# 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 ...
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....
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 $A$. ...
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 ...
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 ...
0answers
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?
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, $\hat{\Lambda}$ ...
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 ...
2answers
154 views

2answers
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### 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 ...
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 $\mathcal{H}$ ...
1answer
83 views

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 $\left|n,0\right>$ ...
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{...
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 ...
1answer
447 views

### Uniqueness of representing POVM using projective measurement

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