##### Leader: Ana Belen Sainz

Post-docs: David Schmid, Roberto Dobal-Baldijao, Jan Głowacki

PhD students: Paulo Cavalcanti, Vinicius Pretti Rossi, Beata Zjawin

MSc student: Robin Saunders

The broad aim of the Foundational Underpinnings of Quantum Technologies Group is to understand the quantum manifestation of nonclassical phenomena, and how harness such nonclassicality for information processing. This is tackled from a novel perspective, combining an operational vision with the process-theoretic framework.

## Activity

Specific goals of the group include:

– Formulate candidate theories that supersede quantum. In particular, explore which possible

deviations of quantum theory are still sensible.

– Study causality within and beyond quantum theory, from a process-theoretic perspective.

– Characterise the quantum manifestation of nonclassical phenomena. The main focus will be on Bell nonlocality, contextuality, steering, and correlations in causal networks.

– Develop resource theories to address quantification.

– Identify current and new forms of nonclassicality as resources for quantum technologies.

– Assess nonclassical speed-up for computation, within and beyond quantum theory.

– Contribute to the development of a systematic approach to quantum program optimisation based on the zx-calculus, by further developing the foundations of the latter.

## Publications

### 2023

- John H. Selby, David Schmid, Elie Wolfe, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Contextuality without Incompatibility.
*Physical Review Letters*, 130(23), 2023. doi:10.1103/PhysRevLett.130.230201

[BibTeX]`@ARTICLE{Selby2023, author = {Selby, John H. and Schmid, David and Wolfe, Elie and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, title ="{Contextuality without Incompatibility}", year = {2023}, journal = {Physical Review Letters}, volume = {130}, number = {23}, doi = {10.1103/PhysRevLett.130.230201}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85161941733&doi=10.1103%2fPhysRevLett.130.230201&partnerID=40&md5=f329d241bdcd7251ed989c13ab4c4231}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 3; All Open Access, Green Open Access} }`

- Lorenzo Catani, Matthew Leifer, David Schmid, and Robert W. Spekkens. Why interference phenomena do not capture the essence of quantum theory.
*Quantum*, 7, 2023. doi:10.22331/q-2023-09-25-1119

[BibTeX]`@ARTICLE{Catani2023, author = {Catani, Lorenzo and Leifer, Matthew and Schmid, David and Spekkens, Robert W.}, title ="{Why interference phenomena do not capture the essence of quantum theory}", year = {2023}, journal = {Quantum}, volume = {7}, doi = {10.22331/q-2023-09-25-1119}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85176543863&doi=10.22331%2fq-2023-09-25-1119&partnerID=40&md5=e42239fec72e80c4c699f0431ecc7fd2}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 1; All Open Access, Gold Open Access, Green Open Access} }`

- Lorenzo Catani, Matthew Leifer, Giovanni Scala, David Schmid, and Robert W. Spekkens. Aspects of the phenomenology of interference that are genuinely nonclassical.
*Physical Review A*, 108(2), 2023. doi:10.1103/PhysRevA.108.022207

[BibTeX]`@ARTICLE{Catani2023aa, author = {Catani, Lorenzo and Leifer, Matthew and Scala, Giovanni and Schmid, David and Spekkens, Robert W.}, title ="{Aspects of the phenomenology of interference that are genuinely nonclassical}", year = {2023}, journal = {Physical Review A}, volume = {108}, number = {2}, doi = {10.1103/PhysRevA.108.022207}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85167873133&doi=10.1103%2fPhysRevA.108.022207&partnerID=40&md5=4dac4874cac4a3b1786a26564b5c5752}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 1; All Open Access, Green Open Access} }`

- Beata Zjawin, David Schmid, Matty J. Hoban, and Ana Belén Sainz. Quantifying EPR: the resource theory of nonclassicality of common-cause assemblages.
*Quantum*, 7, 2023. doi:10.22331/Q-2023-02-16-926

[BibTeX]`@ARTICLE{Zjawin2023, author = {Zjawin, Beata and Schmid, David and Hoban, Matty J. and Sainz, Ana Belén}, title ="{Quantifying EPR: the resource theory of nonclassicality of common-cause assemblages}", year = {2023}, journal = {Quantum}, volume = {7}, doi = {10.22331/Q-2023-02-16-926}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85152550871&doi=10.22331%2fQ-2023-02-16-926&partnerID=40&md5=43f6a23dc49619100fe1dc0883dbe8f2}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 4; All Open Access, Gold Open Access, Green Open Access} }`

- John H. Selby, David Schmid, Elie Wolfe, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality.
*Physical Review A*, 107(6), 2023. doi:10.1103/PhysRevA.107.062203

[BibTeX]`@ARTICLE{Selby2023ab, author = {Selby, John H. and Schmid, David and Wolfe, Elie and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, title ="{Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality}", year = {2023}, journal = {Physical Review A}, volume = {107}, number = {6}, doi = {10.1103/PhysRevA.107.062203}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85161891900&doi=10.1103%2fPhysRevA.107.062203&partnerID=40&md5=cb35e56c68dc3635ab117f3b2355cca3}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 5; All Open Access, Green Open Access} }`

- Beata Zjawin, David Schmid, Matty J. Hoban, and Ana Belén Sainz. The resource theory of nonclassicality of channel assemblages.
*Quantum*, 7, 2023. doi:10.22331/q-2023-10-10-1134

[BibTeX]`@ARTICLE{Zjawin2023aa, author = {Zjawin, Beata and Schmid, David and Hoban, Matty J. and Sainz, Ana Belén}, title ="{The resource theory of nonclassicality of channel assemblages}", year = {2023}, journal = {Quantum}, volume = {7}, doi = {10.22331/q-2023-10-10-1134}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85176400015&doi=10.22331%2fq-2023-10-10-1134&partnerID=40&md5=675f6cb77020d3a0399cd8b3bf8ac414}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 1; All Open Access, Gold Open Access, Green Open Access} }`

- Vinicius P. Rossi, David Schmid, John H. Selby, and Ana Belén Sainz. Contextuality with vanishing coherence and maximal robustness to dephasing.
*Physical Review A*, 108(3), 2023. doi:10.1103/PhysRevA.108.032213

[BibTeX]`@ARTICLE{Rossi2023, author = {Rossi, Vinicius P. and Schmid, David and Selby, John H. and Sainz, Ana Belén}, title ="{Contextuality with vanishing coherence and maximal robustness to dephasing}", year = {2023}, journal = {Physical Review A}, volume = {108}, number = {3}, doi = {10.1103/PhysRevA.108.032213}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85173002700&doi=10.1103%2fPhysRevA.108.032213&partnerID=40&md5=27078c9622a1a9658d9faed8ab865bf8}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 2; All Open Access, Green Open Access} }`

- David Schmid, Thomas C. Fraser, Ravi Kunjwal, Ana Belén Sainz, Elie Wolfe, and Robert W. Spekkens. Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory.
*Quantum*, 7, 2023. doi:10.22331/q-2023-12-04-1194

[BibTeX]`@ARTICLE{Schmid2023, author = {Schmid, David and Fraser, Thomas C. and Kunjwal, Ravi and Sainz, Ana Belén and Wolfe, Elie and Spekkens, Robert W.}, title ="{Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory}", year = {2023}, journal = {Quantum}, volume = {7}, doi = {10.22331/q-2023-12-04-1194}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85180603184&doi=10.22331%2fq-2023-12-04-1194&partnerID=40&md5=b36a2c519f5da9b937eefd6108100308}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 0; All Open Access, Gold Open Access, Green Open Access} }`

### 2022

- Lorenzo Catani, Matthew Leifer, Giovanni Scala, David Schmid, and Robert W. Spekkens. What is Nonclassical about Uncertainty Relations?.
*Physical Review Letters*, 129(24), 2022. doi:10.1103/PhysRevLett.129.240401

[BibTeX]`@ARTICLE{Catani2022, author = {Catani, Lorenzo and Leifer, Matthew and Scala, Giovanni and Schmid, David and Spekkens, Robert W.}, title ="{What is Nonclassical about Uncertainty Relations?}", year = {2022}, journal = {Physical Review Letters}, volume = {129}, number = {24}, doi = {10.1103/PhysRevLett.129.240401}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85143653148&doi=10.1103%2fPhysRevLett.129.240401&partnerID=40&md5=328f17d613b531fe492c4d82b7202a0b}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 5; All Open Access, Green Open Access} }`

