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  • Open Access

Composable Framework for Device-Independent State Certification

Rutvij Bhavsar1,*,†,‡, Lewis Wooltorton2,3,4,*,§, and Joonwoo Bae1,‖

  • *These authors contributed equally to this work.
  • †Contact author: rutvij.bhavsar@kcl.ac.uk
  • ‡Current affiliation: Department of Mathematics, King’s College London, Strand, London, WC2R 2LS, United Kingdom.
  • §Contact author: lewis.wooltorton@ens-lyon.fr
  • ‖Contact author: joonwoo.bae@kaist.ac.kr
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PRX Quantum 6, 040356 – Published 5 December, 2025

DOI: https://doi.org/10.1103/1j5f-qjhx

Abstract

Certifying a quantum state in a device-independent (DI) manner, in which no trust is placed on the internal workings of any physical components, is a fundamental task bearing various applications in quantum information theory. The composability of a state-certification protocol is key to its integration as a subroutine within information-theoretic protocols. In this work, we present a composable certification of quantum states in a DI manner under the assumption that a source prepares a finite sequence of independent quantum states that are not necessarily identical. We show that the security relies on the DI analog of the fidelity, called the extractability. We develop methods to compute this quantity under local operations and classical communication in certain Bell scenarios that self-test the singlet state, which may also be of independent interest. Finally, we demonstrate our framework by certifying the singlet state in a composable and DI manner using the Clauser-Horne-Shimony-Holt inequality.

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Popular Summary

Processing information with quantum resources holds great potential for applications, including secure communication and computing. A key step to achieving this so-called “quantum advantage” is the reliable generation of quantum states. For example, sharing entanglement, a feature not present in the classical theory, enables two parties to exchange secret messages with the strongest form of security known. Given an unknown device, how can we verify that it is actually producing good quality states? As quantum technologies become more widespread, the importance of developing techniques to answer this question has grown significantly. In this work, we provide a recipe for certifying quantum states under very weak assumptions, enabling their reliable use in further protocols.

Our state-certification protocol is device independent, meaning that we can certify the quality of a quantum state without knowing anything about its inner workings. Instead, we rely on observed measurement statistics, and from these alone we can deduce the structure of the state. One of the main features of our protocol is that it is composable: the certified states can be safely used in other quantum protocols without breaking security. This is vital for building scalable quantum networks, where many quantum systems interact in complex ways.

Our results pave the way for more secure, modular quantum technologies—tools that could help realize the promise of a truly quantum Internet.

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PRX Quantum 6, 040356– Published 5 December, 2025
  • Received 11 June 2025
  • Revised 4 September 2025
  • Accepted 16 October 2025
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