Coherent Optical Control of a Single Donor Electron Spin

Abstract

Coherent spin-photon interfaces are essential for scalable quantum networks, but their realization requires optically bright transitions, long-lived spin coherence, and complete coherent control. Donor-bound electron spins in II-VI semiconductors offer an attractive platform for optically active spin qubits because they combine strong donor-bound exciton transitions in a direct-bandgap material with a potentially nuclear-spin-free environment. However, coherent control of an individual optically active donor spin has remained elusive. Here we demonstrate optical initialization, coherent control, and readout of a single donor-bound electron spin in ZnSe. Using spin-selective donor-bound exciton optical transitions, we initialize the electron spin and directly resolve its spin-relaxation dynamics. We achieve coherent spin control with off-resonant picosecond optical pulses, as demonstrated by spin Rabi oscillations and Ramsey interference. We further coherently drive two spin-selective optical transitions and observe high-contrast coherent population trapping, demonstrating the formation of an optically controlled dark state. At higher driving strengths, multiple dark resonances reveal the hyperfine interaction between the electron and the I = 3/2 Cl nucleus. These results establish the first coherent optical control of a single donor electron spin in a II-VI semiconductor and identify isolated optical donors as a promising platform for solid-state optically active qubits.

The paper addresses whether an individual optically active donor electron spin in a II–VI semiconductor can be coherently controlled. It studies a single chlorine donor in a ZnSe/ZnMgSe quantum-well nanopost at 3.6 K, using spin-selective donor-bound-exciton transitions for initialization and fluorescence readout. Off-resonant 2.3-ps optical pulses produce spin-population oscillations versus pulse power, fitted to rotations reaching $5\pi/2$. Two-pulse Ramsey measurements show a $16\pm1$ ps precession period at 4 T, consistent with the measured electron $g$-factor. Coherent population trapping (CPT) under two-frequency optical driving produces a dark resonance with contrast $0.83\pm0.02$. Density-matrix fitting gives an inhomogeneous spin-dephasing time $T_2^*=21.9$ ns, consistent with calculations of dephasing from the natural-abundance host nuclear-spin bath. Stronger driving reveals four CPT dips consistent with hyperfine coupling to the chlorine nucleus, which has $I=3/2$. Pump–delay–probe measurements give a population-loss timescale of $18.5\pm3.8\,\mu\mathrm{s}$. Comparing opposite spin preparations and an unpolarized reference attributes this decay primarily to charge tunneling rather than spin flips; the inferred intrinsic spin-flip time has a one-sided 90% confidence lower bound of $0.51$ ms. The Ramsey delay range is limited to 1.2 ns, so it does not independently resolve the inferred dephasing time. Optical rotations become damped at higher power, and optical linewidths remain substantially above the lifetime limit. Charge stabilization, isotopic purification, nuclear-spin control, cavity integration, and spin–photon entanglement remain prospective developments.