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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The $n$-th root of the NOT gate

I simply can not find material containing facts about the $n$-th root of the NOT gate and it's realization in Q.M. and also in C.M.. Does anyone have material? A comparison of the $n$-th root NOT ...
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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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What is Absorption Grating

I came across the word "absorption grating" in a review article. I googled it tried to find out what it means but couldn't. Could you explain it to me?
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Increase path length for a photon with two mirrors vs optical fiber

Is it possible to build two mirrors in vacuum such that a light pulse (single photon) gets refleted many times in order to increase the distance it travels? For example like that? Say I want the ...
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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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What are typical error rates of quantum computers?

I read in an article that in order to perform error correction on a quantum computer there can only be one error per 10.000 calculations (=unitary transformations). This sounds pretty high but how ...
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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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Wigner-Yanase skew information [closed]

I am reading Eric Carlen's paper on Trace Inequalities and Quantum Entropy. I am currently reading about the Wigner-Yanase skew information which is defined as: $$I_{WY}(\rho)=-\frac{1}{2} ...
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Can a free falling observer localize the event horizon by calculations?

I'm think that in general relativity we can always pass the one curve in one coordinate system for another coordinate system. My intuition say that the free falling observer locate the event horizon ...
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What's the Cause of Quantum Entanglement? [duplicate]

What is the cause of quantum entanglement? When two particles become entangled what property of them basically changes as to establish a link between them and how the information is exchanged between ...
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About Shor's error correcting algorithm

In this paper, http://arxiv.org/abs/1301.4504 in equation 4.1 in what sense are the two states a "9-qubit state"? I did not understand this counting. Can someone explain what are the different $X_i$ ...
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Number of Parameters Required to Specify n-Qubit States and Quantum Operations

How many parameters are required to specify the density matrix of a $n$-qubit system, and how many parameters are required to specify a quantum operation (completely positive maps between states) on ...
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Meaning of the Reduced Density Operator

I am confused about what it is exactly that a reduced density operator describes. To illustrate, I came across the following seemingly paradoxical argument. Consider a biparte system $AB$, described ...
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Landauer's principle vs Wien's displacement law

Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of kTln2? If it is so, can we also argue ...
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830 views

Quantum circuit, two control not gates

Consider the quantum circuit in the picture below: We have a Hadamard gate followed by a CNOT gate, this puts the 2nd & 3rd state in the bell state $\beta_{00}=\frac{1}{\sqrt2}(\mid 00\rangle ...
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Qubit initial state

Suppose that a qubit in an initial state that we don't know was measured, and the result was 1. Is it possible to know the initial state of the qubit by the result measured? And if the result would ...
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Can a classical 50/50 probability be distinguished from a quantum superposition with a single measurement?

$\renewcommand{\ket}[1]{|#1\rangle}$ A "false" (equally superimposed qubit) is created by mechanically firing with 50/50 probability a resonance photon at a Hydrogen atom qubit in the ground state. ...
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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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Condition for quantum error correction based on encoded states

I am trying to understand the properties of quantum error correcting codes. Consider a quantum code on a lattice, with the property that a given region $R$ is correctable (for any error localized to ...
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102 views

When an unitary operator is a quantum gate?

Quantum gates we use like X, Y, Z, H, CNOT, etc. are all unitary. When an arbitary unitary operator can be considered as a quantum gate?
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Can changing representation change the meaning of density operator?

I had posted a question What is the actual meaning of the density operator?. After that I understood that if I have the expression of a density operator $$\rho=\sum_{i=1}^{i=k}p_i|\psi_i\rangle ...
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Quantum mechanics: compatible observables

I am confused about something. If (all what I will write are operators) $x$ is compatible with $p_y$ that means they have the same eigenvectors. However, $x$ is compatible with $y$ which means they ...
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A universe of a finite but increasing number of states of motion?

A limitation of the geometric models of universe is that space locally is considered as a volume, whilst volume is a part of a selected system of inertia. Wouldn't it be more adequate to consider (the ...
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Ultrafast photon detectors

Are there already photon detectors that can resolve the shape of photons in the time domaine if it looks like below where the two humps are separated by a time of the order of femtoseconds? What I ...
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If I have created a polarization-entangled photon source, would I be able to create a qubit?