- Paulo J. Cavalcanti, John H. Selby, Jamie Sikora, and Ana Belén Sainz. Decomposing all multipartite non-signalling channels via quasiprobabilistic mixtures of local channels in generalised probabilistic theories.
*Journal of Physics A: Mathematical and Theoretical*, 55(40), 2022. doi:10.1088/1751-8121/ac8ea4

[BibTeX]`@ARTICLE{Cavalcanti2022, author = {Cavalcanti, Paulo J. and Selby, John H. and Sikora, Jamie and Sainz, Ana Belén}, title ="{Decomposing all multipartite non-signalling channels via quasiprobabilistic mixtures of local channels in generalised probabilistic theories}", year = {2022}, journal = {Journal of Physics A: Mathematical and Theoretical}, volume = {55}, number = {40}, doi = {10.1088/1751-8121/ac8ea4}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85145575825&doi=10.1088%2f1751-8121%2fac8ea4&partnerID=40&md5=0a28190f48bf4f4eb6f1d643f5d2b252}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 0; All Open Access, Green Open Access, Hybrid Gold Open Access} }`

- David Schmid, Haoxing Du, John H. Selby, and Matthew F. Pusey. Uniqueness of Noncontextual Models for Stabilizer Subtheories.
*Physical Review Letters*, 129(12), 2022. doi:10.1103/PhysRevLett.129.120403

[BibTeX]`@ARTICLE{Schmid2022, author = {Schmid, David and Du, Haoxing and Selby, John H. and Pusey, Matthew F.}, title ="{Uniqueness of Noncontextual Models for Stabilizer Subtheories}", year = {2022}, journal = {Physical Review Letters}, volume = {129}, number = {12}, doi = {10.1103/PhysRevLett.129.120403}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85138913163&doi=10.1103%2fPhysRevLett.129.120403&partnerID=40&md5=804da9d6aa9c312ac79d7da9af0dd310}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 11; All Open Access, Green Open Access} }`

- Vinicius P. Rossi, Matty J. Hoban, and Ana Belén Sainz. On characterising assemblages in Einstein-Podolsky-Rosen scenarios.
*Journal of Physics A: Mathematical and Theoretical*, 55(26), 2022. doi:10.1088/1751-8121/ac7090

[BibTeX]`@ARTICLE{Rossi2022, author = {Rossi, Vinicius P and Hoban, Matty J and Sainz, Ana Belén}, title ="{On characterising assemblages in Einstein-Podolsky-Rosen scenarios}", year = {2022}, journal = {Journal of Physics A: Mathematical and Theoretical}, volume = {55}, number = {26}, doi = {10.1088/1751-8121/ac7090}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85132346036&doi=10.1088%2f1751-8121%2fac7090&partnerID=40&md5=dc6318f6c5719f3cc5fa701c3439ccb4}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 0; All Open Access, Green Open Access} }`

- Paulo J. Cavalcanti, John H. Selby, Jamie Sikora, Thomas D. Galley, and Ana Belén Sainz. Post-quantum steering is a stronger-than-quantum resource for information processing.
*npj Quantum Information*, 8(1), 2022. doi:10.1038/s41534-022-00574-8

[BibTeX]`@ARTICLE{Cavalcanti2022aa, author = {Cavalcanti, Paulo J. and Selby, John H. and Sikora, Jamie and Galley, Thomas D. and Sainz, Ana Belén}, title ="{Post-quantum steering is a stronger-than-quantum resource for information processing}", year = {2022}, journal = {npj Quantum Information}, volume = {8}, number = {1}, doi = {10.1038/s41534-022-00574-8}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85133104573&doi=10.1038%2fs41534-022-00574-8&partnerID=40&md5=d6c64ba8e672f73a41e455c20a9c8f3c}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 5; All Open Access, Gold Open Access, Green Open Access} }`

### 2021

- David Schmid, John H. Selby, Elie Wolfe, Ravi Kunjwal, and Robert W. Spekkens. Characterization of Noncontextuality in the Framework of Generalized Probabilistic Theories.
*PRX Quantum*, 2(1), 2021. doi:10.1103/PRXQuantum.2.010331

[BibTeX]`@ARTICLE{Schmid2021, author = {Schmid, David and Selby, John H. and Wolfe, Elie and Kunjwal, Ravi and Spekkens, Robert W.}, title ="{Characterization of Noncontextuality in the Framework of Generalized Probabilistic Theories}", year = {2021}, journal = {PRX Quantum}, volume = {2}, number = {1}, doi = {10.1103/PRXQuantum.2.010331}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85104225742&doi=10.1103%2fPRXQuantum.2.010331&partnerID=40&md5=0e27e9ca26fedd114e0b7c1f14a41479}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 24; All Open Access, Gold Open Access, Green Open Access} }`

- Noam Finkelstein, Beata Zjawin, Elie Wolfe, Ilya Shpitser, and Robert W. Spekkens. Entropic Inequality Constraints from e-separation Relations in Directed Acyclic Graphs with Hidden Variables.
*37th Conference on Uncertainty in Artificial Intelligence, UAI 2021*, page 1045 – 1055, 2021.

[BibTeX]`@article{Finkelstein20211045, author = {Finkelstein, Noam and Zjawin, Beata and Wolfe, Elie and Shpitser, Ilya and Spekkens, Robert W.}, title ="{Entropic Inequality Constraints from e-separation Relations in Directed Acyclic Graphs with Hidden Variables}", year = {2021}, journal = {37th Conference on Uncertainty in Artificial Intelligence, UAI 2021}, pages = {1045 – 1055}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85124306480&partnerID=40&md5=a2596f45c9b1011153a7d1c5428d23d0}, type = {Conference paper}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 1} }`

- Noam Finkelstein, Beata Zjawin, Elie Wolfe, Ilya Shpitser, and Robert W. Spekkens. Entropic Inequality Constraints from e-separation Relations in Directed Acyclic Graphs with Hidden Variables.
*Proceedings of Machine Learning Research*, 161:1045 – 1055, 2021.

[BibTeX]`@article{Finkelstein20211045aa, author = {Finkelstein, Noam and Zjawin, Beata and Wolfe, Elie and Shpitser, Ilya and Spekkens, Robert W.}, title ="{Entropic Inequality Constraints from e-separation Relations in Directed Acyclic Graphs with Hidden Variables}", year = {2021}, journal = {Proceedings of Machine Learning Research}, volume = {161}, pages = {1045 – 1055}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85163409563&partnerID=40&md5=74db7c6b8e854fd3e25718677bc3cfb5}, type = {Conference paper}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 1} }`

- B. Zjawin, M. Bober, R. Ciuryło, D. Lisak, M. Zawada, and P. Wcisło. Engineering the sensitivity of macroscopic physical systems to variations in the fine-structure constant.
*EPL*, 136(5), 2021. doi:10.1209/0295-5075/ac3da3

[BibTeX]`@ARTICLE{Zjawin2021, author = {Zjawin, B. and Bober, M. and Ciuryło, R. and Lisak, D. and Zawada, M. and Wcisło, P.}, title ="{Engineering the sensitivity of macroscopic physical systems to variations in the fine-structure constant}", year = {2021}, journal = {EPL}, volume = {136}, number = {5}, doi = {10.1209/0295-5075/ac3da3}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85126454138&doi=10.1209%2f0295-5075%2fac3da3&partnerID=40&md5=cd168cd76c8ce8ad7279225692e13eee}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 0; All Open Access, Green Open Access, Hybrid Gold Open Access} }`

- Beata Zjawin, Elie Wolfe, and Robert W. Spekkens. Restricted hidden cardinality constraints in causal models.
*Electronic Proceedings in Theoretical Computer Science, EPTCS*, 343:119 – 131, 2021. doi:10.4204/EPTCS.343.6

