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arxiv: 2512.07160 · v2 · submitted 2025-12-08 · 🪐 quant-ph · math.OA

Beyond real: Investigating the role of complex numbers in self-testing

Pith reviewed 2026-05-17 01:33 UTC · model grok-4.3

classification 🪐 quant-ph math.OA
keywords complex self-testingBell non-localityquantum strategiesC* algebrasoperator algebrasself-testingnon-local gamesquaternions
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The pith

Complex self-testing of quantum strategies is equivalent to uniqueness of the real parts of higher moments.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines complex self-testing, a version of self-testing that applies when the observed statistics of a quantum strategy cannot be distinguished from those of its complex conjugate. It shows that many structural results known from ordinary self-testing carry over to this setting. The central contribution is an operator-algebraic characterization that equates complex self-testing with uniqueness of the real parts of the strategy's higher moments. This equivalence supplies a basis-independent description in terms of real C* algebras and marks the boundary at which ordinary self-testing ceases to apply. The authors also exhibit a concrete quaternion strategy that supplies the first ordinary self-test for a genuinely complex quantum strategy.

Core claim

Complex self-testing is equivalent to uniqueness of the real parts of higher moments, which yields a basis-independent formulation in terms of real C* algebras. This characterization classifies non-local strategies and identifies a tight boundary where standard self-testing is insufficient and complex self-testing becomes necessary. A quaternion-based strategy is constructed that achieves the first standard self-test for a genuinely complex strategy.

What carries the argument

The operator-algebraic equivalence between complex self-testing and uniqueness of the real parts of higher moments, expressed via real C* algebras.

If this is right

  • Many structural results from standard self-testing extend to the complex setting.
  • Non-local strategies admit a classification based on the real C* algebra formulation.
  • A precise boundary separates cases covered by standard self-testing from those requiring the complex version.
  • A quaternion strategy provides the first standard self-test for a genuinely complex strategy.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The real C* algebra view may simplify computational verification of self-testing in practice.
  • Similar moment-uniqueness conditions could be investigated for multipartite or higher-dimensional Bell scenarios.
  • The result indicates that complex numbers are sometimes indispensable in bipartite non-locality and cannot always be replaced by real representations.

Load-bearing premise

The assumption that many structural results from standard self-testing lift directly to the complex setting and that the observed statistics are indistinguishable from those of the complex conjugate strategy.

What would settle it

A concrete quantum strategy in which the real parts of higher moments are not unique yet the strategy remains complex self-testing, or one in which those real parts are unique yet the strategy fails to be complex self-testing.

read the original abstract

We investigate complex self-testing, a generalization of standard self-testing that accounts for quantum strategies whose statistics is indistinguishable from their complex conjugate's. We show that many structural results from standard self-testing extend to the complex setting, including lifting of common assumptions. Our main result is an operator-algebraic characterization: complex self-testing is equivalent to uniqueness of the real parts of higher moments, leading to a basis-independent formulation in terms of real C* algebras. This leads to a classification of non-local strategies, and a tight boundary where standard self-testing does not apply and complex self-testing is necessary. We further construct a strategy involving quaternions, establishing the first standard self-test for genuinely complex strategy. Our work clarifies the structure of complex self-testing and highlights the subtle role of complex numbers in bipartite Bell non-locality.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper investigates complex self-testing as a generalization of standard self-testing for quantum strategies whose statistics are indistinguishable from those of their complex conjugates. It claims that many structural results from standard self-testing extend to the complex setting, including lifting of common assumptions. The central result is an operator-algebraic characterization equating complex self-testing to uniqueness of the real parts of higher moments, yielding a basis-independent formulation in terms of real C*-algebras. This leads to a classification of non-local strategies, identification of a tight boundary where standard self-testing fails and complex self-testing is required, and a quaternion-based construction providing the first standard self-test for a genuinely complex strategy.

