Spin-dependent optical properties of Ce-implanted MgAl₂O₄

Defect centers in diamond [1] and SiC [2,3] are common solid-state spin qubits with long coherence times T₂ [4,5] with a variety of potential applications in quantum information sciences. Based on the guidelines for developing a new optically accessible qubit system [2,5], and theoretical predictions of qubit host materials with long T₂ [6,7] we explore alternative qubit platforms. Here, we investigate the optical properties of Ce-implanted MgO and its mixed crystal with Al₂O₃.

We implant Ce into three substrates: bare MgO substrate without cap layer (Sample A), MgO with a 10-nm-thick Al₂O₃ layer (Sample B), and bare MgAl₂O₄ substrate (Sample C). The implantation energy and the dose are 100 keV and 1.0×10¹⁴ atoms/cm², respectively. The substrates are annealed at 1000^oC in Ar atmosphere for 2 hours.

The photoluminescence (PL) spectroscopy signal is measured using a spectrometer with an excitation wavelength of 325 nm. All samples show a broad peak at around 450 nm, consistent with the 4f-5d transition of the Ce³⁺ center. Sample C shows 14 (8) times larger intensity than Sample A (Sample B). This increase of the PL intensity suggests an increase in the formation of the luminescent Ce³⁺ center [8]. Next, we investigate the polarized optical properties of Ce-implanted MgAl₂O₄. The degree of circular polarization (DOCP) reaches ~2% under 500 mT at 4K. This behavior suggests spin-dependent PL [9,10], which is inevitable in optical initialization and readout of the spin qubit state.