Support Measurement Gates

[SalahedeenIssa]

Problem Statement

I would like to create a circuit where I can measure only some of the qudit registers. As far as I know, at the moment, I can only create a circuit and then it automatically measures all qudits when I go to turn it into a qasm circuit or simulate it.

Proposed Solution

Would it be possible to instead have measurement gates that I can add only on my desired registers?

[burgholzer]

Hey 👋🏼
Thanks for your interest in the project!
You are right that, at the moment, all qubits are implicitly measured at the end of the program and there is no explicit measurement instruction.
Adding such an instruction would definitely make sense.
However, we currently do not have the capacity to add that feature.
A pull request adding it is welcome though and would be much appreciated.

[SalahedeenIssa]

Hey Lukas,

Could you please explain further why you do not have the capacity to add that feature? Is it something that would be inherently impossible/super difficult to add, or do you believe it is possible but just haven't gotten around to it yet?

[burgholzer]

It's definitely not super hard or impossible. It just takes time and we have other priorities at the moment across the MQT.
As stated above, a PR adding such a feature would be welcome.

[KevinMTO]

@JanVanDieBos and me solved lots of these things in a private fork
for this paper https://arxiv.org/abs/2507.12614
However, Jan didn't have time and the chance to clean up enough the code to initiate PRs. It depends on him at this point :)
Thanks for using MQT Qudits and feel welcome to continue developing it and make science on it @SalahedeenIssa!
We have some fancy things now for quantum simulations. @JanVanDieBos wanna chime in?

[SalahedeenIssa]

Hey @KevinMTO I can't seem to find the measurement gate you're referring to when I searched through the fork by @JanVanDieBos . Would you mind pointing me to it if possible? Thanks so much in advance, this is super helpful work :)

[JanVanDieBos]

Hi @SalahedeenIssa, so it is a private fork at the moment with support for directly applying the noisy gates using sparse matrix methods. Usually in the noisy case such methods become worse than dense matrix methods very quickly since the noisy means that the unitary circuit becomes dense. However, if one applies measurements, or partial measurements, then these methods are quite useful. I would have to familiarize myself again with the changes we made in this fork, however, since this is not quite what you were asking for, I am not sure it will be of much use to you. Essentially what I did do (which I did not directly integrate into mqt.qudits), and what might also interest you, is to generate the Kraus evolution operators given partial measurements are made at the end of the noisy circuit. By looking at "Appendix B: Post-selection and noise" of https://arxiv.org/abs/2507.12614 you should get a better idea of what I mean here.

[KevinMTO]

Hey @JanVanDieBos and @SalahedeenIssa!

I think creating a PR from the private fork would definitely be worthwhile.
We’ve introduced several major updates — including new noise modeling, sparse simulation methods, new gates, and new language features. The projections and time-evolution improvements are just a few examples.

It would be great to collaborate here to refine the workflow. I’m quite booked at the moment, but feel free to discuss between yourselves, and if you have any questions, I can point you to where specific changes were made.

There is even a huge compilation PR that is still open. There is only one corner case that needs to be debugged and the compiler will be completely finished.

@aaronleesander for viz

[KevinMTO]

@aaronleesander integrating YAQS here would be very impactful. You can all make a difference here I think :)

[KevinMTO]

@JanVanDieBos @SalahedeenIssa @aaronleesander Now that I think about it, it seems a great ground for qudit QCVV
and for classical shadows
If you want to have a call all together let me know :)

[KevinMTO]

@JanVanDieBos @SalahedeenIssa @aaronleesander

if you any of you is still interested I might just take this into my hands in the next weeks and make also the follow up integrations. Let me know if you have feedback or you want to get on baord

[JanVanDieBos]

Hi Kevin, sounds good, please let me know if I can do anything to help!