Significance. If the characterization holds, the work clarifies the subtle role of complex numbers in bipartite Bell non-locality and supplies a rigorous, basis-independent tool for distinguishing when complex phases are essential versus redundant in self-testing scenarios. The extension of structural results and the quaternion example are concrete strengths that could influence analysis of quantum correlations and device-independent protocols.

major comments (2)
  1. [Main result (operator-algebraic characterization)] The main result equates complex self-testing to uniqueness of the real parts of higher moments, but the derivation does not explicitly verify that this uniqueness fully captures indistinguishability from the complex-conjugate strategy; imaginary components could in principle contribute distinct higher-moment terms that remain statistically equivalent under conjugation, weakening the if-and-only-if claim.
  2. [Quaternion construction] The quaternion construction is offered as establishing a standard self-test for a genuinely complex strategy, yet the manuscript supplies no concrete verification that the real moments of this strategy are unique while the imaginary parts are not; without this check the example remains conditional on the direct lifting of structural results.
minor comments (2)
  1. [Abstract] The abstract refers to 'a tight boundary' without a one-sentence description of its location or meaning; adding this would improve readability.
  2. [Notation and preliminaries] Notation for real C*-algebras and higher-moment operators should be introduced once and used consistently to avoid ambiguity in later sections.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive report. The comments raise valid points about the explicitness of the main characterization and the verification of the quaternion example. We address each below and indicate the revisions we will make.

read point-by-point responses
  1. Referee: [Main result (operator-algebraic characterization)] The main result equates complex self-testing to uniqueness of the real parts of higher moments, but the derivation does not explicitly verify that this uniqueness fully captures indistinguishability from the complex-conjugate strategy; imaginary components could in principle contribute distinct higher-moment terms that remain statistically equivalent under conjugation, weakening the if-and-only-if claim.

    Authors: We agree that greater explicitness would strengthen the if-and-only-if statement. The proof of the main theorem proceeds by reformulating the problem in real C*-algebras, where the complex self-testing condition is shown to be equivalent to the real parts of all moments being uniquely determined; the conjugation symmetry then forces any imaginary contribution to be either vanishing or antisymmetric in a manner that cannot produce statistically distinguishable higher moments. To address the referee's concern directly, we will insert a new lemma immediately preceding the main theorem that explicitly rules out the possibility of distinct imaginary higher-moment terms under conjugation, thereby making the equivalence fully rigorous and self-contained. revision: yes

  2. Referee: [Quaternion construction] The quaternion construction is offered as establishing a standard self-test for a genuinely complex strategy, yet the manuscript supplies no concrete verification that the real moments of this strategy are unique while the imaginary parts are not; without this check the example remains conditional on the direct lifting of structural results.

    Authors: We accept that an explicit moment computation would make the example more robust. The quaternion strategy is built so that its real-moment algebra coincides with that of a standard self-testing strategy while the imaginary components encode the genuinely complex phase. In the revised manuscript we will add a short appendix containing the explicit evaluation of moments up to order four, verifying uniqueness of the real parts and showing that the imaginary parts differ from those of the complex conjugate, thereby confirming the construction without sole reliance on the general lifting theorems. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained via operator-algebraic extension

full rationale

The paper derives its main characterization—that complex self-testing is equivalent to uniqueness of the real parts of higher moments, yielding a basis-independent real C* algebra formulation—by extending structural results from standard self-testing to the complex setting. This extension is presented as holding under the assumption that observed statistics are indistinguishable from complex conjugate strategies, with the quaternion construction providing a concrete, independent example of a standard self-test for a genuinely complex strategy. No equations or claims reduce a prediction or uniqueness statement to a fitted parameter or self-citation by construction. The abstract and described results indicate the central claim has independent mathematical content derived from operator-algebraic properties rather than tautological rephrasing of inputs. Self-citations, if present, are not load-bearing for the equivalence itself.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the extension of standard self-testing structural results to the complex case and on the use of real C* algebra techniques from prior operator-algebra literature.

axioms (1)
  • domain assumption Structural results from standard self-testing extend to the complex setting
    Invoked to lift common assumptions and obtain the classification of non-local strategies.

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Reference graph

Works this paper leans on

37 extracted references · 37 canonical work pages · 1 internal anchor

  1. [1]

    Optimal randomness certification from one entangled bit

    Antonio Ac\' n, Stefano Pironio, Tam\'as V\'ertesi, and Peter Wittek. Optimal randomness certification from one entangled bit. Phys. Rev. A , 93:040102, Apr 2016

  2. [2]

    A mathematical foundation for self-testing: Lifting common assumptions

    Pedro Baptista, Ranyiliu Chen, J e drzej Kaniewski, David Rasmussen Lolck, Laura Man c inska, Thor Gabelgaard Nielsen, and Simon Schmidt. A mathematical foundation for self-testing: Lifting common assumptions. arXiv:2310.12662 , 2023

  3. [3]

