More entropy from shorter experiments using polytope approximations to the quantum set

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AbstractWe introduce a systematic method for constructing polytope approximations to the quantum set in a variety of device-independent quantum random number generation (DI-QRNG) protocols. Our approach relies on two general-purpose algorithms that iteratively refine an initial outer-polytope approximation, guided by typical device behaviour and cryptographic intuition. These refinements strike a balance between computational tractability and approximation effectiveness. By integrating these approximations into the probability estimation (PE) framework [Zhang et al., PRA 2018], we obtain significantly improved certified entropy bounds in the finite-size regime. We test our method on various bipartite and tripartite DI-QRNG protocols, using both simulated and experimental data. In all cases, it yields notably higher entropy rates with fewer device uses than the existing techniques. We further extend our analysis to the more demanding task of randomness amplification, demonstrating major performance gains without added complexity. These results offer an effective and ready-to-use method to prove security---with improved certified entropy rates---in the most common practical DI-QRNG protocols. Our algorithms and entropy certification with PE tools are publicly available under a non-commercial license at github. ► BibTeX data@article{Jee2026moreentropyfrom, doi = {10.22331/q-2026-03-10-2019}, url = {https://doi.org/10.22331/q-2026-03-10-2019}, title = {More entropy from shorter experiments using polytope approximations to the quantum set}, author = {Jee, Hyejung H. and Curchod, Florian J. and Almeida, Mafalda L.}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, volume = {10}, pages = {2019}, month = mar, year = {2026} }► References [1] Masahito Hayashi and Toyohiro Tsurumaru. ``More efficient privacy amplification with less random seeds via dual universal hash function''. IEEE Transactions on Information Theory 62, 2213–2232 (2016). https://doi.org/10.1109/TIT.2016.2526018 [2] Cameron Foreman, Richie Yeung, Alec Edgington, and Florian J. Curchod. ``Cryptomite: A versatile and user-friendly library of randomness extractors''. Quantum 9, 1584 (2025). https://doi.org/10.22331/q-2025-01-08-1584 [3] Stefano Pironio, Antonio Acín, Serge Massar, A Boyer de La Giroday, Dzmitry N. Matsukevich, Peter Maunz, Steven Olmschenk, David Hayes, Le Luo, Timothy A. Manning, et al. ``Random numbers certified by Bell’s theorem''. Nature 464, 1021–1024 (2010). https://doi.org/10.1038/nature09008 [4] Stefano Pironio and Serge Massar. ``Security of practical private randomness generation''. Physical Review A 87, 012336 (2013). https://doi.org/10.1103/PhysRevA.87.012336 [5] Serge Fehr, Ran Gelles, and Christian Schaffner. ``Security and composability of randomness expansion from Bell inequalities''. Physical Review A 87, 012335 (2013). https://doi.org/10.1103/PhysRevA.87.012335 [6] Frédéric Dupuis, Omar Fawzi, and Renato Renner. ``Entropy accumulation''. Communications in Mathematical Physics 379, 867–913 (2020). https://doi.org/10.1007/s00220-020-03839-5 [7] Yanbao Zhang, Emanuel Knill, and Peter Bierhorst. ``Certifying quantum randomness by probability estimation''. Physical Review A 98, 040304 (2018). https://doi.org/10.1103/PhysRevA.98.040304 [8] Emanuel Knill, Yanbao Zhang, and Peter Bierhorst. ``Generation of quantum randomness by probability estimation with classical side information''.
Physical Review Research 2, 033465 (2020). https://doi.org/10.1103/PhysRevResearch.2.033465 [9] Lynden K. Shalm, Yanbao Zhang, Joshua C. Bienfang, Collin Schlager, Martin J. Stevens, Michael D. Mazurek, Carlos Abellán, Waldimar Amaya, Morgan W. Mitchell, Mohammad A. Alhejji, Honghao Fu, Joel Ornstein, Richard P. Mirin, Sae Woo Nam, and Emanuel Knill. ``Device-independent randomness expansion with entangled photons''. Nature Physics 17, 452–456 (2021). https://doi.org/10.1038/s41567-020-01153-4 [10] Peter Bierhorst and Yanbao Zhang. ``Tsirelson polytopes and randomness generation''. New Journal of Physics 22, 083036 (2020). https://doi.org/10.1088/1367-2630/aba30d [11] Antonio Acín, Serge Massar, and Stefano Pironio. ``Randomness versus nonlocality and entanglement''.
