Hybrid spin-mechanical systems are at the forefront of quantum information transfer due to their proposed applications in quantum memory registers and transducers to convert information between qubit platforms. Point defects in silicon carbide (SiC) off long-lived, optically addressable spin registers that exist in a wafer-scale material with low acoustic losses from which to make the high-quality mechanical resonators that can couple to the spins.
Integration of the spin with a mechanical excitation in the SiC can be challenging due to the low intrinsic spin-strain coupling of the divacancy spin defect studied in this work. The mechanical waves generated must be contained in as small a volume as possible around the single divacancy spin in order to enable mechanical control of the spin-qubit's states. Thus, after preparing the spin qubit in the ground state, mechanical driving can shift population into the excited state of the qubit without the use of magnetic fields commonly used for these systems.
The mechanical excitations are produced by an interdigitated transducer (IDT) (a) which when driven by an AC voltage source, produce surface acoustic waves (SAWs) on the surface of the piezoelectrici 4H-SiC substrate. Of note, this work uses sputtered aluminum nitride (AlN) to enhance the piezoelectric response of the IDT and a Bragg grating to focus the SAWs around the spin-defect ensemble (b) to make a mechanical cavity that enhances the spin-mechanical coupling. Measurements of the one-port reflection of the Gaussian SAW resonator in (c) reveal a quality factor of ~16,000 at 30 K, probably limited by the AlN substrate quality.
Mechanical driving between all three of the magnetic groundstate sublevels 0, -1, and +1 (a) of the axial divacancy spin defect can be achived with this SAW device. The magnetic transitions where Δms=±1, can be achieved by tuning the axial magnetic field such that the magnetic transitions is tuned to the SAW resonantor's cavity frequency, then a lock-in amplifier technique can show optical contrast when population is driven between 0 and +/-1 by continually driving the mechanical resonator.
For the magnetically forbidden transition between -1 and +1 states, Δms=±2, optical contrast is insufficient to tell if spin population is in either of the states during Rabi oscillations. So we conduct an experiment to observe Autler-Townes splittings when the magnetic transitions are continually driven with an AC magnetic field and the mechanical resonator is driven by the SAW transducer. Figure (b) shows an anti-crossing when the frequency of the microwave drive approaches that of the mechanical resonator and (c) shows the power dependence of the size of the Autler-Townes splitting follows the square-root of the mechanical drive power. Fits are produced by a finite-element model of the inhomogeneous strain field interacting with the spin-defect.
The results in this work show a high-degree of mechanical control of an ensemble of qubits composed of the magnetic sublevels of divacancy defect spins, which provides a more complete picture of spin-strain coupling for various defects. This opens up avenues for mechanical driving of other defects in SiC or coherantly control at the single spin level.
Details can be found in our manuscript: