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What would be the outcome of an experiment wherein the spin of a qubit is measured in two or more orthogonal directions simultaneously?

Out of curiosity, I am seeing the Leonard Susskind’s Theoretical Minimum lecture series on YouTube and have just started watching the second lecture. As I understand so far, the qubit can have a spin ...
Kapil Jhindal's user avatar
5 votes
1 answer
333 views

In hetero- homodyne detection, what does it mean to operate at the Quantum Shot Noise limit?

I am an electrical engineering by trade, working on the analogue part (Transmitter & Receiver) of a quantum optical communication channel. By this, I mean I have not much experience on things ...
MrNotSafe4Work's user avatar
3 votes
1 answer
134 views

Joint measurability in quantum mechanics

Two POVMs $E= \{E_i\}$ and $F= \{F_j\}$ are said to be jointly measurable if there exists a POVM $G= \{G_{ij}\}$ such that $$E_i=\sum_j G_{ij}\quad \text{and}\quad F_j =\sum_i G_{ij}$$ From this ...
veke's user avatar
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Mean energy measurement in an arbitrary quantum state

I've gone through many papers looking for a way to measure a mean energy in an arbitrary state $\langle \psi | H | \psi \rangle$. I am interested in a theoretical protocol or an exemplary experimental ...
Pawel's user avatar
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4 answers
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Why are measurements considered irreversible?

In quantum mechanics, every interaction is described by a unitary Hamiltonian operator. We expect that a measurement is no different from any other interaction, yet in the standard way of treating QM ...
Davyz2's user avatar
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0 answers
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What is the interpretation of the covariance of two quantum observables?

I have been studying the covariance matrix in continuous variable quantum systems and I am struggling to understand the interpretation of this object. In statistics the covariance measures the joint ...
Hermitian_hermit's user avatar
0 votes
2 answers
56 views

Quantum mechanical model of a system together with its measurement device

I am reading Griffith's introduction to quantum mechanics. The book assumes an object quantum system and then assumes that we somehow perform a measurement on it from the outside. However, clearly the ...
user56834's user avatar
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2 votes
3 answers
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Circular setup of Stern–Gerlach experiment and repeated measurements

Let us start with the standard Stern–Gerlach experiment which separates an electron beam either up or down as they pass through and are deflected by an inhomogeneous magnetic field. We can find the ...
QEntanglement's user avatar
4 votes
2 answers
243 views

What is Decoherence? [duplicate]

What precisely is decoherence? Assume familiarity with the density matrix formalism of quantum mechanics. I read this related question, but I am looking for a more precise answer than the one given. I ...
Silly Goose's user avatar
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1 answer
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Is the annihilation operator an observable (it is non-Hermitian)?

In most treatments of quantum mechanics that I have seen, observables of a quantum system are defined using Hermitian operators. The most intuitive reason for this is that Hermitian operators have ...
Biophysman's user avatar
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1 answer
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Why can't we measure different properties of an electron in accordance with its frequency?

sorry if this is a stupid question but its my first one.. Why cant we just observe and electron in accordance to its frequency. Like during one frequency peak we could observe position, next could be ...
omgcchheeessee's user avatar
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Observing a particle [duplicate]

We say that there is uncertainty in position when we observe a particle. But first i want to know in detail about how do we observe a particles position and momentum. Suppose we know that an electron ...
Physics's user avatar
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1 answer
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Conceptual problem with post-measurement density matrix

I'm asked to derive what the density matrix would be after a measure of $|\phi \rangle$ ($\rho \rightarrow \rho'$). At first I thought that, since $\rho$ has an statistical meaning on an ensemble of ...
Akko0907's user avatar
0 votes
1 answer
48 views

Finding literature with quantum tomography during a measurement to "see" the wave function collapse

I was at a journal club some time around 2020 where we discussed a paper that I am trying to find again. The claim was that it had experimentally shown that wave function collapse was not "...
1 vote
1 answer
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Measuring Incompatible observables simultaneously of an entangled electron [duplicate]

Given that a pair of electrons are in the state $$\psi =\frac{ {|00}\rangle \pm {|11}\rangle }{\sqrt{2}}$$ (in the the spin-$x$ eigenbasis) . If we measure the spin along $x$ axis of one of the ...
veke's user avatar
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Can we measure the current created by single electron in spacetime?

Consider single hydrogen atom in the ground state. Does there exist an experimental procedure that would determine the 0th component of the current $J_\mu(x) = \bar{\psi}(x)\gamma_\mu \psi(x)$ created ...
0x11111's user avatar
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0 answers
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How does a heat bath measure a system?

