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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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Relationship between physical and information entropy

This question is inspired by the following example given in Barnett's book on quantum information: (PDF here, starting at the bottom of page 5) I am not a physicist, but I'll accept that the ...
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1answer
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Can we always find a Quasi-Probability distribution representation for density operator of any physical system?

For a pure, quantum optical system, the QPD formalism is straightforward and possible because of the existence of coherent states. The question is whether we can find coherent states for any quantum ...
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How to make two fermions entangled with each other?

How to make two fermions entangled with each other. When we say two particles have to interact with each other to get entangled, what kind of interaction is that?
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Conservation of information

From the classical and quantum mechanical viewpoint, is information conserved in an expanding universe or not?
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What does sub-bosonic behavior of composite boson means? [on hold]

Let say we consider two distinguishable fermions(bi-fermions) in compact form. This pair of fermions make a composite boson. Now we develop formalism to check either our composite bosns is like a pure ...
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68 views

Why does less knowledge imply more correlations?

Given two systems, correlations quantify how much information we can gain about system 1 by measuring system 2. Entanglement is a type of correlations which only occurs in quantum mechanics. There ...
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$k$-local Hamiltonian with long range entangled ground states?

Is it possible, and if yes, is there a relatively simple example of a Hamiltonian that only has k-local terms but its ground state always has entanglement beyond $k$ sites? For instance if $H = H_{...
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1answer
38 views

Negative unity matrix not hermitian? (stabilizer formalism)

I read the section in the attached picture about the stabilizer formalism and was wondering about the last sentence in the pic. It says that all operators of the stabilizer group are hermitian, ...
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1answer
180 views

Can Bell-CHSH inequality be used to prove quantumness of an physical effect?

I would like to apologize if this is the wrong StackExchange site but I couldn't think of anything better. I'm writing my thesis about random number generators. In a section about quantum random ...
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What is the theoretical resolution limit of magnetometers / magnetoencephelography?

I recently read this article about a group of physicists and neurobiologists working on developing advancements to magnetoencephelography, presumably based on new quantum metrology and quantum ...
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Symmetric Quantum Discord

Let's define the symmetric quantum discord in the following way: $$D(A:B):=\frac{1}{2}(D(A|B)+D(B|A))$$ where $D(A|B)$ is the usual quantum discord given as $$D(A|B) = I(A:B) - \mathcal{J}(A|B)$$ ...
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A unitary that induces Entanglement Breaking Channel for any ancillary state

Let's say there is a unitary operator $M:\mathcal{H}_1\otimes\mathcal{H}_2\to\mathcal{H}_A\otimes\mathcal{H}_B$ such that the channel $\Phi$ given as $$\Phi(\rho)=\text{Tr}_BM(\rho\otimes\phi)M^\...
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2answers
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What happens when we cool down the gas of non-identical particles?

For gas of identical particles, when we cool it down to extremely low temperature we can see one of two types of behaviour depending on the symmetry of wavefunction with respect to argument ...
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62 views

Tensor product of Hilbert space and Fock space

If we have system consist of two or more than two subsystems, we can write hilbert space of a system in terms of tensor product of Hilbert spaces of subsystems and we can write state of a system. But ...
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1answer
55 views

Understanding conditional Entropy between two physical states

From the paper: https://arxiv.org/abs/1303.4686 Let us consider an ensemble of N identical quantum systems with Hamiltonian H and density matrix $$\rho^{(N)}$$ which is diagonal in the energy ...
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1answer
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Example of mixd entangled state [duplicate]

I'm looking for specific examples of all combinations of pure, mixed, separable and entangled states, but I would prefer if they all have some physical significance. For pure states, the separable ...
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1answer
80 views

Is it possible to prepare thermal state by performing a single mode quantum operation on vacuum states?

Let me define thermal state as \begin{equation} \rho=\sum_{n=0}^{n=\infty}\frac{\bar{n}^n}{(\bar{n}+1)^{n+1}}\left|n\right>\left<n\right|. \end{equation} is it possible to prepare this state ...
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What is effective mean value? [closed]

For counting the number of particles in any specific state we use effective mean value $\langle \hat{N} \rangle $ instead of number operator $\hat{N}$ in ensemble. I want to know what is the advantage ...
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1answer
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Why is the entropy of a subsystem density matrix equal to that of the other subsystem?

