Coupling diamond spin qubits through ferromagnetic waveguides

We seek to study the magnetic coupling and decoherence mechanisms in a hybrid quantum system composed of diamond spin qubits coupled via ferromagnetic spinwaves. This basic research program will inform on future applications in quantum information processing (QIP). Hybrid diamond-ferromagnet networks could potentially serve as scalable quantum processors operating at higher temperatures than superconducting qubits. Our choice of spin qubit is the diamond nitrogen vacancy (NV) center, which has already been shown to have long coherence times (T2 >1 ms at 300 K) and can be optically initialized, coherently manipulated, and read out with high fidelity on nanosecond timescales [ref]. NV centers will be precisely placed in a micrometer-pitch array near the surface of a diamond chip, and ferromagnetic nanostructures (nanowires, cavities) will be patterned to couple the NV spin qubits via resonant spin-wave interactions, Fig. 1 By separating spin qubits by micrometers and providing externally-controlled inter-qubit interactions we hope to overcome issues of scalability in proposed spin networks based on direct dipolar coupling. We are exploring other defects in SiC and optical cavities to link the spins.

                                                  

Figure 1: Hybrid diamond-ferromagnet quantum system. (upper left) Diamond crystal and unit cell hosting a single NV center. (lower left) Rabi oscillations from a single NV center in isotopically-pure diamond [ref]. (middle) Two NV centers separated by several µm, are connected by a magnetic field-tunable ferromagnetic waveguide. Microwave control lines are placed under a thin diamond membrane to drive the NV electron spin. An out-of-plane magnetic field is applied along one of the N-V axes. A transverse magnetic field sets up an additional tuning between the NV spin qubits and spinwave resonance, (lower right). When the waveguide’s ferromagnetic resonance frequency matches the NV spin frequency, the NV centers interact and can be entangled (for suitable interaction time). When the waveguide is off resonance the spins do not interact due to their large physical separation. In future implementations the ferromagnetic resonance may instead be voltage-tuned using multiferroic materials.