January 2022

A local-realistic theory for fermions

We propose a local model for general fermionic systems, which we express in the Heisenberg picture. To this end, we shall use a recently proposed formalism, the so-called “Raymond-Robichaud” construction, which allows one to construct an explicitly local model for any dynamical theory that satisfies no-signalling, in terms of equivalence classes of transformations that can be attached to each individual subsystem. By following the rigorous use of the parity superselection rule for fermions, we show how this construction removes the usual difficulties that fermionic systems display in regard to the definition of local states and local transformations.

Wald-Zoupas prescription with (soft) anomalies

We show that the Wald-Zoupas prescription for gravitational charges is valid in the presence of anomalies and field-dependent diffeomorphism, but only if these are related to one another in a specific way. The geometric interpretation of the allowed anomalies is exposed looking at the example of BMS symmetries: They correspond to soft terms in the charges. We determine if the Wald-Zoupas prescription coincides with an improved Noether charge. The necessary condition is a certain differential equation, and when it is satisfied, the boundary Lagrangian of the resulting improved Noether charge contains in general a non-trivial corner term that can be identified a priori from a condition of anomaly-freeness. Our results explain why the Wald-Zoupas prescription works in spite of the anomalous behaviour of BMS transformations, and should be helpful to relate different branches of the literature on surface charges.

Generic features of a polymer quantum black hole

Non-singular black holes models can be described by modified classical equations motivated by loop quantum gravity. We investigate what happens when the sine function typically used in the modification is replaced by an arbitrary bounded function, a generalization meant to study the effect of ambiguities such as the choice of representation of the holonomy. A number of features can be determined without committing to a specific choice of functions. We find generic singularity resolution. The presence and number of horizons is determined by global features of the function regularizing the angular components of the connection, and the presence and number of bounces by global features of the function regularizing the time component. The trapping or anti-trapping nature of regions inside horizons depends on the relative location with respect to eventual bounces. We use these results to comment on some of the ambiguities of polymer black hole models.

Quantum communication through devices in an indefinite input-output direction

A number of quantum devices are bidirectional, meaning that exchanging their inputs with their outputs yields valid quantum processes. Bidirectional devices, such as half-wave plates and quarter-wave plates in quantum optics, can be used in a forward mode and a backward mode, corresponding to two opposite choices of the input-output direction. They can also be used in a coherent superposition of the forward and backward modes, giving rise to new operations in which the input-output direction is subject to quantum indefiniteness. In this work we explore the potential of input-output indefiniteness for the transfer of classical and quantum information through noisy channels. We first formulate a model of quantum communication with indefinite input-output direction. Then, we show that the ability to coherently control the input-output direction yields advantages over standard communication protocols in which the input-output direction is fixed. These advantages range from a general reduction of noise in bidirectional processes, to heralded noiseless communication, and, in some special cases, to a complete noise removal. The noise reduction due to input-output indefiniteness can be experimentally demonstrated with current photonic technologies, providing a way to investigate the operational consequences of exotic scenarios characterised by coherent quantum superpositions of forward-time and backward-time evolutions.