[BibTeX]`@article{Zjawin2021119, author = {Zjawin, Beata and Wolfe, Elie and Spekkens, Robert W.}, title ="{Restricted hidden cardinality constraints in causal models}", year = {2021}, journal = {Electronic Proceedings in Theoretical Computer Science, EPTCS}, volume = {343}, pages = {119 – 131}, doi = {10.4204/EPTCS.343.6}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85115859386&doi=10.4204%2fEPTCS.343.6&partnerID=40&md5=d5e3387ae9777aeed2b2ab1eb9c6a3b3}, type = {Conference paper}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 0; All Open Access, Gold Open Access} }`

### 2020

- Elie Wolfe, David Schmid, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Quantifying Bell: The Resource Theory of Nonclassicality of Common-Cause Boxes.
*Quantum*, 4, 2020. doi:10.22331/Q-2020-06-08-280

[BibTeX]`@ARTICLE{Wolfe2020, author = {Wolfe, Elie and Schmid, David and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, title ="{Quantifying Bell: The Resource Theory of Nonclassicality of Common-Cause Boxes}", year = {2020}, journal = {Quantum}, volume = {4}, doi = {10.22331/Q-2020-06-08-280}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85090607975&doi=10.22331%2fQ-2020-06-08-280&partnerID=40&md5=69d0ce8d7a1355344d727565eabeed03}, type = {Review}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 42; All Open Access, Gold Open Access, Green Open Access} }`

### 2019

- Alejandro Pozas-Kerstjens, Rafael Rabelo, Łukasz Rudnicki, Rafael Chaves, Daniel Cavalcanti, Miguel Navascués, and Antonio Acín. Bounding the Sets of Classical and Quantum Correlations in Networks.
*Physical Review Letters*, 123(14), 2019. doi:10.1103/PhysRevLett.123.140503

[BibTeX]`@ARTICLE{Pozas-Kerstjens2019, author = {Pozas-Kerstjens, Alejandro and Rabelo, Rafael and Rudnicki, Łukasz and Chaves, Rafael and Cavalcanti, Daniel and Navascués, Miguel and Acín, Antonio}, title ="{Bounding the Sets of Classical and Quantum Correlations in Networks}", year = {2019}, journal = {Physical Review Letters}, volume = {123}, number = {14}, doi = {10.1103/PhysRevLett.123.140503}, OTurl = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85073050982&doi=10.1103%2fPhysRevLett.123.140503&partnerID=40&md5=e8eec371fc8496053e9292fe6614b99f}, type = {Article}, publication_stage = {Final}, source = {Scopus}, OPTnote = {Cited by: 32; All Open Access, Green Open Access} }`

## arXiv preprints

### 2022

- Ana Belén Sainz. Q-Turn: Changing Paradigms In Quantum Science.
*arXiv:2202.06867 [physics.soc-ph]*, 2022.

[BibTeX]`@article{sainz2022q_arXiv, title={Q-Turn: Changing Paradigms In Quantum Science}, author={Sainz, Ana Bel{\'e}n}, journal={arXiv:2202.06867 [physics.soc-ph]}, year={2022} }`

- Matty J. Hoban, Tom Drescher, and Ana Belén Sainz. A hierarchy of semidefinite programs for generalised Einstein-Podolsky-Rosen scenarios.
*arXiv:2208.09236 [quant-ph]*, 2022.

[BibTeX]`@article{hoban2022hierarchy, title={A hierarchy of semidefinite programs for generalised Einstein-Podolsky-Rosen scenarios}, author={Hoban, Matty J and Drescher, Tom and Sainz, Ana Bel{\'e}n}, journal={arXiv:2208.09236 [quant-ph]}, year={2022} }`

- Paulo J. Cavalcanti, John H. Selby, Jamie Sikora, and Ana Belén Sainz. Simulating all multipartite non-signalling channels via quasiprobabilistic mixtures of local channels in generalised probabilistic theories.
*arXiv:2204.10639 [quant-ph]*, 2022. doi:10.48550/arXiv.2204.10639

[BibTeX] [Download PDF]`@article{Cavalcanti_2022_Simulating, author = {Cavalcanti, Paulo J. and Selby, John H. and Sikora, Jamie and Sainz, Ana Belén}, keywords = {Quantum Physics (quant-ph), FOS: Physical sciences, FOS: Physical sciences}, title = {Simulating all multipartite non-signalling channels via quasiprobabilistic mixtures of local channels in generalised probabilistic theories}, doi = {10.48550/arXiv.2204.10639}, url = {https://arxiv.org/abs/2204.10639}, journal={arXiv:2204.10639 [quant-ph]}, year={2022} }`

- John H. Selby, Elie Wolfe, David Schmid, and Ana Belén Sainz. An open-source linear program for testing nonclassicality. , 2022. doi:10.48550/ARXIV.2204.11905

[BibTeX] [Download PDF]`@article{Selby_linear_program_contextuality, doi = {10.48550/ARXIV.2204.11905}, url = {https://arxiv.org/abs/2204.11905}, author = {Selby, John H. and Wolfe, Elie and Schmid, David and Sainz, Ana Belén}, keywords = {Quantum Physics (quant-ph), FOS: Physical sciences, FOS: Physical sciences}, title = {An open-source linear program for testing nonclassicality}, publisher = {arXiv}, year = {2022}, copyright = {arXiv.org perpetual, non-exclusive license} }`

- Vinicius P. Rossi, Matty J. Hoban, and Ana Belen Sainz. On characterising assemblages in Einstein-Podolsky-Rosen scenarios.
*arXiv:2202.05553 [quant-ph]*, feb 2022. arXiv:2202.05553 [quant-ph]

[BibTeX] [Abstract] [Download PDF]

Characterising non-classical quantum phenomena is crucial not only from a fundamental perspective, but also to better understand its capabilities for information processing and communication tasks. In this work, we focus on exploring the characterisation of Einstein-Podolsky-Rosen inference (a.k.a. steering): a signature of non-classicality manifested when one or more parties in a Bell scenario have their systems and measurements described by quantum theory, rather than being treated as black boxes. We propose a way of characterising common-cause assemblages from the correlations that arise when the trusted party performs tomographically-complete measurements on their share of the experiment, and discuss the advantages and challenges of this approach. Within this framework, we show that so-called almost quantum assemblages satisfy the principle of macroscopic noncontextuality, and demonstrate that a subset of almost quantum correlations recover almost quantum assemblages in this approach.

`@article{rossi_2022_characterising, title = {On characterising assemblages in {Einstein-Podolsky-Rosen} scenarios}, url = {https://arxiv.org/abs/2202.05553}, urldate = {2022-03-29}, abstract = {Characterising non-classical quantum phenomena is crucial not only from a fundamental perspective, but also to better understand its capabilities for information processing and communication tasks. In this work, we focus on exploring the characterisation of Einstein-Podolsky-Rosen inference (a.k.a. steering): a signature of non-classicality manifested when one or more parties in a Bell scenario have their systems and measurements described by quantum theory, rather than being treated as black boxes. We propose a way of characterising common-cause assemblages from the correlations that arise when the trusted party performs tomographically-complete measurements on their share of the experiment, and discuss the advantages and challenges of this approach. Within this framework, we show that so-called almost quantum assemblages satisfy the principle of macroscopic noncontextuality, and demonstrate that a subset of almost quantum correlations recover almost quantum assemblages in this approach.}, journal = {arXiv:2202.05553 [quant-ph]}, author = {Rossi, Vinicius P. and Hoban, Matty J. and Sainz, Ana Belen}, month = feb, year = {2022}, note = {arXiv:2202.05553 [quant-ph]}, keywords = {Quantum Physics} }`

- Lorenzo Catani, Matthew Leifer, David Schmid, and Robert W. Spekkens. Reply to “Comment on ‘Why interference phenomena do not capture the essence of quantum theory’ “. 2022. doi:10.48550/ARXIV.2207.11791

[BibTeX] [Download PDF]`@misc{catani2022reply, doi = {10.48550/ARXIV.2207.11791}, url = {https://arxiv.org/abs/2207.11791}, author = {Catani, Lorenzo and Leifer, Matthew and Schmid, David and Spekkens, Robert W.}, keywords = {Quantum Physics (quant-ph), FOS: Physical sciences, FOS: Physical sciences}, title = {Reply to "Comment on 'Why interference phenomena do not capture the essence of quantum theory' "}, publisher = {arXiv}, year = {2022}, copyright = {Creative Commons Attribution 4.0 International} }`

- Lorenzo Catani, Matthew Leifer, Giovanni Scala, David Schmid, and Robert W. Spekkens. What is nonclassical about uncertainty relations?. , jul 2022.

