R. , G. Mc, U. Ap, . Jg, . Kd et al., defined the mission orbital parameters and estimated the industrial costs. GH, MC, RK, and UJ estimated the vacuum level to be expected. MC, RK, and UJ defined the launch and separation sequence, organized scientific meetings for discussions on the mission design and goals. All authors contributed to central discussions resulting in the presented mission layout and design, and all authors read and approved the final manuscript

U. Laboratoire-kastler-brossel, C. Universités, C. De-france, E. University, and P. , 13 Laboratori Nazionali di Frascati dell'INFN, Via Enrico Fermi, 14 School of Physics and Astronomy Newtonstraße, vol.40, issue.15

Q. Institut-für, 24 Centre for Theoretical Atomic, Molecular and Optical Physics, 26 EN-STI-TCD, CERN - European Organization for Nuclear Research, p.31

C. Street-north, . Waterloo, and C. On, Institute for Quantum Science and Engineering (IQSE), and Department of Physics and Astronomy, 29 Texas A & M University Institute for Advanced Study (TIAS) 30 Vienna Center for Quantum Science and Technology Institut für Luft-und Raumfahrttechnik, p.32

D. Di, F. Astronomia, L. Infn, and U. Di-firenze, Sansone, 1, Sesto Fiorentino, Firenze, Italy. 34 Physics and Astronomy, 35 Atomic and Laser Physics

R. 1. Kaltenbaek, R. Hechenblaikner, G. Kiesel, N. Romero-isart, O. Schwab et al., Macroscopic quantum resonators (MAQRO) Exp Astron, pp.123-64, 2012.

R. Kaltenbaek, G. Hechenblaikner, N. Kiesel, F. Blaser, S. Gröblacher et al., Macroscopic quantum experiments in space using massive mechanical resonators, 2012.

G. Hechenblaikner, F. Hufgard, J. Burkhardt, N. Kiesel, J. U. Aspelmeyer et al., How cold can you get in space? Quantum physics at cryogenic temperatures in space, New Journal of Physics, vol.16, issue.1, pp.13058-13068, 2014.
DOI : 10.1088/1367-2630/16/1/013058

P. Zanoni, A. Kaltenbaek, R. Burkhardt, J. , J. U. Hechenblaikner et al., Performance of a radiatively cooled system for quantum optomechanical experiments in space, 2015.

T. Li, S. Kheifets, and M. Raizen, Millikelvin cooling of an optically trapped microsphere in vacuum, Nature Physics, vol.7, issue.7, pp.527-557, 2011.
DOI : 10.1103/PhysRevA.77.033804

J. Gieseler, B. Deutsch, R. Quidant, and L. Novotny, Subkelvin Parametric Feedback Cooling of a Laser-Trapped Nanoparticle, Physical Review Letters, vol.109, issue.10, 2012.
DOI : 10.1103/PhysRevLett.109.103603

N. Kiesel, F. Blaser, U. Deli´cdeli´c, D. Grass, R. Kaltenbaek et al., Cavity cooling of an optically levitated submicron particle, Proceedings of the National Academy of Sciences, vol.110, issue.35, pp.14180-14185
DOI : 10.1073/pnas.1309167110

J. Millen, P. Fonseca, T. Mavrogordatos, T. Monteiro, and P. Barker, Cavity Cooling a Single Charged Levitated Nanosphere, Physical Review Letters, vol.114, issue.12, 2015.
DOI : 10.1103/PhysRevLett.114.123602

P. Fonseca, E. Aranas, J. Millen, T. Monteiro, and P. Barker, Nonlinear dynamics and millikelvin cavity-cooling of levitated nanoparticles, 2015.

P. Schmid, U. Sezer, J. Horak, M. Aspelmeyer, M. Andt et al., Trapped nanoparticles for space experiments, Study conducted under contract with the European Space Agency, pp.1-6889, 2014.