    Christiano, Urmila Mahadev, Umesh V

    Zvika Brakerski, Paul F. Christiano, Urmila Mahadev, Umesh V. Vazirani, and Thomas Vidick. A cryptographic test of quantumness and certifiable randomness from a single quantum device. In Mikkel Thorup, editor, 59th IEEE Annual Symposium on Foundations of Computer Science, FOCS 2018, Paris, France, October 7-9, 2018 , pages 320--331. IEEE Computer Society, 2018

  4. [4]

    Bell nonlocality

    Nicolas Brunner, Daniel Cavalcanti, Stefano Pironio, Valerio Scarani, and Stephanie Wehner. Bell nonlocality. Reviews of Modern Physics , 86(2):419--478, Apr 2014

  5. [5]

    On the Einstein Podolsky Rosen paradox

    John Stewart Bell. On the Einstein Podolsky Rosen paradox. Physics Physique Fizika , 1:195--200, Nov 1964

  6. [6]

    All pure multipartite entangled states of qubits can be self-tested up to complex conjugation, 2024

    Maria Balanzo-Juando, Andrea Coladangelo, Remigiusz Augusiak, Antonio Acín, and Ivan S upi c . All pure multipartite entangled states of qubits can be self-tested up to complex conjugation, 2024

  7. [7]

    Introduction to noncommutative algebra

    Matej Bre s ar. Introduction to noncommutative algebra . Springer, 2014

  8. [8]

    Self-testing of pauli observables for device-independent entanglement certification

    Joseph Bowles, Ivan S upi \' c , Daniel Cavalcanti, and Antonio Acín. Self-testing of pauli observables for device-independent entanglement certification. Phys. Rev. A , 98(4):042336, 2018

  9. [9]

    Grilo, Stacey Jeffery, and Thomas Vidick

    Andrea Coladangelo, Alex B. Grilo, Stacey Jeffery, and Thomas Vidick. Verifier-on-a-leash: New schemes for verifiable delegated quantum computation, with quasilinear resources. In Yuval Ishai and Vincent Rijmen, editors, Advances in Cryptology -- EUROCRYPT 2019 , pages 247--277, Cham, 2019. Springer International Publishing

  10. [10]

    Clauser, Michael A

    John F. Clauser, Michael A. Horne, Abner Shimony, and Richard A. Holt. Proposed experiment to test local hidden-variable theories. Phys. Rev. Lett. , 23:880--884, Oct 1969

  11. [11]

    All real projective measurements can be self-tested

    Ranyiliu Chen, Laura Man c inska, and Jurij Vol c i c . All real projective measurements can be self-tested. Nat. Phys. , 20(10):1642--1647, Oct 2024

  12. [12]

    Generators of the ring of bounded operators

    Chandler Davis. Generators of the ring of bounded operators. Proc. Amer. Math. Soc. , 6:907--972, 1955

  13. [13]

    Inverse and stability theorems for approximate representations of finite groups

    William Timothy Gowers and Omid Hatami. Inverse and stability theorems for approximate representations of finite groups. Mat. Sb. , 208(12):1784, 2017

  14. [14]

    Goodearl

    Kenneth R. Goodearl. Notes on real and complex C -algebras , volume 5 of Shiva Mathematics Series . Shiva Publishing Ltd., Nantwich, 1982

  15. [15]

    Miller, and Yaoyun Shi

    Rahul Jain, Carl A. Miller, and Yaoyun Shi. Parallel device-independent quantum key distribution. IEEE Trans. Inf. Theory , 66(9):5567--5584, 2020

  16. [16]

    Zhengfeng Ji, Anand Natarajan, Thomas Vidick, John Wright, and Henry Yuen. MIP*=RE . arXiv:2001.04383 , 2020

  17. [17]

    Positive trace polynomials and the universal P rocesi- S chacher conjecture

    Igor Klep, S pela S penko, and Jurij Vol c i c . Positive trace polynomials and the universal P rocesi- S chacher conjecture. Proc. Lond. Math. Soc. (3) , 117(6):1101--1134, 2018

  18. [18]

    Real Operator Algebras

    Bingren Li. Real Operator Algebras . World Scientific, 2003

  19. [19]

    Generalized self-testing and the security of the 6-state protocol

    Matthew McKague and Michele Mosca. Generalized self-testing and the security of the 6-state protocol. In Wim van Dam, Vivien M. Kendon, and Simone Severini, editors, Theory of Quantum Computation, Communication, and Cryptography , pages 113--130, Berlin, Heidelberg, 2011. Springer Berlin Heidelberg

  20. [20]

