Strong hole-photon coupling in planar Ge: probing the charge degree and Wigner molecule states*

Semiconductor quantum dots (QDs) stand as promising candidates for advanced quantum information processing. Notably, hole spins confined in QDs offer long coherence and rapid gate operations achievable through electrical control, leveraging their strong spin-orbit interactions. Among various implementations, planar germanium (Ge) heterostructures with confined holes have recently emerged as front-runners for future quantum processors.

In this study, we present the strong coupling between hole charge qubits, defined in a double quantum dot (DQD) hosted in a planar Ge heterostructure, and microwave photons  confined in a high-impedance superconducting quantum interference device (SQUID) array resonator. Our investigations into various DQD configurations reveal vacuum-Rabi splittings with coupling strengths reaching up to g0/2π = 260 MHz and a charge qubit decoherence rate  as low as Γ/2π = 57 MHz, dependent on the DQD tuning.

Moreover, we demonstrate that the resonator’s frequency tunability is a key resource to investigate multi-hole spin qubits in QDs. This property enables us to explore explore the  quenched energy splitting of strongly correlated Wigner molecule states that emerge in Ge QDs. The observed enhanced coherence of the excited state suggests the presence of  distinct symmetries among related spin functions. These findings facilitate the coherent coupling of remote hole qubits confined within planar Ge, enabling all-microwave quantum state detection and long-range entanglement.