non-deterministic, from the perspective of the ...
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1answer
113 views

State and measurement after a sequence of measurements

Hi I just want to confirm my interpretation of the following question: Let the quantum state be given as $$|\psi_0 \rangle = [\sqrt{2}|\phi_1 \rangle + \sqrt{3}|\phi_2 \rangle + | \phi_3 \rangle + |\...
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1answer
312 views

In quantum weak measurement, what kind of theory replace Copenhagen interpretation?

Here, I denote the initial states of device and quantum system as $|\Phi_\textrm{in}\rangle$ and $|\Psi_\textrm{in}\rangle$. The measurement interval is $[t_i,t_f]$, after measurement, the device and ...
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Quantum entanglement and affecting the particles [duplicate]

I am trying to grasp some aspects of the quantum entanglement, but the existing resources (including some of the links here) seem a bit confusing. I am trying to find an answer to the following ...
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3answers
365 views

What happens when we manipulate the observer in the double slit experiment?

Consider the following alterations to the double slit experiment: turn the observer on and off rapidly move the observer further and further away, until the point at which it can no longer ...
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1answer
117 views

Different postulates and statistical interpreations of quantum mechanics

Hi I have a query about the difference of two aspects of the statistical interpretation of quantum mechanics given in the popular introductory quantum mechanics books "Introduction to Quantum ...
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2answers
630 views

How can an electron's magnetic moment precess around the direction of an external magnetic field?

I am reading this article: The Nature of the Electron by Don Lincoln in The Physics Teacher, Volume 54 (2016), pg. 203, and I ran across the part where he talks about measuring the magnetic moment of ...
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Does spontaneous emission count as a measurement?

It is my understanding that in the density matrix formalism for open quantum systems the environment-induced measurements/collapses/projections are accounted for by the Lindblad superoperator (from ...
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Our choice of basis surely cannot effect possible outcomes of a measurement?

Common sense says that, of course, the outcome of a measurement on a quantum system cannot be affected by what base we choose to represent it in. However, while studying QM text, it seems like they ...
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Do conservation laws contradict quantum mechanics? [closed]

Take for example the double-slit experiment interpreted in the Copenhagen sense. The particle leaves as an object with mass, yet passes through the slits as a massless wave, only to collapse again as ...