J. Gieseler, L. Novotny, and R. Quidant, Thermal nonlinearities in a nanomechanical oscillator, Nature Physics, vol.105, issue.12, pp.806-816, 2013.
DOI : 10.1038/nphys2798

J. Millen, T. Deesuwan, P. Barker, and A. J. , Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere, Nature Nanotechnology, vol.64, issue.6, pp.425-434, 2014.
DOI : 10.1038/nnano.2014.82

P. Mestres, J. Berthelot, M. Spasenovi´cspasenovi´c, J. Gieseler, L. Novotny et al., Cooling and manipulation of a levitated nanoparticle with an optical fiber trap, Applied Physics Letters, vol.107, issue.15, 2015.
DOI : 10.1063/1.4933180.2

O. Connell, A. Hofheinz, M. Ansmann, M. Bialczak, R. Lenander et al., Quantum ground state and single-phonon control of a mechanical resonator, Nature, vol.4, issue.7289, pp.697-703, 2010.
DOI : 10.1038/nature08967

J. Teufel, T. Donner, D. Li, J. Harlow, M. Allman et al., Sideband cooling of micromechanical motion to the quantum ground state, Nature, vol.459, issue.7356, pp.359-63, 2011.
DOI : 10.1038/nature10261

J. Chan, A. Mayer, T. Safavi-naeini, A. Hill, J. Krause et al., Laser cooling of a nanomechanical oscillator into its quantum ground state, Nature, vol.107, issue.7367, pp.89-92, 2011.
DOI : 10.1038/nature10461

R. Kaltenbaek, G. Hechenblaikner, T. Schuldt, A. Pilan-zanoni, N. Kiesel et al., Design and build of a glued cavity with good optical access for experiments in quantum optomechanics In preparation, 2016.

J. Bateman, S. Nimmrichter, K. Hornberger, and H. Ulbricht, Near-field interferometry of a free-falling nanoparticle from a point-like source, Nature Communications, vol.110, pp.4788-4798, 2014.
DOI : 10.1038/ncomms5788

G. Lin, J. Fürst, D. Strekalov, and N. Yu, Wide-range cyclic phase matching and second harmonic generation in whispering gallery resonators, Applied Physics Letters, vol.103, issue.18, 2013.
DOI : 10.1063/1.4827538

F. Gebert, M. Frosz, T. Weiss, Y. Wan, A. Ermolov et al., Damage-free single-mode transmission of deep-UV light in hollow-core PCF, Optics Express, vol.22, issue.13, pp.15388-96, 2014.
DOI : 10.1364/OE.22.015388

A. Bassi and G. Ghirardi, Dynamical reduction models, Physics Reports, vol.379, issue.5-6, pp.5-6257, 2003.
DOI : 10.1016/S0370-1573(03)00103-0
URL : https://hal.archives-ouvertes.fr/hal-01168326

A. Bassi, E. Ippoliti, and S. Adler, Towards Quantum Superpositions of a Mirror: An Exact Open Systems Analysis, Physical Review Letters, vol.94, issue.3, 2005.
DOI : 10.1103/PhysRevLett.94.030401

A. Bassi, K. Lochan, S. Satin, T. Singh, and H. Ulbricht, Models of wave-function collapse, underlying theories, and experimental tests, Reviews of Modern Physics, vol.85, issue.2, pp.471-527, 2013.
DOI : 10.1103/RevModPhys.85.471

G. Ghirardi, A. Rimini, and T. Weber, Unified dynamics for microscopic and macroscopic systems, Physical Review D, vol.34, issue.2, pp.470-91, 1986.
DOI : 10.1103/PhysRevD.34.470

N. Gisin, Stochastic quantum dynamics and relativity. Helv Phys Acta, pp.363-71, 1989.

P. Pearle, Combining stochastic dynamical state-vector reduction with spontaneous localization, Physical Review A, vol.39, issue.5, pp.2277-89, 1989.
DOI : 10.1103/PhysRevA.39.2277

G. Ghirardi, P. Pearle, and A. Rimini, Markov processes in Hilbert space and continuous spontaneous localization of systems of identical particles, Physical Review A, vol.42, issue.1, pp.78-89, 1990.
DOI : 10.1103/PhysRevA.42.78

L. Diósi, Gravitation and quantum-mechanical localization of macro-objects, Physics Letters A, vol.105, issue.4-5, pp.4-5199, 1984.
DOI : 10.1016/0375-9601(84)90397-9

R. Penrose, On Gravity's role in Quantum State Reduction, General Relativity and Gravitation, vol.44, issue.5, pp.581-60010, 1996.
DOI : 10.1007/BF02105068

L. Diósi, Notes on certain Newton gravity mechanisms of wavefunction localization and decoherence, Journal of Physics A: Mathematical and Theoretical, vol.40, issue.12, pp.2989-9510, 2007.
DOI : 10.1088/1751-8113/40/12/S07

M. Jaekel and S. Reynaud, Gravitational quantum limit for length measurements, Physics Letters A, vol.185, issue.2, pp.143-810, 1994.
DOI : 10.1016/0375-9601(94)90838-9

