Origin and Development

MAQRO is a proposal for a medium-sized space mission. The objective of the proposed MAQRO mission is to harness space for achieving long free-fall times, extreme vacuum, nano-gravity, and cryogenic temperatures to test the foundations of physics in macroscopic quantum experiments. This will result in the development of novel quantum sensors and a means to probe the foundations of quantum
physics at the interface with gravity.

The original proposal was submitted in 2010 in response to the “M3” Call of the European Space Agency (ESA), and it was later updated for a proposal to the “M4” call of ESA in 2015. In September 2016, MAQRO was submitted as a proposal in response to the “New Science Ideas” Call of ESA. The science objectives of MAQRO were selected by ESA as a New Science Idea. This led to a detailed feasibility study (QPPF) at ESA’s Concurrent Design Facility (CDF). The final report of that study was published in early 2019. In 2021, MAQRO was submitted as a Research Campaign Whitepaper to the BPS 2023 Decadal Survey of NASA. In 2022, MAQRO will be submitted as a proposal for ESA’s 2021 call for medium sized missions.

Science Case and Motivation

The MAQRO mission intends to address the questions (1) whether there are fundamental limits to the size, complexity or the mass of quantum superpositions, (2) whether we fully understand all fundamental sources of decoherence, which leads to the decay of quantum states, or (3) whether we will eventually see deviations from the predictions of quantum theory due to yet unknown physics. For example, gravitational time dilation, space-time fluctuations, or decoherence due to gravity may result in modifications to the coherent evolution predicted by quantum theory. This could potentially lead to a transition from the sometimes non-intuitive behavior of quantum systems to a behavior more in line with our everyday perception of reality. Identifying such deviations from the predictions of quantum physics would provide new insights for understanding the fundamental laws of Nature. With the space-based platform MAQRO, these questions will be addressed by observing free quantum evolution and interference of massive dielectric test particles with radii of about 100nm and masses up to several 1010 atomic mass units (amu). For comparison, the current record of showing matter-wave interferometry with massive objects lies at several 104 amu for molecules consisting of a few thousand atoms. In the case of MAQRO, the test particles would consist of billions of atoms and are, in principle, visible by eye.

MAQRO will investigate sources of decoherence affecting macroscopic quantum superpositions, such as the scattering of residual gas, or solar/cosmic radiation. MAQRO also has the potential to detect some forms of dark or exotic matter, and for rare scattering processes in a space environment. MAQRO could provide experimental input for the standard model of cosmology, for possible extensions of the standard model of particle physics, and for a better understanding of the origin of the Universe and the foundations of physics.

Science Objectives

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

  1. To test the predictions of quantum physics in parameter regimes that overlap with experiments possible on ground. This is critical to ensure that the measurement results of MAQRO will agree with well-tested and more accessible experiments in Earth-based laboratories.
  2. To test the dependence of standard decoherence mechanisms on varying particle parameters like size, mass and composition. A very precise understanding of these effects will be crucial in order to enable tests for potential small deviations due to other decoherence effects.
  3. To test gravitational decoherence with sufficiently massive test particles in order to see whether gravitational effects will eventually lead to deviations from the predictions of quantum physics.

In the research campaign proposed to NASA, we also proposed to investigate the feasibility of addressing the following additional science objectives:

  1. To use the novel method of monitoring orientational quantum revivals. In contrast to the other experiments proposed in MAQRO which rely on matter-wave interferometry in the center-of-mass position of spherical test particles, this novel approach relies on the interference of orientational states of non-symmetric test particles. This would provide an additional, independent method of testing quantum physics, and it may expand the parameter regime accessible to the tests performed in MAQRO.
  2. To use the high sensitivity of the test particles and of the interferometric experiments in MAQRO as sensors for dark or exotic matter. Some candidates for dark and exotic matter may be shielded by Earth’s atmosphere. In space, the instruments of MAQRO could be sufficiently sensitive to detect such particles.