If we have some system $\rho_{AB}$ we can find the entanglement entropy as $S(\rho_A)=S(\text{Tr}_B(\rho_{AB}))=S(\text{Tr}_A(\rho_{AB}))$ But why are the entropies of the subsystems $\rho_A$ and $\...
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Why the ground state of the Kiteav model almost a singlet?

The interaction strengths J_x=J_y=J_z. I have checked by exact diagonalization.
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1answer
36 views

Is long range entanglement detectable with k-local operators?

Given a state $\psi \in H_1\otimes H_2\otimes ... H_n$, and there is long range entanglement, is it possible to certify this by only using k-local operators where $k < n$? To make it concrete but ...
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Quantum computing explanation for the quantum Zeno effect

I'm playing around with the IBM Q to demonstrate the qunatum Zeno effect. If we prepare a qubit in the $|0\rangle$ state and apply 5 consecutive $R_y(\pi/5)$ gates, we should end up in state $|1\...
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EPR paradox measurement at the same (exactly the same) time [duplicate]

I am currently working on the EPR paradox. I have read the original work and a good basic knowledge of physics. The experiment and the paradox of this experiment as such I have understood. For me, ...
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1answer
66 views

Why do we use matrix product states?

Given a many body $\vert\psi\rangle$, we can express it in terms of a matrix product state. That is, $\vert\psi\rangle = \sum_{i,j..k}\psi_{i,j..k}\vert i,j..k\rangle$ can be rewritten as $\vert\...
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How can massless particles carry information? [on hold]

I've read that even massless particles can carry information. If we look for instance at gluons: they carry flavour charge (so information) but still are massless. We also can store information in ...
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2answers
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Is this entanglement of formation identity true?

Let a tripartite system be given with pure state $\rho_{ABC}$. My adviser said that the entanglement of formation satisfies the identity $${E_F}_{A(BC)}^2={E_F}_{AB}^2+{E_F}_{AC}^2$$ Where ${E_F}_{...
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1answer
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Post-measurement density matrix derivation

This is something standard, by I'm trying to redo this with spectral theory. Suppose we start with the usual postulates of quantum mechanics: States are unit rays on a separable Hilbert space. In ...
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In $e^+e^-\to e^+e^-$ scattering is the final outgoing state with hard and soft particles together known?

In $e^+e^-\to e^+e^-$ scattering there is a production of low energy photons (soft photons). This seems to be connected with infrared divergences. One way to deal with this seems to be the so-called "...
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Faster than light speed transfer of certain event

I am CS/Math person I am not quite sure what it means in physics information cannot be transferred faster than light (FTL). I tried to understand the proof of no transfer theorem but I lacked the ...
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1answer
29 views

Conservation of quantum information in mixed states quantum secret sharing (QSS) schemes?

Consider a $((3,5))$ pure state quantum secret sharing (QSS) scheme. For instance this paper: arXiv:quant-ph/9901025. If I divide any 5 shares to two sets then allways one of those two set are ...
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177 views

How does the “No-communication theorem” prohibit communication? [closed]

The "No-cloning theorem" just stems from the "No-communication" theorem, and it is known there are potential falsifications of the "No-cloning" theorem. Even so, does anything in the more general "No-...
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How to measure off diagonal elements of mixed state density matrix?

Let’s assume I want to do quantum tomography using polarizers, half Waveplates and detectors , it’s obvious for me how we can measure diagonal elements of 2 qubit system density matrix using polarizer ...
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1answer
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Delayed choice experiment for misaligned Mach-Zender interferometer

In quantum Mach-Zender interference, a single photon can be prepared to have a 100 percent probability of traveling through a particular path (that classically would be a 50 percent chance). This can ...
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1answer
86 views

How to bound the dimension of infinite dimensional Hilbert space?

