Paul Skrzypczyk

I am theoretical quantum physicist.

My primary research interest is quantum theory, in particular, the renewed understanding that has been gained with the advent of quantum information. I am interested in many aspects of quantum theory, with a primary focus on are quantum nonlocality, quantum measurements, and quantum thermodynamics.

In quantum theory the way actions in one place effect far-away places is much more intricate and fascinating than in classical physics. In particular, quantum theory allows for ‘nonlocal’ effects, whereby actions in one place seemingly affect another distant place instantaneously, although this can only ever be confirmed later in time. My aim is to obtain a deep understanding of this counter-intuitive phenomenon. This is not only crucial in order to really understand quantum theory and the way the microscopic world behaves, but also for applications in quantum information processing, where it opens up new possibilities which are impossible using only classical physics.

The process of measurement plays a much more fundamental and prominent role in quantum theory compared to much of classical physics. It is through the process of measurement that we gain information about the microscopic world, and in quantum theory this measurement necessarily disturbs the system measured. I am particularly interested in this information gain, and also affects related to the fact that not all properties of a system can be measured simultaneously in quantum theory, known as measurement incompatibility.

The laws of thermodynamics are arguably the most prevalent in the whole of physics. It is fascinating to understand to what extent they apply at the quantum level, especially for small systems - far from their original realm of applicability. Pursuing this line of investigation is not only a way to probe the limits of quantum theory and thermodynamics, but is also relevant for future quantum technologies, where both quantum and thermal effects will ultimately play a role.

News

Jun 28, 2024	Very pleased to once again getting a backlog of papers out this year: Locally inaccessible hidden quantum correlations with Andres Ducuara and Cristian Susa. This is one of a few results Andres obtained during his PhD that we are finally getting round to publishing. Operational Interpretation of the Choi Rank Through k-State Exclusion with Ben Stratton and Chung-Yun Hsieh. Ben’s 2nd paper during his PhD, which I think is a very nice result about quantum channels. Activation of post-quantum steering with Ana Belen Sainz and Matty Hoban. This is a ridiculously overdue paper, which I’m delighted to finally have out. Activation of post-quantumness in bipartite generalised EPR scenarios with Beata Zjawin, Matty Hoban and Ana Belen Sainz. A generalisation of the above paper, which we put out at the same time. Beata did all of the hard work here! Revising the quantum work fluctuation framework to encompass energy conservation with Giulia Rubino and Karen Hovhannisyan. My first paper with Giulia! There will be a few more to come this summer, so I’ll post an update later.
Jun 26, 2024	I am delighted to announce the Ben Jones successfully defended his PhD last week. Many congratulations to Ben. His examiners were Andreas Winter and Tony Short.
Jun 24, 2024	I am thrilled that Chung-Yun Hsieh has been offered a prestigious Leverhulme Early Career Fellowship, which means he will be able to stay in Bristol for a further 3 years! Many congratulations to Chung-Yun on this well-deserved fellowship!
Jun 30, 2023	Very pleased to have started to get a backlog of papers finally out: Maxwell’s Demon walks into Wall Street: Stochastic Thermodynamics meets Expected Utility Theory with Andres Ducuara, Francesco Buscemi, Peter Sidajaya and Valerio Scarani Update: This was published in Phys. Rev. Lett. Fundamental connections between utility theories of wealth and information theory with Andres Ducuara The Dynamical Resource Theory of Informational Non-Equilibrium with Ben Stratton and Chung-Yun Hsieh. This is Ben’s first paper, and Chung-Yun’s first Bristol paper, to congratulations to them both! Update: This was also published in Phys. Rev. Lett.
Mar 13, 2023	My textbook Semidefinite Programming in Quantum Information Science, written with Daniel Cavalcanti, has just been published by IOP Publishing as part of their IOP Series in Quantum Technology.

Selected Publications

Textbook

Semidefinite Programming in Quantum Information Science

P. Skrzypczyk, D. Cavalcanti

Abstract Full Text

Semidefinite programs (SDPs) are a class of optimisation problems that find application in numerous areas of physics, engineering and mathematics. Semidefinite programming is particularly suited to problems in quantum physics and quantum information science. Following a review of the theory of semidefinite programming, the book proceeds to describe how it can be used to address a wide range of important problems from across quantum information science. Specific applications include quantum state, measurement, and channel estimation and discrimination, entanglement detection and quantification, quantum distance measures, and measurement incompatibility. Though SDPs have become an increasingly important tool in quantum information science it’s not yet the kind of mathematics students learn routinely. Assuming only a basic knowledge of linear algebra and quantum physics and quantum information, this graduate-level book provides a unified and accessible presentation of one of the key numerical methods used in quantum information science. Whilst the focus is on the theoretical machinery of SDPs, the authors have provided an accompanying GitHub repository containing example code, covering some of the SDPs studied in this book. Part of IOP Series in Quantum Technology.Key features• Accessible for graduate students in science and mathematics.• A unified and accessible presentation of one of the key numerical methods used in quantum information science.• Written by leading researchers on the topic.• Accompanying GitHub repository with sample code.
Phys. Rev. Lett. Editor’s Suggestion

