## 4 Summary  
We reviewed the progress of our experiments that probe physics in a vacuum, with a focus on those using an XFEL. Various combinations among an XFEL, a high-power laser, and a high-field magnet provided a test for QED and new physics in a different energy scale. The results obtained by our first-phase experiments are as follows. (i) The photon-photon scattering experiment using an XFEL provides the first limits in the X-ray region [19,20]. The sensitivity needs to be gained by 20 orders of magnitude to reach the QED cross-section and now proceeds to the second phase with significant improvements. (ii) The experiment probing laser-induced birefringence and diffraction of X-rays  
Figure 11: **(Left)** Top: time–energy distribution of X-rays measured with the germanium detector. Each circle represents an event. The horizontal axis is the time from the beginning of the first pulse. The energy threshold was set around 3 keV. Bottom: events around the signal region defined by the beam energy of \(\pm 2\sigma\) (\(\sigma\) is the detector resolution) and the time window of 2.1 ms. Adapted from Reference [42]. **(Right)** Upper limit on the axion-like particle (ALP)-two-photon coupling constant \(g_{\sigma\gamma\gamma}\) at 95% C.L. as a function of the ALP mass. The X-ray light shining through a wall (LSW) limits obtained at the ESRF (2010) [44] and SPring-8 (2017) [42] are shown along with the expected sensitivity at the SPring-8 Angstrom Compact Free-Electron Laser (SACLA, dashed).  
Figure 10: Schematic of the experimental setup. (**a**) Layout of components in the optics hutch and in the two experimental hutches. Retractable components are shown with a vertical arrow. (**b**) Side view of the magnet system. The magnets are placed so as to produce parallel fields with respect to the horizontal polarization of the X-ray beam. Two identical discharge sections (DS1 and DS2) that are contained in the same hutch supply pulsed currents to the magnets by \(LC\)-discharge circuits. Adapted from Reference [42].  
requires a new synchronization scheme between the pump and probe lasers. A test measurement using a 2.5-TW pump laser succeeded in the temporal and spatial synchronization with the accuracies of \(\sim\)1 ps and 10 \(\upmu\)m, respectively. We are currently preparing for the first measurement using a PW laser in 2018 [34]. (iii) The VMB measurement features a unique field-generation system with high-repetition pulsed magnets. The use of a high-finesse Fabry-Perot cavity verified the automatic resonance recovery scheme required for continuous data-taking and obtained the first results [37; 38]. A long-term run for a few months will start after some improvements of the optics. (iv) The dedicated search for ALPs using the magnets and continuous X-rays was carried out at SPring-8 and improved the previous limit on the coupling constant by a factor of 5.2 [42]. The preparation for an experiment at SACLA using pulsed X-rays is underway.  
This research work at SACLA and SPring-8 BL19 LXU is approved and supported by JASRI and RIKEN.  
[PERSON] wrote the paper. All authors reviewed and critiqued the manuscript.  
The authors declare no conflict of interest.