[BibTeX] [Abstract]

Uncertainty relations express limits on the extent to which the outcomes of distinct measurements on a single state can be made jointly predictable. The existence of nontrivial uncertainty relations in quantum theory is generally considered to be a way in which it entails a departure from the classical worldview. However, this view is undermined by the fact that there exist operational theories which exhibit nontrivial uncertainty relations but which are consistent with the classical worldview insofar as they admit of a generalized-noncontextual ontological model. This prompts the question of what aspects of uncertainty relations, if any, cannot be realized in this way and so constitute evidence of genuine nonclassicality. We here consider uncertainty relations describing the tradeoff between the predictability of a pair of binary-outcome measurements (e.g., measurements of Pauli X and Pauli Z observables in quantum theory). We show that, for a class of theories satisfying a particular symmetry property, the functional form of this predictability tradeoff is constrained by noncontextuality to be below a linear curve. Because qubit quantum theory has the relevant symmetry property, the fact that it has a quadratic tradeoff between these predictabilities is a violation of this noncontextual bound, and therefore constitutes an example of how the functional form of an uncertainty relation can witness contextuality. We also deduce the implications for a selected group of operational foils to quantum theory and consider the generalization to three measurements.

`@Article{Catani2022, author = {Lorenzo Catani and Matthew Leifer and Giovanni Scala and David Schmid and Robert W. Spekkens}, title = {What is nonclassical about uncertainty relations?}, year = {2022}, month = jul, abstract = {Uncertainty relations express limits on the extent to which the outcomes of distinct measurements on a single state can be made jointly predictable. The existence of nontrivial uncertainty relations in quantum theory is generally considered to be a way in which it entails a departure from the classical worldview. However, this view is undermined by the fact that there exist operational theories which exhibit nontrivial uncertainty relations but which are consistent with the classical worldview insofar as they admit of a generalized-noncontextual ontological model. This prompts the question of what aspects of uncertainty relations, if any, cannot be realized in this way and so constitute evidence of genuine nonclassicality. We here consider uncertainty relations describing the tradeoff between the predictability of a pair of binary-outcome measurements (e.g., measurements of Pauli X and Pauli Z observables in quantum theory). We show that, for a class of theories satisfying a particular symmetry property, the functional form of this predictability tradeoff is constrained by noncontextuality to be below a linear curve. Because qubit quantum theory has the relevant symmetry property, the fact that it has a quadratic tradeoff between these predictabilities is a violation of this noncontextual bound, and therefore constitutes an example of how the functional form of an uncertainty relation can witness contextuality. We also deduce the implications for a selected group of operational foils to quantum theory and consider the generalization to three measurements.}, archiveprefix = {arXiv}, eprint = {2207.11779}, file = {:http\://arxiv.org/pdf/2207.11779v1:PDF}, keywords = {quant-ph}, primaryclass = {quant-ph}, }`

- Lorenzo Catani, Matthew Leifer, Giovanni Scala, David Schmid, and Robert W. Spekkens. What aspects of the phenomenology of interference witness nonclassicality?. , nov 2022.

[BibTeX] [Abstract]

Interference phenomena are often claimed to resist classical explanation. However, such claims are undermined by the fact that the specific aspects of the phenomenology upon which they are based can in fact be reproduced in a noncontextual ontological model [Catani et al. arXiv:2111.13727]. This raises the question of what other aspects of the phenomenology of interference do in fact resist classical explanation. We answer this question by demonstrating that the most basic quantum wave-particle duality relation, which expresses the precise trade-off between path distinguishability and fringe visibility, cannot be reproduced in any noncontextual model. We do this by showing that it is a specific type of uncertainty relation, and then leveraging a recent result establishing that noncontextuality restricts the functional form of this uncertainty relation [Catani et al. arXiv:2207.11779]. Finally, we discuss what sorts of interferometric experiment can demonstrate contextuality via the wave-particle duality relation.

`@Article{Catani2022a, author = {Lorenzo Catani and Matthew Leifer and Giovanni Scala and David Schmid and Robert W. Spekkens}, title = {What aspects of the phenomenology of interference witness nonclassicality?}, year = {2022}, month = nov, abstract = {Interference phenomena are often claimed to resist classical explanation. However, such claims are undermined by the fact that the specific aspects of the phenomenology upon which they are based can in fact be reproduced in a noncontextual ontological model [Catani et al. arXiv:2111.13727]. This raises the question of what other aspects of the phenomenology of interference do in fact resist classical explanation. We answer this question by demonstrating that the most basic quantum wave-particle duality relation, which expresses the precise trade-off between path distinguishability and fringe visibility, cannot be reproduced in any noncontextual model. We do this by showing that it is a specific type of uncertainty relation, and then leveraging a recent result establishing that noncontextuality restricts the functional form of this uncertainty relation [Catani et al. arXiv:2207.11779]. Finally, we discuss what sorts of interferometric experiment can demonstrate contextuality via the wave-particle duality relation.}, archiveprefix = {arXiv}, eprint = {2211.09850}, file = {:http\://arxiv.org/pdf/2211.09850v1:PDF}, keywords = {quant-ph}, primaryclass = {quant-ph}, }`

- Beata Zjawin, David Schmid, Matty J. Hoban, and Ana Belén Sainz. The resource theory of nonclassicality of channel assemblages.
*arXiv e-prints*, pages arXiv:2209.10177, sep 2022.

[BibTeX] [Abstract] [Download PDF]

When two parties, Alice and Bob, share correlated quantum systems and Alice performs local measurements, Alice’s updated description of Bob’s state can provide evidence of nonclassical correlations. This simple scenario, famously studied by Einstein, Podolsky and Rosen (EPR), can be modified by allowing Bob to also have a classical or quantum system as an input. In this case, Alice updates her knowledge of the channel (rather than the state) in Bob’s lab. In this paper, we provide a unified framework for studying the nonclassicality of various such generalizations of the EPR scenario. We do so using a resource theory wherein the free operations are local operations and shared randomness (LOSR). We derive a semidefinite program for studying the pre-order of EPR resources, and discover possible conversions between the latter. Moreover, we study conversions between post-quantum resources both analytically and numerically.

`@Article{Zjawin2022, author = {Zjawin, Beata and Schmid, David and Hoban, Matty J. and Sainz, Ana Belén}, journal = {arXiv e-prints}, title = {The resource theory of nonclassicality of channel assemblages}, year = {2022}, month = sep, pages = {arXiv:2209.10177}, abstract = {When two parties, Alice and Bob, share correlated quantum systems and Alice performs local measurements, Alice's updated description of Bob's state can provide evidence of nonclassical correlations. This simple scenario, famously studied by Einstein, Podolsky and Rosen (EPR), can be modified by allowing Bob to also have a classical or quantum system as an input. In this case, Alice updates her knowledge of the channel (rather than the state) in Bob's lab. In this paper, we provide a unified framework for studying the nonclassicality of various such generalizations of the EPR scenario. We do so using a resource theory wherein the free operations are local operations and shared randomness (LOSR). We derive a semidefinite program for studying the pre-order of EPR resources, and discover possible conversions between the latter. Moreover, we study conversions between post-quantum resources both analytically and numerically. }, archiveprefix = {arXiv}, eid = {arXiv:2209.10177}, eprint = {2209.10177}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://arxiv.org/abs/2209.10177}, }`

### 2021

- John H. Selby, David Schmid, Elie Wolfe, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Contextuality without incompatibility. , 2021. doi:10.48550/ARXIV.2106.09045

[BibTeX] [Download PDF]`@article{Contextuality_without_incompatibility, doi = {10.48550/ARXIV.2106.09045}, url = {https://arxiv.org/abs/2106.09045}, author = {Selby, John H. and Schmid, David and Wolfe, Elie and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, keywords = {Quantum Physics (quant-ph), FOS: Physical sciences, FOS: Physical sciences}, title = {Contextuality without incompatibility}, publisher = {arXiv}, year = {2021}, copyright = {arXiv.org perpetual, non-exclusive license} }`

