Experimental setup for the combined study of spin ensembles and superconducting quantum circuits

A hybrid quantum computing architecture combining quantum processors and quantum memory units allows for exploiting each component’s unique properties to enhance the overall performance of the total system. However, superconducting qubits are highly sensitive to magnetic fields, while spin ensembles require finite fields for control, creating a major integration challenge. In this work, we demonstrate the first experimental setup that satisfies these constraints and provides verified qubit stability. Our cryogenic setup comprises two spatially and magnetically decoupled sample volumes inside a single dilution refrigerator: one hosting flux-tunable superconducting qubits and the other a spin ensemble equipped with a superconducting solenoid generating fields up to 50 mT. We show that several layers of Cryophy shielding and an additional superconducting aluminum shield suppress magnetic crosstalk by more than eight orders of magnitude, ensuring stability of the qubit’s performance. Moreover, the operation of the solenoid adds minimal thermal load on the relevant stages of the dilution refrigerator. Our results enable scalable hybrid quantum architectures with low-loss integration, marking a key step toward scalable hybrid quantum computing platforms.

💡 Research Summary

The paper presents a complete experimental platform that enables the simultaneous operation of flux‑tunable superconducting qubits and a spin‑ensemble quantum memory within a single dry dilution refrigerator, while fully addressing the magnetic incompatibility that traditionally separates these two technologies. The authors divide the refrigerator into two spatially separated sample volumes (approximately 250 mm apart) that are thermally anchored at the mixing‑chamber plate (<10 mK) but electrically and mechanically isolated. Sample volume 1 houses the superconducting quantum circuits; it is surrounded by two layers of Cryophy® (a high‑permeability nickel‑iron‑molybdenum alloy) and an additional inner superconducting aluminum shield. This multilayer shielding attenuates external static fields, including the Earth’s field, by more than eight orders of magnitude, creating a magnetically quiet environment for the qubits. Sample volume 2 contains the spin ensemble together with a custom‑wound NbTi solenoid capable of generating static fields up to 50 mT. The solenoid is 200 mm long, has an inner diameter of 70 mm, and comprises 4 360 turns of 101 µm‑diameter wire, with extra partial windings at both ends to compensate for the distortion introduced by the surrounding magnetic shields. The solenoid is thermally anchored to the 100 mK plate, while its leads run through superconducting wires to the mixing chamber and copper DC lines to the 4 K stage, ensuring that the heat load remains negligible.

The magnetic shielding for the spin‑ensemble volume consists of three concentric Cryophy® cans, each closed at the bottom and open at the top, separated by a 4 mm gap that defines an inner working volume of 88 mm diameter and 256 mm height. The outermost Cryophy® can was later replaced by a superconducting aluminum shield to further improve attenuation. Numerical simulations (COMSOL Multiphysics) guided the geometry and confirmed that the field homogeneity at the intended sample position (designated P₀) is better than 0.1 % over a 10 mm radius, satisfying the stringent requirements for ZEFOZ operation of rare‑earth spin ensembles.

Calibration of the coil constant (CC) was performed in two stages. At room temperature, an axial probe measured a CC of 24.34 mT A⁻¹ at the geometric centre (P₁). At cryogenic temperatures, electron‑spin‑resonance (ESR) on a DPPH reference sample yielded a more accurate CC of 24.57 mT A⁻¹ at the optimal sample position P₀. The discrepancy between room‑temperature and low‑temperature values is attributed to the temperature‑dependent permeability of Cryophy® and the limited field range accessible in the normal‑state measurement of the superconducting coil.

To verify that the magnetic environment does not degrade qubit performance, the authors repeatedly measured the transition frequency, energy‑relaxation time (T₁), and dephasing time (T₂) of a flux‑tunable qubit (FTQ) while sweeping the solenoid current from 0 to the value that generates 50 mT. No statistically significant shifts or coherence degradation were observed, confirming that the shielding suppresses crosstalk to below the qubit’s sensitivity threshold. Additionally, the operation of the solenoid caused less than a 10 mK temperature rise at the mixing chamber, demonstrating that the thermal budget remains compatible with high‑fidelity superconducting circuit operation.

In summary, the authors have engineered a compact, low‑loss, and thermally benign hybrid architecture that reconciles the divergent magnetic requirements of superconducting qubits and spin‑based quantum memories. By combining multilayer Cryophy® shielding, an inner superconducting aluminum shield, and a carefully designed NbTi solenoid, they achieve magnetic isolation exceeding 10⁸, field homogeneity suitable for ZEFOZ points, and negligible thermal impact. This work paves the way for scalable hybrid quantum processors where fast superconducting logic can be directly interfaced with long‑lived spin memories, enabling distributed quantum computing, quantum repeaters, and advanced quantum error‑correction schemes that rely on heterogeneous quantum resources.