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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Quantum entanglement and information transfer

I know it was stated many times that information transfer using entanglement is not possible and I am most probably wrong but I would be glad if you can at least point me to where I am mistaken. If ...
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A question on qbit

As layman I couldn't yet conceive how one could know that a given qbit, if it is said to have certain probability amplitudes alpha and beta, actually have (fairly accurately) these amplitude values, ...
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Where is the one-qubit operation in this graph?

Where is the one-qubit operation on right subfigure of Fig 4.8?
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Does this quote from my textbook imply that not all states are superpositions?

I read this at a book; The difference between bits and qubits is that a qubit can be in a state other than $|0\rangle$ or $|1\rangle$. It is also possible to form linear combinations of ...
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Clustered-Entanglement thermodynamics

I want to understand the tools required to do calculations in entanglement thermodynamics in general, but I'm considering first how to model entanglement mathematically in terms of many-body systems ...
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99 views

Understanding measurement based quantum computing

I am currently reading M. A. Nielsen's review on Cluster-state Quantum Computation (Nielsen, Michael A. "Cluster-state quantum computation." Reports on Mathematical Physics 57.1 (2006): 147-161.). ...
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Is CNOT a one-qubit or two-qubit operation? [closed]

Is CNOT a one-qubit or two-qubit operation? It has inputs of two qubits and needs two quantum wires.
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Nobel Prize challenge: Could neutrinos be used for quantum information processing?

We know that two scientists, Takaaki Kajita and Arthur B. McDonald, working on Neutrinos have just been awarded the 2015 Nobel Prize in Physics. My question as a Nobel Prize Challenge Question is: ...
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Popular books on cutting-edge physics [closed]

I watched this video, in which different types of topics are discussed like Quantum Information, Quantum Computation, Universe as hologram, Hidden information, Quantum Entanglement, Black holes and ...
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Applying a NOT gate on an entangled qubit of equal amplitudes of $|1⟩$ and $|0⟩$

I am currently listening to lecture 11 (Superdense coding) of Michael Nielsen's Quantum Computation for the determined series. It starts off with 2 qubits, both in the $|0\rangle$ state. Then the ...
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45 views

Difference between operators used to represent quantum gates vs that to represent physical observables?

I have learnt that informations about a physical observable property is buried in the state vector of a quantum system. To get the possible value of a property all we need to do is multiply the state ...
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40 views

What determines the anyon fusion outcome (if several are possible)?

Given the fusion rule (for anyons) "A x B = 1 + C" it is possible for the anyons A and B to fuse to the vacuum "1" or to fuse to the anyon C. What determines what will happen if they fuse? Is the ...
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Measuring Qubits in Entangled State

If two people (Alice and Bob) are given a state $|\phi^+\rangle = \frac{1}{\sqrt2}(|00\rangle + |11\rangle)$ from a list of states, then Alice can read the first qubit, see if it is 0 or 1, and send ...
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Quantum teleportation of a macroscopic object

suppose that quantum teleportation of a macroscopic composite object, like say, a grain of rock, was physically and technologically possible The traditional teleportation scheme would have to prepare ...
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why shielding surfaces may not be continuous?

Books say that screening surfaces need not be continuous to work but my question is that counter electric field produced won't be less such as faraday cages is not countinuous then how it works??
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Definition of the “support” of the reduced density matrix

Some of the papers in condensed matter physics use the word "support" (space). For example, the following papers use the support especially for the reduced density matrix. ...
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Receiving Multiple Copies of a Qubit

Suppose you are given one of two non-orthogonal qubits randomly: either $|\phi\rangle$ or $|\psi\rangle$. Using a particular distinguishing procedure, you can guess which of the two qubits you were ...
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59 views

Using LOCC Operations to Distinguish Bell States

Example taken from http://www.quantiki.org/wiki/LOCC_operations: Suppose I have two Bell states $|\phi^+\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$ and $|\psi^+\rangle = ...
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Indirect probing of entanglement measures

Entanglement is a purely quantum mechanical phenomena that e.q. increase precision of quantum interferometers, atomic clocks or have application in quantum computing. There are many "measures" of ...
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How can I derive Fusion-Rules for Anyons?

I am reading Pachos "Introduction to Topological Quantum Computation". Pachos writes that a model for anyons consists of a list of all anyons and a fusion rule for them. Given a model with anyonic ...
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How could memory be organized in quantum computers?

A classical computer has a memory made up of bits, where each bit represents either a one or a zero and its implemented by two-state transistor logic. However, a quantum computer maintains a sequence ...
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Separability of density operators on tensor product spaces

Consider a composite system $\mathcal{H}=\mathcal{H}_{A}\otimes\mathcal{H}_{B}$ where $\mathcal{H}_{A}$ and $\mathcal{H}_{B}$ are Hilbert spaces of constituent components (say two qubits). Let ...
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148 views

Book question positive square root on quantum operator

On p.86 Section 2.2.4 of the Quantum computation and quantum information book by Nielsen, $M_{o}$ is defined as the positive square root of the positive operator. Is the "positive square root" ...
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Purity of a maximally mixed quantum state

The trace of the square of a density matrix(which is also called the Purity of the quantum state) is (lower)bounded by the inverse of the dimension of the Hilbert space and (upper) bounded by 1. I ...
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Quantum probability [duplicate]

I would like to get an idea of what "quantum probability" means and how it differs from classical frequentist or Bayesian probability. Can anyone enlighten me in non-too-technical terms?
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Purification of Gaussian states

Let us consider an $n$-mode Gaussian state $\rho_g$ (mixed state) in the Bosonic Fock space $\Gamma(\mathbb{C}^n)$. My question is, can we have a purification of $\rho_g$ which is itself Gaussian? ...
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dependence of braiding matrix element on the fusion product of anyons

In the case of Majorana fermions (MFs), one knows that if one braids MF $a$ with MF $b$, then braiding matrix element $R^{c}_{ab}$ depends on the state $c$ which is the fusion outcome of $a$ and $b$. ...
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Discrimination of Bell states on unkown qubits

Consider the scenario where Alice is given three qubits, and promised two of them are maximally entangled Bell state $\in_R \{\frac{|00+11>}{\sqrt{2}}, \frac{|00-11>}{\sqrt{2}}\}$ and one of ...
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Does quantum teleportation lead to a change of quantum states?

