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Questions tagged [quantum-information]

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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Why a quantum computer simulation is not a quantum computer?

Is it because the inability to generate true random numbers (I dont think so) in the host machine or something else? My understanding is that the ability to emulate a quantum computer in a ...
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What are the best resources to master Quantum Entanglement?

I am reading Modern Quantum Mechanics by Sakurai for my semester. I read about the Quantum Entanglement, Bell Inequality and Hidden Variables theory, EPR Paradox etc. and got fascinated. I just wanna ...
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Why can time-translation invariant quantum operations never increase coherence between energy eigenspaces?

Set $\hbar =1$. Let $U(t) = e^{-itH}$ be evolution under a Hamiltonian $H$ (for convenience let's assume $H$ is not degenerate). A time-translation invariant quantum operation $\mathcal{E}$ is one ...
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Why do quantum computation models based on open quantum systems receive so little attention?

In almost all research on (universal) quantum computation the common models assumed from the outset are either the quantum circuit model with unitary gates, the measurement-based one-way model or the ...
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Do “delayed choice” experiments send information back in time?

Consider the wave function collapse of a pair of entangled photons: wave function is collapsed, let's call this state '0' normal wave function, let's call this state '1' In a "delayed choice" ...
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How long does photon entanglement last?

After creating entangled photons, how long does the entanglement last?
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What if you turn the photon detector on after the paired photon has already gone through both slits?

Id imagine the answer will be that even if you turn the photon detector on after the paired photon has already hit the screen, the screen will show no interference pattern because the universe sensed ...
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Are photons locked in time, and does this explain the “delayed choice quantum eraser” experiment?

I'm trying to wrap my head around the "Delayed Choice Quantum Eraser" experiment and how events in the future affect light in the past. I'm sure I'm wrong but to me this seems to indicate that ...
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Is there a theoretical limit on the smallest angle that can be used in a quantum rotation gate?

Assuming ideal conditions and no error, what is the smallest $\theta>0$ that can be used in $R_x(\theta)$ or $R_y(\theta)$.
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Why doesn't using quantum error-correction increase the production of entropy and decohrence?

Error-correction in quantum computing is designed to get around the decoherence "washing out" the answer to a computation. But wouldn't the introduction of error-correction procedures or apparatus ...
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How does the Entropy of Entanglement generalize to tripartite and multipartite pure states?

Entropy of Entanglement is a very beautiful measure of entanglement. I wonder how this concept generalizes to three-parite entangled states, such as $$ \left| \psi \right\rangle=\frac{1}{\sqrt{2}}\...
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Unitarily reversing a projective measurement

We start with a particle in a pure superposition state. Let's say it is, $$\vert\psi\rangle = \frac{1}{2}(\vert 0\rangle + \vert 1\rangle)$$ Alice sends this particle inside a box and the box ...
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Spintronics: How can we inject a spin current into/onto a sample without physically touching it, ie: modulation with a field?

So background, I'm imagining a black box scenario where we have some spintronic system. It might be a topological quantum computer that uses spin currents running across it's memory to connect ...
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Can a random product state be expressed as a MPS (Matrix product state)?

$ |\psi\rangle = \prod_{i=1}^{N}|s_{i}\rangle $ where, $|s_{i}\rangle = \cos\left (\frac{\theta_{i}}{2}\right )|\uparrow_{i}\rangle + \exp{(i\phi_{i})}\sin\left (\frac{\theta_{i}}{2}\right )|\...
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Where is the black hole information paradox?

If an object falls into a Schwarzschild black hole and a distant observer watches, they see the object fall slower and slower as it approaches the event horizon, until it is "frozen." (To the observer,...
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What (in silico information) defines the transfer function of a Quantum Amplifier, in the form of a molecule?

