# Questions tagged [quantum-measurements]

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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 ...
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### 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 ...
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### 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 ...
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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 ...
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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 ...
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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 ...
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### 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 ...
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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 ...
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### 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 ...
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1 vote
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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 ...
1 vote
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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 ...
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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 ...
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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 ...
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### 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
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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 ...
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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 ...
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### 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 ...
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### 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 ...
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### 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,...
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### 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 ...
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1 vote
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### 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 ...
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### 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 ...
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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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