Physical Review Letters 108, 100402 (2012). https://doi.org/10.1103/PhysRevLett.108.100402 [12] Armin Tavakoli, Alejandro Pozas-Kerstjens, Peter Brown, and Mateus Araújo. ``Semidefinite programming relaxations for quantum correlations''. Reviews of Modern Physics 96, 045006 (2024). https://doi.org/10.1103/RevModPhys.96.045006 [13] Sandu Popescu and Daniel Rohrlich. ``Quantum nonlocality as an axiom''. Foundations of Physics 24, 379–385 (1994). https://doi.org/10.1007/BF02058098 [14] Robert T. Konig and Barbara M. Terhal. ``The bounded-storage model in the presence of a quantum adversary''. IEEE Transactions on Information Theory 54, 749–762 (2008). https://doi.org/10.1109/TIT.2007.913245 [15] Renato Renner and Stefan Wolf. ``Smooth Renyi entropy and applications''. In IEEE International Symposium on Information Theory — ISIT 2004. Page 233. IEEE (2004). https://doi.org/10.1109/ISIT.2004.1365269 [16] Boris S. Tsirelson. ``Some results and problems on quantum Bell-type inequalities''.
Hadronic Journal Supplement 8, 329–345 (1993). [17] Miguel Navascués, Stefano Pironio, and Antonio Acín. ``Bounding the set of quantum correlations''.
Physical Review Letters 98, 010401 (2007). https://doi.org/10.1103/PhysRevLett.98.010401 [18] Miguel Navascués, Stefano Pironio, and Antonio Acín. ``A convergent hierarchy of semidefinite programs characterizing the set of quantum correlations''. New Journal of Physics 10, 073013 (2008). https://doi.org/10.1088/1367-2630/10/7/073013 [19] Nicolas Brunner, Daniel Cavalcanti, Stefano Pironio, Valerio Scarani, and Stephanie Wehner. ``Bell nonlocality''. Reviews of Modern Physics 86, 419–478 (2014). https://doi.org/10.1103/RevModPhys.86.419 [20] Olmo Nieto-Silleras, Stefano Pironio, and Jonathan Silman. ``Using complete measurement statistics for optimal device-independent randomness evaluation''. New Journal of Physics 16, 013035 (2014). https://doi.org/10.1088/1367-2630/16/1/013035 [21] Jean-Daniel Bancal, Lana Sheridan, and Valerio Scarani. ``More randomness from the same data''. New Journal of Physics 16, 033011 (2014). https://doi.org/10.1088/1367-2630/16/3/033011 [22] John F. Clauser, Michael A. Horne, Abner Shimony, and Richard A. Holt. ``Proposed experiment to test local hidden-variable theories''.
Physical Review Letters 23, 880 (1969). https://doi.org/10.1103/PhysRevLett.23.880 [23] Wenjamin Rosenfeld, Daniel Burchardt, Robert Garthoff, Kai Redeker, Norbert Ortegel, Markus Rau, and Harald Weinfurter. ``Event-ready Bell test using entangled atoms simultaneously closing detection and locality loopholes''.
Physical Review Letters 119, 010402 (2017). https://doi.org/10.1103/PhysRevLett.119.010402 [24] Yanbao Zhang, Lynden K. Shalm, Joshua C. Bienfang, Martin J. Stevens, Michael D. Mazurek, Sae Woo Nam, Carlos Abellán, Waldimar Amaya, Morgan W. Mitchell, Honghao Fu, et al. ``Experimental low-latency device-independent quantum randomness''.
