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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Constructing a Toffoli gate with 2-and 1-qubit gates?
I'm looking through Nielson's book on quantum computation and information and in part of it he says that any $C^2(U)$ gate can be constructed from two qubit and one qubit gates. I can't figure out how ...
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Should it be obvious that independent quantum states are composed by taking the tensor product?
My text introduces multi-quibt quantum states with the example of a state that can be "factored" into two (non-entangled) substates. It then goes on to suggest that it should be obvious1 that the ...
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Is “entanglement” unique to quantum systems?
My text shows (sections 0.2 and 0.3) that the joint "state space" of a system composed of two subsystems with $k$ and $l$ "bits of information", respectively, requires $kl$ bits to fully describe it. ...
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Finding all marked element by Grover search(not in superposition)
Quantum search enables square-sped up search for marked element. When there are multiple maked element, grover search provides only superposition of them. If I want to find all the marked elements, ...
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Quantum Circuit, example of the Bernstein-Vazirani problem
This question is regarding the quantum circuit in the picture below.
Suppose we have the set up below, where U performs the operation $U:\mid x \rangle \mid y \rangle \rightarrow \mid x \rangle\mid y ...
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Equivalence of simple formulations of qubit entanglement
I'm reading some very elementary treatments of quantum computation and am unsure about the correspondence among "definitions" of qubit entanglement.
One definition states that (1) the bits of a ...
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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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Computing with qubits [closed]
We have a qubit in the state $|\psi \rangle= √3/2 |0\rangle + 1/2 |1\rangle$, which we want to measure in the $cos \theta\ |\theta\rangle + sin \theta |1\rangle, sin \theta |\theta\rangle - cos θ ...
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Tracing out an observable vs integrating over unitaries
Let $O$ be an observable on a Hilbert space $\mathcal{H}$, and let $B$ be a subset of the spins composing $\mathcal{H}$, and let $\bar{B}$ be its complement. Now define
$\displaystyle O_B = ...
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Quantum dimension in topological entanglement entropy
In 2D the entanglement entropy of a simply connected region goes like
\begin{align}
S_L \to \alpha L - \gamma + \cdots,
\end{align}
where $\gamma$ is the topological entanglement entropy.
$\gamma$ is ...
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Quantum Computing Power Advantages
Currently, the world's fastest supercomputer runs at 17.59 Petaflops, which consumes 9 megawatts of electricity. A qubit-based quantum computer has the potential to operate much more quickly for some ...
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Non-destructive measurement of qbits
Yale news "New qubit control bodes well for future of quantum computing" (Original paper) says:
"The Yale physicists successfully devised a new, non-destructive measurement system for observing, ...
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Controlled-measurement of a quantum register
Given a state vector $\left[\alpha,\beta,\gamma,\delta\right]$ which is not known a priori, does there exist an operation, which I will call "controlled-measurement", which results in the ensemble
...
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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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133 views
Why must quantum logic gates be linear operators?
Why must quantum logic gates be linear operators? I mean, is it just a consequence of quantum mechanics postulates?
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How is a Qubit in two states under Superposition?
I have read a little about Quantum computing.
From what I understand, Quantum Superposition is when a qubit is in a state $\alpha|0\rangle$ + $\beta|1\rangle$, where $\alpha$ and $\beta$ are ...
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Why is quantum entanglement so important?
Entanglement also allows multiple states to be acted on simultaneously, unlike classical bits that can only have one value at a time. Entanglement is a necessary ingredient of any quantum ...
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Latest advances in experimental quantum processors?
I'm and undergraduate student and I'm doing a report on Quantum computing. As a conclusion of my report I'd like to highlight the latest experimental advances in Quantum Computing, especially in ...
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Are physical probabilities also quantized?
In physics there is quanta and energy occurs per this unit. Is it it then reasonable that probability also is quantized since energy is?
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Why do we want to entangle qubits?
The title is pretty much all I want to ask. Why are qubits entangled? To my knowledge (which isn't that deep) a quantum register can be realized without entangling the qubits.
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Using wavepackets instead of photons in quantum computer
Why does a photonic quantum computer require photons? Why wouldn't wave packets work just as well, better in fact since it would get away from the use of fragile single photons? (Article)
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How many states can a n qubit quantum computer store?
A classical computer composed of '0' or '1' transistors stores $2^n$ states.
Is it true that a quantum computer composed of '0' or '1' or '0 & 1' qubits stores $3^n$ states?
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What is the motivation for the definition of concurrence in quantum information?
What is the motivation for the definition of concurrence in quantum information? On the surface, the definition looks pretty ad hoc.
The definition is often given for the case of 2 qubits only. What ...
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Bell State, if Bob applies a Pauli Gate?
After Alice and Bob share a Bell state, Bob applies a Pauli gate to his qubit. What will be the situation of the Bell state? What happens?
Then Alice applies the same gate to her qubit – again, what ...
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System with no entanglement but consuming quantum discord
I have come across an article which talks about quantum discord (Observing the operational significance of discord consumption. M. Gu et al. Nature Physics 8, 671–675 (2012) doi:10.1038/nphys2376), ...
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The role of context in information theory
Consider Hofstaedter’s jukebox analogy: A jukebox that contains only one record, but many different record players, each of which interprets that one record in a different way to produce an entirely ...