Particles A and B are entangled, quantum information of particle A is transmitted to particle B via quantum teleportation, does particle B's quantum state change? What happens to particle A's quantum ...
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Eigenvalues of density operator under quantum channel

Consider a 2x2 density operator $\rho$ with eigenvalues $\lambda_{1},\lambda_{2}$ (where $\lambda_{1}\geq\lambda_{2}$) evolving into a 4x4 density operator $\rho'$ (with eigenvalues ...
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Quantum and classical scaling of memory

In the literature of Quantum Computation (QC), when discussing the simulation of quantum systems, one usually comes across comparisons with the classical digital analog such as: "Classically the size ...
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Why can you only measure velocity or location in a particle?

I was talking to a family friend in the field of optics at a quantum scale (not sure the proper name for this) and he was explaining to me why you can only determine either the velocity or location of ...
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How many parameters are needed to specify a quantum state?

We have a spin state \begin{align} \ |{\Psi}\rangle=a_u|{U}\rangle+a_d|{D}\rangle \end{align} where $|U\rangle$ and $|D\rangle$ are $up$ and $down$ basis vectors, and $a_u$,$a_d$ are their complex ...
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Ordering of eigenstates in the quantum adiabatic theorem

Suppose we have an 'initial' Hamiltonian $H_{i}$ whose eigenvalues are all non-degenerate, which we order as follows: $ E^{0}_{i} < E^{1}_{i} < \dots < E^{N-2}_{i} < E^{N-1}_{i}$ Suppose ...
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Entanglement entropy for stabilizer states

A stabilizer state for a stabilizer set S for a system of qubits can be written as $\rho=\frac{1}{2^{n}}\sum_{g\epsilon S}g$ . If we take a bipartition A-B of our system and partial trace over A, ...
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The simulation hypothesis and the halting problem [closed]

I have a basic knowledge of classical and quantum information which isn't remotely enough to make me an expert on the subject. That being said, reading some questions about the simulation hypothesis ...
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Photons have a property from the matter they interact with?

When a photon leaves its source and hits our eye, our brain sees the source of the photon (Like a lightbulb or a star). When a photon is ejected from its source and bounces off of an object we see the ...
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Superposition in Quantum Mechanics

First of all, let $V$ be a vector space over the field $\mathbb{F}$. It is possible then to show, by Zorn's Lemma that there is a basis for $V$. The main point is that although basis are quite ...
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What is Quantum Mechanics really about?

This question might sound very silly, so I'm sorry if that's the case. I'll try my best to make my point clear here. Before explaining, just to make clear, I'm not confused because of the Math ...
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What quantum measurement formalism is easiest to implement physically?

As part of my studies and research, I have learned to work with three different measurement formalism which I define to avoid any ambiguity with the nomenclature: General measurements, which are ...
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What is special about quantum entanglement? [duplicate]

Get two pieces of paper. In secret write the same number on both papers. Transfer one paper to the Moon. Look to the paper which is left at the Earth. Voila! We know what is on the Moon paper. The ...
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The black hole paradox

I recently read in the news that Stephen Hawking claims to have solved the Black Hole Information paradox. I researched a bit about the paradox and the research that Stephen Hawking did to solve it. ...
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Are the produced entangled photons in type II SPDC always in phase?

Are the produced entangled photons in type II SPDC (spontaneous parametric down conversion) always in phase? In Kim's delayed choice quantum eraser, two entangled photons are produced by a BBQ ...
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Can one detect a single photon through measuring its impulse/momentum on a mirror?

Can one detect a single photon through measuring its impulse/momentum on a mirror? If the answer if YES or theoretically possible, photon path and interference fringes can be detected simultaneously ...
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Relationship between Toffoli gate and Deutsch gate

I am reading a book called "Quantum Computing for computer scientists" and stuck at this exercise for a day. Bellow is the description of Deutsch gate: And here is the description of Toffoli ...
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Can someone explain why this QM FTL communication setup is wrong?

So I thought I understood the double-slit experiment and EPR paradox until this setup occured to me. It combines EPR entanglement with "which-way" double slit setups. I know it must be wrong, but I ...
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Light (Laser) operated optical switch

I have searched the web and found that optical switches are activated either through mechanical or electro/magnetics. Making them sort of Optomechanical or optoelectronic switches. Lets exclude ...
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How to understand the three state descriptions from 3 observers of such a system?

Assuming an EPR pair $\psi_{AB}=|00>+|11>$ and three observers Alice, Bob and Charlie, who can communicate with each other but they do not do it for the time being. Alice with a qubit $|0>$ ...
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Why these quantum information processing protocols have a similar structure?

I noticed for some quantum information processing protocols such as teleportation, entanglement distillation, bound entanglement activation, entanglement catalysis, the protocols seem to have a ...
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finding generic Quantum circuits for k-local hamiltonians

Let $P_n$ denote the Pauli group on $n$ qubits (think of n as a large number). Let $G=\left<g_1,...,g_n\right> < P_n$ be some abelian subgroup such that each $g_i$ acts on at most $k\ll n$ ...