Questions tagged [quantum-measurement]

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Qubit state probability

Consider the general qubit state $\alpha \begin{pmatrix} 0 \\ 1\end{pmatrix} + \beta \begin{pmatrix} 1 \\ 0\end{pmatrix}$ in a general orthnormal basis with vectors $\begin{pmatrix} \alpha \\ \beta \...
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Measuring momentum in a 2 height well

In a 2 height finite square well we know that qualitatively the position wavefunction will have a lower amplitude in the side of the well with a smaller potential energy (since it will be travelling ...
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1answer
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Settling Copenhagen Interpretation / Measurement in QM & Existence of Quantum Gravity

It is my understanding that there are many interpretations of observation/measurement in Quantum Mechanics (I am only familiar with the Copenhagen one). The Schrodinger's Cat experiment forms a ...
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Why are there so few ion-trap experiments studying quantum trajectories?

The quantum trajectory theory describes the behaviour of a quantum system under continuous monitoring. Initially it is theoretically studied in quantum optics and single atom scenario (e.g., ion trap ...
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Book recommendation for how classical mechanics emerges from quantum mechanics?

Question So I read this interesting link: The most glaring problem is that the state spaces of classical and quantum mechanics are completely different, so you can’t have a simple limiting procedure ...
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What does Schrödinger equation reduce to in the limit of a continuous position measurement?

If we measure position of a quantum particle, we force its wavefunction to collapse into a wavefunction whose probability density is given by a Dirac delta function (all the probability density of ...
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2answers
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Are distributions of position and momentum assumed to be independent in quantum mechanics?

Given a wave-function of a single particle we can calculate probability density for positions. We can also calculate probability density for momenta. Are these probability densities assumed to be ...
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1answer
62 views

Quantum measurement paradox

let's define 'a measurement device' as a system which is highly sensitive to the eigenstate of an observable. The sensitivity is quantified let's say by how irreversible and grand the small changes in ...
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Can one define a quantum measuring detector as a device that nullifies the interference term and other peculiarities of quantum probabilities?

suppose one has a quantum probability: $$ |\psi_1+\psi_2|^2=|\psi_1|^2+|\psi_2|^2+2|\psi_1||\psi_2|\cos (\varphi_2-\varphi_1) \tag{A} $$ Any device whose geometric shape, or otherwise general ...
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41 views

Continuous spectra and measurement

Suppose I have a particle whose momentum I measure to be $p$ with uncertainty $\delta p$. Right after the measurement we know that its wave function is given by $\psi(x)=\int g(p)e^{ipx/\hbar}dp$ (...
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46 views

Pauli matrices Measurements

When performing a measurement On qubits with the Pauli matrices, They all correspond to outcomes +1,-1 because that's their eigenvalues? In my notes it says that the $z$ Pauli matrix can be broken ...
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1answer
48 views

What theories of quantum mechanics can eschew global phase?

Whilst reading the quantum.country essay by Michael Nielsen and Andy Matuschak, they mention (a bit further down from here) that there is formulations of QM that can completely ignore global phase ...
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Identifying Collapse in Wave-function [duplicate]

Is there a machine or instrument which will notify us instantly when the wave-function has collapsed if we have access to only one of the entangled particles? Note that the wave function collapse will ...
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Conservation of energy in quantum tunneling

By this question, a particle can tunnel to inside a potential barrier. One aspect of this process that was not addressed in the answers was the conservation of energy. Motivated by this question, I am ...
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Field measurements in QFT

If I understand it correctly, QFT assigns a field operator to every point in space. For a free scalar field: $$ \hat{\phi}(\mathbf{x},t)=\int \frac{d^3p}{(2\pi)^3} \frac{1}{\sqrt{2\omega_{\mathbf{p}}}}...
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3answers
60 views

Does seeing an object modify it state?

I took a dive into the concept of information in physics and from what I have read, I which someone to confirm if looking at something really modify the state of that thing.(modify state here refers ...
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5answers
161 views

Is it true that we can't measure position and momentum together?

The uncertainty principle states that there always will be mean variance if we measure position or momentum. It does not state that the measurement is wrong. It only states that there always will be a ...
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1answer
64 views

In what sense is it correct to say that a measurement is always electromagnetic?

