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.

Filter by
Sorted by
Tagged with
0
votes
0answers
18 views

Quantum speed limit quantum control

I'd like to know in quantum control, when you want to reach a state from a starting one, what is the notion of the quantum speed limit and why below a given time it is not possible to reach the final ...
6
votes
4answers
362 views

Is information conserved in quantum mechanics (after wave function collapse)?

I have heard in popular science that there is a law of "conservation of information." Some times this is described as: for any event that happens, there is enough information to reconstruct ...
0
votes
1answer
20 views

Hamiltonian on qubit: necessarily induces rotation. What about the reflections? Are they impossible to implement?

Any Hamiltonian dynamic on a qubit is necessarily a rotation. I consider a two level system having a Hamiltonian $H$. Decomposing this Hamiltonian on the Pauli basis and using the fact it is Hermitian,...
0
votes
0answers
12 views

Question about Ekert protocol of quantum key distribution

recently I've self-studied the quantum computing from the book Quantum Computing for Everyone by Chris Bernhardt. Everything is so far so good until I came across with the Ekert protocol which ...
9
votes
1answer
84 views

What's the motivation behind the seemingly artificial setup of the no-deleting theorem?

If you asked me what physical result would be naturally referred to as "the no-deleting theorem", then I would probably guess something like this: Given a designated "blank" state ...
2
votes
0answers
19 views

How is deterministic state transfer related to entanglement generation between two qubits?

From a quantum information theoretical point of view, how is deterministic state transfer related to entanglement generation between two qubits? Or, more specifically, how can you exploit the former ...
-3
votes
0answers
15 views

Anyone looking into using negative mass from 2 state vector formulation of quantum to create wormholes? [closed]

https://youtu.be/19QRiEyBI3M Based on this video, on 1:10 minute, box c is -1. Can it mean negative mass? Negative mass in Casimir effect is too close to their positive mass, but this might be ...
0
votes
1answer
31 views

Calculating Entanglement Spectrum from reduced density matrix

Im reading about Entanglement Spectrum in very nice webiste here. It says: After a system is partitioned into subsystems {A} and {B}, a state of the entire system may now be expressed as a matrix with ...
3
votes
1answer
91 views

Is there a deep connection between the Heisenberg uncertainty principle and entropy?

(Just so you know my background) I have taken a graduate course in quantum mechanics. I have also learned about information entropy in various places (statistical mechanics, information theory, ...
-1
votes
0answers
24 views

Probability density matrix BB84 in $d=2$ [closed]

I have some question: I am trying to used this equation to caudated the threshold for BB84. Can some give some example how to used this equation? what is privacy amplification in simple way and why ...
-1
votes
0answers
32 views

Best Separable Approximation of quantum state

I was reading the paper "Separability and Entanglement of Composite Quantum Systems" Phys. Rev. Lett. 80, 2261 (1998), arXiv:quant-ph/9707043. In this paper I was unable to understand the ...
1
vote
1answer
29 views

Measuring one particle on this entangled state

If we have a normalized entagled state: $$ |a\rangle=N (|1\rangle|2\rangle+|1\rangle|-2\rangle+|1\rangle|2\rangle-|-1\rangle|2\rangle) $$ Where $|2\rangle, |-2\rangle$ describes particles A and $|1\...
0
votes
0answers
37 views

Do commuting observables share the same set of POVMs?

My understanding of observables is that they can be decomposed into a linear combination of POVMs weighted by the eigenvalues. If two observables $\hat{A}$ and $\hat{B}$, commute, then a measurement ...
0
votes
1answer
38 views

Completely positive quantum channel

Let $\mathcal{E} \to CP(A \to B)$ (completely positive linear map) be a trace non-increasing $CP$ map. Show that any operator sum representation $\{M_x\}_{x=1}^{m}$ of $\mathcal{E}$ satisfes $\sum_{x=...
1
vote
1answer
25 views

Rotation of a Photon Polarisation with a Unitary

As an example of a qubit, one take the polarisation of a photon in which the two states can be taken to be vertical and horizontal polarisation, respectively. Rotation of a photon polarisation would ...
-1
votes
1answer
31 views

Implications of the delayed choice quantum eraser experiment [duplicate]

The experiment says that if I split a photon into two coherent entangled photons, one towards a double-slit, one towards a which-way detector, I can control whether the interference pattern appears ...
0
votes
1answer
37 views

Does work need to be done to a quantum state to change it?

