Questions tagged [quantum-measurement]

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Confusion about the definition of the uncertainty principle

I have heard two definitions of the uncertainty principle, and I am quite confused about the uncertainty principle now. The first one is dependent on the wave function of the particle, and it says ...
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1answer
28 views

Can you give an experimental example showing the difference between global and relative phase in QM?

Let's say, that we are in possesion of a very simple quantum system, whose state can be written as $$ |\psi\rangle = c_0 |\psi_0 \rangle + c_1 |\psi_1\rangle.$$ Now, we can change this state in two ...
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1answer
30 views

Measuring overlap of two quantum states

I have a qubit state $|\psi\rangle = a|0\rangle + b|1\rangle$, and another qubit state $|\psi'\rangle = a'|0\rangle + b'| 1 \rangle$. I want, through quantum gates, to measure the overlap $|\langle \...
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1answer
31 views

Understanding time evolution in Von Neumann's pre-measurement (Ozawa model)

I'm studying Quantum Information Theory as a non-physicist and so I'm struggling a bit with simple concepts. Let's say we have a system $\Psi$ composed of two sub-systems: $\Psi = \Gamma \cup \Xi$ ...
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0answers
19 views

Deriving the Bloch Equation of qubit purification for any measurement angle

I am trying to derive the Bloch vector $dr$ for a measurement of a observable in any arbitrary direction $\theta$. For context this is the setup and derivation I have for continuous measurement along ...
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3answers
102 views

Does light travel “quantised”? Or , is it perceived as quantised only when it interacts ,i.e, undergoes measurement?

When light interacts (non self interaction) , it is perceived as photon (I am assuming this to be so) . So, is photon then a collapsed light wavefunction?
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1answer
53 views

Quantum measurement:can we recover evolution operator from measurement?

Suppose there is a quantum system in state $|a\rangle$$\in$ H,an instrument designed to measure some physical quantity, which itself is in state $|b\rangle$ $\in$ H'. If we consider the bigger system "...
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1answer
53 views

How is weak measurement operator formed?

I am new to the topic of weak measurement. While reading this paper ( the open Arxiv version is here ), the authors have just introduced the operators: $$\hat{P}(x)=\sqrt{\frac{1-\tanh(x)}{2}}\hat{P1}+...
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1answer
40 views

Interpretation of Uncertainty After Non-ideal Measurements in QM

Answers to questions like this discuss how real measurements retain uncertainty due to limitations of our instruments. Is this uncertainty quantum or classical? If it's classical, i.e., the wave ...
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1answer
35 views

Help interpret bra-ket notation, quantum measurement equation (neielsen chuang, postulate 3)

I'm reading Quantum Computation and Quantum Information by Nielsen and Chuang (http://mmrc.amss.cas.cn/tlb/201702/W020170224608149940643.pdf). Equation 2.92 on page 85 is a bit confusing to me if ...
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1answer
41 views

Confusion about vacuum fluctuation and the Big Bang

I usually hear physicists say that the Big Bang was probably caused by vacuum fluctuations. However, as I understand from answers here (this one for example), what we refer to by vacuum fluctuations ...
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0answers
27 views

double split experiment, detection by (not at) slit

During double split experiment, a particle wave function passes through both the slit and creates interference pattern. But that means central part of the slit is blocking wave function. Does not ...
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1answer
28 views

Does the energy and/or mass of an electron change during the act of measurement and so the wavefunction collapse?

In the double-slit experiment, if we want to know which slit did the electron go through, we have to use a laser that gives an energy to the electron. Now, I have two questions: 1. Does the energy of ...
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0answers
16 views

Observables and different collider machines

Recently I heard that there is no point in going for the observable $\frac{d\sigma}{dq^2_T}$ in a $e^+ e^-$ collider (e.g. in the process $e^+ e^- \rightarrow Z$, measuring the transverse momenta $q_T$...
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1answer
41 views

Is relativity in action while we keep measuring some observable of a, let's say, neutrino?

Imagine a particle moving with non-zero velocity with respect to us. If I keep measuring an observable of the particle, such as position, then does the particle evolve relativistically? For example, ...
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4answers
107 views

If an object is large enough and slow enough is there a point that we can say we know its position and momentum?

