University of St Andrews

A Josephson junction is a weak link between two superconductors. Andreev bound states (ABSs) in Josephson junctions are nowadays extensively studied both theoretically and experimentally because of the nontrivial combined effect of Rashba spin-orbit coupling and a Zeeman field on ABSs, which gives rise to Majorana states, self-conjugate zero-energy ABSs [1].In this talk, we theoretically analyze the spin structure of ABSs in superconductor-nanowire-superconductor Josephson junctions where the nanowire possesses multichannel structure, Rashba spin-orbit coupling, and Zeeman fields [2]. We show that these ingredients lead to spin-split ABSs, and enable transition between them by microwave irradiation. These features may provide a way for the Andreev spin qubit manipulation.

[1] Y. Oreg, G. Refael, and F. von Oppen, PRL 105, 177002 (2010)[2] S. Park and A. Levy Yeyati, PRB 96, 125416 (2017)

When the effective coupling between matter and light is large enough, a phase transition to a "superradiant'' state can occur.  The archetypal model for this is the Dicke model, describing N emitters coupled to a single photon mode. Such a transition has been realised in a driven system, with ultracold atoms in an optical cavity [1].  These experiments prompt questions about the effects of loss and dissipation on the phase transition. While the effect of photon losses is well understood, there are conflicting statements in the literature about the effects of losses on the emitters [2,3].   We show that the presence of pure dephasing can kill the superradiance transition, but it can be restored by loss terms [4].  The fact that adding atom loss can restore a phase transition to an ordered state is a surprising result, which provides a crucial example where the precise form of dissipation dramatically changes the collective behaviour. To solve this problem, we present a method to give an exact solution for the N emitter problem which scales polynomially with N.  By comparing finite size scaling of our exact solution to a cumulant expansion, we confirm the destruction and restoration of the superradiance transition holds in the thermodynamic limit. We also show how, by adding incoherent pumping processes we are able to distinguish between regions of phase diagram where superradiance occurs and those where coherence is generated by regular lasing. This leads to regions of the phase diagram where the mean field equations show chaotic behaviour.[1] K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger, Nature 464, 1301 (2010)[2] M. H. Szymanska, P. B. Littlewood, and B. D. Simons, Phys. Rev. A 68, 013818 (2003)[3] E. G. Dalla Torre, Y. Shchadilova, E. Y. Wilner, M. D. Lukin, and E. Demler, Phys. Rev. A 94, 061802 (2016).[4] P. Kirton and J. Keeling, Phys. Rev. Lett. 118, 123602 (2017)

The Fauvel Lecture, supported by the British Society for the History of Mathematics

Free, but booking essential 

Evelyn Fox Keller is one of the most internationally respected historians of science. A physicist, author and feminist, she is currently Professor Emerita of the History & Philosophy of Science at MIT. Beginning her career in theoretical physics, she moved on to work in molecular biology before becoming renowned for her work as a feminist critic of science. Her books include Keywords in Evolutionary Biology (1998), The Century of the Gene (2000) and Making Sense of Life: Explaining Biological Development with Models, Metaphors, and Machines (2002). The latter has a particular focus on mathematical biology and in her lecture she will discuss the legacy of D’Arcy Thompson’s work.

This free public lecture is associated with the 3-day conference celebrating the centenary of D'Arcy Thompson's On Growth and Form. Full programme and registration