Spins bound to point defects have been widely studied as a resource for solid-state implementation of quantum information technologies. Motivated by a search for identifying new defect centers that would be an efficient bridge between spin-qubits and photons, we looked at transition metal ion-doped solids. These exhibit a striking range of magnetic, electronic, and optical behaviors, in many similar ways to that of trapped ion systems. Chromium ions in wide-bandgap semiconductors, such as SiC and GaN, are reported to exhibit sharp optical transitions in the near-infrared which couple individually to the spin sub-levels of the ground state. This suggests a straightforward pathway for optical-spin manipulation of these ions.

We study two semiconductors: a 4H-SiC, which is epitaxially grown and doped with Cr4+, and a GaN sample that is unintentionally Cr4+ doped during the growth. The photoluminescence (PL) of these defects in SiC (a) and GaN (c) are measured at T=30K, excited with a 780 nm laser. Cr4+ ions in SiC can be located in two inequivalent lattice sites, which results in two different zero phonon lines: SiC:CrA and SiC:CrC. SiC and GaN samples are then scanned with a narrow line laser and the PL emission is collected through the side band shown in (b) and (e).  On resonance photoluminescence excitation (PLE) scans demonstrating direct excitation of Cr4+ ions are shown in (e) 4H-SiC:CrA, (f) 4H-SiC:CrC and (g) GaN:Cr. Fits to the data are shown in the latter two panels, where spin sub-levels of the electronic ground state can be resolved optically at zero magnetic field.

The PLE scan as a function of the magnetic field at T=30K of the SiC:CrC ZPL is shown below, with B=13 G PLE subtracted for clarity. We observe a stationary dip corresponding to the ms=0 state, and two peaks splitting with a magnetic field corresponding to the ms=±1 spin sub-levels. In separate experiments at T=20K, we find that T1 time is longer than the optical decay time by two orders of magnitude, which enables us to polarize the ensemble of defects by tuning a narrow line laser to one of the transitions and optically pumping the population out of those spin states.

In a two color experiment at T=15K the narrow line laser is tuned to be resonant with ms=0 transition energy. This polarizes the defect to ms=±1 states and thus the PL intensity decreases. We then use an EOM to generate and scan a sideband on the laser. When the sideband is resonant with the ms=±1 energies, it pumps the defect out of the ms=±1 and the PL is recovered.

We can generate spin rotations with microwaves using a coplanar waveguide placed behind the sample. Just like in the previous experiment, we pump the ensemble spins to ms=±1. When the microwaves are resonant with the ground state spin splitting, the spin population gets transferred to ms=0, and thus the PL is recovered and we detect the magnetic resonance optically (ODMR). A magnetic field scan of the ODMR experiment yielded results consistent with previous studies. Similarly combining optical pumping and microwave driving, it is possible to detect coherent Rabi driving of the Cr4+ spins, which demonstrates the coherent control of chromium ions in SiC.

Since the optical transitions are protected from the influence of strain and phonons most of the luminescence from the first excited state is contained within the narrow ZPL optical transitions, which form a very simple lambda structure. The knowledge gained in our studies of Cr4+ ions in SiC and GaN is transferrable to other materials systems that reproduce the basic structural characteristics of the ions. Embedded within semiconductor hosts amenable to advanced optoelectronic device design, Cr4+ ions could be used as quantum emitters that couple efficiently to chip-scale, integrated photonic control structures. Our results present exciting opportunities in the ongoing effort to exploit defect-localized spins for explorations in quantum science and engineering.

To learn more about our studies, please refer to "Resonant optical spectroscopy and coherent control of Cr4+ spin ensembles in SiC and GaN", William F. Koehl, Berk Diler, Samuel J. Whiteley, Alexandre Bourassa, N. T. Son, Erik Janzén, and David D. Awschalom. Phys. Rev. B 95: 035207 (2017).