Interfacing Superconducting Qubits and Resonator Qudit

Joel Strand

Haoxin Zhou, Kangdi Yu, Yashwanth Balaji +5 more

Dissipation remains a central obstacle to improving superconducting quantum circuits, yet the microscopic origins of loss in widely used materials platforms are not fully understood. Here, we report the observation of interface piezoelectricity-induced dissipation in superconducting qubits fabricated on high-resistivity silicon. Our devices use a transmon qubit with a shunt capacitor that simultaneously serves as an interdigital transducer embedded in a surface acoustic wave resonator. By tuning the qubit transition into resonance with discrete mechanical modes, we observe up to a factor-of-two reduction in qubit lifetime, consistent with energy exchange between the qubit and mechanical modes mediated by piezoelectric coupling at the aluminum-silicon interface. Our findings provide direct evidence for interface piezoelectricity as a distinct loss channel in superconducting qubits. Combined with multiphysics simulations, these findings suggest that interface piezoelectric loss can dominate over loss from two-level systems at sufficiently high frequencies.

Javier Serrano-Oiza, Inés Tejedor, Eva Natividad +6 more

Integration of molecular spin qubits onto superconducting circuits is one of the current proposals to build a hybrid quantum processor, in which their coupling to resonators would allow complex coherent quantum operations. As an alternative to large magnetically dilute crystals or concentrated nanoscale deposits of paramagnetic molecules studied so far, we consider here preformed nanoparticles of a Zr(IV)-based 3D MOF doped with Cu(II) and V(IV)O metalloporphyrin. Highly crystalline, phase-pure nanoparticles are obtained with an adjustable number of spin carriers and a homogeneous size that is adequate for nanoconstrictions in superconducting resonators. Continuous-wave and time-domain EPR spectroscopy show that the Cu(II) and V(IV) spins in these particles are potential qubits and qudits, respectively, with robust quantum coherence. These nanoparticles form good dispersions in methanol-chloroform mixtures, which allows the formation of monolayers of densely packed particles at the air-water interface. These Langmuir films are successfully transferred to very different substrates, including Nb resonators, homogeneously covering their most sensitive areas, i.e., the edges of the superconducting lines. Finally, the quantum coherence of the qubit nodes is maintained in the transferred nanoparticles, thereby demonstrating the viability of the present approach for the development of hybrid quantum processors.

Yufeng Wu, Yiyu Zhou, Haoqi Zhao +4 more

Superconducting quantum processors operate at microwave frequencies in millikelvin environments, making it challenging to interconnect distant nodes using conventional microwave wiring. Coherent microwave-to-optical (M2O) transduction enables superconducting quantum networks by interfacing itinerant microwave photons with low-loss optical fiber. However, many state-of-the-art transducers provide efficient conversion only over a narrow frequency span, complicating deployment with heterogeneous superconducting devices that are detuned by gigahertz-scale offsets. Here we demonstrate a frequency-agile microwave-optical interface that overcomes this bandwidth mismatch by cascading an electro-optic M2O transducer with a multimode microwave-to-microwave (M2M) frequency converter, with in situ tunability of the microwave resonances in both stages. Using this architecture, we realize continuous frequency coverage from 5.0 to 8.5 GHz within a single system. As an application relevant to superconducting-qubit networking, we use the cascaded M2M-M2O interface to optically read out a superconducting qubit whose readout resonator is detuned by 1.7 GHz from the native M2O microwave resonance, demonstrating a scalable route toward fiber-linked superconducting quantum nodes.

Ivan N. Zhilyaev

Alexandre Blais