Questions tagged [quantum-measurements]

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Is there any limit to the utility of quantum measurements in the sense of a Lieb-Robinson bound?

So the Lieb-Robinson bound of 1972 derives an emergent maximum speed $v \ll c$ of the propagation of quantum information under time evolution generated by some local Hamiltonian $H(t)$. Basically, ...
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Non-Orthogonal Measurements in Quantum Systems

If we aim to transmit some information using the polarization of photons, lets suppose horizontally polarized = |0> and vertically polarized = |1>. We can decode this information at the receiver ...
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Why is this a correct explanation for the Uncertainty Principle? [duplicate]

There's this really common explanation for Heisenberg's Uncertainty Principle, but I quote it from Classical Dynamics by Marion and Thornton: The wave character of the photon precludes an exact ...
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Ideal Von-Neumann measurement scheme - Do the system states need to be orthogonal too?

This question follows up on this question on the ideal Von-Neumann Measurement scheme - also regarding the way Schlosshauer (978-3-540-35773-4) puts it on page 50 f. In this scheme one has given a ...
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Say, we find two exact systems in a pure state. When Alice does von Neumann measurement on it, the state collapse to a pure state because Alice knows the measurement outcomes. In this case, we have $\... 1 vote 1 answer 70 views Confusion regarding working around with the mathematics of operators acting on the whole tensor product state In 'Quantum Information by Stephen Barnett' (Page 95), we have:$$P(m, l)=\left|{ }_a\langle l|\otimes\langle m|\hat{U}| \psi\rangle \otimes| A\rangle_a\right|^2=\left\langle\psi\left|\hat{\pi}_{m l}\... 2 votes 0 answers 33 views Equivalence of two POVMs in continuous variable system In continuous variable systems, the Gaussian POVM is generally defined as [see e.g. Eq. ($1$) of this article] \Pi_j(\alpha_j) = \frac{1}{\pi} D_{j}(\alpha_j) \Pi_j^0 D_{j}^\dagger(\... 0 votes 3 answers 98 views How do we measure an electron's position by shining light on it? I saw this post which says: The way you do it is just a simple logic. Imagine a tennis ball and imagine you can't really see it, just like you can't see an electron. So the only way you can see where ... • 200 0 votes 0 answers 36 views Calculating Optomechanical coupling rate How do I calculate the optomechanical coupling rate$G$[Hz/m] through thermal motion? I measured the spectrum with a spectrum analyzer which was connected to a rf-source and the laser detector. I get ... • 21 0 votes 1 answer 68 views Does a measurement violate conservation law? [duplicate] (Edited according to the discussion with @naturallyInconsistent. The edited part is highlighted in italic.) We have an experimental bench and we assign a coordinate system$(x,y)$to it. We shall call ... • 256 3 votes 1 answer 143 views How do physicist keep entangled particles entangled while moving them/storing them? Once an entangled particle is measured, it's wave function collapses. From my understanding, any sort of information exchanged to the particles can be considered a mistaken measurement. So how do ... • 61 1 vote 2 answers 122 views How to determine elements of the covariance matrix from homodyne measurements? Suppose we have a source which can repeatedly create an$n$-mode Gaussian state with covariance matrix$\sigma$. How can I use homodyne measurements to completely determine$\sigma$? If the quadratues ... • 505 1 vote 4 answers 83 views Ensemble interpretation and density matrices - why is it impossible to distinguish two equivalent density matrices? I read this in a paper : "... we cannot distinguish them by making measurements because they have the same density matrix". The authors are referring to two different decompositions of the ... 1 vote 1 answer 109 views Why does one need a low resolution when trying to tell$|0\rangle^{\otimes n}$and$|1\rangle^{\otimes n}\$ apart in a coarse grained measurement?

I am looking at a certain measure for macroscopic quantum states, namely the one in http://dx.doi.org/10.1103/PhysRevA.89.012116. I use the notation from https://arxiv.org/abs/1706.06173 (p. 15-16, ...
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