Talk:Quantum logic gate

message: The article asserts that R(\cos^{-1}\begin{matrix} \frac{3}{5} \end{matrix})) along with a couple of other gates form a "universal" set of gates.  What is the significance and/or derivation of 3/5?  Would other angles work as well?  What is the critical property of the particular angle?  --David Battle 01:39, 18 August 2005 (UTC)
replies: text: The fact that it's usable at all is due to the fact that it divided by 2pi is irrational, I believe. For all irrational numbers x all real numbers r in [0,1) and all open intervals that include r, there is an integer n such that nx mod 1 is in the interval. Why this specific angle? Well, e^{\theta i} = \cos x + i \sin x, so e^{\cos^{-1}\begin{matrix} \frac{3}{5} \end{matrix}} = \cos (\cos^{-1}\begin{matrix} \frac{3}{5} \end{matrix}) + i \sin (\cos^{-1}\begin{matrix} \frac{3}{5} \end{matrix}), which I believe is \frac{3}{5} + i \frac{4}{5}, whose absolute value is 1. The associated angle (the argument of this number) is irrational--I think. Actually, using e^i for that term of the matrix should work as well, and it's pretty simple, too. Now my question is why an angle is being multiplied by 2pi. --Ihope127 23:10, 19 June 2007 (UTC)
replies: text:  You're right, multiplying an angle by 2pi in the phase gate example doesn't make sense. I have removed it. --V79 (talk) 17:15, 1 May 2009 (UTC)
thread_title: Comments

message: concerning articles :1:Quantum gate, 2:Quantum Gate and 3:Quantum Gate (PC game)
previously, 3 redirected to 2 which held the info about the game, which i thought was very confusing, as it was clear to me that 2 should redirect to 1, with 3 holding the information about the game, so i moved the game information from 2 to 3, and redirected 2 to one, this will be coppied on the other two articles' talk page 193.60.83.241 (talk) 11:47, 14 May 2008 (UTC)
thread_title: Quantum gate (PC game) confusion

message: Do the logic gates allow superpositional values to act as operators or instructions? If they do, that's not really coming through in the article. If they do not, then is this a fundamental limitation of the quantum phenomena, or is it just much simpler to build a QC with "ordinary" logic gates? While the answers to these questions might turn out to be rather awkward, I think they'd go a long way to making the idea of quantum computing more accessible to a larger number of people. Thank you! -- 17:38, 22 April 2014 (UTC)
replies: text: Superpositions are those values that are not aligned perfectly along the base vectors, which represents the observable states of the variable. It is a medium-large project to 1) figure out *which* pieces of knowledge are the basics of quantum computing, and 2) to wrap the (mine/your) head around them -- so expecting it to just jump out at you and explain itself is a bit.. much to ask. Of course it would be awesome if every article was awesome that way, but we are just humans. Also, ***you*** can enhance it. 2001:2002:51E3:8007:B66D:83FF:FE0E:C298 (talk) 20:36, 7 September 2017 (UTC)
thread_title:  Superpositions within Matrices 

message: Assume Deutsch gates are permitted to take an arbitrary rotation as parameter, e.g. set via a potentiometer-like control attached to each Deutsch gate that can be rotated smoothly about 360 degrees when manually constructing the circuit.  Presumably this would permit realizing any unitary operation on n qubits with finitely many quantum gates.
As a function of n, what is the minimum number of Deutsch gates needed to be sure of being able to implement any unitary operation?
It seems likely this has already been determined.  If so this section of the article should say something about it. Vaughan Pratt (talk) 18:09, 1 December 2015 (UTC)
thread_title:  Universality via Deutsch gates 

message: Where can I learn about physically realizations of these various gates?
Meekohi (talk) 21:01, 2 December 2019 (UTC)
replies: text: Check out the "Physical Implementations" section of the infobox at the bottom of the page. The two most popular models of quantum computing are the superconducting qubit (particularly transmon) and trapped ion computers. Those pages have some information on how gates are implemented, but it will vary by architecture (and potentially even by device). --Rxtreme (talk) 05:41, 10 December 2019 (UTC)
thread_title:  Physical Manifestation? 

message: The section on Unitary inversion of gates claims that the Pauli Y gate is not Hermitian.
The Pauli Y gate is Hermitian because the Pauli y matrix/operator is hermitian (as are all the Pauli matrices).
I have edited the page to give the Ising (XX) gate instead as an example.  65.130.201.235 (talk) 19:33, 9 February 2020 (UTC)
replies: text: that's correct, of course, thanks. Maybe a more standard gate like \pi/8 (T) would be a better example? --Qcomp (talk) 22:20, 9 February 2020 (UTC)

text: Thank you for this correction! I am adding phase shift gates as examples to the article. Omnissiahs hierophant (talk) 20:05, 28 February 2020 (UTC)
thread_title:  Pauli Y operator is Hermitian 