Let’s say you have a 3000 lb vehicle moving very slow at 1 kilometer per hour. It seems obvious that you could simultaneously determine its position and momentum at any point along the way but does ...
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1answer
25 views

Behavior of particles at very small scale of spacetime

If I act $\phi(x)$ on $|0\rangle$ I get a particle created at $x$. Now if I let my system evolve for some time and now I do the measurement on this system at $y$ and my $(x-y)^2$ is small let say of ...
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1answer
37 views

Wave function collapse without finding a particle

The wave function in Quantum Mechanics tells us, among other things, the probability of finding a particle at somme location. According to the Copenhagen interpretation of QM, when we look for and ...
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1answer
32 views

The state of qubit after measuring it

I have a simple question. When a qubit is measured it will collapse into either zero or one state. So how should the notation of the state after measurement be displayed? Is it like classical $0$ or $...
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1answer
43 views

Post measurement state after heterodyne measurement

I want to understand the phase space formulation of quantum mechanics better. Specifically, I am considering the following situation: A quantum state $\rho$ on two modes can be described by its ...
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0answers
31 views

Measurement Puzzle Quantum Mechanics [closed]

I have to solve the following Question: Bob asks you to prepare an electron on which he will perform a single Quantum measurement of either $\sigma_x$ or $\sigma_z$ without telling you which ...
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1answer
40 views

How to detect the entanglement of two qubits by means of a statistical tool?

Is there any robust statistical approach to help us in determining the entanglement of the two qubits based on the results of their so many times measurements? After having the set of measurements ...
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1answer
51 views

Quantum Eigenvalues, measurements, chances not matching up

I've got an assignment with a given quantum state, denoted $\phi=\frac{1}{\sqrt{2}} \begin{bmatrix}0 \\ 1 \\ -1\end{bmatrix}$ and an operator for the observable B, given by $B=b\begin{bmatrix}0 & ...
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1answer
54 views

What is measurements in quantum mechanics

I really can not understand it. For instance, if a machine measures the quantum system. But no one checks this machine. Then the wave function of the quantum system is collapse or not? Besides, is ...
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1answer
93 views

Does exist a (truly) stochastic but causal physical process?

I wonder if it is possible for causality to be held, but not determinism. Does any real-world example exist? If yes, please explain how does that satisfy causality and not determinism. (for example, ...
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1answer
33 views

Is 'measurement' deterministic in the pilot wave theory?

From what I read, measurements in pilot wave theory affect the particles as well as the guiding waves. Is it a random process or is everything deterministic according to the theory?
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Decoherence and measurement in simple terms?

I don't know about physics. Say there are only two electrons in the universe, so there is a probability distribution of their position, speed etc. They are in each other's observable universe so they ...
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33 views

Uncertainty principle and meaning of particle's position

Suppose we have a particle and a detector. The detector will detect electron position not in a sharp peak but according Heisenberg principle in a peak with a deviation. But what that deviation ...
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1answer
37 views

Limits to the precision of quantum measurements

When measuring position the Compton wavelength is believed to be a limit on the precision of the result. If we shine light of a too high frequency on the particle then we will see particle creation ...
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0answers
101 views

Are results of “quantum eraser experiment” same both for particles moving at the speed of light and slower ones?

Recently I've encountered a video about the quantum eraser experiment. I was curious, if results of this experiment are the same for entangled particles moving with the speed of light and slower ones?
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2answers
66 views

How could the momentum of a particle be directly measured in experiment?

What could be done in actual experiment to directly measure the momentum of a particle (i.e. without measuring position) in such a way that would collapse the wave function (i.e. reduce the amount of ...
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4answers
83 views

In QM, what causes a particle to have more probability to be somewhere else when it's found in a less probable position?

For the position of a particle, there's a probability which, somewhere, there's highest probability. And as you move away from it, the probability reduces. But the particle can be anywhere. It's often ...
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1answer
28 views

Continuous variables: Probability of outcome in a quadrature measurement

In the phase-space formulation of QM over continuous variables, how can I determine the probability of obtaining a particular measurement outcome $m$ in the following setting. Given a quantum state $\...
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1answer
93 views

Using the Born rule to measure UV divergence?