B. Lamine, R. Hervé, A. Lambrecht, and S. Reynaud, Ultimate Decoherence Border for Matter-Wave Interferometry, Physical Review Letters, vol.96, issue.5, 2006.
DOI : 10.1103/PhysRevLett.96.050405
URL : https://hal.archives-ouvertes.fr/hal-00004899

C. Riedel, Direct detection of classically undetectable dark matter through quantum decoherence, Physical Review D, vol.88, issue.11, 2013.
DOI : 10.1103/PhysRevD.88.116005

J. Bateman, I. Mchardy, M. A. Morris, T. Ulbricht, and H. , On the Existence of Low-Mass Dark Matter and its Direct Detection, Scientific Reports, vol.78, 2015.
DOI : 10.1038/srep08058

M. Zych, F. Costa, I. Pikovski, and C. Brukne?, Quantum interferometric visibility as a witness of general relativistic proper time, Nature Communications, vol.13, pp.505-515, 1498.
DOI : 10.1038/ncomms1498

I. Pikovski, M. Zych, F. Costa, and C. Brukne?, Universal decoherence due to gravitational time??dilation, Nature Physics, vol.11, issue.8, pp.668-72, 2015.
DOI : 10.1103/PhysRevD.44.1067

B. Abbott, R. Abbott, R. Adhikari, P. Ajith, A. B. Allen et al., Observation of a kilogram-scale oscillator near its quantum ground state, New Journal of Physics, vol.11, issue.7, pp.73032-73042, 2009.
DOI : 10.1088/1367-2630/11/7/073032

M. Armano, M. Benedetti, J. Bogenstahl, D. Bortoluzzi, P. Bosetti et al., the experiment and the route to LISA, Class Quantum Gravity, vol.2626, issue.99, pp.264-9381094001, 2010.
URL : https://hal.archives-ouvertes.fr/in2p3-00713825

S. Anza, M. Armano, E. Balaguer, M. Benedetti, C. Boatella et al., The LTP experiment on the LISA Pathfinder mission, Classical and Quantum Gravity, vol.22, issue.10, pp.125-163, 2005.
DOI : 10.1088/0264-9381/22/10/001

L. Lindegren, C. Babusiaux, C. Bailer-jones, U. Bastian, A. Brown et al., Abstract, Proceedings of the International Astronomical Union, vol.15, issue.S248, pp.217-2310, 1017.
DOI : 10.1051/0004-6361:20077479

M. Drinkwater, R. Haagmans, D. Muzi, A. Popescu, R. Floberghagen et al., The GOCE gravity mission: ESA's first core Earth explorer, Proceedings of 3rd international GOCE user workshop, pp.1-8, 2006.

J. Marque, C. B. Foulon, and B. , In-orbit data of the accelerometers of the ESA GOCE mission, 61st international astronautical congress, pp.10-131, 2010.

P. Touboul and M. Rodrigues, The MICROSCOPE space mission, Classical and Quantum Gravity, vol.18, issue.13, 2001.
DOI : 10.1088/0264-9381/18/13/311
URL : https://hal.archives-ouvertes.fr/hal-00932761

F. Liorzou, D. Boulanger, M. Rodrigues, P. Touboul, and H. Selig, Free fall tests of the accelerometers of the MICROSCOPE mission, Advances in Space Research, vol.54, issue.6, 2014.
DOI : 10.1016/j.asr.2014.05.009

B. Sheard, G. Heinzel, K. Danzmann, D. Shaddock, W. Klipstein et al., Intersatellite laser ranging instrument for the GRACE follow-on mission, Journal of Geodesy, vol.182, issue.9, pp.1083-95, 2012.
DOI : 10.1007/s00190-012-0566-3

B. Christophe, D. Boulanger, B. Foulon, P. Huynh, V. Lebat et al., A new generation of ultra-sensitive electrostatic accelerometers for GRACE Follow-on and towards the next generation gravity missions, Acta Astronautica, vol.117, pp.1-7, 2015.
DOI : 10.1016/j.actaastro.2015.06.021
URL : https://hal.archives-ouvertes.fr/hal-01232474

P. Lightsey and . James, Webb Space Telescope: large deployable cryogenic telescope in space, Opt Eng, vol.51, issue.1, 2012.

