# Questions tagged [quantum-information]

Quantum information is the study of the informational content of quantum states. The most common object of study is the "qubit", the information in a two-state quantum system such as spin-1/2 or photon polarization.

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### Transitions between states

In the time dependent perturbation theory, transition between two states requires time dependent hamiltonian, V(t). The question is, why the constant or time independent perturbation does is not ...
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### Interaction (Dirac) picture and time dependent perturbation theory

Why in many, if not all, references that discuss the time dependent perturbation theory, they start the discussion with the interaction (Dirac) picture, although, what we need is only solving the time ...
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### Creation $n$-particle states

The n-particle states are defined by the creation operators on the vacuum state orthonormalized How we prove that the n particle states are ? How we prove:
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### Generalised measurements and collapse

My reference is Nielsen and Chuang's "Quantum Computation and Quantum Information". When introducing qubits, $|\psi\rangle=\alpha|0\rangle+\beta|1\rangle$ the authors state that (*)When we measure ...
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### Coarse-graining and probabilities in quantum theory

What is the origin of probabilities in quantum theory if it is not postulated? I mean, how can we interpret Born's Rule as a probability and how does it enter in quantum theory? More than this, I want ...
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### Bit threads as noisy quantum channels

In literature on the holographic duality, recently a new way of looking at the Ryu-Takayanagi formula has been presented, where the minimal surface area has been replaced by the maximum number of "bit-...
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### Is there any definition for $(\hat{a}^{\dagger} \pm \hat{a})^{n}\hat{S} \left | 0 \right>$

Specifically, I want to apply the following operators $\hat{P}^n = \left( \hat{a}^{\dagger} - \hat{a} \right)^n \tag{1},$ $\hat{X}^n = \left( \hat{a}^{\dagger} + \hat{a} \right)^n \tag{2},$ (...
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### Difference between multiphoton signal and, $n$-photon signal state

I was reading decoy state quantum key distribution article, by Lo et.al. (PRL paper , PRA paper). In this article, authors have discussed the possibility of photon number splitting attack due to ...
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### Analyzing a quantum circuit

This is an exercise from Nielson Chuang that I am totally stuck at. First I can't understand why the measurement outcomes are related to the operation on the target bit at all. Second, why is the ...
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### Properties of controlled $z$-rotations

Given $n$ qubit gate of the form c-z-z-z... (shorthand for c-z between qubit 1 and 2 followed by c-z between 1 and 3 and so on up to n qubits) it seems to be possible to find local unitaries which ...
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### Homodyne detection as quantum measurement

How is homodyne detection a quantum measurement? In quantum mechanics, the way I'm used to think about obtaining the expectation value of an operator $A$ when the state is $| \psi \rangle$ is as ...
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### Quantum Entropy Venn diagrams

We know that in classical information theory the Shannon Entropies follow such Venn-Diagram approach to summarize its' relations and inequalities between-marginal,joint and conditional entropies. Is ...
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### Experimental Parameters and Unitary in a specific Boson Sampling Experiment

I tried to reproduce the data of one of the four historically first BosonSampling experiments. While they present the information in a very accessible form, unfortunatly I fail to understand some ...
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### Is it possible to use Bayesian method in improving the measuring conditions and accuracy of an electron's (or photon) momentum and position?

for you who need a definition for what a "Bayesian method" is, and as per wiki's easy definition. it's a method of statistical inference in which Bayes' theorem is used to update the probability for a ...
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### What are good books covering information theoretic approaches to theoretical physics?

I am about to finish my undergraduate studies and am very interested in going into the applications of information theory to either general relativity, or quantum mechanics. However I have been ...
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### Conditional probability between time parameter and operator in quantum mechanics?

Question and Background So I came across a question on conditional probability in quantum mechanics: How is conditional probability handled in quantum mechanics? There's an interesting comment which ...
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### Quantum annealing computing

What is Quantum Annealing and quantum annealing computing and what are its advantages and disadvantages with respect to quantum circuit quantum computing/computers?
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### How to define information flow?

If there are two interacting subsystems A and B, how to define the information flow from A to B in quantum regime and classical regime?
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### Bell's experiment - Why doesnt entanglement violate speed of light? [duplicate]

If i understand this correctly, Bell's experiment is trying to distinct between these two things: 1]Entangled particles obtained their spin upon measurement. 2]Entangled particles had their spin ...
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### Why is the diamond norm called like that?

The diamond norm is a measure of the distance between two quantum channels. If $\mathcal{E}_1$ and $\mathcal{E}_2$ are two channels, then the diamond norm is defined as  \delta^{\diamond}(\mathcal{...
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### I don't understand the no-communication theorem

I don't understand the no-communication theorem. Okay, first I'll say the bit I do understand about it: if Alice and Bob both have two atoms, such that Alice's atom 1 is entangled with Bob's atom 1, ...
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### In layman terms: Why can't Quantum Entanglement be used to achieve FTL Communication? [duplicate]

I've found a lot of questions that try to answer this, but I can't quite understand the answers so far. If you can make two particles be entangled, why can't you force one of them into a pattern that ...
How much information is contained in one qubit? A qubit is defined in Wikipedia as $a\left|0\right> +b\left|1\right>$, where a and b are complex numbers subject to $a^2 + b^2 = 1$. One complex ...