PhD defense: Felix Caspar Hahne

Investigating Cobalt Nanomagnets with Single Spins in Diamond for Hybrid Spin-Mechanics

Hybrid spin-mechanics seeks to combine the quantum nature of a single spin qubit with the versatility of mechanical systems with applications in quantum information processing and tests of quantum mechanics with macroscopic objects. One promising approach involves magnetically coupling a spin qubit to a highly coherent mechanical oscillator. However, progress toward the quantum regime is currently limited by the achievable magnetic field gradients in such systems. This thesis presents the development toward a versatile spin-mechanical platform, envisioning the coupling of a single nitrogen-vacancy (NV) center in diamond to a macroscopic, highly coherent silicon nitride membrane oscillator. In particular, the feasibility of cobalt nanomagnets grown by focused electron beam induced deposition (FEBID) is explored as a novel approach of generating strong magnetic gradients for spin-mechanical coupling. In contrast to existing systems, such amorphous FEBID nanomagnets exhibit soft magnetic behavior and require external magnetic biasing. With the presented setup and using a combination of atomic force microscopy and lifted scanning techniques, the dipole-like stray fields of FEBID nanomagnets (hundreds of nanometers in size) deposited on Si substrates were investigated in scanning NV magnetometry measurements. Moreover, by analyzing their response to increasing external bias fields, the magnetic moment magnitude of the individual nanomagnet and gradients up to ∂Bnv/∂z = 170 kT m−1 with the NV center at a height of 250 nm above the nanomagnet were measured at bias fields up to Bbias = 140 mT. Micromagnetic simulations are in agreement with these measurements and predict field gradients reaching 1 MT m−1 at a feasible stand-off distance under saturated magnetization. Furthermore, spin-echo measurements revealed coherent spin-mechanical modulation caused by oscillatory motion of the NV probe in the gradient field. Spin coherence times in the vicinity of the nanomagnet remained steady at T2,echo ∼ 50 μs, showing no increased decoherence induced by the nanomagnet below ∂Bnv/∂z < 10 kT m−1. Ultimately, spin resonance acquisition in all measurements was limited by Zeeman shifts arising from mechanical drift in the presence of strong gradients, predominantly induced by microwave driving. With the integration of SiN membranes, the demonstrated configuration based on FEBID nanomagnets is ready to surpass the single-phonon spin-mechanical coupling rate of existing platforms, marking a significant step toward hybrid spin-mechanical quantum systems.