Macroscopic Quantum Resonators Path Finder Explores Limits of Superposition for Increasingly Larger Objects

The quest to understand the boundary between the quantum and classical worlds drives increasingly ambitious experiments involving ever-larger objects exhibiting quantum behaviour, and a team led by Jack Homans, Laura da Palma Barbara, and Jakub Wardak from the University of Southampton proposes a groundbreaking approach to push these limits. Their work addresses a fundamental challenge in mechanics, namely establishing an absolute upper bound on the size of objects that can exist in a quantum superposition, a state where they simultaneously occupy multiple states. Previous experiments face gravitational limitations that restrict observation times, and this team proposes a space-based optical levitation experiment, dubbed MAQRO-PF, to overcome these constraints and explore the behaviour of macroscopic objects in freefall, potentially revealing deviations from classical predictions and deepening our understanding of the foundations of physics. This innovative design, developed with contributions from Elliot Simcox, Tim M. Fuchs, and Hendrik Ulbricht, represents a significant step towards testing the limits of quantum mechanics at an unprecedented scale.

Particular interest exists in utilising these particles to investigate quantum mechanics. Exploring this limit involves allowing ever-larger objects to freely and coherently evolve, to assess if their behaviour matches quantum or classical theoretical predictions. Space-based platforms therefore represent the next key step in these investigations.

Spaceborne Quantum Tests and Levitation Systems

This document outlines a proposal for a mission dedicated to conducting fundamental physics experiments in space, leveraging microgravity and advanced levitation technologies. The core goal is to perform high-precision tests of fundamental physics, including quantum mechanics, gravity, and the search for new physics beyond the Standard Model. Space provides a near-perfect vacuum, microgravity, and shielding from terrestrial disturbances, enabling experiments with unprecedented sensitivity and precision. Optically levitated microspheres are central to the proposed experiments, allowing for the isolation and manipulation of macroscopic quantum objects and enabling tests of quantum mechanics at larger scales.

The mission aims to test the limits of quantum superposition and entanglement with increasingly massive objects, including creating Schrödinger cat states with larger masses. Precise measurements of gravitational forces and the search for deviations from General Relativity are also planned, potentially involving a more accurate measurement of the gravitational constant. The mission will also search for interactions between dark matter particles and levitated sensors, explore the connection between quantum mechanics and gravity, and search for violations of fundamental symmetries. Improving the accuracy of measurements of fundamental constants is another key objective.

The experiment relies on optically levitated microspheres, requiring precise laser control, high vacuum, and vibration isolation. An ultra-high vacuum system is essential to preserve the coherence of the levitated particles. Vibration isolation is critical to minimize external disturbances. Precise and stable lasers are needed for trapping, cooling, and manipulating the microspheres. High-resolution detectors are required to measure the properties of the levitated particles.

Sophisticated software and hardware are needed to control the experiment, acquire data, and analyze results. The mission aligns with the UK Space Agency’s National Space Strategy and Space Industrial Plan, promoting innovation in space science and technology. The project encourages international collaboration to leverage expertise and resources. The technologies developed for this mission could have applications in other areas, such as precision sensing, metrology, and materials science. This mission offers unprecedented sensitivity, long coherence times, the potential to reveal new physics beyond the Standard Model, and technological advancement.

Optical Levitation Extends Quantum Limits of Mass

Scientists are developing a space-based optical levitation experiment designed to explore the fundamental limits of mechanics and investigate the behaviour of larger objects in superposition. The research aims to determine the maximum size of an object that can exhibit quantum mechanical properties. The core of the mission involves a 200mm diameter vacuum chamber housing a ‘floating optical breadboard’ (FOB), a Zerodur platform designed to decouple the experiment from external vibrations and facilitate drag-free motion. This FOB features identical optical trapping experiments on both sides, enabling simultaneous data collection and increasing experimental efficiency.

The team intends to release and recapture the FOB hundreds of thousands of times over multiple years using a solenoid system, necessitating highly resilient solenoids, potentially incorporating redundant pairs as backup. A crucial element of the experiment is the creation of an optical diffraction grating using a pulsed ultraviolet laser, forming a standing wave through which the trapped particle passes. Precise stability of this grating, maintained to within a few nanometres, is essential for accurate data acquisition, achieved through the use of Zerodur and thermal decoupling from the satellite. The payload’s electronics have demonstrated full operation across a 0°C to 40°C temperature range, with a thermal change rate limited to 0.

1°C per minute to ensure experimental stability. Maintaining an ultra-high vacuum is paramount, with the experiment requiring at least 10⁻⁹ mbar, targeting 10⁻¹³ mbar. To achieve this, the FOB will be enclosed within a vacuum chamber pumped to 10⁻⁹ mbar before launch and coated with ‘getter’ material to continuously remove gas molecules. Scientists plan to develop a compact, low-power vacuum pressure gauge capable of measuring pressure down to 10⁻¹³ mbar, and are also exploring the possibility of using the trapped particle itself to measure surrounding pressure. The optical trapping laser system incorporates an FPGA for data acquisition and direct laser intensity modulation, enabling precise control of the trapped particles.

Space-Based Quantum Superposition of Massive Objects

This research presents a plan for a satellite-based experiment designed to explore the limits of quantum mechanics as applied to increasingly large objects. The team aims to extend the size of objects that can be prepared in a superposition by conducting experiments in the unique environment of space. Current terrestrial experiments are limited by gravitational effects, and a space-based platform offers the extended free-fall times necessary to observe these effects. The proposed experiment utilizes optically levitated particles, allowing researchers to isolate and manipulate these objects without physical contact.

By conducting these experiments in space, the team anticipates overcoming the limitations imposed by gravity and establishing a new upper bound on the size of objects exhibiting quantum mechanical behaviour. The current design focuses on achieving extremely high levels of stability, including linear and rotational control, alongside precise thermal regulation, to minimize external disturbances. Further development is required in areas such as laser coherence testing and payload power optimization. Future work will focus on refining these aspects of the design and aligning the project with broader UK space strategy initiatives, building upon previous proposals. The team received funding from multiple sources, including UKRI, QuantERA II, the EU Horizon Europe EIC Pathfinder project, and the Leverhulme Trust, supporting this ambitious undertaking to push the boundaries of quantum mechanics.