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 maximally entangled qutrit state from $n$ Bell states

I've read that maximally entangled qubit states are a good "unit" of bipartite entanglement since it is possible to create any other entangled state from them using local operations and classical ...
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Can quantum annealing be used for factorization?

It is known that there is a famous quantum factorization algorithm by Peter Shor. The algorithm is thought to be suitable only for quantum gate computer. But can a an adiabatic quantum computer ...
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Relation between classical and quantum information

It is known that for a classical system the amount of information needed to store its state is the same as the amount of information that can be stored in that system. This amount is equal to ...
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What is quantum discord?

What is quantum discord? I stumbled upon this term on Quantum Computing: The power of discord, but have never heard of it before. Can you give a bit more mathematical explanation of the term here?
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How many qubits does it take to specify an event in spacetime?

The title says it all. My understanding is that a qubit is a superposition of $|0\rangle$ and $|1\rangle$, i.e. the answer to a binary question. So I imagine that specifying an event in spacetime ...
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Hayden-Preskill informational mirrors and decryption

I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long ...
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Is a weak measurement the same as an unsharp measurement or POVM?

This is prompted by the strong claims made in Science 332, 1170 (2011) to have observed trajectories of photons, "something all of our textbooks and professors had always told us was impossible". I'm ...
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What can the D-Wave quantum computer do?

The media are reporting the commercially sold 128-bit quantum computer from D-Wave http://news.google.com/news?ned=us&hl=us&q=d-wave+quantum&cf=all&scoring=n which of course ...
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Do quantum states contain exponentially more information than classical states?

Do quantum states contain exponentially more information than classical states? It might seem so at first sight, but what about in light of this talk?
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Is Shor's algorithm a demonstration of the many worlds interpretation?

David Deutsch is very fond of pointing out Shor's integer factorization algorithm is a demonstration of the many worlds interpretation. As he often asked, where else did all the exponentially many ...
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How many bits are needed to simulate the universe?

This is not the same as: How many bytes can the observable universe store? The Bekenstein bound tells us how many bits of data can be stored in a space. Using this value, we can determine the ...
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Is microcausality *necessary* for no-signaling?

There are proofs in the literature that QFT including microcausality is sufficient for it not to be possible to send signals by making quantum mechanical measurements associated with regions of ...
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Bell polytopes with nontrivial symmetries

Take $N$ parties, each of which receives an input $s_i \in {1, \dots, m_i}$ and produces an output $r_i \in {1, \dots, v_i}$, possibly in a nondeterministic manner. We are interested in joint ...
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Quantum Computing and Animal Navigation

Someone sent me this link to a talk by Prof. Klaus Schulten from the University of Illinois: (my emphasis) Quantum Computing and Animal Navigation Quantum computing is all the rage nowadays. ...
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Is it possible to bind an entangled electron to the outer shell of an atom or just a proton?

Say we start with an entangled electron--positron pair and we separate them. I want to take the entangled electron and bind it to a proton or the outer shell of an atom. Is it possible to do this ...
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Is “analog” quantum-computation not useful?

I understand what a qubit-based quantum computer by the current definition is and how they are constructed. I read another thread where someone suggested encoding a computation into a dual-slit ...
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Distinguishability in Quantum Ensembles

Inspired by this question: Are these two quantum systems distinguishable? and discussion therein. Given an ensemble of states, the randomness of a measurement outcome can be due to classical reasons ...
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fermions and quantum gates

Say that I have 2 qubits - 2 spin half fermions. my initial condition is $|00\rangle$ in the spin-wave function and some anti-symmetrical spacial wave function. I'm wondering about what happens when ...
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Representation of quantum transformations as matrices

I was reading Quantum Computation explained to my mother, which makes the following claim Postulate 2 A closed physical system in state V will evolve into a new state W , after a certain ...
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How much is quantum computation changing the interpretation of quantum theory, and, if at all, how?

At the beginning of quantum computation, David Deutsch made a strong claim that the Many Worlds interpretation of quantum theory was at the foundation of his ability to do what he did. There was a lot ...
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Recent breakthroughs in quantum computing?

