MAQRO is a proposal for a medium-sized space mission. The proposal originally was submitted in 2010 in response to the "M3" Call of the European Space Agency (ESA) for a medium-size mission opportunity with a launch in 2022 in the course of the Cosmic Vision programme. Since then, a series of studies have aimed at increasing the technological readiness of key technologies necessary for the realization of MAQRO. In January 2015, an updated version of the proposal was submitted in response to the "M4" Call of ESA for a medium-size mission opportunity with a launch in 2025. In September 2016, we submitted MAQRO as a proposal in response to the "New Science Ideas" Call of ESA in order to potentially enter discussions with ESA with respect to the roadmap for technology developments towards a possible future realization of MAQRO.

MAQRO intends to harness recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal of MAQRO is to test quantum physics in hitherto inaccessible parameter regimes and to compare the predictions of quantum theory for truly macroscopic objects with proposed alternative theoretical models. This will allow probing the vastly unexplored "quantum-classical" transition for increasingly massive objects, and will provide unambiguous tests for so-called collapse models that have been suggested to resolve the quantum measurement problem.

In a more general context, MAQRO aims at addressing the following questions:

  1. do the laws of quantum physics still hold for macroscopic objects – this is at the heart of Schrödinger’s cat paradox – or do gravitation or yet unknown effects set a limit for massive particles?
  2. What is the fundamental relation between quantum physics and gravity?

Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times, and the quality of vacuum and micro-gravity. MAQRO may overcome these limitations and allow addressing those fundamental questions in line with the Fundamental-Laws Theme of Cosmic Vision exploring the limits of contemporary physics.

The central component of MAQRO is an optical bench mounted outside the spacecraft and thermally insulated from the hot spacecraft by three consecutive shields. This design will allow MAQRO to optimally exploit the coldness and vacuum of space, creating a unique environment fulfilling the thermal and vacuum requirements of quantum experiments with macroscopic, dielectric spheres in micro-gravity. A single dielectric sphere is optically trapped in a high-finesse cavity, and its centre-of-mass motion is cooled via feedback and cavity cooling close to the quantum ground state. The sphere then is released from the trap and prepared in a non-classical state. After some time in free fall, this will lead to quantum interference. Repeating this procedure will allow discerning the interference pattern, and its visibility can be used to test the predictions of quantum theory against the predictions of alternative theoretical models.

The hardware for the mission will largely be based on available optical space technology. The spacecraft for MAQRO as well as the mission configuration will build on technological heritage from LISA Pathfinder. In particular, the ideal mission configuration would be a Lissajous orbit around a Lagrange Point L1 or L2 orbit in order to minimize atmospheric drag and gravitational gradients, and to have maximum thermal stability. In order to reach L1/L2, we propose using a Vega launcher including a propulsion module as in LISA Pathfinder, exploiting maximum re-use of already existing equipment. The recent launch and impressively successful operation of LISA Pathfinder has provided a singular demonstration of of richness of technological heritage for the MAQRO mission proposal.