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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Is there a definition of relative Renyi entropy?

Is there a Renyi entropy analogue of ``$H(X \vert Y)$" ? If yes then is there any known meaning to that? Googling around I found a few different notions, equation 18 here, ...
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What is the condition for local operations on bipartite entangled state?

I have an entangled state between Alice and Bob $|\psi\rangle_{AB}$ ( both Alice and Bob have states in Hiblert space of dimension $n$ ). Alice and Bob can only perform local meaurements. I assumed ...
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What is known about Renyi entropy of a probability density function?

I see most discussions about Renyi entropy to be using either of these two kinds of definitions, for $\alpha > 0, \alpha \neq 1$ $H_{\alpha}(p_i)=\frac{1}{1-\alpha}\log \sum p_i^{\alpha}$ for a ...
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Information and entaglement via determination of the first's system state with interaction

Could we have entangled systems (microscopical or macroscopical) and construct a way of altering the state of one of the two entangled parts (let's say by Alice) via interaction and thus making the ...
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Was quantum mechanics made to fit the Bell violations or they just happen to fit them?

Entangled bipartite states can violate the CHSH inequality upto $2\sqrt{2}$ with suitable measurements. Is it that in nature we don't witness violation of CHSH more than this and quantum mechanics ...
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How to apply Controlled-NOT gate?

look at the figure below it is about an example to multiply two qubits by 3 Controlled gate to get the SWAP operation .. I'm trying to follow this step-by-step but I couldn't know how this is ...
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Measuring quantum entanglement in paper by Ma et al [duplicate]

Looking at the links below, could somebody please explain how entanglement between Alice and Bob particles is established/deduced from Victor's choice/measurement? I understand that Alice and Bob can ...
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What is quantum mysticism? [closed]

Most of my questions on stack physics exchange are being commented on as being quantum mystic. The questions I ask are basically related to device independence and how local hidden variable theory ...
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Which bipartite entangled states violate the CHSH maximally?

I am reading the device independent outlook on quantum mechanics. Here the author gives a proof that for two qubit system maximally entangled states violate the CHSH inequality maximally that is upto ...
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Do we have algorithms that are polynomial on a Q-Computer but not poly. on a classical Computer?

I am currently reading “Introduction to Topological Quantum Computation” by J.K. Pachos. In the book the author mentions that Shor’s factoring algorithm is polynomial (with regard to the complexity ...
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How to calculate fidelity of a specific quantum channel?

Let $\gamma$ be a completely trace preserving operator such that $\gamma(\rho) \to (1-\epsilon)\rho+\epsilon(|\phi\rangle \langle\phi|)$. Here $\rho$ is density matrix of two dimensional hilbert space ...
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What do we mean by Unitary Dynamics in Quantum Computing?

In the afterword to the Tenth Anniversary Edition of the book Quantum Computation and Quantum Information the authors say: For many years, the conventional wisdom was that coherent ...
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Delayed choice experiment and weak measurements

My questions relate to this recent delayed choice experiment with a helium atom: http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys3343.html Is there anyway whatsoever - directly or ...
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How to write a generic density matrix for multi qubit system

I was reading the paper device independent outlook on quantum mechanics. The author defines a generic two qubit density matrix as $$ \rho=\frac{1}{4}\left( I \otimes I + \vec{r_{\rho}} \cdot ...
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Quantum Teleportation between Entangled Qubits

My question refers to the experiment described in this article: http://www.sciencemag.org/content/345/6196/532.abstract Here's a popular science description: ...
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101 views

What is the meaning of integrating over the state space?

If $\lvert\psi\rangle$ denotes the state space corresponding to a qubit, then what is the meaning of the $$\int d\psi$$ where the integral is over whole state space of a qubit? How do I evaluate it? ...
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28 views

What does conditional probability mean in case of two party system where no-signalling holds?

Background to the problem: I have two parties ( spatially separated ) $A$ and $B$ each having a set of measurements $M_A$ and $M_B$ respectively, and set of outcomes $m_A$ and $m_B$ respectively. Let ...
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80 views

How powerful would a quantum computer need to be to break RSA encrytion codes?

First off, just to reassure everybody, I have no motive other than pure curiosity for asking this question. I don't want my bank account hacked any more than you want the same done to yours. My ...
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Mermin Inequality

Suppose I want to calculate the maximum of Bell inequality for three parties system. In this case I will have 6 measurement directions (unit vectors). It has been done in the paper PHYSICAL REVIEW A ...
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How asymptotically efficient is quantum state tomography of a flat qubit?

Suppose you receive $n$ copies of a qubit rotated by an unknown angle. That is to say, you're given the state: $$T(\theta) = \left(\sin(\theta) \left|0\right\rangle + \cos(\theta) ...
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34 views

What are the characteristic of a unitary acting on a composite system?

I have a composite system AB, initially the state of the system is $|\psi\rangle_A \otimes |\phi\rangle_B$. $U$ is an operator acting on the composite system. If even after application of the operator ...
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What is Bell Measurement (wrt its use in quantum teleportation)?

Bell measurement is joint quantum-mechanical measurement of two qubits, so that after the measurement the two qubits will be maximally entangled. According to the answer here, this is acceptable ...
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Why does an anti-unitary operator have to be both left and right anti-unitary?

I am reading about anti-unitary operators from here anti-linear operators. They have defined an anti-unitary operator $$K: |\psi\rangle \to K|\psi\rangle$$ $$K(\alpha|\psi\rangle+\beta|\phi\rangle) ...
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How are anti-unitary operators applied?

I was reading about anti-unitary operators from Wikipedia. They give an example of an anti-unitary operator: were $K$ is complex conjugate operation. $\sigma_y$ is defined with respect to two ...
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What kind on transformations can be applied on density matrices?