- John H. Selby, David Schmid, Elie Wolfe, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality.
*arXiv:2112.04521 [quant-ph]*, dec 2021. arXiv: 2112.04521

[BibTeX] [Abstract] [Download PDF]

The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particular experimental setup within a given physical theory. We show that the resulting characterization is not generally a GPT in and of itself-rather, it is described by a more general mathematical object that we introduce and term an accessible GPT fragment. We then introduce an equivalence relation, termed cone equivalence, between accessible GPT fragments (and, as a special case, between standard GPTs). We give a number of examples of experimental scenarios that are best described using accessible GPT fragments, and where moreover cone-equivalence arises naturally. We then prove that an accessible GPT fragment admits of a classical explanation if and only if every other fragment that is cone-equivalent to it also admits of a classical explanation. Finally, we leverage this result to prove several fundamental results regarding the experimental requirements for witnessing the failure of generalized noncontextuality. In particular, we prove that neither incompatibility among measurements nor the assumption of freedom of choice is necessary for witnessing failures of generalized noncontextuality, and, moreover, that such failures can be witnessed even using arbitrarily inefficient detectors.

`@article{selby_accessible_2021, title = {Accessible fragments of generalized probabilistic theories, cone equivalence, and applications to witnessing nonclassicality}, url = {https://arxiv.org/abs/2112.04521}, abstract = {The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particular experimental setup within a given physical theory. We show that the resulting characterization is not generally a GPT in and of itself-rather, it is described by a more general mathematical object that we introduce and term an accessible GPT fragment. We then introduce an equivalence relation, termed cone equivalence, between accessible GPT fragments (and, as a special case, between standard GPTs). We give a number of examples of experimental scenarios that are best described using accessible GPT fragments, and where moreover cone-equivalence arises naturally. We then prove that an accessible GPT fragment admits of a classical explanation if and only if every other fragment that is cone-equivalent to it also admits of a classical explanation. Finally, we leverage this result to prove several fundamental results regarding the experimental requirements for witnessing the failure of generalized noncontextuality. In particular, we prove that neither incompatibility among measurements nor the assumption of freedom of choice is necessary for witnessing failures of generalized noncontextuality, and, moreover, that such failures can be witnessed even using arbitrarily inefficient detectors.}, urldate = {2021-12-10}, journal = {arXiv:2112.04521 [quant-ph]}, author = {Selby, John H. and Schmid, David and Wolfe, Elie and Sainz, Ana Belén and Kunjwal, Ravi and Spekkens, Robert W.}, month = dec, year = {2021}, note = {arXiv: 2112.04521}, keywords = {Quantum Physics}, }`

- Lorenzo Catani, Matthew Leifer, David Schmid, and Robert W. Spekkens. Why interference phenomena do not capture the essence of quantum theory.
*arXiv:2111.13727 [quant-ph]*, nov 2021. arXiv: 2111.13727

[BibTeX] [Abstract] [Download PDF]

Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman even went so far as to proclaim that they are the only mystery and the basic peculiarity of quantum mechanics. Many have also argued that such phenomena force us to accept a number of radical interpretational conclusions, including: that a photon is neither a particle nor a wave but rather a schizophrenic sort of entity that toggles between the two possibilities, that reality is observer-dependent, and that systems either do not have properties prior to measurements or else have properties that are subject to nonlocal or backwards-in-time causal influences. In this work, we show that such conclusions are not, in fact, forced on us by the phenomena. We do so by describing an alternative to quantum theory, a statistical theory of a classical discrete field (the `toy field theory’) that reproduces the relevant phenomenology of quantum interference while rejecting these radical interpretational claims. It also reproduces a number of related interference experiments that are thought to support these interpretational claims, such as the Elitzur-Vaidman bomb tester, Wheeler’s delayed-choice experiment, and the quantum eraser experiment. The systems in the toy field theory are field modes, each of which possesses, at all times, both a particle-like property (a discrete occupation number) and a wave-like property (a discrete phase). Although these two properties are jointly possessed, the theory stipulates that they cannot be jointly known. The phenomenology that is generally cited in favour of nonlocal or backwards-in-time causal influences ends up being explained in terms of inferences about distant or past systems, and all that is observer-dependent is the observer’s knowledge of reality, not reality itself.

`@article{catani_why_2021, title = {Why interference phenomena do not capture the essence of quantum theory}, url = {http://arxiv.org/abs/2111.13727}, abstract = {Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman even went so far as to proclaim that they are the only mystery and the basic peculiarity of quantum mechanics. Many have also argued that such phenomena force us to accept a number of radical interpretational conclusions, including: that a photon is neither a particle nor a wave but rather a schizophrenic sort of entity that toggles between the two possibilities, that reality is observer-dependent, and that systems either do not have properties prior to measurements or else have properties that are subject to nonlocal or backwards-in-time causal influences. In this work, we show that such conclusions are not, in fact, forced on us by the phenomena. We do so by describing an alternative to quantum theory, a statistical theory of a classical discrete field (the `toy field theory') that reproduces the relevant phenomenology of quantum interference while rejecting these radical interpretational claims. It also reproduces a number of related interference experiments that are thought to support these interpretational claims, such as the Elitzur-Vaidman bomb tester, Wheeler's delayed-choice experiment, and the quantum eraser experiment. The systems in the toy field theory are field modes, each of which possesses, at all times, both a particle-like property (a discrete occupation number) and a wave-like property (a discrete phase). Although these two properties are jointly possessed, the theory stipulates that they cannot be jointly known. The phenomenology that is generally cited in favour of nonlocal or backwards-in-time causal influences ends up being explained in terms of inferences about distant or past systems, and all that is observer-dependent is the observer's knowledge of reality, not reality itself.}, urldate = {2021-11-30}, journal = {arXiv:2111.13727 [quant-ph]}, author = {Catani, Lorenzo and Leifer, Matthew and Schmid, David and Spekkens, Robert W.}, month = nov, year = {2021}, note = {arXiv: 2111.13727}, keywords = {Quantum Physics}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. No-go for device independent protocols with Tan-Walls-Collett `nonlocality of a single photon’.
*arXiv:2102.03254 [quant-ph]*, feb 2021. arXiv: 2102.03254

[BibTeX] [Abstract] [Download PDF]

We investigate the interferometric scheme put forward by Tan, Walls and Collett [Phys. Rev. Lett. \\textbackslashbf 66\, 256 (1991)] that aims to reveal Bell non-classicality of a single photon. By providing a local hidden variable model that reproduces their results, we decisively refute this claim. In particular, this means that the scheme cannot be used in device-independent protocols.

`@article{das_no-go_2021, title = {No-go for device independent protocols with {Tan}-{Walls}-{Collett} `nonlocality of a single photon'}, url = {http://arxiv.org/abs/2102.03254}, abstract = {We investigate the interferometric scheme put forward by Tan, Walls and Collett [Phys. Rev. Lett. \{{\textbackslash}bf 66\}, 256 (1991)] that aims to reveal Bell non-classicality of a single photon. By providing a local hidden variable model that reproduces their results, we decisively refute this claim. In particular, this means that the scheme cannot be used in device-independent protocols.}, urldate = {2021-07-28}, journal = {arXiv:2102.03254 [quant-ph]}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, month = feb, year = {2021}, note = {arXiv: 2102.03254}, keywords = {Quantum Physics}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Zukowski. Can single photon excitation of two spatially separated modes lead to a violation of Bell inequality via homodyne measurements?.
*arXiv:2102.06689 [quant-ph]*, feb 2021. arXiv: 2102.06689

[BibTeX] [Abstract] [Download PDF]

We reconsider the all-optical homodyne-measurement based experimental schemes that aim to reveal Bell nonclassicality of a single photon, often termed `nonlocality’. We focus on the schemes put forward by Tan, Walls and Collett (TWC, 1991) and Hardy (1994). In the light of our previous work the Tan, Walls and Collett setup can be described by a precise local hidden variable model, hence the claimed nonclassicality of this proposal is apparent, whereas the nonclassicality proof proposed by Hardy is impeccable. In this work we resolve the following problem: which feature of the Hardy’s approach is crucial for its successful confirmation of nonclassicality. The scheme of Hardy differs from the Tan, Walls and Collett setup in two aspects. (i) It introduces a superposition of a single photon excitation with vacuum as the initial state of one of the input modes of a 50-50 beamsplitter, which creates the superposition state of two separable (exit) modes under investigation. (ii) In the final measurements Hardy’s proposal utilises a varying strengths of the local oscillator fields, whereas in the TWC case they are constant. In fact the local oscillators in Hardy’s scheme are either on or off (the local setting is specified by the presence or absence of the local auxiliary field). We show that it is the varying strength of the local oscillators, from setting to setting, which is the crucial feature enabling violation of local realism in the Hardy setup, whereas it is not necessary to use initial superposition of a single photon excitation with vacuum as the initial state of the input mode. Neither one needs to operate in the fully on/off detection scheme. Despite the failure of the Tan, Walls and Collett scheme in proving Bell nonclassicality, we show that their scheme can serve as an entanglement indicator.