In the information theoretic treatment of say, the canonical ensemble, one must maximise the Shannon entropy $$ S =-\sum_i p_i \ln p_i$$ where $p_i$ is the subjective probability for occupying state $\...
Ghorbalchov's user avatar
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1 answer
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Physically how measurement in quantum mechanics works and how the formula is deduced? [closed]

This question seems a little broad but in fact it is very specified. I read a lot of webpages and papers and books about measurement in quantum computing. But in my opinion, they are just like "...
tangyao's user avatar
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2 votes
3 answers
146 views

How to represent the state of this system after the measurement?

This is a question my professor gave us in the test. Consider the spin-$\frac{1}{2}$. Alice measured the system along the $z$ direction, and she observed the outcome as $+1$. After Alice's measurement,...
Kevin027's user avatar
2 votes
0 answers
51 views

Measurement unit of a tensor product combinations of quantum states in Pusey-Barrett-Rudolph (PBR) theorem proof

I'm dealing with the Pusey-Barrett-Rudolph (PBR) theorem proof, in which we are using a basis built of linear combinations with tensor products. I'm trying to figure out whether measuring those linear ...
Wojciech Szyszka's user avatar
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1 answer
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Conservation of energy after measurement

I thought that when a system was measured, there could be an interaction between the measurement device or environment and the system but overall energy was conserved. However, I came across these 2 ...
OCD's user avatar
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1 answer
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Effects of measurement on the entropy in quantum mechanics

How does measurement in quantum mechanics (QM) impact the system's entropy? The measurement process in QM is considered time-irreversible. Are there principles akin to the second law of thermodynamics ...
Omid's user avatar
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0 votes
3 answers
128 views

What happens if two people have different knowledge about a state in a quantum mechanical system?

Let’s say I measure the spin of an electron, but I don’t tell you what it is and you don’t measure it yourself. Does that change the wave function for you or does it remain the same either way? If it ...
Name's user avatar
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27 votes
2 answers
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Why are quantum effects of the apparatus ignored in quantum experiments?

When reading about various quantum experiments the apparatus often includes things like "semitransparent mirrors", "regular mirrors", "polarization filters" and others. ...
Vilx-'s user avatar
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Why can quantum-classical ensembles be written as $\sigma = \sum_x |x\rangle\!\langle x|\otimes{\rm Tr}_{A}((M^x_A \otimes I_B) \rho_{AB} )$?

I was reading about the entropic uncertainty principle, but there is something that is confusing me. Suppose we have a state $\rho_{AB}$ for a state that belongs to the product of two Hilbert spaces, ...
forgetfuled's user avatar
1 vote
0 answers
51 views

Measurement of the momentum of electron when the momentum is very small

In particle physics, it is usually no problem to determine the momentum of a charged particle such as electron, by using something like a pixel detector. This relies on ionization. There is also ...
0x11111's user avatar
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1 answer
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Questions regarding measurement of a qubit

I am trying to understand the basic concept of observable and state. Say that we have a state $|\psi>=|0>$ which is $(1\ \ \ 0)^T$, we have an observable $\sigma_x= \begin{bmatrix}0&1\\1&...
sett the guy's user avatar
1 vote
1 answer
113 views

The Heisenberg Uncertainty Principle (HUP) and "borrowing energy" [duplicate]

Often when physics students are introduced to the HUP for position and momentum, the interpretation is that you aren't able to measure position and momentum for a particle to arbitrary precision at ...
Depenau's user avatar
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2 votes
0 answers
69 views

How does pilot wave theory explain non-zero momentum measurements? [closed]

The momentum of particles in pilot wave theory is dependent on the phase of the spatial wave function, which is 0 for stationary states. This means that electrons in all excited stationary states of a ...
user avatar
0 votes
1 answer
123 views

Does a Quantum System Really "Jump" to an Eigenstate When Observed?

Warning: This is a highly hypothetical question. I am bothered with Dirac's description of the system when making a measurement. Without quoting his statement (from The Principles of Quantum Mechanics,...
Con's user avatar
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7 votes
1 answer
652 views

On the Measurement Problem

In the orthodox interpretation of quantum mechanics, the following three assumptions are made (please correct me if I am wrong): Every physical system is completely specified by a state $\lvert\psi\...
Davyz2's user avatar
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1 vote
0 answers
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Time evolution of monitored quantum dynamics

Let us assume that the quantum system consists of Hilbert space $H$ with basis vectors $|i\rangle$. The initial state $|\Psi(0)\rangle$ evolves in discrete time steps such that $|\Psi(t+\delta t) \...
Young Kindaichi's user avatar
-1 votes
3 answers
71 views

If I shoots an electron in empty space and never measure it, is the electron still there?

Imagine there is an electron gun that is capable of emitting 1 electron at a time, the gun emits an electron directed at an empty space, no dust, no matter or anything that could interact with the ...
user6760's user avatar
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1 vote
2 answers
55 views

Would quantum superposition stop if we observe the particle in the case of photosynthesis?

