Experimental Investigation of Indefinite Causal Orders in Quantum Mechanics

What sorts of processes without a fixed causal order can be created experimentally? And how does the way such processes manipulate information differ from standard processes? ​A causal order describes the order in which events occur and the effects these events have on each other. Causal orders deal with information in fundamental ways: they describe how an event occurring at one point in space-time can influence another point. All information transfers must be described through such a construct. Since information is transferred by quantum states, the complete physical description of information requires a thorough understanding of causal orders in quantum mechanics. A new description of quantum mechanics has emerged: it makes no assumption about the order of events. This formalism considers events in terms of their ability to exchange information between each other. Applying this new formalism to quantum mechanics has shown that processes without a defined causal order can exist. Nevertheless, information can be transferred through these processes. Extraordinarily, these processes can carry out tasks that are otherwise impossible. This project aims to investigate these processes and thus discover practical ways to build them in a laboratory, characterize them, and develop new applications for them. The research will study processes with an indefinite causal order experimentally, providing a physical platform to test a host of hypotheses. These hypotheses include ideas relating to the characterization of the causal order of quantum processes, protocols to transfer or compute information more efficiently through such processes, and the study of any limitations that nature may place on them. To achieve this, the project will begin with a novel construction of a process—the quantum SWITCH—which lacks a definite causal order. Showing that such processes can be of practical use will bring these issues to the attention of the broader quantum information and computation community. This work will help guide theoretical research on causal orders, bring to light new relevant details, and keep the field grounded in experiment. Beyond this, it will also impact the general physics community by revealing how one can take advantage of new quantum resources.