`@article{das_can_2021, title = {Can single photon excitation of two spatially separated modes lead to a violation of {Bell} inequality via homodyne measurements?}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Zukowski, Marek}, url = {http://arxiv.org/abs/2102.06689}, abstract = {We reconsider the all-optical homodyne-measurement based experimental schemes that aim to reveal Bell nonclassicality of a single photon, often termed `nonlocality'. We focus on the schemes put forward by Tan, Walls and Collett (TWC, 1991) and Hardy (1994). In the light of our previous work the Tan, Walls and Collett setup can be described by a precise local hidden variable model, hence the claimed nonclassicality of this proposal is apparent, whereas the nonclassicality proof proposed by Hardy is impeccable. In this work we resolve the following problem: which feature of the Hardy's approach is crucial for its successful confirmation of nonclassicality. The scheme of Hardy differs from the Tan, Walls and Collett setup in two aspects. (i) It introduces a superposition of a single photon excitation with vacuum as the initial state of one of the input modes of a 50-50 beamsplitter, which creates the superposition state of two separable (exit) modes under investigation. (ii) In the final measurements Hardy's proposal utilises a varying strengths of the local oscillator fields, whereas in the TWC case they are constant. In fact the local oscillators in Hardy's scheme are either on or off (the local setting is specified by the presence or absence of the local auxiliary field). We show that it is the varying strength of the local oscillators, from setting to setting, which is the crucial feature enabling violation of local realism in the Hardy setup, whereas it is not necessary to use initial superposition of a single photon excitation with vacuum as the initial state of the input mode. Neither one needs to operate in the fully on/off detection scheme. Despite the failure of the Tan, Walls and Collett scheme in proving Bell nonclassicality, we show that their scheme can serve as an entanglement indicator.}, urldate = {2021-07-28}, journal = {arXiv:2102.06689 [quant-ph]}, month = feb, year = {2021}, note = {arXiv: 2102.06689}, keywords = {Quantum Physics}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, Bianka Woloncewicz, and Marek Żukowski. On detecting violation of local realism with photon-number resolving weak-field homodyne measurements.
*arXiv:2104.10703 [quant-ph]*, apr 2021. arXiv: 2104.10703

[BibTeX] [Abstract] [Download PDF]

Non-existence of a local hidden variables (LHV) model for a phenomenon benchmarks its use in device-independent quantum protocols. Nowadays photon-number resolving weak-field homodyne measurements allow realization of emblematic gedanken experiments. Alas, claims that we can have no LHV models for such experiments on (a) excitation of a pair of spatial modes by a single photon, and (b) two spatial modes in a weakly squeezed vacuum state, involving constant local oscillator strengths, are unfounded. For (a) an exact LHV model resolves the dispute on the “non-locality of a single photon” in its original formulation. It is measurements with local oscillators on or off that do not have LHV models.

`@article{das_detecting_2021, title = {On detecting violation of local realism with photon-number resolving weak-field homodyne measurements}, url = {http://arxiv.org/abs/2104.10703}, abstract = {Non-existence of a local hidden variables (LHV) model for a phenomenon benchmarks its use in device-independent quantum protocols. Nowadays photon-number resolving weak-field homodyne measurements allow realization of emblematic gedanken experiments. Alas, claims that we can have no LHV models for such experiments on (a) excitation of a pair of spatial modes by a single photon, and (b) two spatial modes in a weakly squeezed vacuum state, involving constant local oscillator strengths, are unfounded. For (a) an exact LHV model resolves the dispute on the "non-locality of a single photon" in its original formulation. It is measurements with local oscillators on or off that do not have LHV models.}, urldate = {2021-07-28}, journal = {arXiv:2104.10703 [quant-ph]}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Woloncewicz, Bianka and Żukowski, Marek}, month = apr, year = {2021}, note = {arXiv: 2104.10703}, keywords = {Quantum Physics}, }`

- David Schmid, John H. Selby, and Robert W. Spekkens. Unscrambling the omelette of causation and inference: The framework of causal-inferential theories.
*arXiv:2009.03297 [quant-ph]*, may 2021. arXiv: 2009.03297

[BibTeX] [Abstract] [Download PDF]

Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell’s notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.

`@article{schmid_unscrambling_2021, title = {Unscrambling the omelette of causation and inference: {The} framework of causal-inferential theories}, shorttitle = {Unscrambling the omelette of causation and inference}, url = {http://arxiv.org/abs/2009.03297}, abstract = {Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell's notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.}, urldate = {2021-07-28}, journal = {arXiv:2009.03297 [quant-ph]}, author = {Schmid, David and Selby, John H. and Spekkens, Robert W.}, month = may, year = {2021}, note = {arXiv: 2009.03297}, keywords = {Quantum Physics}, }`

- John H. Selby, Ana Belén Sainz, and Paweł Horodecki. Revisiting dynamics of quantum causal structures — when can causal order evolve?.
*arXiv:2008.12757 [quant-ph]*, mar 2021. arXiv: 2008.12757

[BibTeX] [Abstract] [Download PDF]

Recently, there has been substantial interest in studying the dynamics of quantum theory beyond that of states, in particular, the dynamics of channels, measurements, and higher-order transformations. Ref. [Phys. Rev. X 8(1), 011047 (2018)] pursues this using the process matrix formalism, together with a definition of the possible dynamics of such process matrices, and focusing especially on the question of evolution of causal structures. One of its major conclusions is a strong theorem saying that, within the formalism, under continuous and reversible transformations, the causal order between operations must be preserved. Here we find a surprising result: if one is to take into account a full picture of the physical evolution of operations within the standard quantum-mechanical formalism, then one can actually draw the opposite conclusion. That is, we show that under certain continuous and reversible dynamics the causal order between operations is not necessarily preserved. We moreover identify and analyse the root of this apparent contradiction, specifically, that the commonly accepted and widely applied framework of higher-order processes, whilst mathematically sound, is not always appropriate for drawing conclusions on the fundamentals of physical dynamics. Finally we show how to reconcile the elements of the whole picture following the intuition based on entanglement processing by local operations and classical communication.

