Telefilters, telemirrors, and causality

Abstract: Mode-selective interactions are ubiquitous in quantum optics and interacting quantum field theories. Such unitaries take as input a multi-mode field but uniquely affect a single mode, compared with those orthogonal to it. Despite their wide application in theoretical and experimental physics, the temporal dimension of such interactions are typically ignored.

Here, I demonstrate how the naïve treatment of a mode-selective unitaries leads to the possibility of acausal signalling, due to the tacit assumption that certain kinds of idealised measurements of quantum fields can be made. I achieve this by introducing new theoretical models for mode-selective mirrors using continuous-variable teleportation, which is able to unveil the temporal aspects of the mode propagation.

The main result is that mode-selective operations must generally enact a fundamental time-delay on input modes that are delocalised in time. This result resolves Sorkin's long-standing "impossible measurement" problem in relativistic quantum field theory, and thus has significant implications for experimental quantum optics and those working in quantum causality and field theory.

mathematical physicshistory and philosophy of physicsquantum physics

Audience: advanced learners

( paper )

Comments: Talk not recorded

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Tricontinental Quantum Fundamentals Seminar

Series comments: The Tricontinental Quantum Fundamentals Seminar (3C-QFS) is an online seminar series connecting researchers from Australia, Europe, and America. Our goal is to facilitate discussion between the three continents and bring together those working on foundational topics in relativistic quantum physics, covering but not limited to:

• Measurement, localisation, and causality in relativistic quantum theory
• Energy conditions and quantum inequalities
• Non-locality and observer-dependence of entanglement
• Quantum reference frames and symmetries in quantum theory
• Holography, AdS/CFT, and analogue gravity

For 3C-QFS, we wish to foster an inclusive community and promote open discussion by encouraging a casual atmosphere during the seminars.

The seminars are held online via Zoom, consisting of 40-60 minute talks followed by additional time for questions. We meet every three weeks on Tuesday, alternating between two times in order to connect research communities across two of the three continents.

For those unable to join the seminars live, talks are recorded and hosted on the 3C-QFS YouTube channel.

The seminar series is primarily for students and academics active in the foundations of physics. To sign up for the mailing list and join the seminars live, please complete the registration form on the 3C-QFS website.

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