Let’s say a team of scientists have published the structure of a molecule they claim acts as a quantum amplifier. Photons travel along a quantum channel carrying qubits in their polarization. The ...
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Eigenstructure of the Dicke Model

I am beginning a study of the Dicke model and found a very interesting publication: "The Dicke model in quantum optics: Dicke model revisited" by Barry M Garraway in Phil. Trans. R. Soc. A (2011). I ...
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Can quantum entanglement provide a faster way of transfer of information than the speed of light? [duplicate]

Can quantum entanglement provide a faster way of transfer of information than the speed of light as it travells at a speed which is 10000 times faster than the speed of light
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Do rotating phases limit the possibility to perform quantum computing?

Let me try to make several statements which I believe to be true - I'm basically hoping that somebody will point out where I make an error (or errors). 1) per wikipedia, any two-level quantum ...
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Two qubit system and the notion of mixed states

In this paper, pages 6-9, the author discusses mixed states for two entangled qubits. After solving for Alice's density matrix, the author keeps referring to Alice's qubit state as a mixed state. But, ...
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Does measurement of quantum system always collapses the state of system?

Does measurement in quantum mechanics always disturb the system? The measurement postulate states that "when we do a measurement on a quantum system, the state of the system is collapsed to one of ...
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About black holes paradox [duplicate]

About the information paradox in the black holes, isn't it possible in principle, that the "singularity" itself doesn't really exists, and its just the horizon plane, where the particles that falls ...
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Can a Controlled-Swap gate control itself in a Quantum Computer?

Let there be a system of three qubits, $a,b,c,d$ where $d$ is an ancilla qubit. We can add more ancilla if required. The operation I want to implement is as follows: "If $a$ and $b$ are both in the '$...
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Symmetry of the scattering super-operator

Suppose we have an initial ensemble described by a density matrix $\rho$ and any given member of the ensemble scatters from one of some set of scattering matrices $\{S_g \equiv O_g S O_g^\dagger : g \...
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Can someone help in applying covariance condition to this unitary transformation?

I am trying to apply covariance property on this Quantum adder transformation in which a two particle input state is mapped onto a two particle output state. I am trying to represent these states into ...
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Three dimensional visualization of a qutrit

My question is in reference to the paper "Three dimensional visualization of a qutrit"(https://arxiv.org/abs/1601.07361). The author's start with a symmetric two qubit density matrix written in the ...
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Quantum Non-Demolition of entangled superposition using an electron with infinite spin

Disclaimer: I have no idea what I am talking about, as I just started college and barely understand quantum physics. If an electron can have a linear velocity of the speed of light, theoretically ...
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Does Quantum Darwinism select against the Frauchiger-Renner inconsistent states?

Zurek's Quantum Darwinism is based on an environment induced superselection of pointer states, within which entanglement plays an essential role. Similar arguments can be made related to Horodecki's ...
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Why does implementing the annihilation operator $\hat a$ not allow action at a distance?

I was reading a paper (link) in which they implement action at a distance using the annihilation operator, and in it they say that "this observation does not imply superluminal signaling because ...
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Is it possible for the measurement to yield a superposition of states in the original space?

Suppose we have a wave function $|\Psi\rangle=\sum_i c_i|\psi_i\rangle$, where the original probability probability amplitude was in a distribution of, i.e. $c_1^*c_1=\frac{1}{2},...,c_i^*c_i=\frac{1}{...
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Reduction from PEPS to MERA possible?

Multi-scale entanglement renormalization ansatz (MERA) includes two kinds of isometric tensors, disentanglers and isometries. Thus tensors in MERA are by definition composed of isometries. Meanwhile ...
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Can elementary particles quantum entangle?

I just want to know if elementary particles, such as a quark, can entangle with another elementary particle of the same kind. If so, does entanglement for elementary particles follow the same ...
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Wave function collapse of photon state transformed by beamsplitter

$\newcommand{\ket}[1]{\left|#1\right>}$I am trying to make sense of the math behind transforming the state of a photon with a beam splitter. Suppose I have a beamsplitter crystal which split by ...
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General structure of parity preserving two qubit gate

I am trying to decompose a set of two qubit gates repecting parity, though I am not sure whether parity is the right word for this. The gate has the following structure: $$\begin{bmatrix} u_{11}&...
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Correlation functions of quantum Ising models

