Talks

While spacetime and quantum theory are crucial parts of modern theoretical physics, the problem of quantum gravity demonstrates that their full relationship is not yet completely understood. In my talk, I report on two recent results that aim to shed light on this relationship via ideas and tools from quantum foundations.

We start with the setting of (semi-) device-independent quantum information protocols. In this scenario one considers abstract black boxes that are characterised by their input-output statistics. Typically, these inputs and outputs are assumed to be abstract labels from a finite set of integers. We replace the abstract inputs with physical inputs that correspond to continuous spatio-temporal degrees of freedom, e.g. angles of polarisers and time-durations of laser pulses. This framework gives new insights about the relation between space, time, and quantum correlations, and it gives rise to new kinds of Bell non-locality witnesses.

We then turn to the topic of quantum reference frames. Specifically, we consider a composite quantum system and an outside experimenter who does not have access to an external reference frame to specify all of the system's properties. We show that for such an observer the possible descriptions of states and observables are related by quantum reference frame transformations that have been independently proposed in recent works. We give an explicit description of the observables that are measurable by agents constrained by such quantum symmetries, and we introduce a relational generalisation of the partial trace that applies to such situations.

We consider monochromatic and isotropic photon emission from circular equatorial Kerr orbiters. We calculate the critical curve delineating the region of photon escape from that of photon capture in each emitter’s sky, allowing us to derive analytic expressions for the photon escape probability and the redshift-dependent total flux collected on the celestial sphere as a function of emission radius and black hole parameters. This critical curve generalizes to finite orbital radius the usual Kerr critical curve and displays interesting features in the limit of high spin. These results confirm that the near-horizon geometry of a high-spin black hole is in principle observable.