`@Article{selby_revisiting_2021, author = {Selby, John H. and Sainz, Ana Belén and Horodecki, Paweł}, journal = {arXiv:2008.12757 [quant-ph]}, title = {Revisiting dynamics of quantum causal structures -- when can causal order evolve?}, year = {2021}, month = mar, note = {arXiv: 2008.12757}, abstract = {Recently, there has been substantial interest in studying the dynamics of quantum theory beyond that of states, in particular, the dynamics of channels, measurements, and higher-order transformations. Ref. [Phys. Rev. X 8(1), 011047 (2018)] pursues this using the process matrix formalism, together with a definition of the possible dynamics of such process matrices, and focusing especially on the question of evolution of causal structures. One of its major conclusions is a strong theorem saying that, within the formalism, under continuous and reversible transformations, the causal order between operations must be preserved. Here we find a surprising result: if one is to take into account a full picture of the physical evolution of operations within the standard quantum-mechanical formalism, then one can actually draw the opposite conclusion. That is, we show that under certain continuous and reversible dynamics the causal order between operations is not necessarily preserved. We moreover identify and analyse the root of this apparent contradiction, specifically, that the commonly accepted and widely applied framework of higher-order processes, whilst mathematically sound, is not always appropriate for drawing conclusions on the fundamentals of physical dynamics. Finally we show how to reconcile the elements of the whole picture following the intuition based on entanglement processing by local operations and classical communication.}, groups = {Pawel_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2008.12757}, urldate = {2021-07-28}, }`

- David Schmid, Thomas C. Fraser, Ravi Kunjwal, Ana Belen Sainz, Elie Wolfe, and Robert W. Spekkens. Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory.
*arXiv:2004.09194 [quant-ph]*, may 2021. arXiv: 2004.09194

[BibTeX] [Abstract] [Download PDF]

A standard approach to quantifying resources is to determine which operations on the resources are freely available, and to deduce the partial order over resources that is induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, i.e., entanglement, then the common assumption is that the appropriate choice of free operations is Local Operations and Classical Communication (LOCC). We here advocate for the study of a different choice of free operations, namely, Local Operations and Shared Randomness (LOSR), and demonstrate its utility in understanding the interplay between the entanglement of states and the nonlocality of the correlations in Bell experiments. Specifically, we show that the LOSR paradigm (i) provides a resolution of the anomalies of nonlocality, wherein partially entangled states exhibit more nonlocality than maximally entangled states, (ii) entails new notions of genuine multipartite entanglement and nonlocality that are free of the pathological features of the conventional notions, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which generalizes and simplifies prior results. Along the way, we derive some fundamental results concerning the necessary and sufficient conditions for convertibility between pure entangled states under LOSR and highlight some of their consequences, such as the impossibility of catalysis for bipartite pure states. The resource-theoretic perspective also clarifies why it is neither surprising nor problematic that there are mixed entangled states which do not violate any Bell inequality. Our results motivate the study of LOSR-entanglement as a new branch of entanglement theory.

`@article{schmid_understanding_2021, title = {Understanding the interplay of entanglement and nonlocality: motivating and developing a new branch of entanglement theory}, shorttitle = {Understanding the interplay of entanglement and nonlocality}, url = {http://arxiv.org/abs/2004.09194}, abstract = {A standard approach to quantifying resources is to determine which operations on the resources are freely available, and to deduce the partial order over resources that is induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, i.e., entanglement, then the common assumption is that the appropriate choice of free operations is Local Operations and Classical Communication (LOCC). We here advocate for the study of a different choice of free operations, namely, Local Operations and Shared Randomness (LOSR), and demonstrate its utility in understanding the interplay between the entanglement of states and the nonlocality of the correlations in Bell experiments. Specifically, we show that the LOSR paradigm (i) provides a resolution of the anomalies of nonlocality, wherein partially entangled states exhibit more nonlocality than maximally entangled states, (ii) entails new notions of genuine multipartite entanglement and nonlocality that are free of the pathological features of the conventional notions, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which generalizes and simplifies prior results. Along the way, we derive some fundamental results concerning the necessary and sufficient conditions for convertibility between pure entangled states under LOSR and highlight some of their consequences, such as the impossibility of catalysis for bipartite pure states. The resource-theoretic perspective also clarifies why it is neither surprising nor problematic that there are mixed entangled states which do not violate any Bell inequality. Our results motivate the study of LOSR-entanglement as a new branch of entanglement theory.}, urldate = {2021-07-28}, journal = {arXiv:2004.09194 [quant-ph]}, author = {Schmid, David and Fraser, Thomas C. and Kunjwal, Ravi and Sainz, Ana Belen and Wolfe, Elie and Spekkens, Robert W.}, month = may, year = {2021}, note = {arXiv: 2004.09194}, keywords = {Quantum Physics}, }`

- Beata Zjawin, David Schmid, Matty J. Hoban, and Ana Belén Sainz. Quantifying EPR: the resource theory of nonclassicality of common-cause assemblages.
*arXiv e-prints*, pages arXiv:2111.10244, nov 2021.

[BibTeX] [Abstract] [Download PDF]

Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering – endorsed by EPR themselves – is that Alice has no causal influence on the physical state of Bob’s system; rather, Alice merely updates her knowledge of the state of Bob’s system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective (from which the very term `steering’ is seen to be inappropriate), and we are led to a resource-theoretic treatment of correlations in EPR scenarios. For both bipartite and multipartite scenarios, we develop the resulting resource theory, wherein the free operations are local operations and shared randomness (LOSR). We show that resource conversion under free operations in this paradigm can be evaluated with a single instance of a semidefinite program, making the problem numerically tractable. Moreover, we find that the structure of the pre-order of resources features interesting properties, such as infinite families of incomparable resources. In showing this, we derive new EPR resource monotones. We also discuss advantages of our approach over a pre-existing proposal for a resource theory of `steering’, and discuss how our approach sheds light on basic questions, such as which multipartite assemblages are classically explainable.

`@Article{Zjawin2021, author = {Zjawin, Beata and Schmid, David and Hoban, Matty J. and Sainz, Ana Belén}, journal = {arXiv e-prints}, title = {Quantifying {EPR}: the resource theory of nonclassicality of common-cause assemblages}, year = {2021}, month = nov, pages = {arXiv:2111.10244}, abstract = {Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering - endorsed by EPR themselves - is that Alice has no causal influence on the physical state of Bob's system; rather, Alice merely updates her knowledge of the state of Bob's system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective (from which the very term `steering' is seen to be inappropriate), and we are led to a resource-theoretic treatment of correlations in EPR scenarios. For both bipartite and multipartite scenarios, we develop the resulting resource theory, wherein the free operations are local operations and shared randomness (LOSR). We show that resource conversion under free operations in this paradigm can be evaluated with a single instance of a semidefinite program, making the problem numerically tractable. Moreover, we find that the structure of the pre-order of resources features interesting properties, such as infinite families of incomparable resources. In showing this, we derive new EPR resource monotones. We also discuss advantages of our approach over a pre-existing proposal for a resource theory of `steering', and discuss how our approach sheds light on basic questions, such as which multipartite assemblages are classically explainable.}, archiveprefix = {arXiv}, eid = {arXiv:2111.10244}, eprint = {2111.10244}, keywords = {Quantum Physics}, primaryclass = {quant-ph}, url = {https://ui.adsabs.harvard.edu/abs/2021arXiv211110244Z}, }`

- Tamoghna Das, Marcin Karczewski, Antonio Mandarino, Marcin Markiewicz, and Marek Żukowski. Optimal interferometry for Bell$-$nonclassicality by a vacuum$-$one$-$photon qubit. , 2021. doi:10.48550/ARXIV.2109.10170

[BibTeX] [Download PDF]`@article{https://doi.org/10.48550/arxiv.2109.10170, doi = {10.48550/ARXIV.2109.10170}, url = {https://arxiv.org/abs/2109.10170}, author = {Das, Tamoghna and Karczewski, Marcin and Mandarino, Antonio and Markiewicz, Marcin and Żukowski, Marek}, keywords = {Quantum Physics (quant-ph), FOS: Physical sciences, FOS: Physical sciences}, title = {Optimal interferometry for {Bell}$-$nonclassicality by a vacuum$-$one$-$photon qubit}, publisher = {arXiv}, year = {2021}, copyright = {arXiv.org perpetual, non-exclusive license} }`

### 2020

- David Schmid, John H. Selby, Matthew F. Pusey, and Robert W. Spekkens. A structure theorem for generalized-noncontextual ontological models.
*arXiv:2005.07161 [quant-ph]*, may 2020. arXiv: 2005.07161

[BibTeX] [Abstract] [Download PDF]

It is useful to have a criterion for when the predictions of an operational theory should be considered classically explainable. Here we take the criterion to be that the theory admits of a generalized-noncontextual ontological model. Existing works on generalized noncontextuality have focused on experimental scenarios having a simple structure, typically, prepare-measure scenarios. Here, we formally extend the framework of ontological models as well as the principle of generalized noncontextuality to arbitrary compositional scenarios. We leverage this process-theoretic framework to prove that, under some reasonable assumptions, every generalized-noncontextual ontological model of a tomographically local operational theory has a surprisingly rigid and simple mathematical structure; in short, it corresponds to a frame representation which is not overcomplete. One consequence of this theorem is that the largest number of ontic states possible in any such model is given by the dimension of the associated generalized probabilistic theory. This constraint is useful for generating noncontextuality no-go theorems as well as techniques for experimentally certifying contextuality. Along the way, we extend known results concerning the equivalence of different notions of classicality from prepare-measure scenarios to arbitrary compositional scenarios. Specifically, we prove a correspondence between the following three notions of classical explainability of an operational theory: (i) admitting a noncontextual ontological model, (ii) admitting of a positive quasiprobability representation, and (iii) being simplex-embeddable.