I have a single technical question regarding a statement on page 7 of the paper "Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence". The paper ...
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2answers
113 views

The overlap of two Gaussian states

According to e.g. Serafini (Quantum Continuous Variables), the Hilbert-Schmidt product ('overlap') of two multimode Gaussian states $\rho_1,\rho_2$ is $$\text{Tr}[\rho_1\rho_2]=|\langle\psi_1|\psi_2\...
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Complementary channel of convex combination of channels

Consider two channels $N_1$ and $N_2$ which have Kraus operators $\{A_k\}$ and $\{B_k\}$ respectively, where the index $k$ runs over the number of Kraus operators. Define a third channel $N$ as a ...
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Given a positive element, $a$, of a $C$*-algebra, why does there exists a pure state, $p$, on $A$ such that $p(a)=||a||$? [duplicate]

I'm reading secondary literature where they make this claim, however, I cannot see why it holds true. This is a reformulation from a previous question that I didn't specify good enough.
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Completeness relation and projective measurement operators on Hilbert space

I have a question about a passage in the Book Quantum Computation and Quantum Information Textbook by I. Chuang and M, Nielsen. My query is about a passage on page 86 of the 10th edition: ...
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Trace of the quantum map $ A^n_m (\rho) = \sum_{ij} | i…i \rangle^n \langle i…i|^m \rho | j…j \rangle^m \langle j…j|^n$

We define some quantum map $ A^n_m (\rho) $ and let it act on density matrix $\rho$: $$ A^n_m (\rho) = \sum_{ij} | i...i \rangle^n \langle i...i|^m \rho | j...j \rangle^m \langle j...j|^n.$$ ...
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Energy dissipation in unitary dynamics

In the context of quantum mechanics it is often (e.g. in several Wikipedia pages, like on Quantum dissipation) stated, that: "If the time evolution of a system is unitary (e.g. always in the ...
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Clarification on DMRG computational complexity

I was reading a paper on the density matrix renormalization group (https://arxiv.org/abs/1008.3477). In DMRG, we gradually grow a chain by inserting a unit cell at the center of the chain (for ...
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Axiomatic Quantum Theory, and the complex numbers

In Lucien Hardy's influential paper "Quantum Theory from five reasonable axioms," Hardy states in the abstract: This work provides some insight into the reasons why quantum theory is the way it ...
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Stinespring dilation of a channel vs Naimark's theorem

I'm trying to understand the connection between the Stinespring dilation of a quantum channel and Naimark's theorem that shows that POVMs can be written as projective measurements in a larger Hilbert ...
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Does $U^T U$ have real eigenvectors if $U$ is unitary?

I was reading a paper by Kraus and Cirac on general two qubit gates. A crucial step is to decompose an arbitrary unitary operator acting on two qubit space into a special form: $$ U_{AB} = U_A\otimes ...
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Two-photon interference inside Mach-Zehnder interferometer

Imagine there's a strong laser beam, not just an attenuated stream of single photons, entering a balanced Mach-Zehnder interferometer. One-photon picture: Each photon interferes with itself on the ...
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Applying a General Lorentz Boost to Multi-Partite Quantum State in Dirac Notation

I would like to apply a General Lorentz Boost to some Multi-partite Quantum State. I have read several papers (like this) on the theory of boosting quantum states, but I have a hard time applying ...
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Von Neumann entropy in terms of the mutual overlap?

I have $N$ pure, but nonorthogonal, states $|\psi_n\rangle$ with density matrix $\rho_n=|\psi_n\rangle\langle\psi_n|$. Say we call the the total density matrix $\rho=\frac{1}{N}\sum_n \rho_n$. Are ...
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Can quantum disentanglement be triggered by time dilation?

The question is really one question that leads to the final one: Is it possible to realize a qubit that naturally flips between two quantum states on a definite and fixed period without any ongoing ...
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Quantum secret sharing

I was reading the article "Multiparty quantum secret sharing". It had 3 parties Bob, Alice and Charlie. They agree on sharing a classical secret quantum mechanically. The steps they follow are for a ...