Robustness of Measurement, Discrimination Games, and Accessible Information

P. Skrzypczyk, N. Linden

Phys. Rev. Lett. 122, 140403 (2019)

Abstract Full Text arXiv

We introduce a resource theory of measurement informativeness. This allows us to define an associated quantifier, which we call the robustness of measurement. It describes how much “noise” must be added to a measurement before it becomes completely uninformative. We show that this geometric quantifier has operational significance in terms of the advantage the measurement provides over guessing at random in a suitably chosen state discrimination game and that it is the single-shot generalization of the accessible information of a certain quantum-to-classical channel. Using this insight, we further show that the recently introduced robustness of asymmetry or coherence is the single-shot generalization of the accessible information of an ensemble. Finally, we discuss more generally the connection between robustness-based measures, discrimination problems, and information-theoretic quantities.
Phys. Rev. Lett. Editor’s Suggestion

All Entangled States can Demonstrate Nonclassical Teleportation

D. Cavalcanti, P. Skrzypczyk, I. Šupić

Phys. Rev. Lett. 119, 110501 (2017)

Abstract Full Text

Quantum teleportation, the process by which Alice can transfer an unknown quantum state to Bob by using preshared entanglement and classical communication, is one of the cornerstones of quantum information. The standard benchmark for certifying quantum teleportation consists in surpassing the maximum average fidelity between the teleported and the target states that can be achieved classically. According to this figure of merit, not all entangled states are useful for teleportation. Here we propose a new benchmark that uses the full information available in a teleportation experiment and prove that all entangled states can implement a quantum channel which cannot be reproduced classically. We introduce the idea of nonclassical teleportation witness to certify if a teleportation experiment is genuinely quantum and discuss how to quantify this phenomenon. Our work provides new techniques for studying teleportation that can be immediately applied to certify the quality of quantum technologies.
Nature Commun.

Detection of entanglement in asymmetric quantum networks and multipartite quantum steering

D. Cavalcanti, P. Skrzypczyk, G. H. Aguilar, R. V. Nery, P. H. Souto Ribeiro, S. P. Walborn

Nature Commun. 6, 7941 (2015)

Abstract Full Text

The future of quantum communication relies on quantum networks composed by observers sharing multipartite quantum states. The certification of multipartite entanglement will be crucial to the usefulness of these networks. In many real situations it is natural to assume that some observers are more trusted than others in the sense that they have more knowledge of their measurement apparatuses. Here we propose a general method to certify all kinds of multipartite entanglement in this asymmetric scenario and experimentally demonstrate it in an optical experiment. Our results, which can be seen as a definition of genuine multipartite quantum steering, give a method to detect entanglement in a scenario in between the standard entanglement and fully device-independent scenarios, and provide a basis for semi-device-independent cryptographic applications in quantum networks.
Nature Commun.

Work extraction and thermodynamics for individual quantum systems

P. Skrzypczyk, A. J. Short, S. Popescu

Nature Commun. 5, 4185 (2014)

Abstract Full Text

Thermodynamics is traditionally concerned with systems comprised of a large number of particles. Here we present a framework for extending thermodynamics to individual quantum systems, including explicitly a thermal bath and work-storage device (essentially a ‘weight’ that can be raised or lowered). We prove that the second law of thermodynamics holds in our framework, and gives a simple protocol to extract the optimal amount of work from the system, equal to its change in free energy. Our results apply to any quantum system in an arbitrary initial state, in particular including non-equilibrium situations. The optimal protocol is essentially reversible, similar to classical Carnot cycles, and indeed, we show that it can be used to construct a quantum Carnot engine.
Phys. Rev. Lett.

Quantifying Einstein-Podolsky-Rosen Steering

P. Skrzypczyk, M. Navascués, D. Cavalcanti

Phys. Rev. Lett. 112, 180404 (2014)

Abstract Full Text

Einstein-Podolsky-Rosen steering is a form of bipartite quantum correlation that is intermediate between entanglement and Bell nonlocality. It allows for entanglement certification when the measurements performed by one of the parties are not characterized (or are untrusted) and has applications in quantum key distribution. Despite its foundational and applied importance, Einstein-Podolsky-Rosen steering lacks a quantitative assessment. Here we propose a way of quantifying this phenomenon and use it to study the steerability of several quantum states. In particular, we show that every pure entangled state is maximally steerable and the projector onto the antisymmetric subspace is maximally steerable for all dimensions; we provide a new example of one-way steering and give strong support that states with positive-partial transposition are not steerable.