Today in Quantum we talked about a 3 component quantum channel; maximally mixed light flowing into a linear filter oriented to 0 degrees, to through a phase retarder, then through another linear ...
1
vote
1answer
35 views

Kraus decomposition [closed]

In the derivation of the Kraus decomposition, it is not clear to me how to get from the LHS to the expression on the RHS: $$ \text{Tr}_{env}[U(\rho_1\otimes \rho_{env})U^\dagger]=\sum_k\langle k|U|0\...
1
vote
2answers
80 views

How does relaxing counterfactual definiteness allows for CHSH inequality violation?

Scenario, I want to play a game with a group of students as a teacher, I ask two teachers to help. The students are from the central classroom, going to room A and room B where the teachers there asks ...
1
vote
2answers
59 views

How can information be preserved if quantum phenomena are uncertain?

Firstly, I am a layman with an interest in physics only so please forgive any ignorance I demonstrate here. This question puzzles me and I'm sure there's a good answer. I've heard it said repeatedly ...
2
votes
0answers
65 views

How to measure a multipartite density matrix?

I consider the tensor product $H_1 \otimes H_2$ for pairs of qubits. they are described by a 4*4 density matrix $\rho$. Bob and Alice share them and measure them randomly in many directions. At the ...
1
vote
1answer
36 views

Bloch sphere representations for multi-qubit quantum systems

Just a short mixed quantum state representation question. Given a single qubit density matrix $\rho$, since the Pauli matrices form a basis for 2x2 complex matrices, the Bloch sphere representation ...
0
votes
1answer
28 views

Can a trace function be represented as a Hilbert-Schmidt product with the density matrix?

Given a trace function, say the von Neumann entropy $S(\rho) = - \rho \log \rho$, can it be express as a matrix $M$ where $S(\rho) = \mathrm{Tr} M^\dagger\rho$ is given by the Hilbert-Schmidt product?...
4
votes
1answer
63 views

Does an entangled pair remain entangled after the first measurement?

Consider an entangled pair described by the wavefunction $$\lvert1,0\rangle = \frac{1}{\sqrt{2}}(\lvert\uparrow_1\downarrow_2\rangle-\lvert\downarrow_1\uparrow_2\rangle)$$ in in the $S_z$-basis. If ...
1
vote
0answers
34 views

Entanglement networks and perspective

So, superposition is supposed to be fragile because, if any information leaks from the system, the wave function collapses. However, the very definition of 'information leaking' must set up an ...
0
votes
0answers
14 views

How does I show how the heat capacity and temperature relate for 2D graphene at low temperature?

I know that for 3D solids, heat capacity and temperature relate as C proportional to T^3. Graphene has 3 acoustic branches and one has a dispersion for which 𝜔 ∝ k^2. It's this branch at low ...
1
vote
1answer
32 views

What is the inverse of the cyclic shift operator?

I have given the cyclic shift operator $X(x) \in \mathbf{R}^{d \times d}$, the generalized pauli operator, for a qudit system. This operator is defined by the action it takes on an arbitary ket vector ...
2
votes
1answer
50 views

How to entangle two superconducting qubits?

I do understand qubits entanglement, and their spins but I do not understand that at circuit level how can we entangle 2 qubits? Is it related to 2 qubits gates?
0
votes
0answers
55 views

Pertubation Theory

For long-range barrier tunneling, consider three qubits governed by the Hamiltonian $H$. $$ H = -J(\sigma_1^+\sigma_2^-+\sigma_1^-\sigma_2^++\sigma_3^+\sigma_2^-+\sigma_3^-\sigma_2^+)+\frac{U}2\sigma^...
1
vote
0answers
21 views

Interpreting the amplitude damping channel

In page 380 of Nielsen and Chuang (Quantum information), there's a statement about the amplitude damping channel, which is defined by the operators $E_0 = \begin{bmatrix}1 & 0\\0 & \sqrt{1-\...
-1
votes
0answers
21 views

Quantum key distribution

Key advantage in quantum key distribution is using quantum channel. (The quantum channel is preserve coherent superposition of quantum bit of information). what is means (The quantum channel is ...
1
vote
0answers
18 views

How to understand the concept of a fluxon, the dual of the Cooper pair?

I've been reading the paper Bifluxon: Fluxon-Parity-Protected Superconducting Qubit by Kalashnikov et al. which deals with a novel type of superconducting qubit. Without going into too much of the ...
6
votes
0answers
49 views

A Venn diagram for: (non-)locality, (non-)realism, (non-)contextuality, (non-)signalling, (dis-)entanglement

I recently asked a (yet-unanswered) question about the relationship between state-dependence and violations of realism. The more I read on the subject, the more I find myself digging deeper in a ...
3
votes
1answer
53 views

How to prove that a state is pure if and only if it cannot be expressed as a convex combination