Completely positive trace preserving maps ( CPTP ) transform a valid density matrix to another, then why do we only talk about unitary transformations on density matrices ( $\rho \to U\rho ...
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Is device independence and non local games only studied for cryptography purposes?

I started reading device independent approach on quantum mechanics from here Device Independent Outlook on QM. I am still a beginner in this field, but out of interest I just browse papers related to ...
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decoherency of number states

The meaning of decoherency is: When a nonclassical state, transitions to a classical state. It means that the state lost it's phase information. I can't understand how it happens in number states.
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How does one measure the state probabilities of a 2 level Hydrogen atom qubit?

I have been struggling with trying to understand how one could measure the probabilities of a superimposed Hydrogen atom's two energy states. Say the atom is in a superimposed state: Psi = a|0> + ...
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Why is Quantum Information Theory till now not ready for a Quantum Computer? [closed]

Quantum Information Theory is a very much discussed term in Theoretical Physics, Information Science and Mathematics. Till now nearly 50 years after first definitions we have no usage in Computer ...
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63 views

Is a zero-energy universe necessarily also a zero-information universe?

The universe needs to be near zero energy to not crumple in on itself. Under the same logic, does it also need to have near zero information content to prevent the passage of time from burning it to ...
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What counts as information?

What counts as information? In e.g. the EPR experiment why is one entangled particle knowing instantaneously the state of the other not counted as 'information'. Edit Following a discussion in the ...
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'schrodinger' picture in measurement based topological quantum computation

I am looking at the measurement processes in topological quantum computation (TQC) as mentioned here http://arxiv.org/abs/1210.7929 and in other measurement based TQC papers. Let's say I start with ...
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Why we compare expectations sometimes and sometimes probabilities?

I am reading device independent approach on quantum mechanics from here device independent outlook on quantum mechanics. In here the author introduces local variable theory. Then he give two results. ...
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Long distance entanglement maintenance for quantum teleportation

Quantum teleportation starts out with an EPR pair being distributed among Alice and Bob i.e. one qubit goes to Alice and the other goes to Bob. The problem that I see in this scenario is the state of ...
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which are the non-abelian anyons for universal quantum computation

I am trying to get a list of non-abelian anyons that can be used for universal quantum computation by implementing gates via braiding. I found that Majorana fermions and para-fermions (not sure about ...
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Operational difference between separable, entangled, PPT and NPT states

Given two parties Alice and Bob, a state $\rho_{AB}$ is said to be separable if it can be written as $\rho_{AB}=\sum_i p_i\rho^i_A\otimes\rho^i_B$, with $p_i$ being probabilities and ...
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Why reduced density operator being same is necessary sufficient for no signalling?

Problem Statement : Two parties $A$ ( Alice ) and $B$ ( Bob ) ( in order ) share an entangled pair $\frac{1}{\sqrt{3}}(|00\rangle+|01\rangle +|11\rangle)$. Bob does a measurement in basis $\{ ...
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Recommendations for Advanced Books on the Interface between CMT and Quantum Information

I am looking for a book/review article/website which covers applications of condensed matter theory to quantum information. In particular, I am interested in such topics as a mathematical description ...
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How to see validity of no signalling principles in case of entangled parties?

From what I understood the density operator $\rho$ is a mathematical tool which tells us about the probabilities of getting a particular output after measurement. I have two parties entangled with ...
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How can a correlation be teleported?

I am aware of how a qubit can be teleported from party $A$ to party $B$ if both the parties are entangled. Is there a similar way to teleport a correlation? I googled and found Entanglement ...
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What is the difference between general measurement and projective measurement?

Nielsen and Chuang mention in Quantum Computation and Information that there are two kinds of measurement : general and projective ( and also POVM but that's not what I'm worried about ). General ...
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Entanglement of Tripartite States

Is there any simple algorithm to determine the entanglement of a tri-partite state? In particular, what is the proof for entanglement of $ |GHZ\rangle $ and $ |W\rangle $ states? $ |GHZ\rangle ...
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Is macroscopic causality an issue in the context of certain quantum experiments?

In order to formulate my question properly I need to explain a few things. Cramer_Herbert Zych_Brukner Reference 1. - John Cramer, Nick Herbert, "An Inquiry into the Possibility of Nonlocal Quantum ...
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Is Interpretation of state vectors and density matrices according to Frequentist or Bayesian interpretation of probability?

I asked a question on math stack exchange what does probability mean. I did not know about Frequentist and Bayesian interpretation of probability previously. So according to which interpretation are ...
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What does it mean physically if pentagon identity or hexagon identity doesn't have any answers?

Imagine I write a fusion rule for some anyons on a paper. Then I try to solve pentagon identity and hexagon identity, imagine finally I find out for example Hexagonal equation doesn't have any ...
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are there known fundamental limits of quantum computer scaling?

Quantum computers provide exponential speedup relative to classical computers. However, it is empirical fact that increasing of number of qubits makes the the computer harder and harder to keep ...
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Why schmidt decomposition only holds for two component composite systems?

According to schmidt decomposition any pure state belonging to a composite system $AB$ can be written as $|\psi\rangle = \sum_i \lambda_i |i_A\rangle |i_B\rangle$ where $\lambda_i$ are non negative ...
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How to physically prepare a qubit in a certain state?

I earlier asked the question about definition of a qubit. From it I understood that its the experimental setup that actually defines the qubit. But I don't get it's physical realization. How a qubit ...
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Question regarding entanglement

Could the paradoxical nature of entanglement be the result of a change in the nature of quantum properties, or an incomplete understanding of such, instead of non-locality?
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Toric Code and the String-Net Model

What, exactly, makes the toric code a quantum error-correcting code as opposed to any other string-net model? What makes it special? The way I understand it, it's a normal string-net model on a torus, ...