SciPost Submission Page
Scalable modular architecture for universal quantum computation
by Fernando Gago-Encinas, Christiane P. Koch
Submission summary
| Authors (as registered SciPost users): | Fernando Gago Encinas · Christiane Koch |
| Submission information | |
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| Preprint Link: | https://arxiv.org/abs/2507.14691v3 (pdf) |
| Date submitted: | Dec. 22, 2025, 6:32 p.m. |
| Submitted by: | Fernando Gago Encinas |
| Submitted to: | SciPost Physics |
| Ontological classification | |
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| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
Universal quantum computing requires the ability to perform every unitary operation, i.e., evolution operator controllability. In view of developing resource-efficient quantum processing units (QPUs), it is important to determine how many local controls and qubit-qubit couplings are required for controllability. Unfortunately, assessing the controllability of large qubit arrays is a difficult task, due to the exponential scaling of Hilbert space dimension. Here we show that it is sufficient to connect two qubit arrays that are evolution operator controllable by a single entangling two-qubit gate in order to obtain a composite qubit array that is evolution operator controllable. The proof provides a template to build up modular QPUs from smaller building blocks with reduced numbers of local controls and couplings. We illustrate the approach with two examples, consisting of 10, respectively 127 qubits, inspired by IBM quantum processors.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Author comments upon resubmission
we are writing in response to the editorial decision made on our article “Scalable modular architecture for universal quantum computation”.
We would like to thank the reviewers for their time and effort spent on our manuscript, the positive assessments and helpful comments they have provided. A new version of the manuscript may be found on arXiv ( arXiv:2507.14691v3 ).
We believe that with these revisions our manuscript may be accepted for publication in SciPost Physics.
Sincerely yours,
Fernando Gago-Encinas, and Christiane P. Koch
List of changes
Changes following report #1
The authors thank the reviewer for their appraisal and feedback. Here is a complete list of the corrections in response to the reviewer’s suggestions.
1) The change of notation has been implemented to match the rest of the article.
2) A comment has been added in Section II to reference some of the latest works on tunable couplings in superconducting qubits and in particular in IBM’s QPUs, complementing the existing references to more conceptual works. 3) A sentence has been added to the beginning of Section III, mentioning algorithms that benefit from a large number of qubits.
Changes following report #2
The authors thank the reviewer for their valuable insights. The typo has been corrected. Here we provide a detailed list of changes to the manuscript following the reviewer’s suggestions:
1) The article has been better put into context by explicitly stating the differences between the result here shown and previous scientific contributions, in particular Zeier & Schulte-Herbrüggen 2011 and Albertini & D’Alessandro 2025. This clarification is found in Section II, after Theorem 1.
2) The discussion of the extensionof the current proof to the case of qudits has been expanded in the last section of the article. In particular, itis pointed out that the result also holds thanks to previous theoretical results and the only missing part is the exact operations needed.
3) We agree that a quantitative analysis of how computational efforts scale with the number of subsystems would be a very interesting topic. However, this question merits a study of its own, far exceeding the scope of the work here presented. Whether studying the problem via quantum speed limits or via the quantum circuit depth after transpilation, the required optimizations need careful tuning and quickly become complex for larger systems.
4) Section IV has been renamed and expanded. In particular, we have added a discussion of our insights regarding quantum speed limit, circuit depth and fidelity.
Changes following report #3
A) Surprisingly, it is not completely straightforward since it is always the joint drift that governs the evolution, even with zero coupling, which implies that the individual drifts H_0,A and H_0,B evolve for the same amount of time. In the particular case where both drifts have the same period, it becomes necessary to prove that they can be decoupled.
C.3) The indices are indeed correct. We start with the term with \sigma_3 but due to the commutator rules it ends up as \sigma_1. However, note that the coefficient in front of it is still c_{3,3}, showing that the original term was the product of two \sigma_3 matrices.