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
24 views

Transitions between states

In the time dependent perturbation theory, transition between two states requires time dependent hamiltonian, V(t). The question is, why the constant or time independent perturbation does is not ...
0
votes
0answers
21 views

Interaction (Dirac) picture and time dependent perturbation theory

Why in many, if not all, references that discuss the time dependent perturbation theory, they start the discussion with the interaction (Dirac) picture, although, what we need is only solving the time ...
-1
votes
0answers
34 views

Creation $n$-particle states

The n-particle states are defined by the creation operators on the vacuum state orthonormalized How we prove that the n particle states are ? How we prove:
1
vote
1answer
31 views

Generalised measurements and collapse

My reference is Nielsen and Chuang's "Quantum Computation and Quantum Information". When introducing qubits, $|\psi\rangle=\alpha|0\rangle+\beta|1\rangle$ the authors state that (*)When we measure ...
1
vote
1answer
73 views
+50

Coarse-graining and probabilities in quantum theory

What is the origin of probabilities in quantum theory if it is not postulated? I mean, how can we interpret Born's Rule as a probability and how does it enter in quantum theory? More than this, I want ...
0
votes
0answers
12 views

Bit threads as noisy quantum channels

In literature on the holographic duality, recently a new way of looking at the Ryu-Takayanagi formula has been presented, where the minimal surface area has been replaced by the maximum number of "bit-...
3
votes
1answer
93 views

Is there any definition for $(\hat{a}^{\dagger} \pm \hat{a})^{n}\hat{S} \left | 0 \right> $

Specifically, I want to apply the following operators $\hat{P}^n = \left( \hat{a}^{\dagger} - \hat{a} \right)^n \tag{1},$ $\hat{X}^n = \left( \hat{a}^{\dagger} + \hat{a} \right)^n \tag{2},$ (...
1
vote
0answers
8 views

Difference between multiphoton signal and, $n$-photon signal state

I was reading decoy state quantum key distribution article, by Lo et.al. (PRL paper , PRA paper). In this article, authors have discussed the possibility of photon number splitting attack due to ...
0
votes
1answer
111 views

Analyzing a quantum circuit

This is an exercise from Nielson Chuang that I am totally stuck at. First I can't understand why the measurement outcomes are related to the operation on the target bit at all. Second, why is the ...
0
votes
1answer
146 views

Properties of controlled $z$-rotations

Given $n$ qubit gate of the form c-z-z-z... (shorthand for c-z between qubit 1 and 2 followed by c-z between 1 and 3 and so on up to n qubits) it seems to be possible to find local unitaries which ...
8
votes
2answers
256 views

Homodyne detection as quantum measurement

How is homodyne detection a quantum measurement? In quantum mechanics, the way I'm used to think about obtaining the expectation value of an operator $A$ when the state is $| \psi \rangle $ is as ...
2
votes
0answers
61 views

Partial trace of matrix product state

I have come accross a formula that puzzles me a bit in the proof of lemma 23 (page 32) of this paper. The authors start from a (translationally-invariant) matrix product state: $$\lvert\psi\rangle := ...
0
votes
0answers
21 views

Viewing quantum teleportation as a channel

Suppose Alice has two qubits $A'$ and $A$ and Bob has one qubit $B$. The system is in the state $$\rho_{A'}\otimes \Phi^+_{AB} $$ where $\rho$ is an arbitrary state and $\Phi^+$ is the maximally ...
1
vote
1answer
79 views

What Prevents Delayed Choice Quantum Eraser Experiment from Being Used to Predict the Future?

Reading about the DCQE experiment commentary by Ross Rhodes. My question is: why can't information about the future state of the idler photon (the one that heads toward the Glen-Thompson prism) be ...
4
votes
3answers
184 views

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 ...
-3
votes
0answers
38 views

What is a non-square density matrix?

At first glance the idea of a non-square density matrix makes no sense because we cannot normalize it to have unit trace. But we can certainly form an outer product $|a\rangle\langle b|$ with $|a\...
1
vote
1answer
254 views

Finding basis of Schmidt decomposition

How do you find the basis for the Schmidt decomposition when given a state of multiple qubits? For example, if you have the systems $A,B$ and $C$, how do they correspond to eigenvectors of a density ...
1
vote
0answers
28 views

what we call “erase” in the delayed choice quantum eraser experiment?

im thinking about the quantum eraser experiment and I got some insight that I want to check. Suppose that I have the system of the experiment (S): a closed box with 2 slits, with a 1 bit detector (D)...
1
vote
1answer
118 views

Can all expectation values be constructed using only tensor products of local Pauli operators?