message: The top of the article has a Template:More citations needed but the article has already many references. It would be better if we could have in-line templates or specific templates per section.--ReyHahn (talk) 18:31, 10 May 2021 (UTC)
replies: text: Thank you so much for your contributions with rotation operator gates/SU(2)! :D It is joyful to read your texts. -- The references can be made better, though. I have been thinking a lot about the references, and basically the entire content of the article is just mathematical consequences derived from 1) Born rule, 2) how serial gates works, 3) how parallel gates works. Given just these three anchors, it's possible although time-consuming to figure the rest out by oneself. ← This was pretty much what I did, and I sometimes flip out and go over the math again, and again, and again, as maybe I got something wrong regarding Measurement and the two sections about entanglement. It HOPE that is possible to read the article from top to bottom without encountering anything upsetting. (Except maybe for "note3" in the section about the Hadamard Transform. I have not found a source for that somewhat wild statement, although honestly I think it's correct.) Also that someone has identified the gates listed in the article, that they are real and not something we made up (although, it does not matter which gates one use, but anyways).  · · · Omnissiahs hierophant (talk) 05:06, 11 May 2021 (UTC)
replies: text: I agree as many of these facts are almost mathematical not everything has to be referenced (like the gate part) but we have to be sure we are using the right conventions (note 3 seems relevant, let me see what can I find). For the basic gates (Clifford gates, T, rotation gates) a simple ref to Nielsen & Chuang would suffice. I have just read up to the universal gates part, I will check the rest later but I see that there are some ref already. For quick verification for quantum gates we can also use Qiskit textbook/documentation, Quantum Inspire knowledge base] or N&C.--ReyHahn (talk) 08:58, 11 May 2021 (UTC)
thread_title:  References missing 

message: I was the one that expanded on the Ising gates and the related complex SWAPs. But maybe it makes the section of the gates longer and does not really add to it. Also we do not have a circuit representation for those gates. Should we remove them from the article for better fluidity?--ReyHahn (talk) 20:09, 6 August 2021 (UTC)
replies: text: No please! They are interesting! It is our only anchor to iSWAP and trapped ion machines, and maybe gates that are somewhat close to hardware gives some depth :)  · · · Omnissiahs hierophant (talk) 23:30, 6 August 2021 (UTC)
thread_title:  Removal of the Ising gates 

message: Is there any reason the Mølmer–Sørensen gate is excluded in this article? There being no objections I could write up a section on MS gates.  — Preceding unsigned comment added by Andrewvh4 (talk • contribs) 19:15, 11 October 2021 (UTC)
replies: text: Go ahead, with the proper references.--ReyHahn (talk) 19:18, 11 October 2021 (UTC)

text: Yes please write about it :) It seems to me from just reading the mapping on the basis states (g = 0, e = 1) that this gate is maybe something like a phase-shifting Hadamard-CNOT? Tried a little to find the matrix that does exactly that mapping, they don't give it as a single matrix in the sources..  · · · Omnissiahs hierophant (talk) 11:01, 15 November 2021 (UTC)
thread_title:  Mølmer–Sørensen Gate 

message: Global phase is weird. I am starting to think that maybe I have been wrong about it even existing, doubting myself. But, in the books they kindof say that it does exist. For example, by saying that gates belong to U(2), and not SU(2). They also reference it by including it in factorizations.. But, however, IBM says that the Hamiltonian for it is useless (link).
It has to exist, otherwise single-qubit gates would not belong to U(2), but SU(2). And intuitively (although I have not tested this), if one inverts the global phase early in the Grover's algorithm, then that would cause the algorithm amplify (with Grover's diffusion operator) all the wrong values, instead of the correct value(s) - essentially inverting the search query. 
Why are people suggesting global phase does not exist?? Is it because it does not matter at all in normal quantum physics?  · · · Omnissiahs hierophant (talk) 16:10, 20 January 2023 (UTC)
replies: text: Global phases do nothing in quantum circuits, as long as it is truly global. Note that you could write all quantum circuits with SU(2) gates and it will work the same. We just use U(2) gates because it is easier to write some gates in this form. For example, we could use iX instead of X as our bitflip gate, but then controlled-iX is not equivalent to CNOT (which is one of the most helpful ones), so we prefer use U(2). I do not see why a global phase would make the Grover's algorithm not work, could you provide more details? --ReyHahn (talk) 16:44, 20 January 2023 (UTC)
replies: text: IDK, Maybe I should not have written that, without proof or anything. And maybe this topic is moot since I think there is no disagreement in that global phase is real and exist. The reason I picked this up and made it a topic is that I see various people saying online that SU(n) is the symmetry group, and not U(n). Because phase (or even relative phase) is not visible anyways. But it is! Global phase can be converted to relative phase (via P(x)Rx(2x)=Ph(x)) and relative phase is visible through usage of alternative basis (slot in the X or Y matrix instead of Z as the projection operator). Maybe these misunderstandings go away if more details are written about measurement. This article always uses computational basis (ie Z), and it is maybe not clear that other basis are equally valid idk.. "The expectation value of an observable A for a system in a state \psi is given by the inner product \langle \psi, A \psi \rangle." // Dirac–von Neumann axioms. IDK maybe I should just be silent, but this has been bothering me for so long, and derp. I've already written this blob of text already now. This whole thing can be explained better in the articles about quantum this-and-that maybe. Also maybe I want to know if my world-view is wrong, if it actually is SU(n) and what the reason is for people saying this. We dont need it, but it is there.  · · · Omnissiahs hierophant (talk) 14:42, 7 October 2023 (UTC)
thread_title:  About global phase 

message: I am pretty lay when it comes to the math behind quantum mechanics. I think a lot of people may have an interest in stuff like this. However, in just a few paragraphs the article jumps into things that only physics grads are going to understand. It would be nice if there was some more general and less intimidating information about how this works and its applications. The kinds of info a lay person could understand. As I understand it this page should really just describe what a Q logic gate is and how they can be applied. There should be another page with types of gates and the more complex descriptions of the mathematical formulas. I imagine that your average person reads of few lines on this page and gives up right away. Since this is an encyclopedia, it should contain the simplest way to understand how things work alongside the complex information. Thanks 162.246.112.154 (talk) 19:51, 21 April 2024 (UTC)
thread_title:  Reads like an advanced quantum mechanics text book.