Design Report Update 2020 for the Einstein Telescope ET Steering Committee Editorial Team released September 2020 ET Editorial Team First release, September 2020 Contents Nomenclature ................................................... 19 1 Introduction .................................................... 29 1.1 The basics of gravitational wave detectors 30 1.2 Scientific targets of the ET observatory 37 1.3 Design of the ET observatory 39 1.3.1 Size, shape and layout . 39 1.3.2 Quantum noise . 40 1.3.3 Thermal noise . 42 1.3.4 Seismic isolation . 43 1.3.5 Terrestrial gravitational noise . 45 1.3.6 Vacuum . 45 1.3.7 Noise budget . 45 1.3.8 Layout of the observatory . 47 I SCIENCE CASE 49 2 Introduction .....................................................51 3 Astrophysics ................................................... 55 3.1 Black hole binaries 55 3.2 Neutron stars 57 3.2.1 Coalescing neutron star binaries . 58 3.2.2 Continuous waves from spinning neutron stars . 60 3.2.3 Burst signals from neutron stars . 62 3.3 Multi-band and multi-messenger astrophysics: synergies with other gravitational-wave detectors and electromagnetic/neutrino observatories 63 3.3.1 Networks of ground-based gravitational-wave detectors . 63 3.3.2 Joint gravitational-wave and electromagnetic observations . 65 3.3.3 Nucleosynthesis, nuclear physics and kilonovae . 66 3.3.4 Realativistic astrophysics and short GRBs . 67 3.3.5 Core collapse supernovae . 69 3.3.6 Multi-messenger astrophysics with neutrinos and cosmic rays . 70 3.3.7 Multi-band gravitational-wave observations with LISA . 71 4 Fundamental Physics and Cosmology .............................. 73 4.1 Physics near the black hole horizon: from tests of GR to quantum gravity 74 4.1.1 Testing the GR predictions for space-time dynamics near the horizon . 74 4.1.2 Exotic compact objects and signals from quantum gravity . 76 4.2 The nature of dark matter 77 4.3 The nature of dark energy 79 4.4 Toward the big bang: stochastic gravitational-wave backgrounds 82 4.4.1 Cosmological backgrounds . 83 4.4.2 Astrophysical backgrounds . 86 5 Summary of key science objectives .................................87 II SITE, INFRASTRUCTURE AND INSTRUMENTATION 91 6 Detector ....................................................... 93 6.1 Optical layout 93 6.1.1 A xylophone design for ET . 94 6.1.2 Arm cavity design . 97 6.1.3 Central interferometer design . 97 6.2 Core optics 98 6.2.1 The substrate materials . 98 6.2.1.1 Fused silica . 99 6.2.1.2 Silicon . 99 6.2.2 Surface polishing . 100 6.2.3 Other core optics . 101 6.3 Coatings 101 6.3.1 Coating procurement . 101 6.3.2 Coating thermal noise - full treatment . 102 6.3.3 Coating requirements . 103 6.3.3.1 ET-LF . 103 6.3.3.2 ET-HF . 103 6.3.3.3 Other requirements . 103 6.3.4 Coating design solutions . 104 6.3.4.1 Multimaterial coatings . 104 6.3.4.2 Nanolayer coatings . 105 6.3.4.3 Coating structure and mechanical loss . 105 6.3.5 Possible coating materials . 105 6.3.5.1 Amorphous silicon . 105 6.3.5.2 Alternative cryogenic low-index layers . 106 6.3.5.3 Silicon nitride . 106 6.3.5.4 Silica-doped hafnia . 107 6.3.5.5 Alumina . 107 6.3.5.6 Other amorphous coatings . 107 6.3.5.7 Crystalline coatings . 107 6.3.5.8 Coating deposition . 108 6.3.6 Coating strategy for ET . 108 6.4 Light sources 109 6.4.1 Requirements . 109 6.4.2 High power laser . 110 6.4.3 Prestabilization . 111 6.5 Quantum noise reduction 112 6.5.1 Review of quantum non-demolition topology options . 114 6.5.2 Frequency dependent squeezing . 116 6.5.2.1 Filter cavity . 117 6.5.2.2 Squeezed light sources . 120 6.6 Input and output optics 122 6.6.1 Input optics . 122 6.6.2 Output optics . 123 6.7 Scattered light mitigation 124 6.7.1 Lessons learned . 125 6.7.2 Stray light control . 125 6.7.3 New and future developments for stray light reduction . 126 6.8 Interferometer control 127 6.8.1 Longitudinal control . 127 6.8.2 Lock acquisition . 129 6.8.3 Steady state control . 130 6.8.4 Alignment control . 130 6.9 Calibration 132 6.9.1 Science-driven calibration requirements . 132 6.9.2 State of the art . 132 6.9.2.1 Photon calibrators . 132 6.9.2.2 Newtonian calibrators . 133 6.9.2.3 Other calibration methods . 133 6.10 Seismic isolation and suspension 133 6.10.1 Seismic isolation systems . 133 6.10.1.1 ET requirements . 134 6.10.1.2 Seismic platform ..