In my laboratory, I have already obtained a polarization-entangled photon source. It was created via the Spontaneous Parametric Downconversion process of 2 BBO crystals. The next question is that ...
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Numerical Computation of Linbald Equation [closed]

Can anybody suggest me a good algorithm for the time evolution of the reduced density matrix using Linbald equation. My Hamiltonian is time dependent. I am aware about Qotoolbox and Qutip. I have ...
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Can matter be converted to information?

I know that matter can be converted to energy through E=mc^2. I also know that engery can be and has been converted to information through Landauer's principle (with Maxwell's demons). Does this ...
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Understanding of measurement in quantum mechanics?

I have a computer science background with basically zero physics background. I am trying to gain a 'high-level' understanding of quantum mechanics to aid me in some computer science work. Is my ...
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Question on quantum computation, entanglement and speed of information propagation

Imagine a following thought experiment. Suppose we have a large amount of entangled particle pairs, several million or billion. Now suppose there are two observers, each carrying one member of ...
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Adiabatic quantum Hamiltonian of variable dimension

Is adiabatic quantum Hamiltonian of variable dimension possible? This is very hypothetical and I am afraid may not have enough merit to belong to this forum. I would still like to elaborate. Here is ...
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Question about entangled states

I have a question about entangled state. Suppose I consider the entangled state $\frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$. I saw an argument for how measurement of the first bit is affected by ...
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58 views

Violation of the Normalization Constraint?

Say we have two qubits $|a\rangle$ and $|b\rangle$ both initialized to $|0\rangle$. We then apply the rotation gate $R_{x}(\frac{\pi}{2})$ of matrix representation $\left( \begin{array}{} ...
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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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Why does a measurement on one qubit force another one into a given state in Simon's algorithm?

This comes from trying to understand the "Simon's algorithm". So we have a set of $2^n$ kets $|x_i >$ one each for $i \in \{0,1\}^n$. Each $x_j \in \{0,1\}^n$. And we have the further constraint ...
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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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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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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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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 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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Entanglement entropy in (1+1)d field theory with dynamical critical exponent $z>1$

It was well known that for (1+1)d CFT(z=1) case, we can use the tool of conformal map to derive the formula of entanglement entropy for a finite interval: S ~ $c \log L$. L is the length of the ...
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Distinguishing density operators with the same diagonal elements

If I have two sources of qubits and one source produces the density matrix: $$\rho_1 = \begin{pmatrix} 1 & 0 \\ 0 & 1\end{pmatrix}$$ and the other source produces: $$\rho_2 = ...
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The Simon's Algorithm, confusing equation

I'm approaching the Simon's Algorithm and have troubles with understanding a logic in an introduction. Above the eq. 6.5.4 they introduce that set S which has 2 elements. As far as I understand, ...
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Would a pair of independent quantum coin tosses be perfectly anti-correlated?

Background Suppose we attach a button to an electronic flip flop such that an LED will toggle when we press the button with 50% probability, where the source of the randomness is a quantum event, ...
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If perfect maximal entanglement is never true, does a remainder invalidate the monogamy of entanglement?

If something is only very nearly (and/or observed to be) maximally entangled, does that remainder allow for a menage trois of hybrid correlation (as it relates to AMPS)?
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A question about entanglement of formation and decomposition

In the answer of this question, the last paragraph says that If you know one decomposition which is optimal for Entanglement of Formation for a given state $\rho$, you can obtain the optimal ...
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What is trivial and non-trivial gate in computing?

My question seems like it should be posted in Computer Engineering section, but below is how I found this word. I was reading the textbook: Quantum Computation And Quantum Information - by Michael A. ...
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What are density matrices and how do they work?

I have looked in Stack Exchange about density matrices but haven't found any answers. What are density matrices and how do they work? What are they used for? (Also, please tell me what is wrong with ...
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“Entangled photons never show interference in the total pattern without coincidence count” implies FTL

In my previous question, the most defended objection to the gedankenexperiment was that "Entangled photons never show interference in the total pattern without coincidence count". Here I show another ...
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Can Quantum Entanglement and Quantum Superposition be considered the same phenomenon?

Quantum entanglement is known to be the exchange of quantum information between two particles at a distance, while quantum superposition is known to be the uncertainty of a particle (or particles) ...
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Change in Shannon entropy of a quantum circuit of Hadamard gate and a loop

The following Q&A about reversible computing is available here. It has listed a number of practical scenarios where a reversible circuit can still be dissipating heat. Let's assume that none of ...