Physical Review Letters 124, 010505 (2020). https://doi.org/10.1103/PhysRevLett.124.010505 [25] Olmo Nieto-Silleras, Cédric Bamps, Jonathan Silman, and Stefano Pironio. ``Device-independent randomness generation from several Bell estimators''. New Journal of Physics 20, 023049 (2018). https://doi.org/10.1088/1367-2630/aaaa06 [26] Ravishankar Ramanathan, Michał Horodecki, Hammad Anwer, Stefano Pironio, Karol Horodecki, Marcus Grünfeld, Sadiq Muhammad, Mohamed Bourennane, and Paweł Horodecki. ``Practical no-signalling proof randomness amplification using Hardy paradoxes and its experimental implementation''. arXiv preprint arXiv:1810.11648 (2018). https://doi.org/10.48550/arXiv.1810.11648 [27] Rotem Arnon-Friedman, Frédéric Dupuis, Omar Fawzi, Renato Renner, and Thomas Vidick. ``Practical device-independent quantum cryptography via entropy accumulation''. Nature Communications 9, 459 (2018). https://doi.org/10.1038/s41467-017-02307-4 [28] ``Quantinuum H1-1. https://www.quantinuum.com/, May, 2023.''. https://www.quantinuum.com/ [29] Pei-Sheng Lin, Denis Rosset, Yanbao Zhang, Jean-Daniel Bancal, and Yeong-Cherng Liang. ``Device-independent point estimation from finite data and its application to device-independent property estimation''. Physical Review A 97, 032309 (2018). https://doi.org/10.1103/PhysRevA.97.032309 [30] Stefano Pironio. ``Lifting Bell inequalities''. Journal of Mathematical Physics 46 (2005). https://doi.org/10.1063/1.1928727 [31] Erik Woodhead, Boris Bourdoncle, and Antonio Acín. ``Randomness versus nonlocality in the Mermin-Bell experiment with three parties''. Quantum 2, 82 (2018). https://doi.org/10.22331/q-2018-08-17-82 [32] Miklos Santha and Umesh V. Vazirani. ``Generating quasi-random sequences from semi-random sources''. Journal of Computer and System Sciences 33, 75–87 (1986). https://doi.org/10.1016/0022-0000(86)90044-9 [33] Lucien Hardy. ``Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories''.
Physical Review Letters 68, 2981–2984 (1992). https://doi.org/10.1103/PhysRevLett.68.2981 [34] Gilles Pütz and Nicolas Gisin. ``Measurement dependent locality''. New Journal of Physics 18, 055006 (2016). https://doi.org/10.1088/1367-2630/18/5/055006 [35] Yanbao Zhang, Honghao Fu, and Emanuel Knill. ``Efficient randomness certification by quantum probability estimation''.
Physical Review Research 2, 013016 (2020). https://doi.org/10.1103/PhysRevResearch.2.013016 [36] Thomas Van Himbeeck and Stefano Pironio. ``Correlations and randomness generation based on energy constraints''. arXiv preprint arXiv:1905.09117 (2019). https://doi.org/10.48550/arXiv.1905.09117 [37] Wassily Hoeffding. ``Probability inequalities for sums of bounded random variables''. The collected works of Wassily HoeffdingPages 409–426 (1994). https://doi.org/10.1080/01621459.1963.10500830 [38] Tony Metger, Omar Fawzi, David Sutter, and Renato Renner. ``Generalised entropy accumulation''. In 2022 IEEE 63rd Annual Symposium on Foundations of Computer Science (FOCS). Pages 844–850. (2022). https://doi.org/10.1109/FOCS54457.2022.00085 [39] Peter Brown, Hamza Fawzi, and Omar Fawzi. ``Device-independent lower bounds on the conditional von Neumann entropy''. Quantum 8, 1445 (2024). https://doi.org/10.22331/q-2024-08-27-1445 [40] Fernando G. S. L. Brandão, Ravishankar Ramanathan, Andrzej Grudka, Karol Horodecki, Michał Horodecki, Paweł Horodecki, Tomasz Szarek, and Hanna Wojewódka. ``Realistic noise-tolerant randomness amplification using finite number of devices''. Nature Communications 7, 11345 (2016). https://doi.org/10.1038/ncomms11345Cited byCould not fetch Crossref cited-by data during last attempt 2026-03-10 14:09:50: Could not fetch cited-by data for 10.22331/q-2026-03-10-2019 from Crossref. This is normal if the DOI was registered recently. Could not fetch ADS cited-by data during last attempt 2026-03-10 14:09:50: No response from ADS or unable to decode the received json data when getting the list of citing works.This Paper is published in Quantum under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Copyright remains with the original copyright holders such as the authors or their institutions. AbstractWe introduce a