The Case for Space

Experiments testing macroscopic quantum superpositions in space have several key advantages, and some science objectives may not even be achievable on ground:
  1. Long coherence times and free-evolution times.
    Observing the evolution of macroscopic quantum states on Earth requires trapping test
    particles, e.g., via optical, electrostatic or magnetic fields. Trapping will inevitably couple the systems to vibrations or to noise in the trapping potential or lead to decoherence due to scattering or absorption. In order to reduce the prohibitively long free-fall times required for high-mass matter-wave interferometry, methods to accelerate the time evolution of the quantum state inside an interferometer have been suggested. However, such accelerated evolution may add excess noise that could mask the small deviations the experiment has to be sensitive to.
  2. Isolation from low-frequency vibration noise.
    Such noise can, e.g., wash out interference patterns or heat the CM motion of trapped
    particles. Space can provide excellent microgravity conditions as was impressively demonstrated by LISA Pathfinder mission.
  3. Avoiding dephasing in gravitational potentials.
    Gravitational time dilation can lead to dephasing between different branches of superpositions in a gravitational field. While this is not an issue for freely falling interfering particles, it may become relevant if guiding potentials are employed.
  4. Avoiding the shielding of dark matter/exotic matter by the atmosphere.
    Some dark and exotic matter candidates within the detection range of MAQRO would be blocked by, or thermalize with, Earth’s atmosphere before reaching terrestrial detectors. MAQRO, in space, would have clean exposure to any dark matter flux coming from outside the solar system, including the anisotropic dark matter “wind” that would result from Earth’s movement through a dark-matter background and would give a directional signal as the Earth orbits around the Sun, and as the Sun moves around the center of the galaxy.

The mission profile

The proposed experiments

MAQRO is based on optomechanics with optically trapped dielectric particles. In particular, three types of experiments are suggested:

  1. monitoring the center-of-mass motion of a weakly trapped test particle.
  2. monitoring the free quantum evolution of a massive test particle.
  3. observing near-field, center-of-mass matter-wave interferometry with massive test particles.

In case (i), the goal is to detect scattering events between the test particle and other particles. This could,  on the one hand, have expected causes like the scattering of residual gas particles, cosmic radiation, secondary radiation, or the interaction with electromagnetic fields. On the other hand, such a sensor could be sensitive to scattering events with dark or exotic matter.

In case (ii), we want to trap a particle, and then cool its center-of-mass motion close to the quantum ground state. After that, the particle is released, and it will evolve freely for some time T. After that time, we measure the position of the particle. The whole process is repeated many times, and the variance of the measured positions will then provide an estimate for the width of the quantum wavepacket of the test particle. This result will be compared with the predictions of quantum physics. If there are unknown decoherence effects or interactions, they would lead to a faster or slower expansion of the wavepacket compared to the predictions of quantum theory.

In case (iii), we also trap a particle, and then cool its center of mass motion. Then we release the particle and let it evolve freely for some time T1. At that time, a short-wavelength standing-wave grating is applied that will imprint a periodic phase on the particle wavefunction. After that, there will again be free evolution for a time T2. Then the position of the test particle is determined. The whole process is repeated many times. The distribution of the particle positions measured should then show an interference pattern. The form of this pattern as well as its visibility will be compared to the predictions of quantum physics. Any unforeseen decoherence effects will reduce the interference visibility or could lead to a qualitative change in the structure of the interference pattern. Again, this will be compared with the predictions of quantum physics.

Scientific and top-level requirements

To allow for these experiments to be sufficiently sensitive to small deviations from the predictions of quantum physics, the test particles have to be extremely well isolated from their environment. In particular, we are looking for deviations from the predictions of quantum physics that are proportional to the “decoherence parameter” Λ. The size of that parameter determines the strength of decoherence effects. If it vanishes, we have coherent quantum evolution. The weakest forms of decoherence that MAQRO aims to be sensitive to are specific forms of gravitational decoherence. For particles with a mass of a few 1010 amu and a radius of a few hundred nanometers, these models predict a value of Λ on the order of around 1011 Hz/m2.

