Quantum information Definition and 104 Threads

Hello, kind souls. I am planning to finish my bachelor in the start of next year (early February to be precise). Before I go for my PhD (I will skip the master, something not unusual in my country, especially after a long bachelor like I did), I wish to get some more hands-on research...

I was watching John Michael Godier and he mentioned that since quantum measurements can change the measurement in the past (known as a delayed choice quantum eraser); that you potentially could send a message back in time. You would however have to hope the someone does the same measurement in...

I am not sure how to show this inequality, I guess what is the relation between ##Trace(\sigma^2)## and ##Trace(\sigma)##? Here's my latest attempt, let me know how to proceed? $$D(\sigma, I/2^m)\le \sqrt{1-F(\sigma,I/2^m)^2}=\sqrt{1-(Tr(\sqrt{\sigma }))^2/2^m}$$

Why is the quantum state of the Bose-Einstein distribution mixed and not pure? Is there any mathematical proof on this?

I considered an operator ##X \in \mathcal{L}(\mathcal{X} \otimes \mathcal{K})##, that is positive, ##X \geq 0##. And I defined it as it follows: ##X = \sum_{i,j} a_{ij} ∣x_i \rangle \langle x_i ∣ \otimes ∣k_j \rangle \langle k_j∣ ## Where ##x_i## are basis for ##\mathcal{X}## and ##k_j## basis...

In the context of a single phase estimation problem of a quantum photonics experiment. For example consider a 3-photon quantum circuit (such as the Mach-Zehnder which depends on some phase shift operator which encodes a parameter ##\theta##) with a photon counting measurement (two detectors) at...

Hey all, I was just wondering where I could find a current review of the major problems that are being discussed at the intersection of condensed matter theory and high energy theory (+maybe quantum information)? Just looking for some inspiration on what the most "popular" problems being...

I've make progress in obtaining the values for the mutual information using the following: $I(\rho_A:\rho_B) = S(\rho_A) +S(\rho_B) - S(\rho_{AB}) = 1 + 1 - 0 = 2.$ I would like to compute the expectation but I'm facing a problem in the case of $\langle\psi |\mathcal{O}_A|\psi \rangle$ since...

Does it make sense to explain entanglement as a consequence of the ‘singularity' of the early universe? If proto matter/energy was in some mathematically definable ’small’ space, perhaps information was likewise compressed, and entanglement is a consequence of this early cosmological locality...

Ballentine, in his Chapter 8.1, appears to give the attached recipe for *in principle* preparing an (almost) arbitrary (pure) state (of a particle with no internal degrees of freedom) by the method of "waiting for decay to the energy ground state". My questions are fourfold: 1) From (8.1), we...

TL;DR Summary: Imagine you have infinite funds and cooperation to build a very ideal curriculum and student organization, both dedicated to quantum information science, at a university. What does this look like pedagogically? How would this be structured? What resources do students need access...

Hello to everyone, I would like to ask you to brief questions. The first one is whether you could recommend any pedagogical books on Quantum Information and Computation. I tried Nielsen and Chuang but I found it too dense for a beginner in the field. The second question is the following: to...

Alain Aspect, John Clauser & Anton Zeilinger have rightfully received the Nobel prize for their contributions to quantum information, as they were three of the main pioneers of quantum information. However, is it now impossible or very unlikely that other physicists working on this field (e.g...

Hello, I am currently studying about entanglement on spin-1/2 chains and I was able to find some information about the mathematical point of view of concurrence but I can't understand the physical meaning of it . Can somebody help me, please?

Hi, I'm happy to change ideas in Physics Forums, especially in my interesting fields, include quantum fisher information, and quantum entanglement detection and quantum metrology!

Hello, I am a undergraduate student of physics from nepal. I am passionately curious to understand about the fundamental physical laws of nature. Selectively ,I am interested and passionate to understand quantum physics . I would like to work and research in the field of quantum information...

The K-L condition has projection operators onto the codespace for the error correction code, as I understand it. My confusion I think comes primarily from what exactly these projections are? As in, how would one find these projections for say, the Shor 9-qubit code?

