Industrial Silicon Metal-Oxide-Semiconductor Spin Qubits as Quantum Sensors for Single-Molecule Magnet Qudits

Single-molecule magnets (SMMs) provide chemically engineered, long-lived spin states that are attractive as qudits and quantum memories, but fast, local readout at the single-molecule level remains experimentally challenging. This thesis examines whether industrial silicon metal-oxide-semiconductor (SiMOS) spin qubits can serve as quantum sensors for SMMs, and which advances are required to approach single-molecule sensitivity.

Industrial SiMOS quantum dots in enriched 28Si are operated as spin qubits with reliable spin-selective readout and coherence times of $T_2^*= (3.1 ± 0.1)$ μs (Ramsey) and $T_2^\mathrm{HE} = (92 ± 10)$ μs (Hahn echo), establishing them as a suitable platform for quantum magnetometry. Quantitative estimates of the dipolar coupling identify the qubit-molecule separation imposed by the industrial gate stack as the main limitation for single-molecule readout. To amplify the molecular signal into the detectable range for current SiMOS devices, an ensemble of the SMM terbium bis(phthalocyanine), TbPc$_2$, heavily diluted in a YPc$_2$ matrix, is deposited on the chip surface. In this hybrid architecture, TbPc$_2$ serves as a prototypical molecular qudit with long-lived spin states, while the SiMOS qubit provides fast, electrically controlled nanoscale readout. ... mehrUsing a compact rapid adiabatic passage during spin-selective tunnelling protocol, the qubit resonance can be followed in a robust and time-efficient manner across a 100 MHz window, enabling measurements of magnetic hysteresis, angular anisotropy, and slow relaxation of the TbPc$_2$ ensemble, with relaxation times of (107 ± 2) min at 48 mK and (0.8 ± 0.3) min at 140 mK. These results provide the first demonstration of a hybrid SiMOS-SMM sensing architecture.

Beyond single-dot sensing, double quantum dot schemes based on singlet-triplet qubits and Pauli spin blockade are developed, enabling magnetic-field-gradient sensing and low-field measurements of the absolute Zeeman splitting. Finally, self-consistent Schrödinger-Poisson simulations of the industrial gate stack show that the qubit-surface separation can be reduced to well below 10 nm with realistic design modifications, and a first generation of such sensing-optimised devices is fabricated in the 300 mm industrial cleanrooms of imec, outlining a concrete pathway towards single-molecule sensitivity.