I started looking up more into quantum biology from photosynthesis to genetics mutation and how they are explained by quantum properties. my question is about the case of photosynthesis: quantum ...
Rihab's user avatar
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0 votes
2 answers
149 views

Kraus Decomposition of Orthogonal Projections

More specific, short form of my question: POVMs assign expectation values of positive operators (summing to identity) to probabilities of events, but not a particular measurement scheme, leading to no ...
Theoreticalhelp's user avatar
5 votes
4 answers
830 views

Exact time evolution of Stern-Gerlach (SG) apparatus

Background: I was always under the impression that when considering the Stern-Gerlach (SG) Experiment, the interpretation of the split of the beams is that the spin $1/2$ particle get measured the ...
2000mg Haigo 's user avatar
0 votes
3 answers
79 views

Information leakage due to superposition [closed]

My question is going to be a bit sketchy so please forgive me. Imagine the following experiment: -We have a particle that is in a superposition of space, such that $\psi(x)$ has the form of two ...
Sonoda's user avatar
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0 votes
2 answers
85 views

How to see the measurement operator in action?

I understand that an measurement operator always returns its eigenvalues, with a probability. I also understand that it is a postulate of QM, so why can't prove it Why is the measured value of some ...
Minh Triet's user avatar
2 votes
1 answer
71 views

What are the similarities between a beam splitter and a double-slit?

More generally, my question is about the similarity between an $N$-slit and an $N$-mode beam splitter. Conceptually speaking, would it be accurate to say that the beam splitter is just the discretized ...
Tfovid's user avatar
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1 vote
0 answers
37 views

Can other objects feeling the gravitational effect of a particle that is in a state of superposition cause its wavefunction to collapse? [duplicate]

When I feel the slight gravitational pull of one particle that is in a state of superposition and I measure the exact pull at any given instant does the wavefunction collapse? If particle A constantly ...
The Burger King's user avatar
4 votes
1 answer
142 views

Understanding Wigner’s friend scenario

I recently watched this video https://youtu.be/Wsjgtp9XZxo?si=sIRlAbvAm2wjXRNP and tried (unsuccessfully, because i don’t have the knowledge required) to read Bruckner’s paper. What i am missing, is ...
Marco Fabbri's user avatar
0 votes
4 answers
100 views

Did the detector change the spin states?

I am wondering why every detectors that try to measure the spin states of quantum particle had only 2 outcomes for example either spin up or spin down, can we measure the actual spin states or are we ...
user6760's user avatar
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1 vote
1 answer
84 views

Question regarding measurement induced phase transitions

It is often claimed in the literature of random quantum hybrid circuits that a measurement induced phase transition is invisible in trajectory averages of linear observables and can only be seen in ...
Pugs's user avatar
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2 votes
0 answers
86 views

Homodyne and heterodyne detection for light statistics

I'm learning about homodyne and heterodyne detection in quantum optics, but I'm having some trouble understanding how these two detection schemes are used in practice to measure the statistics of your ...
Chan's user avatar
  • 73
2 votes
1 answer
72 views

Long term dispersion of the wavepacket

Consider a one-dimensional wavepacket $\psi(x,t)$ that evolves according to the free particle Schrödinger's equation. It is initially localized around $x=0$ with a certain width $\Delta x$. This ...
Phineas Nicolson's user avatar
3 votes
1 answer
117 views

Assigning Measurement Operators to POVM Operators

I'm trying to understand how different authors assign measurement operators to POVM operators so we can define the post-measurement state. In particular, it's not clear for me if both assignments are ...
ConorP's user avatar
  • 31
2 votes
1 answer
84 views

Does a particle still behave as a wave after being detected as a particle?

I just started learning about the intricacies of quantum mechanics in high school and aim to improve my understanding of the wave-particle duality of particles. I was wondering about different ...
spaghettyii's user avatar
1 vote
1 answer
73 views

Measurement values affects probabilities in QM?

Consider a non-degenerate operator $\Omega$ with discrete eigenvalues $\omega_i$, where $i=1,2,3,...$. We can write $\Omega = \sum_i \omega_i~|\omega_i\rangle \langle \omega_i|$, where $|\omega_i\...
Deep's user avatar
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0 votes
0 answers
53 views

Is there a general construction for three-outcome qutrit POVMs?

For qubits, I can consider the General POVM elements: $M_{\pm} = \frac{1}{2}(I \pm \hat{n}\cdot\overline{\sigma})$ where $\sigma $ is a vector containing the Pauli matrices and $\hat{n}$ a vector with ...
fszba's user avatar
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0 votes
0 answers
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"Jump" operators from Lindblad equation where the external system is measurements

How do we derive the "jump operators" for the Lindblad equation if the external system is measurements? For example in this article for the Bose Hubbard system the Lindblad operators ...
poliroika's user avatar

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