In general, if R is the recovery channel of an error channel ε, with state ρ, then and according to these lecture slides, we get the final result highlighted in red for a bit flip error channel. I am simply asking how one reaches this final result. Thank you (a full-ish derivation can be found...

Hi, I'm going through Nielsen and Chuang's Quantum Computation and Quantum Information textbook and I don't really understand this part about quantum parallelism: Shouldn't the resulting state be (1/sqrt(2^4)) * (|0, f(0)> + |0, f(1)> + |1, f(1)> + |1, f(0)>), since the resulting state would...

I studied at UBC, U of Leiden, in quantum statistical mechanics and kinetic theory. I worked in chemistry at McGill University for many years doing the theory of NMR which involves spin theory. In 1997 I gave up NMR and started to study the EPR paradox. I do not follow the party line because I...

I have used the Lagrange multiplier way of answering. So I have set up the equation with the constraint that ## \sum_{x}^{} p(x) = 1## So I have: ##L(x,\lambda) = - \sum_{x}^{} p(x)log_{2}p(x) - \lambda(\sum_{x}^{} p(x) - 1) = 0## I am now supposed to take the partial derivatives with respect...

What would you say is essential reading for those of us who want to understand how exactly is QFT benefiting from QI? Can anyone give a summary of what is happening, and where to start reading? (at postgraduate/entry-research-level).

In this paper, on quantum Ising model dynamics, they consider the Hamiltonian $$\mathcal{H} = \sum_{j < k} J_{jk} \hat{\sigma}_{j}^{z}\hat{\sigma}_{k}^{z}$$ and the correlation function $$\mathcal{G} = \langle \mathcal{T}_C(\hat{\sigma}^{a_n}_{j_n}(t_n^*)\cdot\cdot\cdot...

I've read these two pages that discuss going from qubit to continuous variable - https://arxiv.org/abs/quant-ph/0008040 and https://arxiv.org/abs/1907.09832 . I'm curious if anyone knows some papers that discuss going the other way around? I.e. qubitizing a continuous variable model? Any insight...

Consider a VCSEL laser that emits photon pulses with Poisson distribution for the number of photons per pulse. The power of the VCSEL has been set low so the mean photon number "u" is u<1. Consider this photon pulses can take two non-orthogonal states of polarization (for example: state 0 with...

I want to pursue research in Quantum Information Theory. I am surveying research topics before applying for PhD in Physics. Can somebody please enlighten me on the latest broad research problems being pursued in Quantum Information?

Here's what I think I understand: First off, the GHZ state ##|GHZ \rangle = \frac {|000\rangle+|111\rangle} {\sqrt 2}##, and ##\sigma_x## and ##\sigma_y## are the usual Pauli matrices, so the four operators are easy to calculate in Matlab. I'm thinking the expectation values of each operator...

Am I correct in thinking that the system measures the probability |<f|1>|^2 for some state <f|? Then the probabilities for each of the six states would be: |<0|1>|^2= 0 |<1|1>|^2= 1 |<+x|1>|^2= |(1/√2)|^2 = 1/2 |<-x|1>|^2= |(-1/√2)|^2 = 1/2 |<+y|1>|^2= |(-i/√2)|^2 = 1/2 |<-y|1>|^2= |(i/√2)|^2...

Part a: Gate H X Y Z S T R_x R_y Theta pi pi pi pi pi/2 pi/4 pi/2 pi/2 n_alpha (1/sqrt(2))*(1,0,1) (1,0,0) (0,1,0) (0,0,1) (0,0,1) (0,0,1) (1,0,0) (0,1,0) Using the info from the table and equation 1, I find: U_H=(i/sqrt(2))*[1,1;1,-1] U_X=i*[0,1;1,0] U_Y=i*[0,-i;i,0] U_Z=i*[1,0;0,-1]...

Given that operator ##S_M##, which consists entirely of ##Y## and ##Z## Pauli operators, is a stabilizer of some graph state ##G## i.e. the eigenvalue equation is given as ##S_MG = G## (eigenvalue ##1##). In the paper 'Graph States as a Resource for Quantum Metrology' (page 3) it states that...

Reduced graph states are characterized as follows from page 46 of this paper: Proposition: Let ##A \subseteq V## be a subset of vertices for a graph ##G = (V,E)## and ##B = V\setminus A## the corresponding complement in ##V##. The reduced state ##\rho_{G}^{A}:= tr_{B}(|G\rangle\langle G|)## is...

