We have developed a software library that simulates noisy quantum logic circuits. We represent quantum states by their density matrices in the Pauli basis, and incorporate possible errors in initialization, logic gates, memory and measurement using simple models. Our quantum simulator is implemented as a new backend on IBM’s open-source Qiskit platform. In this paper, we provide its description, and illustrate it with some simple examples.

Synopsis We detail a benchmark model aimed at improving the implementation of Monte Carlo codes for simulations of positrons propagating through interstellar media. The benchmark highlights the key features of cross sections sets which lead to major differences in astronomical observables. One of these is the energy-differentiation of the ionisation cross section. This leads us to call for improved cross section sets for all constituents of interstellar media.

Relativistic positrons (>1 MeV) are produced from many sources in the Galaxy. Observations of positrons rely on the characteristic gamma-rays emitted from their annihilation with an electron, and the dominant annihilation pathways are determined by low-energy (<1 keV) processes. Thus, detailed information on the transport of positrons from high to low energies is required to accurately determine the propagation of positrons in the interstellar medium. This work considers model cross sections with variable parameters to provide a benchmark for several quantities, including the positronium formation fraction and time to reach subpositronium threshold energies. We simulate these processes using Monte Carlo codes within the cold collisional regime and carefully consider the energy sharing in ionization events that leads us to highlight the need for accurate ionization cross sections, which are differential in energy loss. We include tabulated values so that our benchmark values can be reproduced by other transport codes.

A classical solution where the (scalar) field value moves by an O(1) range in Planck units is believed to signal the breakdown of Effective Field Theory (EFT). One heuristic argument for this is that such a field will have enough energy to be inside its own Schwarzschild radius, and will result in collapse. In this paper, we consider an inverse problem: what kind of field ranges arise during the gravitational collapse of a classical field? Despite the fact that collapse has been studied for almost a hundred years, most of the discussion is phrased in terms of fluid stress tensors, and not fields. An exception is the scalar collapse made famous by Choptuik. We re-consider Choptuik-like systems, but with the emphasis now on the evolution of the scalar. We give strong evidence that generic spherically symmetric collapse of a massless scalar field leads to super-Planckian field movement. But we also note that in every such supercritical collapse scenario, the large field range is hidden behind an apparent horizon. We also discuss how the familiar perfect fluid models for collapse like Oppenheimer-Snyder and Vaidya should be viewed in light of our results.