Enhanced Molecular Spin-Photon Coupling at Superconducting Nanoconstrictions.

We combine top-down and bottom-up nano- lithography to optimize the coupling of small molecular spin ensembles to 1.4 GHz on-chip superconducting resonators. Nanoscopic constrictions, fabricated with a focused ion beam at the central transmission line, locally concentrate the microwave magnetic field. Drops of free-radical molecules have been deposited from solution onto the circuits. For the smallest ones, the molecules were delivered at the relevant circuit areas by means of an atomic force microscope. The number of spins N-eff effectively coupled to each device was accurately determined combining Scanning Electron and Atomic Force Microscopies. The collective spin-photon coupling constant has been determined for samples with N(eff )ranging between 2 x 10(6) and 10(12) spins, and for temperatures down to 44 mK. The results show the well-known collective enhancement of the coupling proportional to the square root of N-eff. The average coupling of individual spins is enhanced by more than 4 orders of magnitude (from 4 mHz up to above 180 Hz), when the transmission line width is reduced from 400 mu m down to 42 nm, and reaches maximum values near 1 kHz for molecules located on the smallest nanoconstrictions.