E. Schrödinger, Die gegenw???rtige Situation in der Quantenmechanik, Die Naturwissenschaften, vol.23, issue.48, p.807, 1935.
DOI : 10.1007/BF01491891

C. Davisson and L. Germer, The Scattering of Electrons by a Single Crystal of Nickel, Nature, vol.119, issue.2998, pp.558-60, 1927.
DOI : 10.1038/119558a0

G. Thomson, The Diffraction of Cathode Rays by Thin Films of Platinum, Nature, vol.120, issue.3031, p.802, 1927.
DOI : 10.1038/120802a0

S. Eibenberger, S. Gerlich, M. Arndt, M. Mayor, and J. Tüxen, Matter???wave interference of particles selected from a molecular library with masses exceeding 10???000 amu, Physical Chemistry Chemical Physics, vol.17, issue.35, pp.14696-700, 2013.
DOI : 10.1039/c3cp51500a

S. Adler and A. Bassi, Is Quantum Theory Exact?, Science, vol.325, issue.5938, pp.275-281, 2009.
DOI : 10.1126/science.1176858

M. Aspelmeyer, T. Kippenberg, and F. Marquardt, Cavity optomechanics, Reviews of Modern Physics, vol.86, issue.4, pp.1391-452, 2014.
DOI : 10.1103/RevModPhys.86.1391

B. Collett and P. P. , Wavefunction collapse and random walk. Found Phys, pp.1495-541, 2003.

M. Bahrami, M. Paternostro, A. Bassi, and H. Ulbricht, Proposal for a Noninterferometric Test of Collapse Models in Optomechanical Systems, Physical Review Letters, vol.112, issue.21, 2014.
DOI : 10.1103/PhysRevLett.112.210404

R. Kaltenbaek and M. Aspelmeyer, Optomechanical Schrödinger cats -a case for space Erwin Schrödinger -50 years after, Vienna: Eur. Math. Soc

S. Bera, B. Motwani, T. Singh, and H. Ulbricht, A proposal for the experimental detection of CSL induced random walk, Scientific Reports, vol.243, pp.7664-7674, 2015.
DOI : 10.1038/srep07664

W. Zurek, Decoherence and the Transition from Quantum to Classical, Physics Today, vol.44, issue.10, 1991.
DOI : 10.1063/1.881293

M. Schlosshauer, Decoherence and the quantum-to-classical transition, 2007.

D. Ariano, G. Yuen, and H. , Impossibility of measuring the wave function of a single quantum system, Phys Rev Lett, vol.76, issue.16, 1996.

J. Ellis, Search for violations of quantum mechanics, Nuclear Physics B, vol.241, issue.2, pp.381-40510, 1984.
DOI : 10.1016/0550-3213(84)90053-1

J. Ellis, S. Mohanty, and D. Nanopoulos, Quantum gravity and the collapse of the wavefunction, Physics Letters B, vol.221, issue.2, pp.113-910, 1989.
DOI : 10.1016/0370-2693(89)91482-2

S. Adler, Lower and upper bounds on CSL parameters from latent image formation and IGM heating, J Phys A, Math Theor, vol.404012, issue.12, pp.2935-5710, 2007.

K. Hornberger, S. Gerlich, P. Haslinger, S. Nimmrichter, and M. Arndt, : Quantum interference of clusters and molecules, Reviews of Modern Physics, vol.84, issue.1, pp.157-73, 2012.
DOI : 10.1103/RevModPhys.84.157

B. Brezger, M. Arndt, and A. Zeilinger, Concepts for near-field interferometers with large molecules, Journal of Optics B: Quantum and Semiclassical Optics, vol.5, issue.2, pp.82-910, 2003.
DOI : 10.1088/1464-4266/5/2/362

P. Haslinger, N. Dörre, P. Geyer, J. Rodewald, S. Nimmrichter et al., A universal matter-wave interferometer with optical ionization gratings in the time domain, Nature Physics, vol.9, issue.3, pp.144-152, 2013.
DOI : 10.1051/0004-6361:20040541

K. Hornberger, S. Gerlich, H. Ulbricht, L. Hackermüller, S. Nimmrichter et al., Theory and experimental verification of Kapitza???Dirac???Talbot???Lau interferometry, New Journal of Physics, vol.11, issue.4, pp.43032-43042, 2009.
DOI : 10.1088/1367-2630/11/4/043032

D. Chang, C. Regal, S. Papp, D. Wilson, J. Ye et al., Cavity opto-mechanics using an optically levitated nanosphere, Proceedings of the National Academy of Sciences, vol.107, issue.3, pp.1005-1015, 2010.
DOI : 10.1073/pnas.0912969107

O. Romero-isart, M. Juan, R. Quidant, and J. Cirac, Toward quantum superposition of living organisms, New Journal of Physics, vol.12, issue.3, p.33015, 2010.
DOI : 10.1088/1367-2630/12/3/033015

S. Nimmrichter, Macroscopic matter wave interferometry Switzerland: Springer

L. De-broglie, Waves and Quanta, Nature, vol.112, issue.2815, p.540, 1923.
DOI : 10.1038/112540a0

R. Kaltenbaek, Testing quantum physics in space using optically trapped nanospheres, Proc SPIE, vol.8810, pp.10-1117, 2013.