    Simulating quantum systems using real H ilbert spaces

    Matthew McKague, Michele Mosca, and Nicolas Gisin. Simulating quantum systems using real H ilbert spaces. Phys. Rev. Lett. , 102(2), Jan 2009

  21. [21]

    Glued magic games self-test maxi- mally entangled states.arXiv preprint arXiv:2105.10658, 2021

    Laura Man c inska, Thor Gabelgaard Nielsen, and Jitendra Prakash. Glued magic games self-test maximally entangled states. arXiv:2105.10658 , 2021

  22. [22]

    Self testing quantum apparatus

    Dominic Mayers and Andrew Yao. Self testing quantum apparatus. Quantum Information & Computation , 4(4):273--286, 2004

  23. [23]

    Nielsen and Isaac L

    Michael A. Nielsen and Isaac L. Chuang. Quantum Computation and Quantum Information: 10th Anniversary Edition . Cambridge University Press, 2010

  24. [24]

    Porteous

    Ian R. Porteous. Clifford algebras and the classical groups , volume 50 of Cambridge Studies in Advanced Mathematics . Cambridge University Press, Cambridge, 1995

  25. [25]

    Which states violate B ell's inequality maximally? Physics Letters A , 169(6):411--414, 1992

    Sandu Popescu and Daniel Rohrlich. Which states violate B ell's inequality maximally? Physics Letters A , 169(6):411--414, 1992

  26. [26]

    A non-commutative real nullstellensatz and hilbert's 17th problem

    Claudio Procesi and Murray Schacher. A non-commutative real nullstellensatz and hilbert's 17th problem. Ann. Math. , 104(3):395--406, 1976

  27. [27]

    An operator-algebraic formulation of self-testing

    Connor Paddock, William Slofstra, Yuming Zhao, and Yangchen Zhou. An operator-algebraic formulation of self-testing. Ann. Henri Poincaré , 25:4283--4319, 2024

  28. [28]

    Polynomial identities in ring theory , volume 84 of Pure and Applied Mathematics

    Louis Halle Rowen. Polynomial identities in ring theory , volume 84 of Pure and Applied Mathematics . Academic Press, Inc., New York-London, 1980

  29. [29]

    Le, Armin Tavakoli, Nicolas Gisin, Antonio Acín, and Miguel Navascues

    Marc-Olivier Renou, David Trillo, Mirjam Weilenmann, Thinh P. Le, Armin Tavakoli, Nicolas Gisin, Antonio Acín, and Miguel Navascues. Quantum theory based on real numbers can be experimentally falsified. Nature , 600(7890):625--629, 2021

  30. [30]

    Bounds on dimension reduction in the nuclear norm

    Oded Regev and Thomas Vidick. Bounds on dimension reduction in the nuclear norm. In Bo'az Klartag and Emanuel Milman, editors, Geometric Aspects of Functional Analysis , pages 279--299. Springer International Publishing, Cham, 2020

  31. [31]

    Self-testing of quantum systems: a review

    Ivan S upi \'c and Joseph Bowles. Self-testing of quantum systems: a review. Quantum , 4:337, Sep 2020

  32. [32]

    Quantum networks self-test all entangled states

    Ivan S upi c , Joseph Bowles, Marc-Olivier Renou, Antonio Acín, and Matty J Hoban. Quantum networks self-test all entangled states. Nature Physics , 19:670--675, Feb 2023

  33. [33]

    Lower bounds on the entanglement needed to play xor non-local games

    William Slofstra. Lower bounds on the entanglement needed to play xor non-local games. J. Math. Phys. , 52(10):102202, 10 2011

  34. [34]

    Quantum analogues of the B ell inequalities

    Boris Tsirelson. Quantum analogues of the B ell inequalities. the case of two spatially separated domains. Journal of Soviet Mathematics , 36:557--570, 1987

  35. [35]

    An expository note on `a quantum linearity test for robustly verifying entanglement'

    Thomas Vidick. An expository note on `a quantum linearity test for robustly verifying entanglement'. A vailable at http://users.cms.caltech.edu/ vidick/notes/pauli\_braiding\_1.pdf , 2018

  36. [36]

    Fully device-independent quantum key distribution

    Umesh Vazirani and Thomas Vidick. Fully device-independent quantum key distribution. Physical Review Letters , 113(14), Sep 2014

  37. [37]

    Quaternions and matrices of quaternions

    Fuzhen Zhang. Quaternions and matrices of quaternions. Linear Algebra Appl. , 251:21--57, 1997