Quantum Circuits with Classical and Quantum Control of Causal Orders

Speaker: 
Alastair Abbott, University of Geneva

Abstract: The process matrix formalism allows us to go beyond the quantum circuit framework and study processes for which there may be no
global causal order between events. While the notion of causal (non)separability has been well studied in the bipartite scenario, multipartite scenarios present a variety of new phenomena. In such scenarios, the definition of causal separability itself is subtle; I will discuss how it should be defined and show how such processes can be characterised.

Causally nonseparable processes that can be physically implemented – such as the quantum switch – are of particular practical interest. I will show how processes that satisfy a sufficient condition for causal separability can be interpreted as "quantum circuits with classical control of causal order". I will then introduce a natural generalistion: "quantum circuits with quantum control of causal order", and provide a characterisation of such processes. These processes are physically realisable and have the potential to go beyond switch-type processes when more parties are considered. [Joint work with Julian Wechs, Hippolyte Dourdent and Cyril Branciard] 

The process matrix formalism allows us to go beyond the quantum circuit framework and study processes for which there may be no global causal order between events. While the notion of causal (non)separability has been well studied in the bipartite scenario, multipartite scenarios present a variety of new phenomena. In such scenarios, the definition of causal separability itself is subtle; I will discuss how it should be defined and show how such processes can be characterised.

Causally nonseparable processes that can be physically implemented – such as the quantum switch – are of particular practical interest. I will show how processes that satisfy a sufficient condition for causal separability can be interpreted as "quantum circuits with classical control of causal order". I will then introduce a natural generalistion: "quantum circuits with quantum control of causal order", and provide a characterisation of such processes. These processes are physically realisable and have the potential to go beyond switch-type processes when more parties are considered. [Joint work with Julian Wechs, Hippolyte Dourdent and Cyril Branciard]

Biography: 
Alastair Abbott did his PhD jointly between the University of Auckland and ENS Paris, working on quantum randomness and contextuality as an nterdisciplinary thesis (part computer science, party physics, part philosophy). From 2015 he was in Grenoble with Cyril Branciard for 3 years, and since September he has been with Nicolas Brunner in Geneva. 

Host:
Prof. Stefan Wolf