Questions tagged [quantum-measurements]

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Are two states with the same measurement probabilities necessarily equal up to unitary equivalence?

Let $\rho$ and $\rho'$ be $n\times n$ density matrices, and suppose that for every observable $A$ and every $\lambda$ in the spectrum of $A$ we have $$ \text{tr}(\rho P_{\lambda})=\text{tr}(\rho' P_{\...
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Does this derivation of the reduced density matrix really prove it containing "all measurement statistics"?

This question is a follow up to this question on the derivation of the reduced density matrix. What this question is about: According to Schlosshauer (ISBN: 978-3540357735), the reduced density ...
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6 answers
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Schrödinger's cat experience: how many cats are alive?

In the Schrödinger cat experiment, I know that the cat is in the superposition of two states, let us say: $$\alpha(t)\, |\mbox{alive}\rangle + \beta(t)\, |\mbox{dead}\rangle$$ After a measurement, ...
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7 votes
2 answers
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Do quantum measurements violate conservation laws?

When we measure the spin angular momentum of a particle in an axis different to its current spin, we change the direction of its spin, which taken by itself would be a violation of the law of ...
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Can a quantum measurement breach the third law of thermodynamics?

The third law of thermodynamics states, in one rather intuitive formulation, that: It is impossible to refrigerate a system to a temperature of exactly $0$ $K$ using a finite process (finite energy ...
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-2 votes
1 answer
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Detecting when a particle has been measured [closed]

Is it possible to entangle two particles so that when one of them is measured, somehow, that can be detected by the other particle? No matter whether or not it is necessary to measure the second ...
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How was kinetic energy for a particle measured?

To my understanding, the original objective of the Schrödinger equation was to find the total amount of energy within the system. So, for the time independent particle in a box, which has 0 potential ...
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2 votes
0 answers
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Could one, in principle, make any predictions using the wavefunction of the universe? [closed]

Do physicists talk about the wavefunction of the universe? What does that wavefunction even mean? Usually, wavefunctions describe probabilities of measurements of a system. But in this case, every ...
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In quantum experiments are we measuring the quantum state of a system or are we measuring the effect of the measurement on the quantum state?

At the macro classical world, measurements in experiments when done correctly interfere very little with the state of the system under investigation. As an example a digital ohmmeter with very high ...
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Understanding spin 1/2 in a projection valued measure perspective

Situation: I am looking at a 2-level or spin 1/2 system from the perspective of a projection valued measure. I thus get a set $\Omega = \{ \uparrow, \downarrow \}$, a pretty simple $\sigma$ algebra ${\...
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1 vote
1 answer
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Can one measure two components of spin exactly by measuring two components of entangled, say, electrons?

Of a single electron, two different components of spin can't be have simultaneously well defined values. But what if we entangle two of them and we measure, say, $(S_1)_z$ and $(S_2)_x$ simultaneously....
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Continuous Collapse Interpretation

Suppose we interpret quantum mechanics to mean that the collapse randomly happens at every single moment. Wouldn't this be identical to the many worlds interpretation except much less objectional ...
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Under what conditions is a POVM a von Neumann measurement?

I want to know a definition of a von Neumann measurement. Because I can't find this concept referenced correctly in internet, and what differentiates it from a POVM, that by definition is the ...
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Measure probability of collapse instead of collapse itself

I was researching quantum mechanics recently. A particle can be in two states at the same time, with a probability of "collapsing" to one or the other. The example I learned was an electron'...
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Von Neumann measurement - the only reasonable way to describe a measurement?

Introductory thoughts: In many books on quantum mechanics, the Von Neumann measurement process (where the measurement device is modeled quantum mechanically) is described. For example by Schlosshauer (...
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Unmeasurable observables in quantum mechanics

Let's consider a single particle in 1D harmonic oscillator for definiteness. In standard QM, we say that any Hermitian operator on the Hilbert space is an "observable". It seems that (in ...
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What is the theoretical limit on accuracy of an idealized photon detector?

