Macroscopic superpositions and the quantum-classical boundary

4 problems

• Leggett-Garg violation with a massive mechanical oscillator

  Open

  Macrorealism is the everyday assumption that a large object always has a definite state that can be checked without disturbing it. A Leggett-Garg test checks this by measuring one object at several times; a convincing test with a heavy mechanical object is missing.

• Spatial superposition of a levitated nanoparticle larger than its size

  Open

  A glass bead about a hundred nanometers across can be held by laser light and cooled to its lowest quantum state of motion. The next step is a superposition of two places farther apart than the bead's own size, confirmed by interference.

• Matter-wave interference with particles above one million atomic mass units

  Open

  The heaviest particles shown to interfere in a way classical physics cannot mimic weigh about 170,000 atomic mass units. Reaching a million or more, the mass of a small virus, requires slower beams and longer coherence.

• Does the superposition principle fail above some mass or complexity?

  Open

  Is there a size beyond which an object cannot be in two places at once even when perfectly isolated from its surroundings? No such limit has been found, but free-flight tests reach only about 170,000 atomic mass units.