`@article{schmid_structure_2020, title = {A structure theorem for generalized-noncontextual ontological models}, url = {http://arxiv.org/abs/2005.07161}, abstract = {It is useful to have a criterion for when the predictions of an operational theory should be considered classically explainable. Here we take the criterion to be that the theory admits of a generalized-noncontextual ontological model. Existing works on generalized noncontextuality have focused on experimental scenarios having a simple structure, typically, prepare-measure scenarios. Here, we formally extend the framework of ontological models as well as the principle of generalized noncontextuality to arbitrary compositional scenarios. We leverage this process-theoretic framework to prove that, under some reasonable assumptions, every generalized-noncontextual ontological model of a tomographically local operational theory has a surprisingly rigid and simple mathematical structure; in short, it corresponds to a frame representation which is not overcomplete. One consequence of this theorem is that the largest number of ontic states possible in any such model is given by the dimension of the associated generalized probabilistic theory. This constraint is useful for generating noncontextuality no-go theorems as well as techniques for experimentally certifying contextuality. Along the way, we extend known results concerning the equivalence of different notions of classicality from prepare-measure scenarios to arbitrary compositional scenarios. Specifically, we prove a correspondence between the following three notions of classical explainability of an operational theory: (i) admitting a noncontextual ontological model, (ii) admitting of a positive quasiprobability representation, and (iii) being simplex-embeddable.}, urldate = {2020-09-04}, journal = {arXiv:2005.07161 [quant-ph]}, author = {Schmid, David and Selby, John H. and Pusey, Matthew F. and Spekkens, Robert W.}, month = may, year = {2020}, note = {arXiv: 2005.07161}, keywords = {Quantum Physics}, }`

- Łukasz Czekaj, Ana Belén Sainz, John Selby, and Michał Horodecki. Correlations constrained by composite measurements.
*arXiv:2009.04994 [quant-ph]*, sep 2020. arXiv: 2009.04994

[BibTeX] [Abstract] [Download PDF]

How to understand the set of correlations admissible in nature is one outstanding open problem in the core of the foundations of quantum theory. Here we take a complementary viewpoint to the device-independent approach, and explore the correlations that physical theories may feature when restricted by some particular constraints on their measurements. We show that demanding that a theory exhibits a composite measurement imposes a hierarchy of constraints on the structure of its sets of states and effects, which translate to a hierarchy of constraints on the allowed correlations themselves. We moreover focus on the particular case where one demands the existence of an entangled measurement that reads out the parity of local fiducial measurements. By formulating a non-linear Optimisation Problem, and semidefinite relaxations of it, we explore the consequences of the existence of such a parity reading measurement for violations of Bell inequalities. In particular, we show that in certain situations this assumption has surprisingly strong consequences, namely, that Tsirelson’s bound can be recovered.

`@Article{czekaj_correlations_2020, author = {Czekaj, Łukasz and Sainz, Ana Belén and Selby, John and Horodecki, Michał}, journal = {arXiv:2009.04994 [quant-ph]}, title = {Correlations constrained by composite measurements}, year = {2020}, month = sep, note = {arXiv: 2009.04994}, abstract = {How to understand the set of correlations admissible in nature is one outstanding open problem in the core of the foundations of quantum theory. Here we take a complementary viewpoint to the device-independent approach, and explore the correlations that physical theories may feature when restricted by some particular constraints on their measurements. We show that demanding that a theory exhibits a composite measurement imposes a hierarchy of constraints on the structure of its sets of states and effects, which translate to a hierarchy of constraints on the allowed correlations themselves. We moreover focus on the particular case where one demands the existence of an entangled measurement that reads out the parity of local fiducial measurements. By formulating a non-linear Optimisation Problem, and semidefinite relaxations of it, we explore the consequences of the existence of such a parity reading measurement for violations of Bell inequalities. In particular, we show that in certain situations this assumption has surprisingly strong consequences, namely, that Tsirelson's bound can be recovered.}, groups = {Michal_H}, keywords = {Quantum Physics}, url = {http://arxiv.org/abs/2009.04994}, urldate = {2021-07-28}, }`

### 2019

- Paul Skrzypczyk, Matty J. Hoban, Ana Belén Sainz, and Noah Linden. Complexity of compatible measurements.
*arXiv:1908.10085 [quant-ph]*, 2019.

[BibTeX]`@article{skrzypczyk2020complexity, title={Complexity of compatible measurements}, author={Skrzypczyk, Paul and Hoban, Matty J and Sainz, Ana Bel{\'e}n and Linden, Noah}, journal={arXiv:1908.10085 [quant-ph]}, year={2019} }`

- Sandu Popescu, Ana Belén Sainz, Anthony J. Short, and Andreas Winter. Reference frames which separately store non-commuting conserved quantities.
*arXiv:1908.02713 [quant-ph]*, 2019.

[BibTeX]`@article{popescu2019reference, title={Reference frames which separately store non-commuting conserved quantities}, author={Popescu, Sandu and Sainz, Ana Bel{\'e}n and Short, Anthony J and Winter, Andreas}, journal={arXiv:1908.02713 [quant-ph]}, year={2019} }`

- Ana Belén Sainz, Matty J. Hoban, Paul Skrzypczyk, and Leandro Aolita. Bipartite post-quantum steering in generalised scenarios.
*arXiv:1907.03705 [quant-ph]*, 2019.

[BibTeX]`@article{sainz2019bipartite, title={Bipartite post-quantum steering in generalised scenarios}, author={Sainz, Ana Bel{\'e}n and Hoban, Matty J and Skrzypczyk, Paul and Aolita, Leandro}, journal={arXiv:1907.03705 [quant-ph]}, year={2019} }`

- Elie Wolfe, David Schmid, Ana Belén Sainz, Ravi Kunjwal, and Robert W. Spekkens. Quantifying Bell: the Resource Theory of Nonclassicality of Common-Cause Boxes.
*arXiv:1903.06311 [quant-ph]*, 2019.

[BibTeX]`@article{wolfe2019quantifying, title={Quantifying Bell: the Resource Theory of Nonclassicality of Common-Cause Boxes}, author={Wolfe, Elie and Schmid, David and Sainz, Ana Bel{\'e}n and Kunjwal, Ravi and Spekkens, Robert W}, journal={arXiv:1903.06311 [quant-ph]}, year={2019} }`

### 2018

- Thomas Van Himbeeck, Jonatan Bohr Brask, Stefano Pironio, Ravishankar Ramanathan, Ana Belén Sainz, and Elie Wolfe. Quantum violations in the Instrumental scenario and their relations to the Bell scenario.
*arXiv:1804.04119 [quant-ph]*, 2018.

[BibTeX]`@article{van2018quantum, title={Quantum violations in the Instrumental scenario and their relations to the Bell scenario}, author={Van Himbeeck, Thomas and Brask, Jonatan Bohr and Pironio, Stefano and Ramanathan, Ravishankar and Sainz, Ana Bel{\'e}n and Wolfe, Elie}, journal={arXiv:1804.04119 [quant-ph]}, year={2018} }`

## Group members

###### Get to know the people behind ICTQT.

## Former members

Victoria J Wright (post-doc in 2019-2020), Marcin Karczewski (post-doc in 2020-2023)

__Keywords:__ quantum nonlocality, quantum contextuality, steering, causality, Bell’s theorem, process theories, generalised probabilistic theories, quantum computational speedup, quantum networks, resource theories, post-quantum theories, post-quantum nonclassicality, zx-calculus.