Room temperature solid-state quantum emitters in the telecom range

Abstract

On-demand, single-photon emitters (SPEs) play a key role across a broad range of quantum technologies. In quantum networks and quantum key distribution protocols, where photons are used as flying qubits, telecom wavelength operation is preferred because of the reduced fiber loss. However, despite the tremendous efforts to develop various triggered SPE platforms, a robust source of triggered SPEs operating at room temperature and the telecom wavelength is still missing. We report a triggered, optically stable, room temperature solid-state SPE operating at telecom wavelengths. The emitters exhibit high photon purity (~5% multiphoton events) and a record-high brightness of ~1.5 MHz. The emission is attributed to localized defects in a gallium nitride (GaN) crystal. The high-performance SPEs embedded in a technologically mature semiconductor are promising for on-chip quantum simulators and practical quantum communication technologies.

Fig. 1. Infrared single-photon emission in GaN.

(A) Schematic illustration of GaN crystal structure and an optical image of the GaN wafer. (B) Confocal PL mapping with a single emitter SPE1 in the center of the map. (C) PL spectra of six infrared emitters, revealing that the PL ranges from 1085 to 1340 nm. PL spectra from three emitters are taken at 4 K (top) and 300 K (bottom), respectively. Note that the emitters at 4 K and RT are different. (D) Second-order correlation measurement of the emission from SPE1 under 950-nm cw laser excitation. The blue dots are the raw data without any background correction, and the red curve is the fitting to a three-level system, yielding g²(0) = 0.05 ± 0.02. (E) Second-order correlation measurement of SPE1 excited by pulsed laser with a 1-ps pulse width and a 80-MHz repetition rate, yielding g²(0) = 0.14 ± 0.01. The g²(τ) measurements were recorded at RT. arb. units, arbitrary units.

Fig. 2. Optical properties of SPE1.

(A) Saturation curve of SPE1 yielding a saturation power P_s = 2.32 mW and I_∞ = 0.69 Mcounts/s. (B) Photon stability measurement at three different excitation powers of 0.1 mW (black curve), 0.8 mW (red curve), and 1.5 mW (blue curve), respectively, over a period of 2 min. The time resolution is 100 ms, and no obvious blinking has been observed. (C) Fluorescence lifetime measurement of SPE1 (black curve) fit with a single exponent (red curve) yielding a lifetime τ₁ = 736 ± 4 ps. The blue curve is the instrument response of the superconducting detector. (D) Schematic diagram of a three-level system used to describe the emitter. Detailed analysis of the transition rate can be found in the Supplementary Materials. (E) Polarization measurement of excitation (red open circle) and emission (blue open circle). The solid lines are the fitting with cos²(θ).

Fig. 3. Enhancement of SPEs in GaN using a PSS.

(A) SEM image of the cross section of the GaN grown on a PSS. Cone shapes of the PSS are seen. (B) Far-field radiation pattern of the in-plane dipole in the pristine GaN wafer. The circles represent collection half-angles from 100 to 900 (inner to outer circles). (C) Far-field radiation pattern of the in-plane dipole in the GaN grown on top of a PSS. Note that the maximum intensity in (C) is two times higher than that in (B). (D) Confocal scan map of an SPE in GaN grown on PSS, with the blue circles corresponding to the PSS cones. (E) Saturation curves comparing between an SPE in a pristine GaN (black squares) with an SPE embedded in a GaN grown on a PSS (blue circles). The saturated emission (I_∞) for SPE with PSS reaches 2.33 × 10⁶ counts/s compared to only 1.13 × 10⁶ counts/s from pristine GaN. The red curve is the fitting function of the raw data. (F) Comparison of count rate at 10 mW for five emitters from pristine GaN (black squares) and five emitters from GaN grown on the PSS (blue circles). The shaded rectangles serve as visual guides. The full saturation curves are presented in the Supplementary Materials.

Fig. 4. Numerical modeling of the SPEs.

(A and B) Theoretical spectral distribution resulting from point defects distributed between cubic inclusions in a 2 + 1 bilayer configuration and a continuous 3 cubic bilayer configuration, respectively. The blue columns correspond to the spectral position of the emitters’ ZPL. The arrows connect possible positions of the point defect (distributed uniformly in each GaN bilayer in the neighborhood of the cubic inclusion) and the resulting spectral positions of the ZPL. (C) If the point defects are located solely at the interface, then only two distinctive ZPLs are visible at ~1100 and 1350 nm. Our experimental data matches well with the scenario in (A) and a particular configuration in (C). Two spectra are shown to exemplify the match between the predicted and the observed PL. CBM and VBM denote the conduction band minimum and valence band maximum, respectively, and five emitters from GaN grown on the PSS (blue circles). The shaded rectangles serve as visual guides. The full saturation curves are presented in the Supplementary Materials.