Can anyone explain to me why we have had no major breakthroughs in the theory of quantum computation in the past 15 years? Shor's algorithm set the standard, since then we've had Grover's algorithm ...
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Can PEPS explain the holographic principle in quantum gravity?

Condensed matter physicists have shown using quantum information that in many condensed matter systems, entanglement entropy only scales as the area of the boundary, and not the volume. This is the ...
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Size of a quantum computer to effectively calculate macroscopic reality from quantum mechanics

Assuming the correctness of QM: Would the size of such a computer be smaller than the observable universe? If it were to represent all available information in the universe it seems that it's minimal ...
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Conservation of energy in quantum teleportation

Consider the quantum state teleportation protocol of Bennett et. al. How does one prove that this protocol would never violate the conservation of energy? At the face of it, it doesn't seem to be ...
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Probabilistic quantum cloning

I have a question with regard to probabilistic quantum cloning - see for example http://prl.aps.org/abstract/PRL/v80/i22/p4999_1. It does seems like I can use the proof for no-cloning theorem to ...
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Are there any applications of quantum information theory to physics?

Are there any applications of quantum information theory to physics?
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The complementary variable to the qubit and spin-1/2

The qubit is a big topic of quantum information theory. A qubit is a single quantum bit. Physical examples of qubits include the spin-1/2 of an electron, for example, see page 39 of Preskill: ...
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How many stabilised qubits have been achieved in Quantum Computing?

The latest I read is 3 but that was in Oct. With Lene Hau of Harvard's "frozen light" and with quantum donuts, newer strategies for stabilization are appearing, but the problem of keeping the qubit in ...
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How many bytes can the observable universe store?

Is the number of states in the Universe countable? What framework could be used to answer the question in the title?
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Informational capacity of qubits and photons

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 ...
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Do Category Theory and/or Quantum Logic add value in physics?

I know they have their adherents, but do more or less esoteric branches of mathematics such as Category Theory and/or Quantum Logic provide powerful tools for new theory development or are they just ...
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What are the benefits of quantum information “teleportation”?

I read occasionally popular science articles and from time to time encounter issues about quantum information teleportation. (this one for example http://www.physorg.com/news193551675.html) So far I ...
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Using delayed choice interference experiments as a computing device

I had an idea how to design a "quantum computer": How about designing interference-experiments where the design of the experiments itself represents algorithmical or mathematical problems that are ...
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Why quantum entanglement is considered to be active link between particles?

From everything I've read about quantum mechanics and quantum entanglement phenomena it's unobvious for me, why quantum entanglement is considered to be active link. I.e. it's stated every time that ...
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Is there such a thing as “Action at a distance”?

What ever happened to "action at a distance" in entangled quantum states, i.e. the Einstein-Rosen-Podolsky (EPR) paradox? I thought they argued that in principle one could communicate faster than ...
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Why are von Neumann Algebras important in quantum physics?

At the moment I am studying operator algebras from a mathematical point of view. Up to now I have read and heard of many remarks and side notes that von Neumann algebras ($W^*$ algebras) are important ...
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Can I parameterize the state of a quantum system given reduced density matrices describing its subparts?

As the simplest example, consider a set of two qubits where the reduced density matrix of each qubit is known. If the two qubits are not entangled, the overall state would be given by the tensor ...
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Which qubit states are accessible with linear optics operations?

Given a quantum state of $n$ qubits, and being restricted to linear optics (that is, the output annihilation operators are linear combinations of the input annihilation operators): Which states are ...
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Is there a simple way to express the 2ⁿ+1 mutually unbiased bases for n qubits?

The title says it. An explanation for only 2 qubits would already be interesting, since I already have difficulties to find the 5 MUBs in this simple case.
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Unambiguous distinguishing of quantum states by local measurement

Let's have two orthogonal n-particle quantum states: $|\psi \rangle$ and $|\phi \rangle$. In theory it is always possible to make an unambiguous measurement. However, things get complicated when one ...