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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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when I take two measure in same time, probability is product of probability?

let |φ› be a state, A the associate operator of "a" and B the associate operator of "b". I want to measure value "a" and then measure value "b", What is the probability? How is this related to the ...
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The 'Conservation of Information' myth

PBS Spacetime recently released a video talking about 'why information is never destroyed in Quantum Mechanics' I'm surprised to see this myth to persist even to this day. We know that unitarity and ...
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253 views

Going from stochastic Schrödinger equation to master equation

I am currently reading the book "Quantum measurement and control" by Wiseman and Milburn (https://www.amazon.ca/Quantum-Measurement-Control-Howard-Wiseman/dp/1107424151) and something is really ...
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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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Is this assertion by Landau and Lifshitz's QM equivalent to the axiom that state spaces are vector spaces?

Many expositions of Quantum Mechanics begin by stating axioms of the following type: "The state of a quantum mechanical system is given by a nonzero vector in a complex vector (or Hilbert space) ...
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Doubt about fifth postulate of QM degenerate case

I think I didn't not really understand a comment found on "Quantum Mechanics" by Claude Cohen-Tannoudji. Talking about the fifth postulate of quantum mechanics in the case in which $a_n$ is a ...
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59 views

When does the wave function collapse in the EPR paradox experiment? Shouldn't this depend on the reference frame?

Suppose you have two entangled particles. Particle A, and Particle B. If the spin of particle A is up, then the spin of particle B must be down and vice versa. Now suppose you measure the spin of A ...
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Generalisation of the measurement postulate in quantum mechanics

Given an observable that has a partially discrete and partially continuous spectrum of eigenvalues associated to it with the order of the spectrum's degeneracy being greater than 1, how would you ...
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Particles trajectories in Stern-Gerlach experiment

I red this sentence on "D.J. Griffiths, Introduction to Quantum Mechanics" talking about the using of heavy particles for the experiment. "...heavy so we can construct localized wave packets and ...
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Which experiment clearly shows us that Heisenberg Uncertainty Principle is ontological and not epistemological?

I have heard that the Heisenberg Uncertainty Principle(HUP) is not because of our limitations in measuring and the apparatus used but instead because of the very way things are I searched online in ...
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86 views

Measurement of position in quantum mechanics

I know that when you perform a measurement of position in quantum mechanics, the wave function collapses to something proportional to it, but in a small range of values of positions, depending on the ...
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121 views

What is the relation between a measurement and an observable?

Observables are represented by Hermitian operators. First of all, it's a little strange (to me) that some measurable physical quantity is represented by a transformation (or linear map), given that I ...
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Uncertains of diffraction mesaurement with a photodiode

I have to study the diffraction curve of a laser passed through a slit. To measure the intensity i am using a photodiode and a voltmeter; I move the photodiode and measure the intensity every $200\mu$...
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Does the quantum eraser double slit experiment disprove many worlds interpretation?

The quantum eraser double slit experiment shows that interference pattern is destroyed not by interacting with the quantum particle, but rather by getting which path information. In this experiment ...
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138 views

Finding amplitude of probability

The mathematical structure of quantum mechanics, follows almost inevitably from the concept of a probability amplitude. For James Binney and David Skinner: "With every value in the spectrum of a ...
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Superposition of position eigenstates after measurement of position?

In a discussion with someone recently, the person made the following statement: Even when we observe a particle, it is still in superposition of position states within the resolution of the ...
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247 views

All of the postulates of quantum mechanics without additional information [closed]

$\newcommand{\braket}[2]{\left<#1|#2\right>}% \newcommand{\bra}[1]{\left<#1\right|}% \newcommand{\ket}[1]{\left|#1\right>}% $As an undergraduate who's trying to learn quantum mechanics it'...
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Why quantum mechanic didn't work well when length approach Planck length?

Information paradox with regard to quantum entanglement when width approach Planck length Context: Suppose I have a 3 d particle placed in a spherical box of infinite well from [-r,r] in all three ...
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Is the Heisenberg uncertainty principle based on experimental imprecision in measurement? [duplicate]

As far as I understand, the Heisenberg uncertainty principle states that the more one of the conjugate quantities are known, the less the other is known. So for instance, if the velocity of a particle ...
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Measurement and probability

In the $\left\{\left|j_1j_2;jm\right\rangle=\left|ls;jm\right\rangle\right\}$ basis, I've got the state ket $$\left|\alpha\right\rangle=\frac{1}{2\sqrt{2}}\left(\sqrt{\frac{2}{3}}\left|11;20\right\...
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Quantum foundations and QM interpretation - reading material

I recently wrote an answer on a question and I would infer from the responses that there is not widespread awareness of the context of the development of QM and its various interpretations. Instead, ...
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Indeterminacy in Quantum Mechanics

Heisenberg's uncertainty principle tells us that there is a lower limit on the accuracy with which we can determine both the momentum and the position of a particle simultaneously. $\Delta$$x$ $\...
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Can Quantum Zeno effect stabilize unstable particles? [duplicate]

Quantum Zeno effect says that a continuous measurement makes the state to be "unmoved". Could we use (at least in principle) this effect to make stable unstable particles?
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What is the physical meaning of exciting a single-electron transition in a many-electron system?

