Questions tagged [quantum-measurement]

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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 ...
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1answer
44 views

Exponent of tensor product of operators (context: weak measurement)

I'm stuck on some identity for an exponentiated tensor product of operators, $e^{\hat A\otimes\hat B}$. I'm learning weak measurement, reading the review by Kofman et al (2012, DOI: 10.1016/j.physrep....
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1answer
60 views

Equivalence of POVM and projective measurement

Suppose I have a POVM whose elements are given by $\{M_i^\dagger M_i\}$ such that $\sum_i M_i^\dagger M_i = I_A$. Let it act on some state $\rho_A$. Everything here happens in the Hilbert space $A$. ...
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4answers
116 views

Using conserved quantities to violate the Uncertainty Principle

If I have a two-body system (particles A and B) which I measure the total momentum of, can I measure the position of particle A very accurately and the momentum of particle B very accureately, and ...
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2answers
71 views

When formulating and performing experiments for quantum theory do physicists account for us constantly moving through non-zero energy vacuum?

Problem: If we suppose that each smallest possible point in vacuum is a size of Planck length and that point is somehow affecting that particle with spontaneously emerging non-zero energy, then as our ...
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19 views

Is the Quantum Zeno Effect real? [duplicate]

When a decaying particle is observed using the Quantum Zeno Effect, can the decay of this particle be delayed or prevented?
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1answer
25 views

Comparing the purity of the unconditional state of a system (post projective measurement) with that of the a-priori state

The context of the question: This is exercise 1.9 in the book Quantum Measurement and Control (Wiseman, H. and Milburn, G., 2010). We have some operator acting on our system, $\hat{\Lambda}$ ...
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1answer
84 views

Probabilities in Quantum Mechanics: Measurement Outcomes or More?

In all treatments of quantum mechanics, the probabilistic nature of the theory enters via the Born rule for the statistical properties of the measurement outcomes of some observable. In short, this ...
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2answers
154 views

If the wavefunction is continuous how can the many-worlds be discrete?

Preamble for clarity: The many worlds interpretation is usually used to explain the measurement of a 2 level system ($|0\rangle$ or $|1\rangle$) as: $$\frac{1}{\sqrt{2}}(|0\rangle+|1\rangle)|\text{...
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2answers
83 views

Hard boundary of Heisenberg's uncertainty principle

Heisenberg's uncertainty principle states that we cannot know the position and momentum of subatomic particles simultaneously...but what exactly is the boundary of size of such a particle? Does such a ...
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0answers
40 views

What is the characteristic of a property?

Background: The following two observations are , in my understanding, pretty much accepted in quantum theory: Location is a property which is not preexisting but is established by measurement. It is ...
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0answers
160 views

Experimental time-series for quantum particle-in-a-box or simple harmonic oscillator?

I would like to see experimental results for repeated measurement of a single-particle, quantum system that is approximately either particle-in-a-box or simple harmonic oscillator. If particle-in-a-...
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3answers
57 views

Quantum Zeno effect and Heisenberg uncertainty principle

The Heisenberg uncertainty principle forbids the existence of static atoms But in the Quantum Zeno effect observing atoms makes them stand still does the Quantum Zeno effect violate the Heisenberg ...
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3answers
2k views

Do we or do we not observe (measure) superpositions all the time?

This is not a duplicate, the other answers do not specifically solve the contradiction, nor do they give an exact answer. I have read this question: Are we so sure about superposition? How do we ...
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40 views

Measuring a quantum state for entanglement

I am new to the concept of quantum entanglement and measurement. The scarce knowledge about them that I have is As soon as you measure, the system collapses into one of the vectors of the eigenbasis ...
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66 views

Does this thought experiment proves that Standard Quantum Mechanics and Pilot Wave Theory make different predictions?

Here is a thought experiment that is supposed to show that standard quantum mechanics and pilot wave theory do not make the same prediction : Take the double slit experiment, and add a detector in ...
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21 views

Unitary operators after measurement?

