Photonic Indistinguishability of the Tin-Vacancy Center in Nanostructured Diamond

Tin-vacancy centers in diamond are promising spin-photon interfaces owing to their high quantum efficiency, large Debye-Waller factor, and compatibility with photonic nanostructuring. Benchmarking their single-photon indistinguishability is a key challenge for future applications. Here, we report the generation of single photons with ${99.7}_{-2.5}^{+0.3}\%$ purity and 63(9)% indistinguishability from a resonantly excited tin-vacancy center in a single-mode waveguide. We obtain quantum control of the optical transition with 1.71(1)-ns-long $\pi$ pulses of 77.1(8)% fidelity and show it is spectrally stable over 100 ms. A modest Purcell enhancement factor of 12 would enhance the indistinguishability to 95%.

Figure 1

(a) Microscope photograph of the lensed fiber aligned to a single waveguide using a three-axis nanopositioning stack. (b) Diagram of the excitation and collection geometry. (c) Electronic structure of the SnV center with no magnetic field. Off-resonant 532-nm and on-resonance 619-nm lasers generate emission into the ZPL and PSB radiative decay pathways.

Figure 2

(a) Histogrammed PSB fluorescence during a 20-ns-long resonant laser pulse. Curves are offset proportionally to the saturation parameter $s$. Solid curves are fits to a master equation [37]. At the highest saturation parameter, the excited-state population saturates to 0.5 at long times [37]. (b) Optical Rabi frequency $\mathrm{\Omega }$ extracted from (a) as a function of $\sqrt{s}$. The black line is a linear fit with zero intercept. (c) Quality factor $Q$ as a function of $s$. The solid black curve is a fit to a master equation including a pure dephasing rate proportional to $\mathrm{\Omega }$. The dashed black curve shows the absolute coherence limit. (d) Detuning $\delta$ dependence of the Rabi oscillations at $s=102(4)$ for the same excitation scheme as (a). Color coding with blue (yellow) corresponds to low (high) fluorescence.

Figure 3

(a) Measured Ramsey (teal) and Hahn-echo (orange) contrast decay envelopes with master-equation fits (solid curves) [37]. The dashed black curve is the absolute coherence limit, $T_2=2T_1$. The contrast at 0 ns is normalized to one. (b) Left: shot-to-shot evolution of the PLE line shape. Each vertical cut is the average of fast PLE scans over 500 ms. Off-resonant 532-nm excitation is applied between line cuts. The solid white line follows the emitter resonance frequency. Right: example line cut (gray) and inhomogeneous distribution of all line cuts (black), with Lorentzian and Gaussian fits, respectively. (c) Average PLE intensity as a function of time following a 532-nm off-resonant pulse for multiple saturation parameters. Solid curves are fits to biexponential decays. The experimental protocol is identical to that of (b). The dark-state pumping time is normalized by accounting for the fraction of time spent on resonance.

Figure 4

(a) Time-resolved ZPL fluorescence under excitation with a 4-ns-long resonant laser pulse, at $s=45(20)$. Red and teal shadings correspond to applied laser excitation and ZPL fluorescence collection, respectively. The average background (dashed line) is extracted with a far-detuned ($\delta /2\pi =6\mathrm{GHz}$) laser pulse [37]. (b) Second-order autocorrelation of the ZPL fluorescence under pulsed resonant excitation. The recorded value $g^{(2)}(0{)}_{\mathrm{raw}}=0.067(23)$ corresponds to a background-corrected photon purity of ${99.7}_{-2.5}^{+0.3}\%$. (c) Experimental setup for the HOM measurement. Abbreviations: quarter-wave plate ($\lambda /4$), half-wave plate ($\lambda /2$), polarizer (Pol.), beam splitter (BS), and dichroic mirror (DC). (d) Pulsed two-photon interference measurement for the parallel (blue) and perpendicular (red) polarization configurations, where the coincidence counts are integrated for each pulse. The background-corrected photon indistinguishability is 73(13)% for photons collected within the teal region in (a). Inset: time-resolved distribution of coincidences around $\tau =0$.