A. Leger, DARWIN mission proposal to ESA, 2007.

M. Loose, J. Beletic, J. Garnett, and N. Muradian, Space qualification and performance results of the SIDECAR ASIC, Space Telescopes and Instrumentation I: Optical, Infrared, and Millimeter, 2006.
DOI : 10.1117/12.672705

Y. Bai, J. Bajaj, J. Beletic, M. Farris, A. Joshi et al., Teledyne imaging sensors: silicon CMOS imaging technologies for X-ray, UV, visible, and near infrared In: High energy, optical, and infrared detectors for astronomy III, Proceedings of SPIE. vol, vol.7021, 2008.

A. Kubanek, M. Koch, C. Sames, A. Ourjoumtsev, P. Pinkse et al., Photon-by-photon feedback control of a single-atom trajectory, Nature, vol.101, issue.7275, pp.898-901, 2009.
DOI : 10.1038/nature08563

Z. Yin, T. Li, and M. Feng, Three-dimensional cooling and detection of a nanosphere with a single cavity, Physical Review A, vol.83, issue.1, 2011.
DOI : 10.1103/PhysRevA.83.013816

L. Lafargue, M. Rodrigues, and P. Touboul, Towards low-temperature electrostatic accelerometry, Review of Scientific Instruments, vol.73, issue.1, 2002.
DOI : 10.1063/1.1416103

M. Tröbs, P. Weßels, C. Fallnich, M. Bode, I. Freitag et al., Laser development for LISA, Classical and Quantum Gravity, vol.23, issue.8, pp.151-159, 2006.
DOI : 10.1088/0264-9381/23/8/S20

L. Hildebrandt, R. Knispel, S. Stry, J. Sacher, and F. Schael, Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy, Applied Optics, vol.42, issue.12, 2003.
DOI : 10.1364/AO.42.002110

S. Vasilyev, A. Nevsky, I. Ernsting, M. Hansen, J. Shen et al., Compact all-solid-state continuous-wave single-frequency UV source with frequency stabilization for laser cooling of Be+ ions, Applied Physics B, vol.68, issue.1, pp.27-3310, 2011.
DOI : 10.1007/s00340-011-4435-1

R. Pound, Electronic Frequency Stabilization of Microwave Oscillators, Review of Scientific Instruments, vol.17, issue.11, 1946.
DOI : 10.1063/1.1770414

R. Drever, J. Hall, F. Kowalski, J. Hough, G. Ford et al., Laser phase and frequency stabilization using an optical resonator, Applied Physics B Photophysics and Laser Chemistry, vol.17, issue.2, pp.97-105, 1983.
DOI : 10.1007/BF00702605

F. Benabid, J. Knight, and P. Russell, Particle levitation and guidance in hollow-core photonic crystal fiber, Optics Express, vol.10, issue.21, 2002.
DOI : 10.1364/OE.10.001195.m002

O. Schmidt, T. Euser, and P. Russell, Mode-based microparticle conveyor belt in air-filled hollow-core photonic crystal fiber, Optics Express, vol.21, issue.24, pp.29383-91, 2013.
DOI : 10.1364/OE.21.029383.m002

D. Grass, Optical trapping and transport of nanoparticles with hollow core photonic crystal fibers

B. Lenoir, A. Lévy, B. Foulon, B. Lamine, C. B. Reynaud et al., Electrostatic accelerometer with bias rejection for gravitation and Solar System physics, Advances in Space Research, vol.48, issue.7, pp.1248-57, 2011.
DOI : 10.1016/j.asr.2011.06.005
URL : https://hal.archives-ouvertes.fr/hal-00542129

L. Biermann, 25. Solar Corpuscular Radiation and the Interplanetary Gas, Observatory, vol.77, pp.109-119, 1957.
DOI : 10.4159/harvard.9780674366688.c28

O. Romero-isart, A. Pflanzer, M. Juan, R. Quidant, N. Kiesel et al., Optically levitating dielectrics in the quantum regime: Theory and protocols, Physical Review A, vol.83, issue.1, 2011.
DOI : 10.1103/PhysRevA.83.013803