[Paper Review] Testing quantum physics in space using high-mass matter-wave interferometry

Rainer Kaltenbaek|arXiv (Cornell University)|Aug 31, 2015

Cold Atom Physics and Bose-Einstein Condensates2 references3 citations

TL;DR

This paper proposes the MAQRO space mission to test quantum mechanics using high-mass matter-wave interferometry in microgravity, leveraging a spacecraft at Earth-Sun L1/L2 Lagrange points to achieve ultra-high vacuum and low temperatures. By optically trapping and cooling dielectric nanospheres (up to ~10^10 amu), the mission aims to probe quantum superposition and decoherence effects at macroscopic scales, with predicted coherence times of ~100 s and interference visibility suitable for testing collapse models and quantum gravity effects.

ABSTRACT

Quantum superposition is central to quantum theory but challenges our concepts of reality and spacetime when applied to macroscopic objects like Schrödinger's cat. For that reason, it has been a long-standing question whether quantum physics remains valid unmodified even for truly macroscopic objects. By now, the predictions of quantum theory have been confirmed via matter-wave interferometry for massive objects up to $10^4\,$ atomic mass units (amu). The rapid development of new technologies promises to soon allow tests of quantum theory for significantly higher test masses by using novel techniques of quantum optomechanics and high-mass matter-wave interferometry. Such experiments may yield novel insights into the foundations of quantum theory, pose stringent limits on alternative theoretical models or even uncover deviations from quantum physics. However, performing experiments of this type on Earth may soon face principal limitations due to requirements of long times of flight, ultra-low vibrations, and extremely high vacuum. Here, we present a short overview of recent developments towards the implementation of the proposed space-mission MAQRO, which promises to overcome those limitations and to perform matter-wave interferometry in a parameter regime orders of magnitude beyond state-of-the-art.

Motivation & Objective

• To test the validity of quantum mechanics for macroscopic objects beyond current ground-based limits, approaching the mass scale of Schrödinger’s cat.
• To overcome fundamental limitations of ground-based experiments, such as short free-fall times, limited vacuum quality, and mechanical vibrations.
• To probe deviations from quantum theory predicted by collapse models (e.g., CSL, Diósi-Penrose), quantum gravity effects, and time-dilation-induced decoherence.
• To explore sensitivity to dark matter and gravitational-wave backgrounds via high-mass matter-wave interference.
• To achieve coherence times of ~100 s and test masses up to ~10^10 atomic mass units using space-based microgravity and ultra-low temperatures.

Proposed method

• Utilize a spacecraft in a Lissajous orbit around the Earth-Sun L1 or L2 Lagrange point to access microgravity, ultra-high vacuum (~10^13 Pa), and passive cooling to ~12 K in the test volume.
• Mount an optical bench outside the spacecraft with three thermal shields to isolate it from spacecraft heat, using reflective optics to minimize thermal loading.
• Employ ion and optical trapping to guide dielectric nanospheres (10^10 amu) via hollow-core fibers from inside the spacecraft to the external experimental region.
• Cool the center-of-mass motion of trapped particles close to the quantum ground state using optomechanical techniques.
• Perform near-field matter-wave interferometry by releasing the particle, allowing wavefunction expansion for ~50 s, applying a UV phase grating (~200 nm), and measuring position after a second evolution period.
• Repeat the experiment with different particle materials and sizes to systematically test quantum predictions and model deviations.

Experimental results

Research questions

• RQ1 Can quantum superposition be observed for objects with masses approaching 10^10 atomic mass units in space-based interferometry?
• RQ2 To what extent do collapse models like CSL or Diósi-Penrose predict deviations from quantum theory at these macroscopic mass scales?
• RQ3 Can the interference visibility in high-mass matter-wave interferometry be used to detect metric fluctuations from quantum gravity or gravitational-wave backgrounds?
• RQ4 Does time dilation in Earth's gravitational field induce measurable decoherence in massive quantum superpositions?
• RQ5 Can high-mass matter-wave interferometry serve as a probe for weakly interacting dark matter candidates?

Key findings

• The MAQRO mission design achieves a predicted test volume temperature of ~12 K and environmental temperature of ~25 K via passive thermal shielding, enabling long coherence times.
• The mission targets a coherence time of ~100 s, which is essential for observing matter-wave interference with test masses up to ~10^10 amu.
• The use of reflective optics on the optical bench reduces the test volume temperature from ~16 K to ~12 K, improving thermal stability.
• The updated MAQRO proposal uses near-field interferometry with a UV phase grating (~200 nm) and 1064 nm lasers, enabling a shorter total measurement time (t1 ≈ t2) and higher visibility compared to far-field double-slit schemes.
• The mission leverages heritage from LISA Pathfinder, GAIA, and Microscope, including microthrusters and inertial sensors, to ensure technological feasibility.
• The mission is designed for a nominal two-year lifetime with potential extension, allowing sufficient data accumulation for statistically robust tests of quantum foundations.