Coherent acoustic control of a single silicon vacancy spin in diamond

Abstract

Phonons are considered to be universal quantum transducers due to their ability to couple to a wide variety of quantum systems. Among these systems, solid-state point defect spins are known for being long-lived optically accessible quantum memories. Recently, it has been shown that inversion-symmetric defects in diamond, such as the negatively charged silicon vacancy center (SiV), feature spin qubits that are highly susceptible to strain. Here, we leverage this strain response to achieve coherent and low-power acoustic control of a single SiV spin, and perform acoustically driven Ramsey interferometry of a single spin. Our results demonstrate an efficient method of spin control for these systems, offering a path towards strong spin-phonon coupling and phonon-mediated hybrid quantum systems.

Fig. 1. Layout of surface acoustic wave (SAW) devices, and structure of the silicon vacancy (SiV) center.

a Schematic of our diamond SAW device. A microwave signal applied to one of the interdigital transducers (IDTs) generates acoustic waves due to the piezoelectric response of aluminum nitride (AlN). SiVs in the diamond are probed using a focused laser beam. (Inset) A scanning electron microscope (SEM) image of a pair of transducers. The scale bar corresponds to $20\mu \mathrm{m}$. b Molecular structure of the SiV. c Electronic structure of the SiV under non-zero external magnetic field. The solid red arrow indicates the optical transition used for spin initialization and readout, and the dashed red arrows indicate other optical transitions. The blue arrow indicates the acoustic transition between the two levels of the spin qubit. d Optical fluorescence spectrum of the C-transition of the SiV under resonant optical excitation, showing fine structure. The four peaks correspond to the transitions C1–C4.

Fig. 2. Characterization of SAW transducers.

a Total displacement profile of the acoustic mode in a cross-section of the device, obtained from finite element simulation. The white horizontal line indicates the interface between AlN and diamond. SiVs are located $100\mathrm{nm}$ below this line. b Room temperature measurement of electrical S-parameters between the transmitter and receiver IDTs. The $S_{11}$ plot is magnified $100\times$ along the vertical axis. c Microwave impedance microscopic image showing the surface electric potential at the focus of the transducers under continuous-wave excitation. The potential is proportional to the SAW amplitude and demonstrates focusing of the SAW on the surface.

Fig. 3. Optically detected acoustic resonance (ODAR) measurements of the SiV spin transition.

a Optical and acoustic pulse sequences used for ODAR measurement. The laser is resonant with the C1 transition and initializes the SiV into $∣\uparrow ⟩$. A $20\mathrm{ns}$ duration SAW pulse drives population between $∣\uparrow ⟩$ and $∣\downarrow ⟩$. b Time-resolved histogram of photon detection events corresponding to the pulse sequence in a. The height of the peaks at the beginning of the initialization and readout pulses is proportional to the population in $∣\downarrow ⟩$. Photon detections are integrated over $10\mathrm{ns}$ (indicated by the shaded region) to determine initialization and readout signals. c Normalized population in the $∣\downarrow ⟩$ state as the center frequency of the acoustic pulse is varied, calculated as the ratio between the readout and initialization signals. A maximum is obtained at $3.43\mathrm{GHz}$, indicating the resonance frequency of the SiV spin transition. The error bars represent the standard deviation of the normalized population.

Fig. 4. Coherent acoustic control of the SiV spin qubit.

a Pulse sequence used for Rabi oscillation measurements. An acoustic pulse of frequency $3.43\mathrm{GHz}$, which is resonant with the SiV spin transition and generated with $2\mathrm{mW}$ peak microwave input power, is used to coherently drive the qubit. b Normalized population in the $∣\downarrow ⟩$ state as the acoustic pulse duration is varied. A fit to a theoretical model is shown in red. (Inset) The dependence of Rabi frequency on the square root of peak microwave input power indicates the expected linear behavior. The errors in the Rabi frequencies are of the order of $0.1\mathrm{MHz}$. c Pulse sequence used for Ramsey interferometry measurements. Two $\pi ∕2$ acoustic pulses, each detuned by $50\mathrm{MHz}$ and generated with $4\mathrm{mW}$ peak input power, are separated by a varying free precession time. d Normalized population in the $∣\downarrow ⟩$ state as the free precession time is varied. A fit to a theoretical model is shown in red. The time constant of the exponential decay of the oscillations gives a spin coherence time $T_2^{*}=33\pm 5\mathrm{ns}$. The error bars in b and d represent the standard deviation of the normalized population.