Publications

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• Numerical evaluation of two-time correlation functions in open quantum systems with matrix product state methods: a comparison

  Stefan Wolff, Ameneh Sheikhan, Corinna Kollath

  SciPost Phys. Core 3, 010 (2020) · published 6 November 2020 | Toggle abstract · pdf

  We compare the efficiency of different matrix product state (MPS) based methods for the calculation of two-time correlation functions in open quantum systems. The methods are the purification approach [1] and two approaches [2,3] based on the Monte-Carlo wave function (MCWF) sampling of stochastic quantum trajectories using MPS techniques. We consider a XXZ spin chain either exposed to dephasing noise or to a dissipative local spin flip. We find that the preference for one of the approaches in terms of numerical efficiency depends strongly on the specific form of dissipation.

  6 citations

• How to GAN event subtraction

  Anja Butter, Tilman Plehn, Ramon Winterhalder

  SciPost Phys. Core 3, 009 (2020) · published 5 November 2020 | Toggle abstract · pdf

  Subtracting event samples is a common task in LHC simulation and analysis, and standard solutions tend to be inefficient. We employ generative adversarial networks to produce new event samples with a phase space distribution corresponding to added or subtracted input samples. We first illustrate for a toy example how such a network beats the statistical limitations of the training data. We then show how such a network can be used to subtract background events or to include non-local collinear subtraction events at the level of unweighted 4-vector events.

  16 citations

• The high-temperature expansion of the thermal sunset

  Andreas Ekstedt, Johan Löfgren

  SciPost Phys. Core 3, 008 (2020) · published 22 October 2020 | Toggle abstract · pdf

  We give a prescription for calculating the high-temperature expansion of the thermal sunset integral to arbitrary order. We derive all terms odd in $T$, and rederive previous results up to $\mathcal{O}(T^0) $ for both bosonic and fermionic thermal sunsets in dimensional regularisation. We perform analytical and numerical cross-checks. Intermediate steps involve integrals over three Bessel functions.

• Gravitational wave probes of dark matter: challenges and opportunities

  Gianfranco Bertone, Djuna Croon, Mustafa A. Amin, Kimberly K. Boddy, Bradley J. Kavanagh, Katherine J. Mack, Priyamvada Natarajan, Toby Opferkuch, Katelin Schutz, Volodymyr Takhistov, Christoph Weniger, Tien-Tien Yu

  SciPost Phys. Core 3, 007 (2020) · published 15 October 2020 | Toggle abstract · pdf

  In this white paper, we discuss the prospects for characterizing and identifying dark matter using gravitational waves, covering a wide range of dark matter candidate types and signals. We argue that present and upcoming gravitational wave probes offer unprecedented opportunities for unraveling the nature of dark matter and we identify the most urgent challenges and open problems with the aim of encouraging a strong community effort at the interface between these two exciting fields of research.

  51 citations

• Density resolved wave packet spreading in disordered Gross-Pitaevskii lattices

  Yagmur Kati, Xiaoquan Yu, Sergej Flach

  SciPost Phys. Core 3, 006 (2020) · published 8 October 2020 | Toggle abstract · pdf

  We perform novel energy and norm density resolved wave packet spreading studies in the disordered Gross-Pitaevskii (GP) lattice to confine energy density fluctuations. We map the locations of GP regimes of weak and strong chaos subdiffusive spreading in the 2D density control parameter space and observe strong chaos spreading over several decades. We obtain a renormalization of the ground state due to disorder, which allows for a new disorder-induced phase of disconnected insulating puddles of matter due to Lifshits tails. Inside this Lifshits phase, the wave packet spreading is substantially slowed down.

  6 citations

• The influence of spacetime curvature on quantum emission in optical analogues to gravity

  Maxime J. Jacquet, Friedrich Koenig

  SciPost Phys. Core 3, 005 (2020) · published 30 September 2020 | Toggle abstract · pdf

  Quantum fluctuations on curved spacetimes cause the emission of pairs of particles from the quantum vacuum, as in the Hawking effect from black holes. We use an optical analogue to gravity to investigate the influence of the curvature on quantum emission. Due to dispersion, the spacetime curvature varies with frequency here. We analytically calculate for all frequencies the particle flux, correlations and entanglement. We find that horizons increase the flux with a characteristic spectral shape. The photon number correlations transition from multi- to two-mode, with close to maximal entanglement. The quantum state is a diagnostic for the mode conversion in laboratory tests of quantum field theory on curved spacetimes.

