All-Optical Preparation of Coherent Dark States of a Single Rare Earth Ion Spin in a Crystal

All-optical addressing and coherent control of single solid-state based quantum bits is a key tool for fast and precise control of ground-state spin qubits. So far, all-optical addressing of qubits was demonstrated only in a very few systems, such as color centers and quantum dots. Here, we perform high-resolution spectroscopic of native and implanted single rare earth ions in solid, namely, a cerium ion in yttrium aluminum garnet (YAG) crystal. We find narrow and spectrally stable optical transitions between the spin sublevels of the ground and excited optical states. Utilizing these transitions we demonstrate the generation of a coherent dark state in electron spin sublevels of a single ${\mathrm{Ce}}^{3+}$ ion in YAG by coherent population trapping.

Figure 1

(a). Unit cell of a YAG crystal. (b) Energy levels of ${\mathrm{Ce}}^{3+}$ ions in YAG. (c) SEM image of a SIL on the surface of the YAG crystal. (d) Laser scanning microscopy image of ${\mathrm{Ce}}^{3+}$ ions underneath the SIL. The ${\mathrm{Ce}}^{3+}$ ion is excited by a 440 nm pulsed laser through the phonon absorption sideband. Bright spots correspond to individual ${\mathrm{Ce}}^{3+}$ ions.

Figure 2

(a) Emission spectrum of a single ${\mathrm{Ce}}^{3+}$ ion at cryogenic temperature, showing a sharp ZPL and broad phonon absorption sideband. The single $\mathrm{Ce}:\mathrm{YAG}$ is excited by a 440 nm femtosecond laser with a 7.6 MHz repetition rate. The spectrometer has 1800 grooves/mm grating and the integration time is 5 min. (b) Excitation spectrum of the single ${\mathrm{Ce}}^{3+}$ ion. The cw diode laser with a wavelength of 489.15 nm is swept, while the pulsed laser at 440 nm repumps the ion. An additional microwave (MW) field frequency of 1.15 GHz is applied simultaneously. The frequency matches the splitting of the lowest ground-state spin transitions. (c) 20 successive photoluminescence excitation sweeps of a native single Ce ion. (d) Consecutive frequency sweeps of a single Ce ion created by focused ion implantation.

Figure 3

(a) Fluorescence intensity of a single Ce ion under pulsed and cw laser excitation. (b) Scheme of the laser pulse sequences. The pulsed laser is used to bring the ${\mathrm{Ce}}^{3+}$ back and the $\mathrm{Ce}:\mathrm{YAG}$ is ionized by cw laser excitation when the pulsed laser is switched off. The femtosecond pulsed laser wavelength is 448 nm. The repetition rate is 2.5 MHz and the average laser power is $10\mu \mathrm{W}/{\mathrm{cm}}^2$. A blue diode laser (451 nm) with $150\mu \mathrm{W}$ power is used as the cw laser.

Figure 4

(a) ODMR of the single ${\mathrm{Ce}}^{3+}$ ion. During the MW frequency sweep, the cw laser is turned on to resonantly excite the $\mathrm{Ce}:\mathrm{YAG}$ through the $D3$ transition. (Inset) Scheme of a single ${\mathrm{Ce}}^{3+}$ ion ODMR. (b) $CPT$ of the single ${\mathrm{Ce}}^{3+}$ ion. The frequency of the pump laser is the same as the one in the $D1$ transition. The probe laser is passed through three acousto-optic modulators to have a 980 to 1250 MHz tuning range to sweep around the $D3$ transition. The pulsed laser is electrically chopped with a 2.5 kHz repetition rate to avoid ionizing the Ce ion to the ${\mathrm{Ce}}^{4+}$ state. The red curve corresponds to the fitting of this $CPT$ process. The blue curve shows the result of a Rabi splitting simulation. (c) The linewidth of $CPT$ dips as a function of the pump laser induced Rabi frequency.