I am trying to teach myself quantum computation, and have managed to confuse myself by asking the following: After a measurement operation has been performed on a qubit, can some unitary operator put ...
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2answers
92 views

Heisenberg's Uncertainty Principle about electrons [closed]

According to Heisenberg's Uncertainty Principle when we shoot or project photons on an electron from one side then there is 100% possibility that there is some error in calculating the position and ...
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2answers
56 views

Why can't measuring a system $B$ change a correlated system $A$?

I am reading Quantum computation & quantum information from Nielsen & Chuang. On page 187, he talks about the principle of implicit measurement. He says that if we have two system $A$ and $B$...
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1answer
92 views

Spatial wave-function of a single photon and its measurement

In the last decade there were several papers claiming that they've measured a "transverse quantum state" / "quantum wave-function" / "spatial Wigner function" of a single photon: Measurement of the ...
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1answer
60 views

Wave function. Measurement of the absence

Imagine we have a particle in an eigenstate of a Hamiltonian, as time passes it will remain in that state. We suppose in this question that the position can take a continuum of values. If we measure ...
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1answer
79 views

In QM does the observer really perturb the system to make a measurement?

It is commonly taught in introductory QM courses that in order to get to know the position or momentum of a particle, be it by "sending a photon" or similar experiments, the measurement necessarily ...
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2answers
52 views

Wavefunction collapse in Stern-Gerlach experiment

Consider silver atoms coming from an S(X) apparatus, after S(X) apparatus we place an S(Z) apparatus $$ |\mathrm{SX+}\rangle= \frac{1}{2}|\mathrm{SZ+}\rangle + \frac{1}{2}|\mathrm{SZ-}\rangle, $$ ...
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2answers
49 views

Wave function collapse and a general wave function

Action of an operator on a state vector collapses the wave function to any of the eigenstate of that operator , So we get resulting state of the system as some base ket. But mathematically action of ...
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1answer
82 views

Is the Process of Projection of a Generic State Onto a Subspace Impossible? [closed]

I can define (in a standard way) the process of projection of a generic state onto the subspace $\mathcal{G}$ as a process that takes a generic state $|\psi\rangle$ of the Hilbert space $\mathcal{H}$ ...
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1answer
83 views

LOCC vs separable measurement

Could you give me an example of a measurement which is LOCC (Local Operations Classical Communication) but not separable? Or better, one which is separable but not LOCC? Given an ensable of states $\...
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1answer
80 views

Is it true that in practice quantum fields are *always* measured by means of coupling to particle detectors?

I'm watching some lectures on Relativistic Quantum Information and in one of them - available here - the instructor is talking about measuring quantum fields. He then says basically that in real ...
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95 views

Measurement problem: Origin of probabilities in Many-Worlds Interpretation

As far as I can tell there appears to be an active group of academics (including the likes of Sean Carrol) who believe in the Many-Worlds Interpretation of quantum mechanics, but feel that the origin ...
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2answers
180 views

Energy measurement from position eigenstate

Given that the eigenstates of the position operator can be written as $\delta(x-x')$, and suppose we measure a particle in an infinite potential with walls at $x=0$ and $x=L$. I measure the particle ...
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1answer
45 views

Measuring the Momentum of a Quantum System using Position Measurements

A basic way to measure the momentum of a charged particle is to know that (classically) it follows a circular trajectory in a uniform magnetic field perpendicular to the plane of its trajectory, with ...
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0answers
31 views

Quantum Measurement in Different Basis [duplicate]

How can you make measurements on a qubit that is in a different basis than your measurement qubit? (i.e. You want to measure a single qubit in the $|+\rangle$/$|-\rangle$ basis but you can only make ...
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2answers
100 views

Is this decoherence?

I have a very basic understanting of decoherence (i.e. I,ve read the Wikipedia page), but I was recently reading Heisenberg's The Physical Principles of the Quantum Theory and I came across a thought ...
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1answer
34 views

How in experimental practice does a momentum measurement reduce a state to a momentum eigenfunction?

