This tag is for questions about the quantum mechanical measurement problem, such as e.g. the wavefunction collapse/decoherence. DO NOT USE THIS TAG for non-quantum measurements.

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Is the momentum of a particle both uncertain and, independently, also random?

Is momentum of a particle "random" because it is uncertain, or is it uncertain in addition to being random? Is the uncertainty principle and quantum randomness different names the same physical ...
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465 views

How can we know the state of a quantum system?

One of the postulates of QM states that given a system in a state $|\psi\rangle$ and given an observable $A$ whose eigenstates are $|\phi_i\rangle$, then the state of the system can be expressed as a ...
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1answer
32 views

Measuring compatible observables in quantum mechanics

Suppose a particle that is under a quantum oscillator potential and is, initially, in the state $\Psi(x,0)=\frac{1}{\sqrt3}\phi_1(x)+\sqrt{\frac23}\phi_2(x)$, where $\phi_1(x)$ and $\phi_2(x)$ are ...
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1answer
43 views

Wavefunction Collapse

I believe my Lecturer and the textbook have contradicted one another. My lecturer gave the example that if the spatial part of the wavefunction of a particle is given by $\psi(x) = c_1\psi_1(x) + ...
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If I repeated a quantum measurement, would it be the same? [closed]

I was thinking about the probabilistic nature of quantum mechanics and that if I measured the position of an electron twice in succession, the outcomes would depend on a probability. However, what if ...
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27 views

Detecting position of electrons [duplicate]

To detect particles like electrons, why would the accuracy of the position determined be affected by the wavelength of EM wave used?
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1answer
71 views

What is a weak value really?

There have been a lot of recent experiments performing weak measurements. Some of the conclusions seem to be quite surprising (e.g. this paper) and there is still debate if the weak measurement is ...
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2answers
56 views

Linear Combinations of Energy Eigenfunctions in 1D

Given that a particle is in a state defined by the wavefunction: $$\Psi (x,t) = \psi_0(x)e^{-iE_0t/\hbar}+\psi_1(x)e^{-iE_1t/\hbar}$$ where $\psi_0(x)$ and $\psi_1(x)$ are the energy eigenfunctions of ...
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1answer
28 views

Given any two quantum states and the information that the system is in one of these two states

Given any two quantum states and the information that the system is in one of these two states, one cannot reliably devise a single measurement which could determine with certainty which state the ...
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1answer
51 views

Quantum non-locality with commuting measurements?

We consider the Bell scenario, in which Alice and Bob share an entangled pure quantum state $\mid \Psi \rangle_{AB}$. Alice gets an input in the set $\{1,2\ldots X\}$ and Bob gets an input in the set ...
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4answers
138 views

“Randomness” versus “uncertainty”

Highly rated PhysicsSE contributor @CuriousOne regularly makes the following claim about quantum mechanics (e.g. here): There is no randomness in quantum mechanics, there is only uncertainty. I ...
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0answers
49 views

How does the two slit experiment work?

Whilst going through 'A brief history of time', I faced difficulty in understanding the two slit experiment. How can an individual electron cause fringes on screen? I was unable to understand it? ...
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1answer
99 views

Quantum mechanics - measuring position

I am watching Susskind's Stanford Lectures on quantum mechanics. The eigenvectors (eigenfunctions) of the position operator are of the form $\delta(x-k)$. But $$\int\delta^{*}(x-k)\delta(x-k)\, ...
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2answers
70 views

How does really the “wave function collapse” work? [closed]

It's usually said that "the direct observation of a process makes the wave function of the system to collapse". How does really that process happen? What exactly means for a wave function to ...
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3answers
48 views

Finding the random uncertainty of a set of values [closed]

Ok, for the switch-on voltage of a red LED I have the readings as follows, all in volts: $$ 1.45, 1.46, 1.46, 1.44, 1.45 $$ The mean of these readings, in volts, is $1.45$ (I rounded up to $2$ decimal ...
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4answers
999 views

Is it possible to determine the slit a photon went through in the double slit experiment by measuring its flight time?

In the double-slit experiment, quantum mechanics states that if you try to determine which slit the photon goes through, you won't have a resulting wave pattern. But, knowing the time it took for the ...
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4answers
2k views

Uncertainty in Uncertainty?

