Authors: Rafael Sánchez, Janine Splettstoesser, Robert S. Whitney

Maxwell demons are creatures that are imagined to be able to reduce the entropy of a system without performing any work on it. Conventionally, such a Maxwell demon’s intricate action consists in measuring individual particles and subsequently performing feedback. Here we show that much simpler setups can still act as demons: we demonstrate that it is sufficient to exploit a non-equilibrium distribution to seemingly break the second law of thermodynamics. We propose both an electronic and an optical implementation of this phenomenon, realizable with current technology.

Authors: Sreenath K. Manikandan, Andrew N. Jordan

We propose an analogy between the quantum physics of a black hole in its late stages of the evaporation process and a superfluid Bose Einstein Condensate (BEC), based on the Horowitz and Maldacena quantum final state projection model [JHEP 2004(02), 008]. The superfluid region is considered to be analogous to the interior of a black hole, and the normal fluid/superfluid interface is compared to the event horizon of a black hole. We theoretically investigate the possibility of recovering the wavefunction of particles incident on a superfluid BEC from the normal fluid, facilitated by the mode conversion processes occurring at the normal fluid/superfluid BEC interface. We also study how the correlations of an infalling mode with an external memory system can be preserved in the process, similar to Hayden and Preskill’s “information mirror” model for a black hole [JHEP 2007(09), 120]. Based on these analogies, we conjecture that the quantum state of bosons entering a black hole in its final state is the superfluid quantum ground state of interacting bosons. Our analogy suggests that the wavefunction of bosons falling into a black hole can be recovered from the outgoing Hawking modes. In the particular case when a hole-like quasiparticle (a density dip) is incident on the superfluid BEC causing the superfluid to shrink in size, our model indicates that the evaporation is unitary.

Authors: Jonathan Oppenheim

We present a consistent theory of classical gravity coupled to quantum field theory. The dynamics is linear in the density matrix, completely positive and trace-preserving, and reduces to Einstein’s equations in the classical limit. The constraints of general relativity are imposed as a symmetry on the equations of motion. The assumption that gravity is classical necessarily modifies the dynamical laws of quantum mechanics — the theory must be fundamentally stochastic involving finite sized and probabilistic jumps in space-time and in the quantum field. Nonetheless the quantum state of the system can remain pure conditioned on the classical degrees of freedom. The measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical space-time necessarily causes collapse of the wave-function. More generally, we derive a form of classical-quantum dynamics using a non-commuting divergence which has as its limit deterministic classical Hamiltonian evolution, and which doesn’t suffer from the pathologies of the semi-classical theory.

Klevgard, Paul (2018) Wave-Particle Duality: A New Look from First Principles. [Preprint]

Crowther, Karen and Linnemann, Niels and Wuthrich, Christian (2018) What we cannot learn from analogue experiments. [Preprint]

Castellani, Elena and Dardashti, Radin (2018) Symmetry breaking. [Preprint]

Lazarovici, Dustin (2018) On Boltzmann vs. Gibbs and the Equilibrium in Statistical Mechanics. [Preprint]

Leegwater, Gijs (2018) When Greenberger, Horne and Zeilinger meet Wigner’s Friend. [Preprint]

Meincke, Anne Sophie (2018) The Disappearance of Change: Towards a Process Account of Persistence. [Preprint]

Quenching our thirst for universality

Quenching our thirst for universality, Published online: 07 November 2018; doi:10.1038/d41586-018-07272-6

Understanding the dynamics of quantum systems far from equilibrium is one of the most pressing issues in physics. Three experiments based on ultracold atomic systems provide a major step forward.

Le Bihan, Baptiste (2018) Space Emergence in Contemporary Physics: Why We Do Not Need Fundamentality, Layers of Reality and Emergence. [Preprint]

Gao, Shan (2018) Unitary quantum theory is incompatible with special relativity. [Preprint]

Quantum 2, 104 (2018).

https://doi.org/10.22331/q-2018-11-06-104Using the existing classification of all alternatives to the measurement postulates of quantum theory we study the properties of bi-partite systems in these alternative theories. We prove that in all these theories the purification principle is violated, meaning that some mixed states are not the reduction of a pure state in a larger system. This allows us to derive the measurement postulates of quantum theory from the structure of pure states and reversible dynamics, and the requirement that the purification principle holds. The violation of the purification principle implies that there is some irreducible classicality in these theories, which appears like an important clue for the problem of deriving the Born rule within the many-worlds interpretation. We also prove that in all such modifications the task of state tomography with local measurements is impossible, and present a simple toy theory displaying all these exotic non-quantum phenomena. This toy model shows that, contrarily to previous claims, it is possible to modify the Born rule without violating the no-signalling principle. Finally, we argue that the quantum measurement postulates are the most non-classical amongst all alternatives.

Crowther, Karen (2018) What is the point of reduction in science? [Preprint]