Talk:Quantum Fourier transform

This page is to me lacking an explanation of how the quantum fourier transform is in practice built up out of a combination of Hadamard and controlled rotation gates, a good explanation can be found at sbandrews 18:37, 18 December 2006 (UTC)

I agree that the page lacks information about the quantum circuit implementation of the QFT. Also, it seems to me that the definition of the QFT given here is backwards to the usual definition of a Fourier transform. That is, the definition here looks like it is actually for the inverse QFT. To see what I mean, compare this article with the Discrete Fourier Transform, or with just the Fourier Transform articles. It makes sense to me that the FT should have a minus sign. The whole point of the FT is to extract a frequency spectrum. However, this is further confused by the fact that the definition given here matches the definition in Nielsen & Chuang's book. Karadoc** 04:08, 2 July 2007 (UTC)


 * I noticed that too about the minus sign, here and in other QFT papers, perhaps it is just a covention amongst QFT researchers, as it will (i think) still give a fourier transform, just with the coefficients reflected about k=0, if it *is* a convention it would be worth referring to it in the article sbandrews (t) 05:37, 2 July 2007 (UTC)

What happened to the old circuit diagram? The recursive implementation of QFT is so much nicer. And whose idea was it to include screen-grabs of messing around in q-kit. Call me a conspiracist, but methinks someone might be trying to promote q-kit via wikipedia. — Preceding unsigned comment added by 134.160.214.34 (talk) 05:06, 15 June 2018 (UTC)

Advertisement
An account called Dev.qkit has added three references in the article to their website, along with some terrible low resolution gifs of their product. I have removed them. They are not even constructive to the article. I hope they do not come back. — Preceding unsigned comment added by 2610:148:1F02:7000:72BF:18D6:948:D41B (talk) 02:27, 7 October 2019 (UTC)

Circuit
Shouldn't the second wire have the transformations $$H, R_2, R_3, ... R_{n-1}$$? The diagram makes it seems like it doesn't continue after $$R_2$$. --132.69.202.250 (talk) 17:58, 7 February 2020 (UTC)

Inverse QFT
Is the matrix for the Inverse QFT the same as the "forward" QFT, just with negative exponents? Just asking for clarification if I interpreted the notation correctly - I find the matrix form easier to read than the summation formula. Abinmorth (talk) 02:38, 5 October 2021 (UTC)

More footnotes
As further improvements are made, please attach inline references to help the reader figure out what content is cited to which source. Thank you. Jehochman Talk 11:04, 19 October 2022 (UTC)

Numer of amplitudes in the discrete Fourier transform
It is not clear to me why the number of amplitudes is $$2^n$$, for a general DFT the number of operations required is only $$n^2$$. UrielSeptim (talk) 07:38, 11 April 2023 (UTC)

Some problems

 * 1) In the definition of the Fourier transform I wouldn't use n for the index in the \Sigma, as n is already denoting the number of bits.
 * 2) There is an error in the definition of the R_n gates: the definition does not depend on n. I believe the correct definition should have R_n(2,2) = e^2\pi*i / 2^n
 * 3) Also, again, I wouldn't use n for the indexing of the R_n's.

Doobiefletzet (talk) 13:37, 30 May 2023 (UTC)