Towards Enhanced Strong-Coupling in Quantum Dot Circuit QED using High-impedance Superconducting Resonators

Abstract

The development of silicon spin qubits plays a critical role in pursuing a large-scale quantum processor, as Si spins exhibit superior coherence properties that outperform other qubit types. Among the various types of spin qubits, resonant exchange (RX) qubits have gained substantial interest due to their potential for scalability and high-fidelity operations. The RX qubit, however, requires a triple dot configuration, which has not yet been fully explored. To achieve a better understanding of triple quantum dots (TQDs) and gain insights into the valley physics in silicon, we study an accumulation mode Si/SiGe TQD with a single electron electrically coupled to microwave photons hosted in a superconducting cavity. Here we explore two different coupling schemes. By coupling the electric dipole moment of a TQD qubit to the cavity, we conduct high-resolution dispersive readout of the cavity and observe valley-induced features. From the data, we extract valley splittings and the inter-valley and intra-valley tunnel coupling rates. In the second experiment, to prepare for the future implementation of an RX qubit, we couple the cavity directly to the middle dot in a TQD system and explore the quadrupolar coupling mechanism. Both coupling mechanisms offer valuable tools for quantum state manipulation, qubit control, and multi-qubit gate operations in various quantum systems. Spin-photon coupling is a critical aspect of spin qubit technology, as it enables entanglement between distant qubits, facilitating long-range communication. The spin-photon coupling rate, denoted as $g_{s}$, is proportional to the charge-photon coupling rate, $g_{c}$, which can be enhanced using high-impedance cavities. A promising way to improve $g_{c(s)}$ is through the use of a high kinetic inductance ($L_k$) film, such as niobium nitride (NbN). To characterize this new material, we conduct direct current resistivity measurements and microwave investigations on NbN films of different thicknesses. Using the film characteristics, we redesign the half-wavelength resonator, comparing it to our previous niobium (Nb) resonator in terms of reduced footprint. To mitigate microwave leakage through dc gate lines on a cQED-QD chip, we fabricate LC filters and evaluate their attenuations through transmission measurements. These results will lead to a drastic increase in the coupling rates, forming a critical step towards realizing high-fidelity two-qubit gates between distant spins and building a scalable quantum processor.