Indefinite causal order as a resource for quantum technology

Indefinite causal order (ICO) describes situations, where causality is broken, where it is no longer clear, which event or process within a sequence preceded the other. Recent fundamental research has developed initial theoretical protocols and realised first proof of principle experiments that are based on such ICO - where events within a sequence are happening both in order and in reverse order. While these initial demonstrations are of a fundamental nature, ICO could hold tremendous potential for technological advances across quantum communication, metrology and computation.

ICO is a rapidly emerging concept, which has been developed largely outside of the UK. Moreover, so far experiments are scarce and concerned mainly with conceptual proofs. It is clear, however, that ICO provides an alternative resource for quantum technologies, with advantages predicted to be comparable, and sometimes possibly surpassing, those offered by quantum entanglement.

Through our network we aim to develop innovative competencies based on the joint expertise of UK researchers and our international colleagues in the EU and beyond, who have pioneered the field of ICO. Our network will bring together researchers offering a wealth of expertise in key experimental skills and theoretical approaches for quantum technological applications. This will allow us to identify the most promising applications for ICO. We will promote and facilitate links with industrial partners to explore use cases and applications around ICO with relevant stakeholders. This will allow us to concentrate our efforts on developing ICO in areas with the most immediate returns.

Potential technological advantages notwithstanding, adapting our world view to ICO constitutes a paradigm shift in our thinking - we are trained and conditioned in causal chains of reasoning. Our network will provide a platform to explore and challenge the concept of ICO and specifically its relation to entanglement, and to test it on a broad range of applications, benefitting from the wide range of expertise provided by the network participants.

We are deliberately starting with a limited number of participants, all bringing a keen desire to explore, and where possible apply, this emerging concept. Our core participants will bring a wealth of experiencein the most promising experimental platforms, including high-dimensional optics, integrated quantum optics, fibres and waveguides, atom optics and nanophotonics, as well as theoretical expertise in quantum logic, probabilistic quantum theory, quantum information and retrodiction. In our experience, collaborations work best by combining formal networking with a plethora of personal connections between researchers with joint interests. The mechanisms to achieve this include monthly virtual meetings for seminars, tutorials, discussions and networking, 2 workshops, and most importantly visits between participants.

The main objective of our proposed network is to explore and quantify the advantages of ICO for the second technological quantum revolution, and to identify specific applications that would benefit from ICO in the short to medium term. We are cautiously optimistic about this because ICO is a technology-friendly resource. Our long-term vision is to boost the status of indefinite causal order from the level of an intriguing conceptual idea to that of a true quantum resource, that can be exploited in developing technologies fit for the second quantum revolution.