Let's say I have a gas of Li atoms in the $1s^2 2s^1$ ground state configuration and I want to excite the $2s \to 3s$ transition of the outermost electron with a laser pulse on resonance. (That is the ...
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Probability of obtaining general quantum measurement outcome

The Fundamental Theorem of Quantum Measurement is stated as follows: Every set of operators $\{ A_n \}$ $n =1,...,N$ that satisfies $\sum_n A_n^{\dagger}A_n = I$ describes a possible measurement on a ...
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Measurements in the phase space picture of quantum mechanics

Suppose we are dealing with non relativistic quantum mechanics of point particles, that is we are in the realm of 'classic' quantum mechanics (no quantum fields ect.). In the Heisenberg picture, ...
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Realism and the Copenhagen interpretation [closed]

In the Copenhagen interpretation, if some device measures the state of a quantum system, finding some property of it to be equal to some value, X, then it is then assumed that all other quantum ...
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745 views

Probability of measuring eigenvalue of non-normalised eigenstate

This came up while working on a question about measuring the angular momentum of a particle in a superposition of angular momentum eigenstates: Given that: $$\langle\theta,\phi|\psi\rangle \propto \...
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1answer
715 views

Probability of finding a particle with a particular value of momentum

Given a particle's wave function, what is the general method of finding the probability distribution of momentum (i.e., the probability of finding that particle with a particular value of momentum)? ...
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1answer
164 views

Defining Kraus operators with normal distribution

I am interested in defining Kraus operators which allow you to define quantum measurements peaked at some basis state. To this end I am considering the Normal Distribution. Consider a finite set of ...
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1answer
48 views

Discarding a quantum system in joint state

If you have a joint quantum state given by the density operator $$\rho^{(XYZ)} = \sum_{k}p_k\rho_{k}^{(XY)} \otimes |k^{(Z)} \rangle \langle k^{(Z)}|$$ then am I correct in stating that if we want to ...
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What is interaction and measurement in QM? [duplicate]

My understanding of "measurement in quantum mechanics" was that any interaction causes the probability "fuzziness" to get "resolved" to a particular value for an observable. However, unlike that, a ...
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Does a photon's spin wavefunction collapse when detected by a photon detector?

Assume two entangled photons A and B have the same spin but travel in different directions. When Photon A is detected by photon detector (e.g. photomultiplier) on its path, does Photon B's spin wave ...
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Is it possible to directly measure the expectation value of a physical observable without measuring the entire probability distribution?

The expectation value of a physical observable in a given state is just a single real scalar quantity. On the other hand, the probability distribution of the eigenvalues of the observable is a set of $...
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Double Slit Experiment. What effect does the detector actually cause?

When a quantum of light arrives at a double slit, it passes through both slits as a wave and arrives upon a second screen with the interference pattern of a single wave that has been split into two ...
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1answer
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Wave function collapse, EPR paradox and information transfer

For a classical formulation of the EPR paradox, two particles are produced, with total momentum zero and separated by a long distance. So say we measure the momentum of one particle first, and measure ...
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3answers
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Is there an objective asymmetry between a collapsed and un-collapsed wave function?

In a quantum double slit experiment, one starts at t0 with a wave function that propagates through two slits, interferes, and probabilities for various positions at the final stage at t1 can be ...
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What are the strongest objections to be made against decoherence as an explanation of “collapse?”

When we measure an observable A of a quantum system, we get an eigenvalue of A. Without worrying about connotations of Copenhagen vs. MWI, etc., let's just call this "collapse." Question: Among ...
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Measurement of wavefunction collapse

I read in Griffith's quantum mechanics that In a particular system, the second time measurement of the position (say) would yield the same result (the same collapse or the same spike) given that ...
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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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Resource recommendation for interpretation of quantum mechanics?

The jumping of the quantum state to one of the eigenstates of $Q$ is the process referred to as state-vector reduction or collapse of the wave function (where $Q$ is an observable). I used to think ...
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What initiates quantum entanglement?

In a previous question, I asked about the measurement problem in quantum mechanics. The sense I got is that the best interpretation we have is the many worlds interpretation, and one of the answers ...
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What is the quantum mechanical definition of a measurement?

I hear the word "measurement" thrown around a lot in quantum mechanics, and I have yet to hear a scientific definition that makes sense. How do we define it?
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How does one actually measure the position or momentum of a quantum object?

How is the position or momentum of a Quantum particle is measured experimentally in laboratory? Suppose we want to know the position or momentum of quantum particle which is kept in a box i.e. an ...
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Does the first part of the measurement postulate imply the second?

Common formulations of the general measurement postulate of quantum mechanics have two parts, one establishing the probability of the outcome $m$ corresponding to an observable $M_m$ for a system in ...
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How to measure a qubit state?

I'm trying to work out how to do a non-demolition measurement on a qubit using an ancilla photon. I start with an entangled electron and nucleus in the state $$ \frac{1}{\sqrt{2}}(\alpha|\uparrow\...
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Measuring the polarisation of light

I've been thinking about the standard and easy "Three-polarising-filters experiment" as a demonstration of quantum phenomenon: If light is passed through two polarising filter and the filters have ...
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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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Specific Question about Particle Entanglement

I understand that in particle entanglement, if you have two entangled particles, if you measure one particle it immediately gives that particle (particle A) a certain spin, and it's paired particle (...
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Do we experience wave function collapse even though we don't read the detector?

In quantum double slit experiment would the the wave function collapse if we place a particle detector at the slit but do not read it and just keep looking at the screen?