systematic method for constructing polytope approximations to the quantum set in a variety of device-independent quantum random number generation (DI-QRNG) protocols. Our approach relies on two general-purpose algorithms that iteratively refine an initial outer-polytope approximation, guided by typical device behaviour and cryptographic intuition. These refinements strike a balance between computational tractability and approximation effectiveness. By integrating these approximations into the probability estimation (PE) framework [Zhang et al., PRA 2018], we obtain significantly improved certified entropy bounds in the finite-size regime. We test our method on various bipartite and tripartite DI-QRNG protocols, using both simulated and experimental data. In all cases, it yields notably higher entropy rates with fewer device uses than the existing techniques. We further extend our analysis to the more demanding task of randomness amplification, demonstrating major performance gains without added complexity. These results offer an effective and ready-to-use method to prove security---with improved certified entropy rates---in the most common practical DI-QRNG protocols. Our algorithms and entropy certification with PE tools are publicly available under a non-commercial license at github. ► BibTeX data@article{Jee2026moreentropyfrom, doi = {10.22331/q-2026-03-10-2019}, url = {https://doi.org/10.22331/q-2026-03-10-2019}, title = {More entropy from shorter experiments using polytope approximations to the quantum set}, author = {Jee, Hyejung H. and Curchod, Florian J. and Almeida, Mafalda L.}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, volume = {10}, pages = {2019}, month = mar, year = {2026} }► References [1] Masahito Hayashi and Toyohiro Tsurumaru. ``More efficient privacy amplification with less random seeds via dual universal hash function''. IEEE Transactions on Information Theory 62, 2213–2232 (2016). https://doi.org/10.1109/TIT.2016.2526018 [2] Cameron Foreman, Richie Yeung, Alec Edgington, and Florian J. Curchod. ``Cryptomite: A versatile and user-friendly library of randomness extractors''. Quantum 9, 1584 (2025). https://doi.org/10.22331/q-2025-01-08-1584 [3] Stefano Pironio, Antonio Acín, Serge Massar, A Boyer de La Giroday, Dzmitry N. Matsukevich, Peter Maunz, Steven Olmschenk, David Hayes, Le Luo, Timothy A. Manning, et al. ``Random numbers certified by Bell’s theorem''. Nature 464, 1021–1024 (2010). https://doi.org/10.1038/nature09008 [4] Stefano Pironio and Serge Massar. ``Security of practical private randomness generation''. Physical Review A 87, 012336 (2013). https://doi.org/10.1103/PhysRevA.87.012336 [5] Serge Fehr, Ran Gelles, and Christian Schaffner. ``Security and composability of randomness expansion from Bell inequalities''. Physical Review A 87, 012335 (2013). https://doi.org/10.1103/PhysRevA.87.012335 [6] Frédéric Dupuis, Omar Fawzi, and Renato Renner. ``Entropy accumulation''. Communications in Mathematical Physics 379, 867–913 (2020). https://doi.org/10.1007/s00220-020-03839-5 [7] Yanbao Zhang, Emanuel Knill, and Peter Bierhorst. ``Certifying quantum randomness by probability estimation''. Physical Review A 98, 040304 (2018). https://doi.org/10.1103/PhysRevA.98.040304 [8] Emanuel Knill, Yanbao Zhang, and Peter Bierhorst. ``Generation of quantum randomness by probability estimation with classical side information''.
Physical Review Research 2, 033465 (2020). https://doi.org/10.1103/PhysRevResearch.2.033465 [9] Lynden K. Shalm, Yanbao Zhang, Joshua C. Bienfang, Collin Schlager, Martin J. Stevens, Michael D. Mazurek, Carlos Abellán, Waldimar Amaya, Morgan W. Mitchell, Mohammad A. Alhejji, Honghao Fu, Joel Ornstein, Richard P. Mirin, Sae Woo Nam, and Emanuel Knill. ``Device-independent randomness expansion with entangled photons''. Nature Physics 17, 452–456 (2021). https://doi.org/10.1038/s41567-020-01153-4 [10] Peter Bierhorst and Yanbao Zhang. ``Tsirelson polytopes and randomness generation''. New Journal of Physics 22, 083036 (2020). https://doi.org/10.1088/1367-2630/aba30d [11] Antonio Acín, Serge Massar, and Stefano Pironio. ``Randomness versus nonlocality and entanglement''.
