Coherent Microwave Control of Shallow Donor Qubits in ZnO

Neutral shallow donors (D⁰) in ZnO, such as Al, Ga, or In substituting a Zn site, can serve as semiconductor donor spin qubits with efficient optical access. They exhibit narrow inhomogeneous linewidths and long longitudinal spin relaxation times (T­₁ > 0.5 s at 2 K) [1, 2]. An inhomogeneous dephasing time (T₂^*) of 17 ns was shown using optical qubit control via stimulated Raman transitions [3]. However, only small angle rotations of low fidelity were accessible with optical control due to laser induced dephasing, motivating alternative qubit-control methods [3]. In this talk, we present fast, coherent microwave (MW) control of an ensemble of implanted In donor qubits by embedding our sample within a microstrip resonator. 

            Utilizing this MW control, we perform pulsed optically detected magnetic resonance (ODMR) and resolve 10 peaks spaced by ~100 MHz due to the strong hyperfine coupling between the In donor electron spin of ½ with the In nuclear spin of 9/2. Nuclear spin polarization is also observed. We demonstrate fast driving of the electron spin states, achieving Rabi frequencies up to Ω/2 π = 36 MHz, corresponding to a p-pulse time of  14 ns. We measure an inhomogeneous dephasing time of T₂^* = 14 ns, consistent with the prior optical study [3]. With dynamical decoupling, we can extend the coherence time to T₂ ~ 400 ns. This is two orders of magnitude shorter than the T₂ previously measured optically [3] and may be caused by MW heating. 

Access to the full hyperfine manifold revealed in ODMR opens a pathway to utilizing the In nuclear spins as long-lived quantum memories. Together with our demonstration of fast, coherent microwave-driven rotations, this work lays the foundation for hybrid electron–nuclear spin qubits in ZnO.

References:

[1] E.R. Hansen, V. Niaouris, et al., Phys. Rev. Lett. 133, 146902 (2024)

[2] V. Niaouris, et al., Phys. Rev. B 105, 195202 (2022)

[3] X. Linpeng et al., Phys. Rev. Appl. 10, 064061 (2018)