Experimental tests of quantum reality

Quantum mechanics is often said to be a ‘strange’ theory: but what exactly is meant by this? Often, what is meant is the failure of our ability to apply certain classical notions to the atomic or molecular scale. I will discuss two such notions of classicality, i) the idea that objects have definite properties independent of measurement and ii) that uncertainty can be thought of as merely imperfect knowledge. The first notion is amenable to test via the Leggett-Garg inequality [1]: I will describe a recent experiment applying a dramatically simplified version in a superconducting flux qubit [2]. The second notion applies to the concept of a psi-epistemic theory: an interpretation of quantum mechanics intended to solve many age-old quantum mysteries. Such interpretations are also open to experimental tests, such as those associated with Pusey, Barrett and Rudolph (PBR) [3] and more recently with Barrett, Cavalcanti, Lal and Maroney (BCLM) [4,5]. I will tell a little of ongoing efforts to improve such tests (so far applied in photonic and solid-state spin systems) in respect of robustness to imperfection and interpretation of their results. I hope to draw comparisons between the two notions of classicality, and to convince the audience that the experimental results can place important restrictions on possible future theories of physics.

1. Quantum Mechanics versus macroscopic realism: is the flux there when nobody looks?, A. J. Leggett and A. Garg. Phys. Rev. Lett. 54, 857 (1985)
2. A strict experimental test of macroscopic realism in a superconducting flux qubit, G. C. Knee et al. arXiv:1601.03728
3. On the reality of the quantum state, PBR, Nature Physics 8, 475–478 (2012)
4. No ψ-epistemic model can fully explain the indistinguishability of quantum states, BCLM, Phys. Rev. Lett. 112, 250403
5. Measurements on the reality of the wavefunction, Ringbauer et al., Nature Physics 11, 249–254 (2015)