A Fiber-pigtailed Quantum Dot Device Generating Indistinguishable Photons at GHz Clock-rates

TL;DR

This work tackles the need for robust, plug-and-play quantum-light sources by deterministically fabricating fiber-pigtailed QD-hCBG cavities that are directly coupled to UHNA3 single-mode fibers. The approach achieves GHz-rate single-photon emission with strong Purcell enhancement (FP≈9) and sub-Poissonian statistics (g(2)(0)<1%), while delivering substantial end-to-end fiber coupling (up to ~53%) and photon-indistinguishability near 0.8 in the 80 MHz regime. Importantly, the device operates at a clock rate of $1.28$ GHz, producing antibunched photons with $V_{HOM}$ around 0.68 after corrections, demonstrating practical potential for field-deployable quantum information systems. The results point to a scalable path toward high-performance, fiber-integrated quantum light sources, with future improvements including coherent excitation schemes, further reduction of $T_1$, and telecom-wavelength operation to enhance deployment in real-world quantum networks.

Abstract

Solid-state quantum light sources based on semiconductor quantum dots (QDs) are increasingly employed in photonic quantum information applications. Especially when moving towards real-world scenarios outside shielded lab environments, the efficient and robust coupling of nanophotonic devices to single-mode optical fibers offers substantial advantage by enabling "plug-and-play" operation. In this work we present a fiber-pigtailed cavity-enhanced source of flying qubits emitting single indistinguishable photons at clock-rates exceeding $1\,$GHz. This is achieved by employing a fully deterministic technique for fiber-pigtailing optimized QD-devices based on hybrid circular Bragg grating (hCBG) micro-cavities. The fabricated fiber-pigtailed hCBGs feature radiative emission lifetimes of $<80\,$ps, corresponding to a Purcell factor of $\sim$9, a suppression of multi-photon emission events with $g^{(2)}(0)<1\%$, a photon-indistinguishability $>80\%$ and a measured single-photon coupling efficiency of 53$\%$ in a high numerical aperture single-mode fiber, corresponding to 1.2 Megaclicks per second at the single-photon detectors under $80\,$MHz excitation clock-rates. Furthermore, we show that high multi-photon suppression and indistinguishability prevail for excitation clock-rates exceeding $1\,$GHz. Our results show that Purcell-enhanced fiber-pigtailed quantum light sources based on hCBG cavities are a prime candidate for applications of quantum information science.

Figures (15)

• Figure 1: Device schematic and simulated performance. (a) Illustration of the fiber-pigtailed quantum light source with an UHNA3 fiber aligned to the center of the hCBG with embedded QD. The fiber-to-hCBG distance $h$ and the lateral misalignment between CBG and fiber $\Delta_\mathrm{XY}$ are indicated. (b) FEM simulation of Purcell factor $F_\mathrm{P}$ (red) and single-photon fiber-coupling efficiency $\eta_\mathrm{FC\text{-}SPS}$ (blue) of QD emission into the UHNA3 fiber at $h=350$ nm. The free space performance with $F^\mathrm{no-fiber}_\mathrm{P}$ and lens-efficiency $\eta^\mathrm{no-fiber}_\mathrm{NA0.8}$ (green) into a lens with NA=0.8 is indicated. (c) Simulated $F_\mathrm{P}$ (red) and $\eta_\mathrm{FC\text{-}SPS}$ (blue) for varying $h$-values at $\lambda$=939.5 nm. The simulated $F^\mathrm{no-fiber}_\mathrm{P}$ of the QD-hCBG without the fiber is indicated. The target fiber-hCBG distance of $h=350(50)$ nm is marked. The simulated $Q$ and $V_\mathrm{M}$ for $h$=350, 500, 800 nm, as well as without fiber are listed. (d) Deviation from the maximum simulated $\eta_\mathrm{FC\text{-}SPS}$ value at $h=350$ nm and $\lambda$=939.5 nm for varying lateral displacements $dX_\mathrm{fiber}$/$dY_\mathrm{fiber}$ of the fiber relative to the hCBG cavity's center. The dotted circle denotes a lateral misalignment of $\Delta_\mathrm{XY}=\pm200$ nm corresponding to the experimentally achieved precision, while the solid line indicates the size of the hCBG's central disc with radius $R$=360 nm.
• Figure 2: Emission properties of the QD-hCBG device before and after fiber-pigtailing. Shown are measurements from four different cooldowns (FC$_\mathrm{CD}$) #1 (red), #2 (blue), #3 (green) and #4 (orange) of the fiber-pigtailed device and the hCBG before fiber-coupling as reference (grey). Dashed grey lines indicate the long-wavelength cut-off of an bandpass filter used in the measurements. (a) Normalized emission intensity of the fiber-pigtailed device under pulsed off-resonant excitation ($\lambda_\mathrm{exc}~=~793$ nm) Respective QD states are indicated. (b) Normalized emission intensity under pulsed p-shell excitation. (c) Normalized white-light reflection spectra for linear H- (solid) and V-polarisation (dashed line).
• Figure 3: Time-resolved measurements before and after the pigtailing. (a) Lifetime-measurement with extracted $T_1$-times from exponential fits for X$^+$ in p-shell excitation before (grey) and after (blue) the fiber-pigtailing. The measurement for the X$^-$ after the pigtailing (purple) in off-resonant excitation is also shown. (b) Measured $T_1$-times of X$^+$, mode $Q$-factors and QD-mode spectral mismatch $\Delta\lambda$. (b) Second-order-auto-correlation $g^{(2)}(\tau)$-measurements and $g^{(2)}(0)$-values from comparing the integrated events at $\tau=0$ to the integrated neighbouring peaks at 12.5 ns time-window. Measurements for X$^+$ and X$^-$ for respective excitation conditions are shown. The integration time-window around $\tau=0$ ns is indicated. (d) $g^{(2)}(0)$-values of the X$^+$ under p-shell excitation before the fiber-pigtailing and for pigtailed cooldowns.
• Figure 4: Two-photon interference measurements for the FC-device. The $g^{(2)}_\mathrm{HOM}(\tau)$ measurements were taken during FC$_\mathrm{CD\#3}$ under 80 MHz p-shell excitation at half the saturation power $P_\mathrm{sat}$. Co-polarized measurements are shown in green, cross-polarized measurements in grey. (a) for a separation of $\delta{t}=2$ ns for the exciting pulses. (b) for a separation of $\delta{t}=12.5$ ns for the exciting pulses. The HOM visibility $V_\mathrm{HOM}$ is obtained by comparing the co- and cross polarized peak areas at $\tau=0$. The respective integration time-windows are indicated with dashed lines.
• Figure 5: Performance of the fiber-pigtailed QD device (X$^+$ emission) under p-shell excitation at a clock-rate of $f$=1.28 GHz. The temporal delay between consecutive single-photon amounts to $1/f=781$ ps. (a) Time-resolved trace of a single-photon pulse train in logarithmic scaling. (b) Photon-autocorrelation $g^{(2)}(\tau)$-measurement. (c) Two-photon interference $g^{(2)}_\mathrm{HOM}(t)$ histograms measured in co- (orange) and cross- (grey) polarized configuration at $P_\mathrm{exc}$=$P_\mathrm{sat}/16$. Dashed lines indicate the $781$ ps repetition period.