Optical and microwave control of germanium-vacancy center spins in diamond

A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum-optical and quantum-information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a germanium-vacancy ($\mathrm{GeV}$) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of $20\mu \mathrm{s}$ is demonstrated. We report resonant microwave control of spin transitions, and use this as a probe to measure the Autler-Townes effect in a microwave-optical double-resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about $19\mathrm{ns}$ was observed at a temperature of 2.0 K.

Figure 1

Stable narrow optical transitions. (a) PL spectrum of GeV at 35 K, demonstrating the four-line ZPL structure. Inset: Molecular structure of the GeV center in diamond showing a Ge atom taking the place of two adjacent carbon atoms. (b) Scanning electron microscopy (SEM) image of a solid immersion lens fabricated on the sample surface. (c) Fluorescence image of a single $\mathrm{GeV}$ center located off center under the SIL, recorded with a long-pass filter (sideband only) and without any filter on the detector. (d) In the low-intensity limit, linewidths as narrow as $42\mathrm{MHz}$ were recorded. (e) The $\mathrm{GeV}$ transitions were stable in frequency for measurements of several hours. (f) Saturation curve for a single GeV center recorded under resonant excitation (only photons from the phonon sideband are recorded). The inset shows a single-sweep PLE spectrum recorded without a long-pass filter (entire emission band is recorded), yielding up to 0.6 Mc/s at saturation intensity.

Figure 2

Optical lambda schemes. (a) The diamond sample was mounted over neodymium magnets, giving a field in the plane of the $\left\{100\right\}$ surface. (b) PLE spectrum shows transition 2-3 split into four peaks, with the outer two being strongest. (c) PLE spectrum shows transition 1–3 split into four peaks, with the inner two being strongest. (d) The magnetic field lifts the spin degeneracy and produces doublets from each of the ground and excited state branches (A and B subscripts describe two spin states). (e) CPT was performed on the $1_A\text{−}{3}_A\text{−}{1}_B\mathrm{\Lambda }$ scheme, demonstrating coherent optical spin manipulation. The inset depicts the narrowest dip, which was found to be $8.6\pm 0.5\mathrm{MHz}$ wide corresponding to a coherence lifetime of $19\pm 1\mathrm{ns}$.

Figure 3

Microwave manipulation of $\mathrm{GeV}$ spin. (a) Excitation of the spin-conserving transition $1_A-3_A$ polarizes the spin due to optical pumping into $1_B$. The time-resolved fluorescence for an excitation pulse shows spin polarization of 59% (limited by laser intensity). High optical Rabi frequencies lead to Autler-Townes splitting of levels $1_A$ and $3_A$. (b) Microwaves resonant to $1_A-1_B$ return the population to $1_A$ and restore the higher fluorescence. The width of the microwave transition is $9\mathrm{MHz}$ corresponding to a dephasing time of about $19\mathrm{ns}$. The microwave-transition contrast of about 35% is limited by the 59% contrast possible from spin polarization. (c) The microwave transition splits at increased laser intensity. The splitting scales linearly with the square root of laser intensity, as expected for the Autler-Townes effect.

Figure 4

Spin relaxation. Pulsed excitation was used to polarize the spin into $1_B$ via optical pumping as in Fig. 3. The dark interval $\tau$ between laser pulses was varied to obtain the spin relaxation time (lines are exponential fits). For the $\sim {80}^{\circ }$ field misalignment in the optically detected magnetic resonance (ODMR) and CPT measurements, $T_1=0.34\mu \mathrm{s}$ was measured. Improving the field alignment produced longer spin relaxation times ($3.33\mu \mathrm{s}$ at ${70}^{\circ }$ and $25\mu \mathrm{s}$ at ${54}^{\circ }$) but lower spin polarization.