Constraints on a Spin-Dependent Exotic Interaction between Electrons with Single Electron Spin Quantum Sensors.

A new laboratory bound on the axial-vector mediated interaction between electron spins at micrometer scale is established with single nitrogen-vacancy (NV) centers in diamond. A single crystal of p-terphenyl doped pentacene-d_{14} under laser pumping provides the source of polarized electron spins. Based on the measurement of polarization signal via nitrogen-vacancy centers, we set a constraint for the exotic electron-electron coupling g_{A}^{e}g_{A}^{e}, within the force range from 10 to 900  μm. The obtained upper bound of the coupling at 500  μm is |g_{A}^{e}g_{A}^{e}/4πℏc|≤1.8×10^{-19}, which is one order of magnitude more stringent than a previous experiment. Our result shows that the NV center can be a promising platform for searching for new particles predicted by theories beyond the standard model.