I understand that the Pauli operators and the identity matrix span the space of complex 2x2 matrices. Let's say you have two qubits and you can perform projective meausurements. So you can measure ...
0
votes
1answer
49 views

Can any bipartite state be written as $\rho_{AB}=\sum_{ij} p_{ij}\rho_A^i\otimes \rho_B^j$?

I'm having a silly doubt. If $\rho_{AB}$ is separable then we can write it as $$ \rho_{AB}=\sum_i p_i\rho_A^i\otimes \rho_B^i$$ but can we write a general (maybe entangled) state as $$ \rho_{AB}=\sum_{...
-1
votes
0answers
16 views

Quantum Entropy Venn diagrams

We know that in classical information theory the Shannon Entropies follow such Venn-Diagram approach to summarize its' relations and inequalities between-marginal,joint and conditional entropies. Is ...
3
votes
0answers
61 views

Experimental Parameters and Unitary in a specific Boson Sampling Experiment

I tried to reproduce the data of one of the four historically first BosonSampling experiments. While they present the information in a very accessible form, unfortunatly I fail to understand some ...
3
votes
2answers
44 views

Is it possible to use Bayesian method in improving the measuring conditions and accuracy of an electron's (or photon) momentum and position?

for you who need a definition for what a "Bayesian method" is, and as per wiki's easy definition. it's a method of statistical inference in which Bayes' theorem is used to update the probability for a ...
9
votes
0answers
172 views

What are good books covering information theoretic approaches to theoretical physics?

I am about to finish my undergraduate studies and am very interested in going into the applications of information theory to either general relativity, or quantum mechanics. However I have been ...
4
votes
1answer
141 views

Conditional probability between time parameter and operator in quantum mechanics?

Question and Background So I came across a question on conditional probability in quantum mechanics: How is conditional probability handled in quantum mechanics? There's an interesting comment which ...
3
votes
1answer
193 views

Quantum annealing computing

What is Quantum Annealing and quantum annealing computing and what are its advantages and disadvantages with respect to quantum circuit quantum computing/computers?
3
votes
2answers
174 views

How to define information flow?

If there are two interacting subsystems A and B, how to define the information flow from A to B in quantum regime and classical regime?
1
vote
1answer
54 views

When does the identity $\|\mathrm{Tr}_B UXU^\dagger\|_1=\|\mathrm{Tr}_B X\|_1$, with $U$ unitary, hold?

Let $\mathcal{H}_{AB}$ be a bipartite, complex Euclidean space, and let $U\colon\mathcal{H}_{AB}\to\mathcal{H}_{AB}$ be a unitary operator. Define the trace norm as $$ \lVert X\rVert_1 = \text{Tr}(\...
0
votes
1answer
124 views

Can one test an octonionic interpretation for a quantum-information conjecture, apparently valid in the real, complex and quaternionic settings?

For the values $\alpha = \frac{1}{2},1, 2$, corresponding to real, complex and quaternionic scenarios, the formulas (https://arxiv.org/abs/1301.6617, eqs. (1)-(3)) \begin{equation} \label{Hou1} P_1(\...
0
votes
1answer
32 views

Hamiltonian for 2-qubit system

What is the general form of a Hamiltonian for a 2 qubit system? The Hamiltonian for D-Wave system is: $H = K_1\sigma_x^1 + K_2\sigma_x^2 + H_1\sigma_z^1 + H_2\sigma_z^2 + J_{12}\sigma_z^1\sigma_z^2 ...
0
votes
0answers
47 views

What is the nature of the quantum state that could be assigned to a large solid object, that could describe all it's degrees of freedom?

Consider a metallic object. To study it's electrical and thermal properties, we could construct the band structure arising from the Bloch wave vectors of individual electrons in periodic potential. ...
0
votes
1answer
47 views

Mean value of a discrete operator

Given an orthonormal basis $\{|i\rangle\}$, I am dealing with an operator that looks like this: $$L=\sum_{j=1}^N|j\rangle\langle j|+\sum_{j\ne k}|j\rangle\langle k|.$$ I would like to calculate its ...
1
vote
2answers
365 views

Measuring polarization - problem with understanding

Let's assume that we have 2 polarizing filters. First with vertical (1) orientation and second with horizontal (0). I want to measure probability that photon passes through those 2 filters. I have: $...
0
votes
1answer
128 views

Partial trace question: how does this term become zero?