Physical Review Letters 108, 100402 (2012). https://doi.org/10.1103/PhysRevLett.108.100402 [12] Armin Tavakoli, Alejandro Pozas-Kerstjens, Peter Brown, and Mateus Araújo. ``Semidefinite programming relaxations for quantum correlations''. Reviews of Modern Physics 96, 045006 (2024). https://doi.org/10.1103/RevModPhys.96.045006 [13] Sandu Popescu and Daniel Rohrlich. ``Quantum nonlocality as an axiom''. Foundations of Physics 24, 379–385 (1994). https://doi.org/10.1007/BF02058098 [14] Robert T. Konig and Barbara M. Terhal. ``The bounded-storage model in the presence of a quantum adversary''. IEEE Transactions on Information Theory 54, 749–762 (2008). https://doi.org/10.1109/TIT.2007.913245 [15] Renato Renner and Stefan Wolf. ``Smooth Renyi entropy and applications''. In IEEE International Symposium on Information Theory — ISIT 2004. Page 233. IEEE (2004). https://doi.org/10.1109/ISIT.2004.1365269 [16] Boris S. Tsirelson. ``Some results and problems on quantum Bell-type inequalities''.
Hadronic Journal Supplement 8, 329–345 (1993). [17] Miguel Navascués, Stefano Pironio, and Antonio Acín. ``Bounding the set of quantum correlations''.
Physical Review Letters 98, 010401 (2007). https://doi.org/10.1103/PhysRevLett.98.010401 [18] Miguel Navascués, Stefano Pironio, and Antonio Acín. ``A convergent hierarchy of semidefinite programs characterizing the set of quantum correlations''. New Journal of Physics 10, 073013 (2008). https://doi.org/10.1088/1367-2630/10/7/073013 [19] Nicolas Brunner, Daniel Cavalcanti, Stefano Pironio, Valerio Scarani, and Stephanie Wehner. ``Bell nonlocality''. Reviews of Modern Physics 86, 419–478 (2014). https://doi.org/10.1103/RevModPhys.86.419 [20] Olmo Nieto-Silleras, Stefano Pironio, and Jonathan Silman. ``Using complete measurement statistics for optimal device-independent randomness evaluation''. New Journal of Physics 16, 013035 (2014). https://doi.org/10.1088/1367-2630/16/1/013035 [21] Jean-Daniel Bancal, Lana Sheridan, and Valerio Scarani. ``More randomness from the same data''. New Journal of Physics 16, 033011 (2014). https://doi.org/10.1088/1367-2630/16/3/033011 [22] John F. Clauser, Michael A. Horne, Abner Shimony, and Richard A. Holt. ``Proposed experiment to test local hidden-variable theories''.
Physical Review Letters 23, 880 (1969). https://doi.org/10.1103/PhysRevLett.23.880 [23] Wenjamin Rosenfeld, Daniel Burchardt, Robert Garthoff, Kai Redeker, Norbert Ortegel, Markus Rau, and Harald Weinfurter. ``Event-ready Bell test using entangled atoms simultaneously closing detection and locality loopholes''.
Physical Review Letters 119, 010402 (2017). https://doi.org/10.1103/PhysRevLett.119.010402 [24] Yanbao Zhang, Lynden K. Shalm, Joshua C. Bienfang, Martin J. Stevens, Michael D. Mazurek, Sae Woo Nam, Carlos Abellán, Waldimar Amaya, Morgan W. Mitchell, Honghao Fu, et al. ``Experimental low-latency device-independent quantum randomness''.