I was reading about how the Planck's Constant can be measured with LEDs, which made me think about this question. The Heisenberg Uncertainty Principle states that: $$\Delta x \Delta p \ge ...
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2answers
83 views

Is this mention of the double-slit phenomenon broadly correct?

I'm reading a book of philosophy (Less Than Nothing by Slavoj Žižek) and a chapter in it (called The Ontology of Quantum Mechanics) frequently references the double-slit phenomenon. At one point it ...
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0answers
56 views

Position Measurement of Quantum Harmonic Oscillator, to a Position Eigenstate

I read that if one takes a quantum harmonic oscillator system, not externally driven, and performs a position measurement (measurement in position basis) that reduces the oscillator to an eigenstate ...
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1answer
27 views

Distinguish statistical uncertaincy from noise disturbance relation in slit gedankenexperiment

I am just reading the "Statistical Interpretation of Quantum Mechanics" from L. Ballentine. In section 3.2. he discusses the difference between the observer effect and the statistical ...
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1answer
68 views

Is the probability current an observable?

Is the probability current in Quantum Mechanics an observable? If so, how can it me measured (directly or indirectly)?
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1answer
59 views

Reason behind the uncertainty principle [duplicate]

I know that Heisenberg Uncertainty principles states that the momentum and position of a quantum object can not be determined at the same time. This is very strange to me. I want the basic reason ...
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1answer
215 views

Did Leggett and Caldeira solve the measurement problem?

In 1983 Leggett and Caldeira published a paper (see also here) that shows the evolution of the density matrix in a dissipative system. Follow-up work by Zurek and others shows the relevance to ...
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1answer
105 views

What does it mean that an electron's position is described by probability?

I just read an answer to a Phys.SE question about why electrons don't collide with protons where an answering user said: In quantum mechanics, an electron doesn't have a definite position or ...
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0answers
39 views

Continuous spectra and quantum decoherence

Suppose that some quantum wave function $\psi = \int a_i \,i\rangle \,\,di$ where pseudo-spanning ket "vectors" of $\psi$, $i \rangle$, are continuous. (thus the use of integral.) By normalization, ...
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2answers
275 views

Does Heisenberg's Uncertainty Principle describe real uncertainty or measured uncertainty?

Some derivations of Hesenberg's Uncertainty principle are based on the momentum of light or particles used in measuring an object's position and momentum. Does this lower limit on uncertainty describe ...
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1k views

What does it mean “not to have a definite trajectory”?

In a comment to my question someone stated the following: "photons do not travel at some definite number of oscillations per second. In fact, they do not "travel" at all, no more than electrons ...
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1answer
165 views

Quantum Measurement of Entangled Time-Varying Systems

Suppose System A has a dynamic, continuous variable $O_A(t)$, which we wish to measure via a quantum probe. Assume system A has a self-Hmiltonian, $H_{SA}$ so that the evolving wave function of ...
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25 views

Measurement without collapse

Does anybody have a reference for Cini's model of Quantum Measurement theory. I have a background of Quantum Mechanics (Ballentine level). Also I don't have access to Cini's paper.
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Why do $\hat{X}$ and $\hat{P}$ have to correspond to position and momentum?

As far as I understand, in QM we treat observables as operators, and the eigenvalues of these operators are the possible values we can measure of the observables. It is usually simpler to work in the ...
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1answer
45 views

What exactly does the expectation value of $x$ mean in quantum mechanics?

When I learn quantum mechanics (by reading Griffith's book Introduction to quantum mechanics 2ed edition (Page 15)), I was confused by the concept of the expectation value of $x$, i.e. $\langle ...
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1answer
112 views

Understanding the delayed choice quantum eraser from a quantum information stand point

A continuation of Understanding the quantum eraser from a quantum information stand point What quantum circuit would correspond to delayed choice? In particular, the decision of whether to not to ...
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1answer
142 views

Understanding the quantum eraser from a quantum information stand point

Is there a way to understand the quantum eraser experiment in terms of quantum information? In particular, is there a quantum circuit that would function as a quantum eraser experiment? The issue I'm ...
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4answers
122 views

Heisenberg's uncertainity principle

In the Heisenberg uncertainty principle, $$\Delta x \cdot \Delta p \geq \frac{h}{4\pi}$$ The values of $\Delta x$ and $\Delta p$ are the standard deviations which we get from the probability ...
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Which is the best place to learn about Measurement theory in Quantum Mechanics?