A typical use of cutoffs is to prevent singularities from appearing during calculation. If some quantities are computed as integrals over energy or another physical quantity, these cutoffs ...
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1answer
32 views

Does quantum measurement uncertainty still apply when conjuate measurements are inferred?

Consider an apparatus which emits a particle at A which then travels through free space to B. I measure some aspect of that particle at A and transmit that information to B in a light beam. Let's say ...
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1answer
41 views

Quantum mechanical measurement

Suppose, a particle has non zero probability to be in any between x = 1 and x = 10. Let, we measure its position with a very low energy photon. It will collapse it’s wave function, since it disturbs ...
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1answer
55 views

What do we see when we observe a molecular orbital?

My understanding is that when two hydrogen atoms bond (for example) the wavefunctions describing each atom combine into a single molecular wavefunction. Assuming the Copenhagen interpretation, does ...
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1answer
33 views

Is every emitted particle in a one-particle double slit experiment the same?

Lately, I was thinking about the one-particle version of the double slit experiment, e.g. in which an electron or a buckyball is emmited. The precise description of what I mean can be found here. ...
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1answer
44 views

How to find the probability of a state from non-degenerate spectral decomposition

I have just begun learning the topics of time evolution in quantum mechanics, and I'm having trouble understanding how to calculate the probabilities of certain eigenvalues of an operator at a later ...
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1answer
115 views

Quantum description of intensity measurement

The traditional way to formulate the measurement of the intensity in an optical field is by using the number operator $$ \hat{n} = a^{\dagger} a . $$ In effect one can say that the intensity, which is ...
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2answers
66 views

What can be measured in quantum mechanics and how?

This is a question about bridging reality to theory. In learning QM theory, we learn a bunch of stuff about observables, like their operators, that two quantities can be measured at once if their ...
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1answer
61 views

Unitarily reversing a projective measurement

We start with a particle in a pure superposition state. Let's say it is, $$\vert\psi\rangle = \frac{1}{2}(\vert 0\rangle + \vert 1\rangle)$$ Alice sends this particle inside a box and the box ...
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1answer
67 views

Measurement postulate and black hole information paradox: are they related somehow?

The measurement postulate states that whenever we make a quantum measurement, we select (projection) from the general superposition state a single pure state. Thus, as a general quantum state is a ...
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2answers
341 views

Expectation value vs most probable value of radius in ground state of hydrogen

What is the difference when we calculating expectation value for example in hydrogen atom in the ground state? The expectation value is 1.5*Bohr radius while the probability density has a max at Bohr ...
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1answer
118 views

How is “the collapse postulate is also present in QFT, only hidden inside the LSZ formula?”

Background So I am reading the following here (Blog: Not Even Wrong, Blog post: Not So Spooky Action at a Distance, Commenter: vmarko) "The collapse postulate is also present in QFT, only hidden ...
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3answers
80 views

Is it possible for the measurement to yield a superposition of states in the original space?

Suppose we have a wave function $|\Psi\rangle=\sum_i c_i|\psi_i\rangle$, where the original probability probability amplitude was in a distribution of, i.e. $c_1^*c_1=\frac{1}{2},...,c_i^*c_i=\frac{1}{...
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0answers
332 views

Does the Copenhagen interpretation (+ “quasi-classical apparatus”) allow one to bypass a derivation's objection? [closed]

Background I asked this question, where I'm thinking about this joshphysics's answer: And the gist of what I get is (from the answers and the chatroom) within interpretations of quantum mechanics ...
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2answers
100 views

Completeness relation and projective measurement operators on Hilbert space

I have a question about a passage in the Book Quantum Computation and Quantum Information Textbook by I. Chuang and M, Nielsen. My query is about a passage on page 86 of the 10th edition: ...
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1answer
73 views

Stinespring dilation of a channel vs Naimark's theorem

I'm trying to understand the connection between the Stinespring dilation of a quantum channel and Naimark's theorem that shows that POVMs can be written as projective measurements in a larger Hilbert ...
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1answer
53 views

What methods do we use to measure position/momentum of quantum systems in the lab?

I've seen this question asked before but couldn't find a satisfactory answer. What is the difference between measuring position vs. momentum in the lab? Is it just something to do with the energy of ...