Physical Review Letters 124, 010505 (2020). https://doi.org/10.1103/PhysRevLett.124.010505 [25] Olmo Nieto-Silleras, Cédric Bamps, Jonathan Silman, and Stefano Pironio. ``Device-independent randomness generation from several Bell estimators''. New Journal of Physics 20, 023049 (2018). https://doi.org/10.1088/1367-2630/aaaa06 [26] Ravishankar Ramanathan, Michał Horodecki, Hammad Anwer, Stefano Pironio, Karol Horodecki, Marcus Grünfeld, Sadiq Muhammad, Mohamed Bourennane, and Paweł Horodecki. ``Practical no-signalling proof randomness amplification using Hardy paradoxes and its experimental implementation''. arXiv preprint arXiv:1810.11648 (2018). https://doi.org/10.48550/arXiv.1810.11648 [27] Rotem Arnon-Friedman, Frédéric Dupuis, Omar Fawzi, Renato Renner, and Thomas Vidick. ``Practical device-independent quantum cryptography via entropy accumulation''. Nature Communications 9, 459 (2018). https://doi.org/10.1038/s41467-017-02307-4 [28] ``Quantinuum H1-1. https://www.quantinuum.com/, May, 2023.''. https://www.quantinuum.com/ [29] Pei-Sheng Lin, Denis Rosset, Yanbao Zhang, Jean-Daniel Bancal, and Yeong-Cherng Liang. ``Device-independent point estimation from finite data and its application to device-independent property estimation''. Physical Review A 97, 032309 (2018). https://doi.org/10.1103/PhysRevA.97.032309 [30] Stefano Pironio. ``Lifting Bell inequalities''. Journal of Mathematical Physics 46 (2005). https://doi.org/10.1063/1.1928727 [31] Erik Woodhead, Boris Bourdoncle, and Antonio Acín. ``Randomness versus nonlocality in the Mermin-Bell experiment with three parties''. Quantum 2, 82 (2018). https://doi.org/10.22331/q-2018-08-17-82 [32] Miklos Santha and Umesh V. Vazirani. ``Generating quasi-random sequences from semi-random sources''. Journal of Computer and System Sciences 33, 75–87 (1986). https://doi.org/10.1016/0022-0000(86)90044-9 [33] Lucien Hardy. ``Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories''.
Physical Review Letters 68, 2981–2984 (1992). https://doi.org/10.1103/PhysRevLett.68.2981 [34] Gilles Pütz and Nicolas Gisin. ``Measurement dependent locality''. New Journal of Physics 18, 055006 (2016). https://doi.org/10.1088/1367-2630/18/5/055006 [35] Yanbao Zhang, Honghao Fu, and Emanuel Knill. ``Efficient randomness certification by quantum probability estimation''.
Physical Review Research 2, 013016 (2020). https://doi.org/10.1103/PhysRevResearch.2.013016 [36] Thomas Van Himbeeck and Stefano Pironio. ``Correlations and randomness generation based on energy constraints''. arXiv preprint arXiv:1905.09117 (2019). https://doi.org/10.48550/arXiv.1905.09117 [37] Wassily Hoeffding. ``Probability inequalities for sums of bounded random variables''. The collected works of Wassily HoeffdingPages 409–426 (1994). https://doi.org/10.1080/01621459.1963.10500830 [38] Tony Metger, Omar Fawzi, David Sutter, and Renato Renner. ``Generalised entropy accumulation''. In 2022 IEEE 63rd Annual Symposium on Foundations of Computer Science (FOCS). Pages 844–850. (2022). https://doi.org/10.1109/FOCS54457.2022.00085 [39] Peter Brown, Hamza Fawzi, and Omar Fawzi. ``Device-independent lower bounds on the conditional von Neumann entropy''. Quantum 8, 1445 (2024). https://doi.org/10.22331/q-2024-08-27-1445 [40] Fernando G. S. L. Brandão, Ravishankar Ramanathan, Andrzej Grudka, Karol Horodecki, Michał Horodecki, Paweł Horodecki, Tomasz Szarek, and Hanna Wojewódka. ``Realistic noise-tolerant randomness amplification using finite number of devices''. Nature Communications 7, 11345 (2016). https://doi.org/10.1038/ncomms11345Cited byCould not fetch Crossref cited-by data during last attempt 2026-03-10 14:09:50: Could not fetch cited-by data for 10.22331/q-2026-03-10-2019 from Crossref. This is normal if the DOI was registered recently. Could not fetch ADS cited-by data during last attempt 2026-03-10 14:09:50: No response from ADS or unable to decode the received json data when getting the list of citing works.This Paper is published in Quantum under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Copyright remains with the original copyright holders such as the authors or their institutions.