I can't seem to wrap my head around the notion of conservation of quantum information. One thing that might help that is if someone can tell me what the associated symmetry is. For example, phase symmetry leads to conservation of electric charge according to Noether's theorem; a fact that...

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 this theory to concrete examples. Let us take a ##|\Phi^+\rangle## Bell State as an example, and...

I am 3rd year physics student (actually I have just finished it). I have good knowledge on basics of quantum mechanics: I had 1 semester of Introduction to Quantum Physics and then 2 semesters of Quantum Physics. Our literature were Griffiths (Introduction to Quantum Mechanics) and ''Gennaro...

I am an undergraduate doing research on QC/QI. My current topic to learn is continuous-time quantum walks, but first I must learn the random quantum walk. That being said, I was wondering if someone could simply explain what a random quantum walk is and then explain how they could be useful with...

I have been asked to draft a (informal) research proposal for a PhD thesis. I have some background in quantum information and my interests are leaning towards the AdS/CFT correspondence. I was wondering if you could suggest a few (preferably recent) theory papers at the junction of quantum...

I have recently been reading some stuff on quantum information in the physics literature which refers to 'a mechanism by which a measurement in A determines quantum coherences in B', where A and B are subsystems of a larger system. I am aware of the meaning of the terms 'decoherence' and...

##U_1 \otimes U_2 = (1- i H_1 \ dt) \otimes (1- i H_2 \ dt)## We can write ## | \phi_i(t) > \ = U_i(t) | \phi_i(0)>## where i can be 1 or 2 depending on the subsystem. The ## U ##'s are unitary time evolution operators. Writing as tensor product we get ## |\phi_1 \phi_2> = (1- i H_1 \ dt) |...

I am a master student in theoretical physics from Italy and I would like to know more about fields related to "Quantum Information". I've been to some seminars and I think that these fields are very interesting, but I need to understand before applying for a PhD. What should I do?

I was wondering how to measure the first or even the second qubit in a quantum computing system after for example a Hadamard Gate is applied to the system of these qubits: A|00>+B|01>+C|10>+D|11>? A mathematical and intuitive explanation would be nice, I am a undergraduate sophomore student...

I'm a math major interested in the field of quantum information science and quantum computing, but I'm unclear on what path to take to explore the field. The field is presented as being at the intersection of physics, computer science, and mathematics but it seems that a majority of the experts...

I'm having trouble understanding the phase shift produced by a beam splitter. I seem to be finding conflicting information. I'm specifically looking to understand a 50/50 beam splitter where one side has a dielectric mirror, as shown in this figure from wikipedia: I understand the pi...

Hello, I am close to finishing my undergraduate degree in Computer Engineering, and I am very interested in pursuing graduate studies. For a long time, I have been passionate about computer science and I've been looking into the research done in various labs in the schools that I'm considering...

Homework Statement J-coupling term between two spins is HJ = ħJ/4 σz(1) σz(2) In the measured magnetization spectrum of the spins, this leads to the splitting of the individual spin lines by frequency J, which we’ll now derive. We can write the magnetization of spin 1 as: <M1(t)> =...

The following, regarding quantum measurement, is stated in the paper "Limitation on the amount of accessible information in a quantum channel" : "Our discussion of measurement will be based on a specific physical model of measurement, to which we now turn. Suppose we have a quantum system ##Q##...

I am applying to phd programs in physics right now. I am interested in doing research in quantum information science. I see there is a lot of interest and funding for quantum information science right now, but does anyone have any advice on whether or not this research will be well funded in the...

Homework Statement I am supposed to construct a controlled Hadamard gate using only single qubit and CNOT gates. Homework Equations [/B] We know that any arbitrary unitary Operator U can be written as the Martrix product U=AXBXC, where X is the NOT-Matrix and ABC=1 (identity matrix) I've...

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I am well aware that QC-related graduate programs are competitive so I am preparing myself for a rejection. Not because I'm unconfident. But because everyone should have a backup plan just in case. I haven't applied yet because I'm about to take the GRE. I really do enjoy both quantum physics...