Is it true or wrong that the interaction between the system to be measured and the measurement apparatus is always electromagnetic - even when looking at gravity or at nuclear interactions? Is it ...
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1answer
57 views

Measuring qubit's state in a real experiment

In theory, it is fairly simple to measure a qubit's state. Let us consider a Ramsey experiment, where the qubit is in the ground state $|0\rangle$ initially. Then by applying a Hadamard gate the qubit ...
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38 views

Repeatedly measuring the same observable on the same system

Suppose we have got some operator $A$ which does not commute with the Hamiltonian, for example $\hat{p}_{x}$ and $\hat{H}$, where $\hat{p}_{x}$ does not commute with the $\hat{H}$ and we make the ...
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A question regarding the commutators of operators

Suppose we have got a triple of observables $A,B$ and $C$. Suppose furthermore, that $[A,B]=0$ and $[B,C]=0$ but $[A,C]\neq 0$ . Suppose, also now we do a measurement of $A$ then accordingly we would ...
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Can we measure $x$ and $p_x$ simultaneously by measuring $p_y$ and $y$ as well?

Suppose our plan is to measure experimentally the position $(x,y)$ in the plane and the momentum $(p_{x}, p_{y})$ of a quantum particle. Assuming the canonical commutation relation between $x$ and $p_{...
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How to specify a general quantum measurement? [closed]

I know that a general quantum measurement is described by a set of measurement operators $\{M_j\}$ satisfying the completeness relation $\sum_j M_j^\dagger M_j = 1$ and that the outcome j occurs with ...
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1answer
85 views

Confusion regarding bra-ket notation and proof of a ket equation

My question is two-part. First, imagine a bipartite quantum state $|\Phi \rangle_{AB}$, made of $2n$-qubits, shared between Alice and Bob (with $n$-qubits each). Alice performs some unitary operation $...
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2answers
82 views

Is causality violated in QFT?

I'm studying QFT, and Peskin is his book takes a couple of paragraphs to talk about causality in QFT, using the Klein-Gordon field as an example. The book says on p. 28: To really discuss causality, ...
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Book about the measurement energy of the Hamiltonian

I am searching for a book about the measurement energy of hamiltonian in adiabatic quantum computing. Have you ever seen good resources? I need good references for my work.
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60 views

The master equation and continuous measurements

To derive the master equation under a continuous measurement we first define the two measurement operators $$M_0=\mathbb{I}-\left(R~/~2+iH\right)dt \tag 1$$ $$M_1=c\sqrt{dt}, \tag 2 $$ where $M_0$ ...
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Can a particle stop disintegrating by zeno effect? [duplicate]

Can a free neutron be prevented from disintegrating by observing it continuously? (Zeno effect).
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1answer
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How is the position of an electron made? [duplicate]

The position of an electron is often discussed in physics. My question is simple. How does one go about measuring an electron’s position ? It seems like an impossible task.
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1answer
18 views

How many time can we measure the same entangled pair?

We know that the entanglement state is a very fragile state and it can be disturbed by very minimum interaction with environment, so how many times can we measure(spin direction) the same entangled ...
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1answer
52 views

State of an entangled state after measurement

I have this general state $\lvert \psi \rangle_{AB} = \sum_{\alpha\beta}c_{\alpha\beta}\lvert\alpha \rangle \otimes \lvert \beta\rangle$ of two qbits. I want to write the state of the system after the ...
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Why is quantum measurement not happening all the time? [duplicate]

I have a question which may be very naive yet I have no answer. I studied undergraduate quantum mechanics 4 years ago now and even if I studied more advanced stuff like QFT I feel like I don't ...
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2answers
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Is it possible to differentiate between pure states and mixed ones in the laboratory?

I am wondering if it is possible to differentiate between pure states and mixed ones in the laboratory? For example consider a quantum system define relative to the orthonormal basis {up,down}. We can ...
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How can this measurement operator be proportional to a projector?