In order for MAQRO to be sensitive to such small deviations from the predictions of quantum physics, the test particle as well as its environment must be at cryogenic temperatures (around 20K for a fused-silica test particle), and the experiment needs to be performed in extremely high vacuum (less than 10mHz scattering rates or 10-15 mbar or less for a helium gas at 20K). In addition, temperature fluctuations must be minimal to avoid too large difference in heat expansion in different parts of the setup, vibrations must be minimized, e.g., by performing the experiment during times when there are no micro-thrusters active to avoid vibrations due to their force noise.

The mission configuration

Thermal shields and outgassing to space

MAQRO proposes to harness the environment of deep space and recent progress in space technology to help achieving these strict requirements. In particular, the experiments in MAQRO are to be performed on an optical bench outside the spacecraft service module, and the optical bench is isolated from the (hot) service module by three consecutive thermal shields. These thermal shields allow purely passive cooling to cryogenic temperatures by radiating most heat directly to deep space. The optical bench itself is proposed to be open to space to (1) allow outgassing directly to space to allow achieving extreme vacuum conditions, and (2) to allow the interaction with the deep-space environment, e.g., the scattering of exotic or dark matter. The challenge is that this also exposes our optical bench and the test particle to the harsh radiation environment of deep space. Cosmic and solar radiation could potentially scatter off our test particles or other sensitive parts of the experimental setup. For that reason, we are currently investigating methods for shielding the test particle from harmful radiation while, at the same time, maintaining the advantages of an open platform. For comparison, in the QPPF feasibility study by the European Space Agency, it was proposed to enclose the optical bench in a protective cover. That would reduce the risks from radiation, but it would also suppress outgassing to space and diminish the vacuum achievable. The following figure shows a schematic representation of what the spacecraft and the shield structure were envisioned to look like for the MAQRO mission proposal in 2015 – back then still for an orbit around Lagrange point L1:

Artistic impression of the MAQRO spacecraft at the L1 Lagrange Point

An orbit around Earth-Sun Lagrange point L2

During the QPPF study, several possible orbits were considered for MAQRO comparing the data rates achievable in the communication with Earth as well as the thermal stability of the environment, possibilities for extending the mission lifetime, gravitational gradients, and the expense and complexity of station-keeping maneuvers. Based on these consideration, the study concluded that a Lissajous orbit around the L2 Earth-Sun Lagrange point would be best suited for MAQRO.

The optical bench

The central component of MAQRO is the optical bench, on which all experiments are to be performed at the cryogenic temperatures and at the vacuum levels required for macroscopic quantum experiments. One of the main purposes of thermal shields and the spacecraft is to ensure that those conditions are met. In order to allow achieving cryogenic temperatures via passive radiative cooling to space, it is crucial to minimize heat dissipation on the optical bench. That means, only components that need to be close to the test particles will be on the optical bench. Everything else will be inside the service module, and the light and electrical signals will be transported along optical fibers and cables, while the lasers etc will be inside the service module. Even the test particles will be guided on demand from inside the spacecraft along linear Paul traps. They will then be discharged to become net neutral and handed over to an optical trap once they are on the optical bench. The figure below shows an illustration of the top view of the optical bench and indicates which directions will be more sensitive to displacements:

The dashed lines are 1cm apart. The complete bench is 20×20 cm2. The test particle is trapped inside a cavity that is formed by mirrors M1 and M2. Light is guided onto and from the optical bench using fibers. These fibers are then connected to fiber-coupling assemblies IRCx. Light is focused into the cavity using elliptical off-axis mirrors EM1 and EM2. In order to observe matter-wave interferometry, a short laser pulse is applied perpendicular to the cavity mode. The path of this deep-UV pulse is indicated in violet. It is back-reflected at a dichroic mirror DM4 to implement a phase grating. An additional infrared beam is used to detect the position of the test particle at the end of the experiment by monitoring the scattered light. On the one hand, we look at a spatial homodyne signal of the back-reflected light, on the other hand, we monitor the coarse position of the particle with a CMOS chip beneath the optical bench, which is insulated by a filter that reflects thermal infrared light and transmits near infrared light.