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Hamilton operator in absence of causal order?
I hope, this question isn't too broad or vague.
In a recent paper, Ognyan Oreshkov et al. worked out a theory of quantum correlations in absence of any causal order, dropping the assumptions of a ...
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using a unitary matrix to transpose
A unitary matrix U is a matrix such that the conjugate transpose of U, when multiplied on the right with U, yields identity. My question is, is it possible to obtain the transpose of any density ...
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Quantum cryptography: encryptions
I am studying quantum cryptography and I have a very basic question. Suppose A and B share a secret key k, where k=0 or 1. A wants to send one qubit to B. What A does is, if k=1, she 'flips' the qubit ...
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The definition of entropy in quantum mechanics
I have seen entropy with several different definitions. Like Von Neumann entropy and Rényi entropy, etc.
So I am curious why there are so many different definitions in quantum mechanics while only ...
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Where is noncommutativity in the state-effect formalism of quantum mechanics?
In quantum information theory, one can adopt the basic formalism where every system is given by an operator algebra, state preparation procedures correspond to linear functionals on that algebra ...
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Why are the equal probabilities for Bell state measurement outcomes essential for “quantum teleportation”?
I've recently been introduced to the basics of finite-dimensional quantum mechanics from a purely mathematical point of view (with a quantum-information theme to it). When discussing quantum ...
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Uncertainty Principle for Information?
I'm not familiar (yet) on how Information theory can be emerged/used in QM/QFT but I was thinking about this question:
While we have Heisenberg uncertainty principle on measuring coupled observables, ...
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Is there a symmetry associated to the conservation of information?
Conservation of information seems to be a deep physical principle.
For instance, Unitarity is a key concept in Quantum Mechanics and Quantum Field Theory.
We may wonder if there is an underlying ...
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Bloch-sphere-like representation of two-qubit density operators
The Bloch sphere is an excellent way to visualize the state-space available to a single qubit, both for pure and mixed states. Aside from its connection to physical orientation of spin in a spin-1/2 ...
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Understanding a Physics Paper on Quantum Teleportation of Continuous variables
The paper I am trying to understand is here:
http://pra.aps.org/abstract/PRA/v49/i2/p1473_1
The paper describes the quantum teleportation protocol in a general case with continuous dynamical ...
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OAM states in communications [duplicate]
Possible Duplicate:
OAM states for wireless communications
Can someone give me an overview of how OAM states are used in communications?
Using Orbital Angular Momentum States seems like a ...
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Mathematically challenging areas in Quantum information theory and quantum cryptography
I am a physics undergrad and thinking of exploring quantum information theory. I had a look at some books in my college library. What area in QIT, is the most mathematically challenging and rigorous? ...
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fiber optic second order PMD as an operator on the tensor product Hilbert space
Second order polarization mode dispersion (SOPMD) is a coupling mechanism between polarization and frequency. Take our photon to be the following tensor product:
$\psi = \int \gamma_{\omega} | ...
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Experimental realization of Quantum Teleportation of Spin, not polarization, not ions or atoms
I've looked everywhere in databases my school provides, to google searches, to the questions asked in physics forums, and here. As I understand, the original QT (quantum teleportation) protocol ...
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OAM states for wireless communications
Can someone give me an overview of how OAM states are used in communications?
Using Orbital Angular Momentum States seems like a hot topic for communications. I read a few articles about the basic ...
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decoherence free subspace of a single photon
Take the state vector for a single photon as
$\psi = \int \gamma_{\omega} | \omega \rangle \otimes (\alpha |H \rangle + \beta | V \rangle )d \omega$
$H, V, \omega$ are the horizontal polarization, ...
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Can we make a Maxwell's Demon using Quantum Computers?
Although I'm reasonably sure that quantum computing advances will not lead to the ability to construct a machine that globally violates the 2nd law of thermodynamics, it feels like a difficult ...
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Using Quantum Teleportation in a way to have the effect of matter teleportation
So, I understand that quantum teleportation is the transfer of a quantum state from one particle or system of particles and its correlations to another receiving system consisting of particle(s). ...
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Classical vs qubits: Superposition
Since a quantum information lecture today I have been wondering what does it really mean for a state to be in superposition? Is this something that is answerable?
This is what we learnt (or what I ...
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Concurrence in higher dimensions?
Does anybody know of any calculation of the concurrence for some mixed state other than the qubit-qubit case (which was solved by Wootters)?
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Extending the idea of superdense coding
I was reading through the superdense coding protocol, that lets A convey two classical bits to B by sending one qubit (assuming B sends A a qubit beforehand). So B creates a 2-qubit state and sends ...
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Quantum Mechanics in terms of *-algebras
I'm currently trying to find my way into the geometric description of Quantum Mechanics. I therefor started reading:
Geometry of state spaces. In: Entanglement and Decoherence (A. Buchleitner et ...
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Question on the preservation of information via mapping to free field states
In Hawking's paper, "Breakdown of predictability in gravitational collapse", the crux of Hawking's argument is as follows:
...,one can extend the principle to treatments in which the gravitational ...
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knowledge of an internal observer
I would like to discuss the consequences of the concept of an internal observer in quantum theory. If we assume that we have a universe that evolves unitarily at a global scale and an observer is ...
