The quantum vacuum shows an intensity-dependent nonlinear index of refraction. Consequently, we may consider vacuum as a non-linear medium enabling, at sufficiently high field strengths, a direct interaction of light with light. In this context, we advocate an experiment to measure light-by-light scattering by the means of self-focussing of an x-ray beam in vacuum. Although hard to measure, we argue that for laser beams consisting of highly energetic photons the beams' power distribution carries a distinctive enough signature of self-focussing such that an observation at modern free-electron laser facilities appears possible. Self-focussing also imposes a limit on the most powerful x-ray beams that may propagate in vacuum being on the order of $10$ PW for $10$ keV x-rays.
Propagation of intense squeezed vacuum light in non-linear media
ICFO – Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology（光子科学研究所，巴塞罗那科技研究院）；Foundation for Research and Technology-Hellas, Institute of Electronic Structure & Laser（希腊研究与技术基金会，电子结构与激光研究所）；Atominstitut, Technische Universität Wien（维也纳工业大学原子物理研究所）；Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, im Forschungsverbund Berlin e.V.（柏林联合研究中心马克斯·玻恩非线性光学与超快光谱学研究所）；ICREA（加泰罗尼亚高级研究学院）；Center for Quantum Science and Technologies (FORTH-QuTech)（量子科学与技术中心（FORTH-QuTech））；ELI-ALPS, ELI-Hu Non-Profit Ltd.（极端光基础设施-阿尔帕斯，匈牙利极端光基础设施非营利有限公司）
Single-shot pulse retrieval of femtosecond bright squeezed vacuum
Technion – Israel Institute of Technology（以色列理工学院）；Solid State Institute（固态研究所）；Helen Diller Quantum Center（海伦·迪勒量子中心）；The Russell Berrie Nanotechnology Institute（罗素·贝里纳米技术研究所）；Max Planck Institute for the Science of Light（马克斯·普朗克光科学研究所）；Friedrich–Alexander Universität Erlangen–Nürnberg（埃尔朗根-纽伦堡弗里德里希-亚历山大大学）
Generation of bright quantum high-order harmonic driven by combined coherent and bright squeezed vacuum light
Institute of Atomic and Molecular Physics, Jilin University（吉林大学原子与分子物理研究所）；Jilin Provincial Key Laboratory of Transient Quantum Process Control and Application, Jilin University（吉林大学瞬态量子过程控制与应用吉林省重点实验室）；College of Physics, Jilin University（吉林大学物理学院）
High-order correlations and ultrafast Wigner negativities in bright-squeezed-vacuum-driven high-harmonic generation
Max Planck Institute for the Structure and Dynamics of Matter（马普固体与结构研究所）；Guangdong Technion-Israel Institute of Technology（广东以色列理工学院）；Technion–Israel Institute of Technology（以色列理工学院）；Initiative for Computational Catalysis (ICC), Flatiron Institute（Flatiron研究所计算催化倡议）