I have recently been introduced to the partial trace but I have a question about how my lecturer recognises 'zero' and 'non-zero' terms. Consider the state: $\psi_{AB} = c_{00}|00\rangle + c_{01}|01\...
4
votes
1answer
129 views

Is HaPPY code a certain type of MERA?

Pastawski, Yoshida, Harlow, and Preskill introduced the HaPPY code in their (now famous) paper, arXiv:1503.06237, as a way to model the AdS/CFT correspondence as a quantum error-correcting code. ...
0
votes
0answers
42 views

What's the meaning of a new quantum state vector obtained from an operator acting on original quantum state vector

Let's say we have a single photon state vector, i.e., $\left|1\right>$. We know that the state vector is normalized since $\left<1\right|\left.1\right>=1$. If we are taking a creation ...
3
votes
1answer
121 views

Can scatter interference patterns from entangled pairs carry information?

I am having problems wrap around why you can't send information faster than light with a similar setup to the quantum eraser experiment. ( https://arxiv.org/abs/quant-ph/9903047v1 ) What I mean is, ...
5
votes
3answers
5k views

How many Bits are in a Qubit?

How many bits of information can be stored in a qubit? On making a measurement on a qubit, one can catch it in either $|0\rangle$ or $| 1\rangle$. Seems that measurement on a qubit can not reveal more ...
-1
votes
0answers
25 views

Distance and speed of quantum entanglement

I'm missing a lot of basics to be honest, but as far I understand quantum entanglement if you manipulate a particle the second one will have the same quantum state. If that is true, how is that about ...
0
votes
1answer
78 views

Understanding the density matrix for pure states vs. mixed states

So, there's a question in my textbook that explains the following scenario: A physicist runs two experiments A and B to prepare quantum systems in a variety of initial states. In experiment A he ...
7
votes
1answer
109 views

Fusion of anyons

I have been studying anyons and I have found the algebraic approach rather abstract and I am struggling to understand it as it seems quite different to the usual procedure of quantum mechanics. I do ...
1
vote
1answer
50 views

What observable(s) does a mirror operate on?

I'm reading this introduction to the Mach-Zehnder interferometer, and I'm confused about what the mirrors are doing. Suppose we're describing the path (upper / lower) and polarization (vertical / ...
1
vote
1answer
32 views

Which spin-1/2 photon states can be used in the context of Bell's inequality violations?

In textbooks, the violation of Bell's inequality is usually demonstrated for spin-1/2 systems in the state $$| \psi \rangle = \frac{1}{\sqrt{2}} \left( | +- \rangle - | -+ \rangle \right).$$ Most ...
1
vote
1answer
80 views

How to determine the purity and nature of a composite system?

So for the combined state of a pair of two-level atoms, A and B, with a density matrix $$\rho =\frac{1}{2}\lvert g_A, g_B \rangle\langle g_A, g_B \rvert + \frac{1}{2}\lvert g_A, e_B \rangle\langle ...
-2
votes
0answers
67 views

Bell's experiment - Why doesnt entanglement violate speed of light? [duplicate]

If i understand this correctly, Bell's experiment is trying to distinct between these two things: 1]Entangled particles obtained their spin upon measurement. 2]Entangled particles had their spin ...
2
votes
1answer
50 views

Why is the diamond norm called like that?

The diamond norm is a measure of the distance between two quantum channels. If $\mathcal{E}_1$ and $\mathcal{E}_2$ are two channels, then the diamond norm is defined as $$ \delta^{\diamond}(\mathcal{...
3
votes
2answers
2k views

I don't understand the no-communication theorem

I don't understand the no-communication theorem. Okay, first I'll say the bit I do understand about it: if Alice and Bob both have two atoms, such that Alice's atom 1 is entangled with Bob's atom 1, ...
5
votes
2answers
455 views

In layman terms: Why can't Quantum Entanglement be used to achieve FTL Communication? [duplicate]

I've found a lot of questions that try to answer this, but I can't quite understand the answers so far. If you can make two particles be entangled, why can't you force one of them into a pattern that ...
12
votes
3answers
1k views

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 ...
3
votes
0answers
64 views

Can we create “asymmetric” EPR pairs in experiments?

Asymmetric here is just for lack of a better word. What I am looking for are experiments that can create a simple EPR pair but without 50% 50% chance of each state. As I understand from standard ...