It's easy to think of ways to reduce the state of a particle to a position eigenfunction (or at least a narrow spread in position space), whether by trapping the particle in a potential well or by ...
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2answers
109 views

What happens when measuring a subsystem of a product bipartite state?

I have an horrible doubt, as I found that in my books the question is never directly addressed. How is collapse defined to work for measures of composite system? I always assumed that when we ...
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64 views

Measurement formalisms - POVM formalism vs Hermitian observables

I am thinking in following way of thinking about measurements in quantum mechanics. Please correct any false statements I may be making below. We start with POVMs. Let our POVM be a set of positive ...
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1answer
32 views

When does a POVM correspond to projective measurements (without ancillas)? [closed]

Consider a POVM given by the following set of elements $\{E_i = M^\dagger_i M_i\}$ such that $\sum_i E_i = I$ Wikipedia says that the condition $M_i^2 = M_i$ implies that the measurement is a ...
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1answer
27 views

Confusion of measuring two quantities on a quantum system

Let's say there are two observables corresponding to two operators A and B, and let's say my system is in a state Phi where with probability 1 if I measure A I get 3 (let's say 3 Joules), If I ...
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227 views

Conceptual understanding of operators in QM

Do operators in QM represent in some fashion the action of the measurement apparatus on a state being measured? Usually operators in QM are introduced as abstract transformations whose eigenvectors/...
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1answer
92 views

Kraus operators of a POVM

I would like to know how one finds the Kraus operators of a channel corresponding to a POVM. Consider a POVM of the form $M_i$ such that $\sum_i M_i = \mathbb{I}$. I can represent this by a quantum ...
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2answers
44 views

Is there experimental evidence for the irreversibility of the collapse of the state of a system due to a measurement?

It has been pointed out by one of my professors that, in quantum mechanics, the collapse of the state of the system by measurement is not time-reversible, i.e once the system collapses due to a ...
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26 views

Measurement in QM [duplicate]

I understand that measurement collapses the wave function of a particle, but what exactly does measurement mean here? In other words, how is a measurement defined? Does it imply interacting/disturbing ...
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2answers
72 views

Doing Stern-Gerlach experiment without blocking the atoms in the $-x$ direction

In quantum mechanics, in general, it is stated that the act of measurement changes the state of the system. For example, consider the following Stern-Gerlach setup; A beam of silver atoms first ...
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1answer
71 views

Heisenberg uncertainty principle meaning and using it to determine approximate sizes confusion

I was always under the impression that the Heisenberg uncertainty principle tells us to what precision we can make a momentum measurement given a position one. However, when doing quantum mechanics ...
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1answer
74 views

Quantum computing explanation for the quantum Zeno effect

I'm playing around with the IBM Q to demonstrate the qunatum Zeno effect. If we prepare a qubit in the $|0\rangle$ state and apply 5 consecutive $R_y(\pi/5)$ gates, we should end up in state $|1\...
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1answer
107 views

Projective measurement using two mode squeezed state?

Let me define two mode squeezed states as $ \left | \xi \right>_n=\exp\left(\xi \hat{a}^\dagger \hat{b}^\dagger-\hat{a} \hat{b} \xi^\star\right)\left | n,0 \right>$ where $\left|n,0\right>$ ...
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1answer
76 views

References in diffusion of quantum state

I would like to know if there are books, articles or any other type of references where a (heuristic) derivation of the equation: \begin{eqnarray} \textrm{d}|\psi(t)\rangle=-\frac{i}{\hbar}H_{\textrm{...
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1answer
61 views

Measurement in QM - positivity criterion

Let me describe my understanding of measurements in quantum mechanics before getting to the part that I'm unclear about. If we have a quantum state $\rho$ and we do a projective measurement in some ...
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1answer
447 views

Uniqueness of representing POVM using projective measurement

$\newcommand\tr{\operatorname{tr}} \newcommand\ket[1]{\lvert#1\rangle} \newcommand\bra[1]{\langle#1\rvert} $[Skip to the conjecture for a self-contained mathematical formulation of the question.] ...