Define a pure state to be one that can be expressed in the form $ \rho = | \psi \rangle \langle \psi |$. How can we show that this is equivalent to $ \rho \neq p \sigma_0 + (1-p) \sigma_1 $ for any ...
1
vote
1answer
32 views

Dual of IC POVM cannot be positive

I've read a claim about POVMs in my lecture notes, which I fail to prove. Hence, I would be grateful if some of you have some hints for me /can help me. Let $\{N_i\}_{i=1}^{d^2}$ be an informationally ...
0
votes
0answers
12 views

How to filter from $N$-fold coincidence to fourfold coincidence

Let's suppose that i have as a result, 8-fold coincidence, something like $[1,0,1,0,0,0,0,1]$ and we are just interested only in the first two and the last two of this output how can i filter it to be ...
1
vote
1answer
49 views

Lindblad from infinitesimal Kraus sum representation

I have a few basic queries regarding a proof in the set of notes MIT: Open Quantum Systems, the following is stated: We can derive the Lindblad equation from an infinitesimal evolution described by ...
0
votes
0answers
10 views

Is the $N$-fold coincidences relative to the number of quantum relays?

I'm working on a project where i have to use quantum relays to ensure the entanglement swapping, and i want to know if for one relay i'll get fourfold coincidences, am i supposed to get 8-fold ...
1
vote
0answers
26 views

Why “diagonal” polarization doesn't decohere?

It is often said, that qubit should be maintained under very low temperature and kept highly isolated not to decohere. But we can represent $|0\rangle$ with vertical polarization of light and $|1\...
1
vote
1answer
43 views

Is the Von Neumann entropy of two initially non-interacting systems always increasing?

Let $A$ and $B$ be two systems that does not interact initially ($t=0$), i.e., the density matrix of the initial total system is given by $\rho(0) = \rho_A (0) \otimes \rho_B (0)$. Suppose that ...
4
votes
0answers
71 views

Can disentangled states violate local realism?

I (re-)learned about Bell tests from the ground up based on this very intuitive lecture by Spekkens. In a nutshell, realism is violated whenever measurement statistics cannot be reproduced by ...
0
votes
0answers
12 views

Transversal Hadamard gates for self-dual CSS quantum codes

I have come across a statement saying that a self-dual CSS quantum code has a transversal Hadamard gate. Initially I thought that there was a fairly elegant reasoning for this by considering only that ...
1
vote
1answer
56 views

Quantum Computation and Quantum Information - Exercise 4.16 [closed]

Exercise Description In the book "Quantum Computation and Quantum Information - 10th Anniversary Edition", there is the following exercise ($H$ being the Hadamard matrix and $x_1$,$x_2$ ...
0
votes
1answer
47 views

Average over the Bloch sphere

Consider you have a function of a two-level wave quantum state $f(\vert \psi \rangle ) $, with $\vert \psi \rangle = \alpha \vert 0 \rangle + \beta e^{\rm i \phi} \vert 1 \rangle$. With no loss of ...
3
votes
1answer
63 views

Finding operator sum decomposition of $T(X)=AX+XA$

Suppose $A$ is a positive definite $d\times d$ matrix and $T$ is a positive map over such matrices defined as follows $$T(X)=AX+XA$$ I'm wondering if if it possible to get a decomposition of this ...
0
votes
0answers
28 views

During electron-positron collision could both spins be same oriented? If not, the more of the question is how than why

Photons, as a result of annihilation should carry information regarding the law of conservation of momentum and spin of the particles that produced them but I can not figure out what force should ...
1
vote
0answers
13 views

What is the recursion step being used for the saturation of mutual information in the area law?

I am trying to understand Lemma 16 of this paper, "Exponential Decay of Correlations Implies Area Law", and I'm getting stuck on the recursion step. The essential bit is going from Equation (...
1
vote
2answers
72 views

Show completeness of general Gaussian POVM

In Continuous-Variable quantum information, a general Gaussian measurement is described by the POVM elements (say on a single mode for simplicity) $$ \Pi(\alpha) = \frac{1}{\pi} D(\alpha) \Pi^0 D^{\...
2
votes
1answer
42 views

Proof Rényi entropy is non negative

The Rényi entropy is defined as: \begin{equation} S_\alpha = \dfrac{1}{1-\alpha}\log(\text{Tr}(\rho^\alpha)) \end{equation} for $\alpha \geq 0$. This can be rewrited in terms of $\rho$ eigenvalues, $\...
1
vote
2answers
102 views

How can we show that an arbitrary state $|\Psi\rangle\in \mathbb{C}^{2}$ is maximally entangled?

I was reading about of entangled states and I encountered a concept which is called "fully entangled state" according to the following definition: Consider a two-qubit state $|\Psi\rangle$. ...

1
2 3 4 5
49