Coupling nanoscale spin ensembles to a SQUID embedded in superconducting circuits

Sebastian Probst, A. Tkalčec, D. J. Rieger +6 more

Jamie Potter, Laith Meti, Gemma Chapman +3 more

SQUID-embedded superconducting resonators are of great interest due to their potential for coupling highly scalable superconducting circuits with quantum memories based on solid-state spin ensembles. Such an application requires a high-Q, frequency-tunable resonator that is both resilient to magnetic field and able to operate at millikelvin temperatures. These requirements motivate the use of a higher Hc metal such as niobium; however, the challenge then becomes to sufficiently reduce the operating temperature. We address this by presenting a monolithic Nb nanoSQUID-embedded resonator, where neon focused-ion-beam fabrication of the nanoSQUID results in a device displaying frequency tunability at T=16 mK. In order to assess the applicability of the device for coupling to small spin clusters, we characterize the flux sensitivity as a function of microwave drive power and externally applied magnetic field and find that the noise is dominated by dielectric noise in the resonator. Finally, we discuss improvements to the device design that can dramatically improve the flux sensitivity, which highlights the promise of Nb SQUID-embedded resonators for hybrid superconductor-spin applications.

Xueqin Li, Zhao Yun-Fang, Tang Yan-ni +1 more

Quantum entanglement is a kernel of quantum computation and quantum communication. We introduce a theoretical scheme to achieve the entanglement between two separated quantum nodes in a hybrid system. The proposed hybrid system based on diamond nitrogen-vacancy (NV) center spin ensemble is coherently coupled to a superconducting quantum circuit consisting of two quantum nodes and a quantum channel. Each node in our setup is composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator. The NV center spin ensemble composed of N identical and non-interacting NV spins, is placed in the magnetic field antinode of the superconducting coplanar resonator where the coupling is maximized. An array of superconducting quantum interference devices (SQUIDs) is inserted in the central conductor of resonator to make its frequency tunable with the magnetic flux threading through the SQUID loops. This flux is generated by passing current through an on-chip wire, so that the resonator can be brought in resonance with the NV center spins without changing their Zeeman splitting. Quantum qubits encoded into two separate nodes are connected by a vacuum superconducting coplanar resonator that is used as a quantum channel. This setup can potentially take the best elements of each individual system:NV center spin ensemble with longer coherence time capable of preparing, storing and releasing photonic quantum information, and the superconducting quantum circuits are easy to manipulate externally and can perform quantum logic gates to control quantum information rapidly. In order to realize the entanglement between two separated quantum nodes, firstly, we make a canonical transformation and obtain the Hamiltonian of the system that is reduced to two NV center spin ensembles resonantly coupled to a single mode of the superconducting coplanar resonator. Then we put forward the hybrid NV center spin-photon qubit encoding. In this hybrid encoding, the NV center spin and photon degrees of freedom enter on an equal footing into the definition of the qubit, in which case, quantum channel will switch on when three superconducting coplanar resonators are in resonance with each other, and all the manipulations can perform simply by tuning the frequencies of the superconducting coplanar resonators. Under the precise control of the evolution time, high fidelity entanglement between two separated quantum nodes is achieved. We show that this proposal can provide high fidelity quantum entanglement under realistic conditions, both in the resonant and the dispersive interaction cases. This hybrid quantum system will exhibit long coherence time and possess features like easy fabrication, integratability, and potential scalability. Furthermore, the quantum node composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator can be respectively integrated, which has practical applications in the realization of quantum information transmission and quantum entanglement among multiple quantum nodes.

Emily Pritchett, Michael R. Geller

Yuimaru Kubo, Cécile Grèzes, Andreas Dewes +15 more

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.

Alejandro E. Baptista, Jinwoong Kim, Sonia Rani +2 more

Efficient quantum state transfer between superconducting circuits and solid-state spins would unlock high-coherence quantum memories for superconducting quantum processors. We demonstrate dynamically controlled strong coupling between a Josephson circuit and a rare-earth spin ensemble. Using a parametric pump, we realize on-demand coupling of several MHz, which will enable faithful state transfer between quantum circuits and spins. Our architecture enables quantum control of spin ensembles, and paves the way for hybrid memories with coherence far beyond those of superconducting circuits alone.