Using spins to search for dark matter and test linearity of quantum evolution

Ensembles of electron and nuclear spins have found applications in a number of precision experiments, such as SHAFT and CASPEr, that search for ultralight dark matter. I will describe the new limits, placed by these experiments on all three possible interactions of axion-like dark matter. We also use this precision-measurement approach to experimentally search for causal non-linear state-dependent terms in quantum field theory. Our apparatus correlates a binary macroscopic classical voltage with the outcome of a projective measurement of a quantum bit, prepared in a coherent superposition state. Measurement results are recorded in a bit string, which is used to control a voltage switch. The presence of a non-zero voltage reading in cases of no applied voltage is the experimental signature of a non-linear state-dependent shift of the electromagnetic field operator. We implement blinded measurement and data analysis with three control bit strings. Control of systematic effects is realized by producing one of the control bit strings with a classical random-bit generator. Our measurements find no evidence for electromagnetic quantum state-dependent non-linearity. We set an upper bound on the parameter that quantifies this non-linearity: |ε_γ| < 4.7 × 10^-11, at 90% confidence level.