Strong coupling of a Gd³⁺ multilevel spin system to an on-chip superconducting resonator

Rare-earth elements diluted in non-magnetic crystals constitute a promising spin memory due to their long coherence times and limited Hilbert space that can be probed by engineered quantum control sequences [1]. Such  spin memories are embedded into hybrid quantum architectures as superconducting qubits, with fast logic-gate operations and high fidelity [2], via transmission lines or a superconducting resonator to harness the strength of each platform [3]. We report the coupling of a coplanar stripline superconducting resonator with the electronic multiplet ⁸S_7/2 of Gd³⁺ diluted in a CaWO₄ single crystal in the weak and strong coupling limit. In the weak coupling limit, continuous-wave spectroscopy of the cavity resonance perturbation allows us to detect the forbidden electro-nuclear transition of the ^155,157Gd isotopes by applying a static field close to ⊥ to crystal c-axis [4]. By increasing the coupling of the spin ensemble to the resonator we observe spin-cavity dressed states with a large mode splitting of ~150 MHz. Numerical simulations based on Dicke model shows a strong hybridization of the first excited level in the presence of a photon and the second excited level with no photon as well as a strong perturbation of the spin ground state generated by photons.

 

[1] S. Bertaina, H. Vezin, H. De Raedt, and I. Chiorescu, Experimental protection of quantum coherence by using a phase-tunable image drive, Scientific Reports 10, 1 (2020)

[2] M. Kjaergaard, M. E. Schwartz, J. Braumüller, P. Krantz, J. I. Wang, S. Gustavsson, and W. D. Oliver, Superconducting Qubits: Current State of Play, Annual Review of Condensed Matter Physics 11, 369 (2020), 1905.13641.

[3] M. Blencowe, Quantum computing: Quantum RAM, Nature 468, 44 (2010).

[4] G. Franco-Rivera et al, submitted