I am trying to show that the measurement operator $\langle x_1|\langle x_2|\hat{U}|d_2\rangle|d_1\rangle$ (for the Arthur-kelly measurement model with unitary $U=\exp(-i[\hat{X}\hat{P_1}+\hat{P}\hat{...
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1answer
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Measurement with any number of outcomes can be performed as a sequence of measurements with two outcomes

I was reading this paper, at the bottom of the first page, it mentions "It is easy to show that measurement with any number of outcomes can be performed as a sequence of measurements with two outcomes....
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55 views

Can a balanced homodyne detector measure instantaneous change in frequency?

I am reading a paper(https://arxiv.org/abs/1906.03540) where a Balanced Homodyne detector is producing a homodyne signal that is the instantaneous change in the oscillator's frequency. I know that a ...
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1answer
62 views

Inverse of Wave Reduction

Let's consider a system of three apparatuses. Sequentially these act as A device that measures momentum of an electron Parallel plates where electric field between them changes randomly. Same device ...
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Measurement of current and measurement of momentum of the electrons

Is a measurement of the current flowing through some material a measurement of the momentum of the electrons? Does their wavefunction collapse to an (approximate?) wavefunction of momentum?
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How to experimentally perform a POVM measurement?

Reading through Nielsen and Chuang, I came across the following example. Consider a POVM containing three elements, $$ E_1 \equiv \frac{\sqrt{2}}{1 + \sqrt{2}} |1⟩⟨1|,\\ E_2 \equiv \frac{\sqrt{2}}...
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46 views

Measuring a state in a basis other than eigenbasis

Suppose I have a state expressed in its eigenbasis as follows. $\rho = \sum_i\lambda_i\vert i\rangle\langle i\vert$. It is now measured in some other basis $\{\vert x\rangle\}$ that is distinct from ...
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Why can't this be a post measurement state for position measurement?

We know that the post measurement states after a projective measurement has the form $$\frac{\hat{\Pi}|\Psi(t)\rangle}{\sqrt{\langle\psi(t)|\hat{\Pi}|\psi(t)\rangle}} \qquad (1) $$ When a position ...
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How does the spin of a particle vary if it is measured several times? [closed]

I have three questions about 3 successive measurements of the spin of an electron. In the first measurement the spin is always measured to be +1/2 on the z-axis. Measurement 2 and measurement 3 are ...
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Is it ever possible to test whether an object is in a quantum superposition (versus a mixed state)?

Today I came across a paper, "Experiments testing macroscopic quantum superpositions must be slow," by Mari et al., which proposes and analyzes a thought experiment involving a first mass mA placed in ...
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The superposition principle in quantum mechanics

This is a question parallel to this question The importance of the phase in quantum mechanics. In introductionary quantum mechanics I have always heared the mantra The superposition principle ...
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The importance of the phase in quantum mechanics

In introductory quantum mechanics I have always heard the mantra The phase of a wave function doesn't have physical meaning. So the states $| \psi \rangle$ and $\lambda|\psi \rangle$ with $|\lambda|...
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1answer
23 views

How to determine the measurement operators of a two qubit system? [closed]

I'm reviewing material for my quantum information module and I came across the following question: "A projective measurement in the standard computational basis is to be performed on the first qubit ...
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3answers
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What is meant by “maximum amount of information is available” in quantum measurements?

I’m reading Isham’s Lectures on Quantum Theory, and in Chapter 5, General Formalism of Quantum Theory, Section 5.1.2, he states the following rule. Rule 1. The predictions of results of ...
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How do you experimentally realise the operator of the form $\hat{A}\hat{B}+\hat{B}\hat{A}$?

I was reading a paper in which the authors use the operator of the form $\hat{A}\hat{B}+\hat{B}\hat{A}$ and it is implied to be experimentally realisable. (i.e either creating an apparatus to ...
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LOCC vs. separable measurements

I am trying to understand a difference between LOCC and separable measurements. If I get it right in the paper Quantum Nonlocality without Entanglement it was given a set of pure states, which cannot ...
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1answer
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What is the Effect of Gravity in Preventing Macroscopic Quantum Superpositions?

I am trying to understand how gravity might prevent the creation of macroscopic quantum superpositions, such as a Schrodinger Cat state. I am not asking about decoherence. Also, Penrose has a great ...