I know that Heisenberg's Uncertainty Principle and the Observer Effect mean there is a fundamental limit to the accuracy with which the values for certain pairs of physical quantities of particles (...
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2 answers
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What are measurable physical quantities?

The book "Fundamentals of many body physics" by Wolfgang Nolting at the beginning of chapter 3 says: Measurable physical quantities are: the eigenvalues of observables the expectation ...
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Making aweak measurement

Weak measurements are used to probe a quantum system that is still in a superposition of states. My question is how do physicists proceed to (cautiously) make such a measurement, eg in the double-slit ...
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If you measure one "share" of an entangled pair, will the resulting pair be a product state?

If you do a partial measurement on one "share" of en entangled pair, will the resulting pair no longer be entangled, i.e will be a product state?
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2 votes
1 answer
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How does particle indistinguishability reconcile with the quantum Zeno effect?

In non-relativistic quantum mechanics, two particles are said to be indistinguishable if the wavefunction for the system consisting of just those two particles satisfies $|\psi(x_{1}, x_{2}, t)|^{2} =...
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Constructing a measurement operator using only unitary evolution

Related to this question I asked, I want to construct a simple unitary operator that performs a measurement operation between two states. I asked about creating an operation that entangles two states ...
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3 votes
0 answers
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Observer Effect for Infinitesimal Measurements [duplicate]

In quantum mechanics, the spin of a particle corresponds to a magnetic moment - and so a particle in a magnetic field gradient, e.g. a Stern-Gerlach experiment will have its spin measured by the ...
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1 answer
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Can quantum Zeno dynamics be written as a Lindblad master equation?

The interaction of the system with the environment is considered weak and Markovian in the Lindblad master equation. Whereas the measurements involved in the Zeno effect seem quite drastic, I think. ...
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0 votes
1 answer
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What does energy, momentum, etc mean in quantum mechanics?

I see all those nice operators that are used to give the expectation values of dynamical varaibeles, but what does it actually mean to measure kinetic energy, momentum, etc of a particle? How is it ...
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1 answer
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How can one measure the exact momentum of a particle in a quantum mechanical context?

According to quantum mechanics you can't measure both the position and momentum of a particle at the same time. A particle has an associated wave function. If you want to measure the momentum of a ...
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0 answers
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Projection postulate and the state of a system

Quantum Mechanics, McIntyre states the projection postulate as: After a measurement of $A$ that yields the result $a_n$, the quantum system is in a new state that is the normalized projection of the ...
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Calculating probability of quantum measurement [closed]

Consider the measurements of an observable $A$, with orthonormal eigenstates $$f_1=\frac{1}{\sqrt{6}}\begin{bmatrix} {-i} \\ {2} \\ {1} \end{bmatrix},\enspace ...
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1 vote
1 answer
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Classically measuring position without disturbing the momentum

Let's assume that the world is classical. We have an electron somewhere in a room. I am trying to understand how one can experimentally measure the position or the momentum of this electron without ...
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5 votes
2 answers
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Measurement on mixed states

I have a conflict between my lecture notes on quantum mechanics, where it is stated that the probability of measuring an eigenvalue $a_i$ on a mixed state with desnsity matrix $\rho$ is $$ \...
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13 votes
3 answers
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Why are wavefunction collapses instantaneous?

From my understanding of quantum mechanics, when a wavefunction is observed, it collapses into a single state instantaneously (or at least in the length of a Planck time.) Is there a reason it has to ...
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QM:: Two independent measurements of orbital angular momentum

Of course, quantum mechanics has all kinds of things to say about what an observer would measure if measuring the position, momentum, etc., of some particle, or of some set of particles. But I have ...
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How the problem of the preferred basis aries?

