Enhanced Molecular Spin-Photon Coupling at Superconducting Nanoconstrictions

Area of Science:

• Quantum physics
• Nanoscience
• Materials science

Background:

• Coupling molecular spin ensembles to superconducting resonators is crucial for quantum information processing.
• Optimizing this coupling requires precise control over nanoscale structures and microwave field confinement.

Purpose of the Study:

• To optimize the coupling of molecular spin ensembles to 1.4 GHz on-chip superconducting resonators.
• To investigate the effect of nanoscopic constrictions on microwave magnetic field concentration and spin-photon coupling.

Main Methods:

• Combined top-down and bottom-up nanolithography techniques.
• Fabricated nanoscopic constrictions using focused ion beam milling.
• Deposited free-radical molecules onto superconducting circuits using solution deposition and atomic force microscopy.
• Quantified the effective number of spins (N_eff) using Scanning Electron and Atomic Force Microscopies.
• Measured collective spin-photon coupling constants at temperatures down to 44 mK.

Main Results:

• Demonstrated collective enhancement of spin-photon coupling, proportional to the square root of N_eff.
• Achieved over 4 orders of magnitude enhancement in average individual spin coupling (from mHz to >180 Hz).
• Observed maximum coupling values near 1 kHz for molecules on the smallest nanoconstrictions (42 nm width).

Conclusions:

• Nanolithography, particularly nanoscopic constrictions, effectively concentrates microwave magnetic fields, significantly enhancing spin-photon coupling.
• The study validates the collective enhancement of coupling with increasing spin ensemble size.
• The achieved enhancement demonstrates a promising pathway for developing highly sensitive quantum devices and sensors.