  12 citations

• Honeycomb rare-earth magnets with anisotropic exchange interactions

  Zhu-Xi Luo, Gang Chen

  SciPost Phys. Core 3, 004 (2020) · published 4 September 2020 | Toggle abstract · pdf

  We study the rare-earth magnets on a honeycomb lattice, and are particularly interested in the experimental consequences of the highly anisotropic spin interaction due to the spin-orbit entanglement. We perform a high-temperature series expansion using a generic nearest-neighbor Hamiltonian with anisotropic interactions, and obtain the heat capacity, the parallel and perpendicular spin susceptibilities, and the magnetic torque coefficients. We further examine the electron spin resonance linewidth as an important signature of the anisotropic spin interactions. Due to the small interaction energy scale of the rare-earth moments, it is experimentally feasible to realize the strong-field regime. Therefore, we perform the spin-wave analysis and study the possibility of topological magnons when a strong field is applied to the system. The application and relevance to the rare-earth Kitaev materials are discussed.

  14 citations

• Lyapunov exponents and entanglement entropy transition on the noncommutative hyperbolic plane

  Sriram Ganeshan, Alexios P. Polychronakos

  SciPost Phys. Core 3, 003 (2020) · published 20 August 2020 | Toggle abstract · pdf

  We study quantum dynamics on noncommutative spaces of negative curvature, focusing on the hyperbolic plane with spatial noncommutativity in the presence of a constant magnetic field. We show that the synergy of noncommutativity and the magnetic field tames the exponential divergence of operator growth caused by the negative curvature of the hyperbolic space. Their combined effect results in a first-order transition at a critical value of the magnetic field in which strong quantum effects subdue the exponential divergence for {\it all} energies, in stark contrast to the commutative case, where for high enough energies operator growth always diverge exponentially. This transition manifests in the entanglement entropy between the `left' and `right' Hilbert spaces of spatial degrees of freedom. In particular, the entanglement entropy in the lowest Landau level vanishes beyond the critical point. We further present a non-linear solvable bosonic model that realizes the underlying algebraic structure of the noncommutative hyperbolic plane with a magnetic field.

  1 citation

• The non-rational limit of D-series minimal models

  Sylvain Ribault

  SciPost Phys. Core 3, 002 (2020) · published 31 July 2020 | Toggle abstract · pdf

  We study the limit of D-series minimal models when the central charge tends to a generic irrational value $c\in (-\infty, 1)$. We find that the limit theory's diagonal three-point structure constant differs from that of Liouville theory by a distribution factor, which is given by a divergent Verlinde formula. Nevertheless, correlation functions that involve both non-diagonal and diagonal fields are smooth functions of the diagonal fields' conformal dimensions. The limit theory is a non-trivial example of a non-diagonal, non-rational, solved two-dimensional conformal field theory.

• Out-of-equilibrium phase transitions induced by Floquet resonances in a periodically quench-driven XY spin chain

  Sergio Enrique Tapias Arze, Pieter W. Claeys, Isaac Pérez Castillo, Jean-Sébastien Caux

  SciPost Phys. Core 3, 001 (2020) · published 22 July 2020 | Toggle abstract · pdf

  We consider the dynamics of an XY spin chain subjected to an external transverse field which is periodically quenched between two values. By deriving an exact expression of the Floquet Hamiltonian for this out-of-equilibrium protocol with arbitrary driving frequencies, we show how, after an unfolding of the Floquet spectrum, the parameter space of the system is characterized by alternations between local and non-local regions, corresponding respectively to the absence and presence of Floquet resonances. The boundary lines between regions are obtained analytically from avoided crossings in the Floquet quasi-energies and are observable as phase transitions in the synchronized state. The transient behaviour of dynamical averages of local observables similarly undergoes a transition, showing either a rapid convergence towards the synchronized state in the local regime, or a rather slow one exhibiting persistent oscillations in the non-local regime, where explicit decay coefficients are presented.

  10 citations