I'm reading a Paper by Maximilian Schlosshauer on Decoherence, the measurement problem, and interpretations of quantum mechanics. Here, One considers a system and apparatus. The whole system and ...
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3 answers
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Problems with Schrödinger's cat

I wanted to check my understanding of why exactly the whole "cat in a superposition of alive-and-dead" would never actually happen in the situation described in the Schrödinger's cat thought ...
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0 answers
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Interpretations for Interaction-free measurements

So I read several papers on IFM by Vaidman, Dicke, and many others, In all of them I think the Pilot wave theory is able to adequately justify the observations, but then I came across several papers ...
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2 votes
0 answers
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Are quantum experiments superdeterministic? [closed]

Lately this recent video of Dr. Sabine Hossenfelder made me think if this theory deserves a serious review and has substantial ground. What are the rigorous arguments against this theory today? Please ...
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Heterodyne measurement on Gaussian state

How can one justify/prove that the Heterodyne measurement on an N-mode Gaussian state with expectation value $\mu$ and covariance matrix $\textbf{V}$ leads to Gaussian probability distribution given ...
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8 votes
2 answers
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How is quantum "measurement" actually done?

There is a lot of talk in the fundamentals of QM about "measurement" and "observation", but never much specificity in what is going on at a basic level. For example, an electron ...
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3 votes
1 answer
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Trying to understand post-measurement density matrices in a state that spans 2 Hilbert spaces

What I would like to understand mathematically is the following situation: Prepare a quantum state that spans two Hilbert spaces Operate on one space with observable operator $\hat{O}$. Obtain ...
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Measure Position of Electron Directly

What is the most directly way to measure the position of free electron in an expirement? I don't asking about theoretical suggestion but rather on practical expirement which people have done.
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1 vote
0 answers
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Proof that joint-measurability means commutativity

For $i=1,2$, two measurements $m_{i}:\mathcal{X}_{i}\to\mathcal{L}(\mathcal{H})$, from alphabet $\mathcal{X}_{i}$ to set of bounded linear operators on Hilbert space $\mathcal{H}$, are compatible or ...
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3 votes
2 answers
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Why Do We Ever Get Interference in a Quantum Eraser?

There are many questions on this site about the quantum eraser, but I think mine is not quite answered by any of the other answers on the topic. Here's the setup: My understanding of this experiment ...
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0 answers
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LOCC distinguishability of a mixture of Bell states

Consider the Bell states \begin{align} |\psi^0\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle),& \quad |\psi^1\rangle = \frac{1}{\sqrt{2}}(|00\rangle - |11\rangle), \\ |\phi^0\rangle = \frac{...
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5 votes
1 answer
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More general quantum measurements than Nielsen and Chuang's general measurement

Background Nielsen and Chuang give a postulate for "general measurements" on quantum systems (which are provably equivalent to unitary time evolution + projective measurements on ancillas) ...
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Does (forced) measurment of one subsystem decreases entanglement entropy?

Problem Perhaps this is simple question, but I failed to find the mathematical proof. Suppose there is a tripartite pure state $\rho_{ABC}$ (A,B,C are non-overlapping). The von Neumann entanglement ...
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8 votes
2 answers
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Proof that you can't disentangle two parties if you only operate on one

Let $A$ and $B$ be two entangled systems. Can someone prove or sketch a proof of why you cant unentangle $A$ and $B$ by only acting on $A$ or $B$ alone? i.e. by only applying $\mathbb{I}_A\otimes U_B$,...
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-1 votes
1 answer
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What exactly is meant by "observing" a quantum?

When a quantum is observed, it falls into a specific state. Is just looking somewhere also considered "observing" or does this have to be with electronic devices, or does something have to ...
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1 vote
0 answers
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Continuous-variable teleportation circuit using finite squeezing

I am studying the continuous variable teleporting circuit using a vacuum squeezed state showed by Weedbrook, pag. 39-40. Section: Gaussian errors from finite squeezing; in that, they propose an ...
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1 vote
0 answers
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How will the wavefunction of a QM particle collapse after a measurement of position depending on the uncertainty of the measurement?

(As far as im aware) No exact measurement of position is or ever will be possible. Suppose we made a measurement of the position of a particle using an instrument which measures the particle to be ...
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-1 votes
1 answer
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Any unique or unified definition of measurement in physics?

I have not found any unique or unified definition of measurement in physics, and it seems that there isn't one. What is a unique or unified definition of measurement in physics?
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