I have read Leslie Ballentine's book on quantum mechanics. I want to learn measurement theory. I am aware of the book Foundation and interpretations of Quantum Mechanics by Auletta. But I want ...
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2answers
135 views

Meaning of “realism” in quantum mechanics

When physicists doing work in quantum measurement, decoherence, Bells' inequalities etc. use the term "realism" what exactly do they mean? I'm looking for answers targeted towards, say, someone whose ...
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42 views

Quantum mechanics of electron beam and measurement

I'm trying to understand the following phenomenon quantum mechanically: Problem statement If I'm shooting an electron beam (using a vacuum tube for example) and I have a measuring device at a ...
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1answer
70 views

Does a particle “exist” between emission and detection?

In considering a simple Young's double slit setup: The classical idea that a particle must exist with defined position and momentum between the source and detection plate leads to philosophical ...
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3answers
104 views

Double slit experiment without observer

If we used an instrument to record the double slit experiment but never looked at the footage so to speak, what would the mathematics show us?
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1answer
156 views

Hydrogen atom in superposition of energy eigenstates

Suppose a single hydrogen atom is in a superposition of energy eigenstates: $$ \psi = \frac{1}{\sqrt{2}}\psi_{100} + \frac{1}{\sqrt{2}}\psi_{200} \,.$$ Then energy will be $E = ...
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64 views

How does one intrepret probabilites in the many-worlds interpretation?

Let's say I flip a coin, and don't look. From the copenhagen interpretation, the state of the coin is: $\frac1{\sqrt2}(i|\text{heads} \rangle - |\text{tails}\rangle)$ If I observe the coin, there is ...
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1answer
103 views

How to understand the measurement on entangled state in the following cases? [closed]

Assuming an EPR pair AB, event MA is a measurement on A. My questions are: (1) At MB and MB' (depending on where B is located), if we try to describe the state of B (but not measure B yet), ...
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2answers
62 views

Operator and apparatus in quantum mechanics? [closed]

If $L$ is an Hermitian matrix associated with an apparatus acting on state $\Psi$, how does the state vector $\Psi$ collapse or change according to matrix arithmetic?
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2answers
70 views

Varying the Predictability in a Double Slit Experiment

I know that the Englert-Greenberger-Yasin Duality Relation states that as the predictability of finding a photon at a slit in a double slit experiment increases, the visibility of the interference ...
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4answers
105 views

Why the word “measurement” all the time?

I'm trying to learn more about the properties of light. In all the youtube videos and related to the two-slit experiment, the explanations always say that "measuring" can change the outcome. Why do ...
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1answer
57 views

How is the Born rule consistent with unitary evolution?

Consider a system $|\Psi_T \rangle_{t = 0} = |\Psi_E \rangle \otimes |\Psi_S \rangle$ where $|\Psi_S \rangle$ is a system that collapses into an eigenstate upon measurement. $|\Psi_E \rangle$ is the ...
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1answer
55 views

Quantum Backaction Question

Question about quantum back action in hypothetical scenario: We know that, at $t_0$, a certain kind of particle, with spin initially prepared to be “spin right” in the x basis, goes through a ...
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2answers
85 views

Measurement of energy apparently violating the position-momentum Uncertainty Principle in a potential that does not depend on distance?

I am taking a beginning course in QM and I have learnt that the measurement of energy collapses the wavefunction of a particle to one of its energy eigenstates. But some misconceptions regarding this ...
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3answers
177 views

Wave function collapse and Schrodinger's equation without measurement

Will wave function collapse without measurement? Since all matters are described by wave functions, then in principle, I should be able to describe wave function collapse by Schrodinger's equation. ...
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What would be the abilities of a wave function collapsing oracle? [closed]

Although the Bell's theorem dismiss local hidden variables as an explanation of the quantum variability, there is nothing to prevent an observer to see the occurrence of a wave function collapse as ...