Bridging spin Qbits to superconducting circuits through carbon nanotubes

Abstract

Hybrid systems based on superconducting planar resonators strongly coupled to magnetic molecules are promising candidates for future large-scale quantum processors. Superconducting microwave resonators allow the strong confinement of the electromagnetic fields mode volume, which is crucial for reaching a strong and coherent coupling with the spin qubits. However, achieving control over a few or single spins remains challenging. In this sense, two main issues need to be overcome. First, the cavity field effective volume needs to be further concentrated to maximize the addressability of a single spin in a controlled way. Second, as the microwave field is localized in a nanoscopic region, developing a method to deliver the magnetic sample with sufficient spatial accuracy is crucial. As a first approach, I. Gimeno et al. showed that reducing the width of a superconducting transmission line locally leads to an enhancement of the microwave magnetic field and, therefore, of the achieved coupling. Utilizing dip pen nanolithography, they can place nanodeposits in a controlled way, reducing the number of coupled spins by several orders of magnitude. However, new approaches must be developed to achieve strong coupling to a single spin. We propose to downscale and reach the single-spin sensitivity limit by replacing the constriction of the superconducting resonator with a carbon nanotube (CNT). This approach presents two significant advantages: first, it concentrates further the microwave magnetic volume, increasing the coupling rate to a single spin; second, as CNT can be directly functionalized with molecular species, it allows addressing a single magnetic molecule in a localized position. Preliminary results of a hybrid superconducting/CNT resonator are presented, where the controlled positioning of the carbon nanotube on the nanometer scale has been achieved by dielectrophoresis. Moreover, optimized superconducting lumped element resonators (LERs) have been developed and coupled to CNT functionalized with magnetic porphyrin rings (mMINT) to estimate its feasibility as qubits measuring its collective coupling and decay rates.