Lets say I have a density matrix $\rho$ encoded into a physical system. Lets say I can have access to as many copy of $\rho$ as I want. I perform hmodyne detection on the copies in phase and out of ...
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1answer
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Projective measurement using two mode squeezed state?

Let me define two mode squeezed states as $ \left | \xi \right>_n=\exp\left(\xi \hat{a}^\dagger \hat{b}^\dagger-\hat{a} \hat{b} \xi^\star\right)\left | n,0 \right>$ where $\left|n,0\right>$ ...
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1answer
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If the particles show uncertainity ,then why the body made of particles cant show that? [duplicate]

What i want to know is that if the properties of the quantumn nature reduses if the particles combine? Or were we missing any fundamental relation to particle or boby made up of particles?
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What happens to the quantum information of a particle and an antiparticle when they annihilate?

I understand that the quantum no-deleting theorem dictates that it's impossible to delete quantum information, so what happens to the quantum information of a particle and an antiparticle when they ...
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Good books on Quantum Complexity and its application to Theoretical Physics?

I am about to complete my undergraduate studies and am trying to investigate some areas I might like to study in postgrad. I have read some very interesting things about the application of quantum ...
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1answer
52 views

Constructing a Hamiltonian for $N$-qubits

Let us assume we have a qubit with an internal Hamiltonian $H_0 = \sum_i \varepsilon_i |i\rangle\langle i|$. Now let's assume we have 2 such qubits. How would their joint Hamiltonian look like? I ...
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How to construct a process matrix?

I have been reading Quantum correlations with no causal order by O. Oreshkov et al. They have introduced the framework of ‘process matrices’ where the causal structure can be partially relaxed. Here, ...
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1answer
71 views

No-S-$B^+$ Theorem like Results

Classification of multipartite quantum state is an interesting topic in quantum information and there have been many accomplishments in the field. For example, according to the result of Thapliyal, ...
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Is the eavesdropper detected by chance in QKD?

So I was starting to familiarise myself with QKD when I came across this video on YouTube breaking down the process (I believe this is the E91 protocol but I could be mistaken) using an animation that ...
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1answer
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Quantum entanglement and Bekenstein Bound

In this article Bekenstein writes that the amount of information could store in a sphere with diameter of a cm is ~$10^{66}$ bits. If one were to take a set of 1000 entangled electrons and put them ...
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1answer
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Measurement in QM - positivity criterion

Let me describe my understanding of measurements in quantum mechanics before getting to the part that I'm unclear about. If we have a quantum state $\rho$ and we do a projective measurement in some ...
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Can we broke the trace of system into $Tr_A$ and $Tr_B$ like we did in above equation, How?

Let say we have entanglesd state for system consist of subsystems A and B. $| \psi \rangle $ and some operator $A$ is acting on state. We can write $ \langle \hat{A} \rangle=Tr(\langle \hat{A} \...
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2answers
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Rank of a density matix

I was just trying to understand the meaning of rank of a density matrix. I came across the following post, which says that the rank of density matrix is the number of non-zero eigenvalues. And for a ...
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Non markovian Bloch Redfield equation

The master equation for density matrix in case of markovian approximation, there is Bloch-Redfield equation \begin{equation} \frac{d}{dt}\rho_{S}(t)=-\int^{\infty}_{0}ds tr_{\epsilon} \left\lbrace [...
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1answer
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Maximum number of bits in one single qubit? [duplicate]

How many bits can a qubit hold? I know there are probabilities involved in a qubit, but there must be some limit to how many bits a qubit can take. If different qubits can have different amounts of ...
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Is there a physical limit to memory reading and writing speed?

Is there a limit to the speed at which a memory, quantum or classical, can be read or written into? I've seen this interesting question : Is there a physical limit to data transfer rate?. Here I'm ...
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Treating $-E_J \vert{\cos{(\hat{\theta}/2)}}\vert$ in the charge basis

I'm trying to write down the Hamiltonian for a Cooper pair box in the charge basis in the scenario where the Josephson potential term is not $-E_J \cos{\hat{\theta}}$, but $-E_J \vert{\cos{(\hat{\...