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

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.

💡 Research Summary

Solid‑state quantum light sources based on semiconductor quantum dots (QDs) have become indispensable for photonic quantum technologies, yet their deployment outside laboratory environments faces two major hurdles: the need for cryogenic cooling and the requirement for a robust, alignment‑free interface to optical fibers. In this work the authors present a fully deterministic, fiber‑pigtailed quantum light source that overcomes both challenges by integrating InAs/GaAs QDs into hybrid circular Bragg grating (hCBG) micro‑cavities and directly coupling the cavities to an ultra‑high numerical aperture (NA 0.35) single‑mode fiber (SMF).

The device fabrication starts with a flip‑chip process that bonds a 170 nm thick GaAs membrane containing the QD layer to a SiO₂ dielectric layer and a backside gold mirror. Deterministic placement of pre‑selected QDs into numerically optimized hCBG structures is achieved via marker‑based cathodoluminescence mapping and electron‑beam lithography. The fiber‑pigtailing procedure is noteworthy for its precision: an interferometric alignment technique uses the interference between light reflected from the sample and the fiber facet to locate the target cavity with sub‑200 nm lateral accuracy and ±50 nm axial control. After physical contact, a focused xenon ion beam etches a shallow trench to set the fiber‑to‑cavity distance (target h ≈ 350 nm). Finally, UV‑curable adhesive secures the fiber ferrule, yielding a permanently attached, vibration‑resistant assembly.

Finite‑element method (FEM) simulations (JCMsuite) predict that for h = 350 nm the cavity supports a Purcell factor exceeding 30 and a single‑photon fiber‑coupling efficiency (η_FC‑SPS) above 80 %. The simulations also show that η_FC‑SPS is relatively insensitive to small variations in h (±50 nm) and lateral misalignment (±200 nm), remaining above 70 % across the explored parameter space.

Experimental characterization is performed in a closed‑cycle helium cryostat at ≈ 4.8 K. Time‑resolved photoluminescence under off‑resonant (λ_exc = 793 nm) and p‑shell resonant excitation reveals radiative lifetimes T₁ < 80 ps, corresponding to a measured Purcell factor of ≈ 9. Second‑order correlation measurements give g²(0) = 0.007 ± 0.002 at an 80 MHz excitation rate, indicating near‑perfect single‑photon emission. Two‑photon Hong‑Ou‑Mandel (HOM) interference yields indistinguishability values of 82 % (2 ns delay) and 79 % (12.5 ns delay) under pulsed p‑shell resonant excitation.

Crucially, the source maintains high performance at gigahertz clock rates. When driven at 1.28 GHz, g²(0) rises modestly to 0.035 ± 0.011, and HOM visibility remains at 68 % ± 7 %, demonstrating that both multi‑photon suppression and photon indistinguishability survive rapid pulsing. The measured fiber‑coupling efficiency per excitation pulse reaches 53.7 % ± 0.2 %, which translates to a detected count rate of 1.2 Mclick s⁻¹ at 80 MHz. Even at the GHz regime, the count rate scales proportionally, confirming that the Purcell‑enhanced short lifetime enables operation well beyond the traditional 100 MHz barrier.

Thermal analysis shows that attaching the fiber introduces an additional heat load of ~0.8 K, yet the QD emission spectrum remains stable, indicating adequate thermalization through the gold mirror and substrate. Long‑term stability tests across multiple cooldown cycles reveal that the fiber‑to‑cavity alignment stays within the designed tolerance, and no degradation of optical performance is observed.

Overall, the paper delivers a compelling proof‑of‑concept for a plug‑and‑play quantum light source that combines (i) deterministic, high‑yield fabrication, (ii) a cavity design (hCBG) that offers strong Purcell enhancement and broadband collection, and (iii) a robust, high‑efficiency fiber interface. The achieved metrics—sub‑80 ps lifetime, g²(0) < 1 %, >80 % indistinguishability, >50 % fiber coupling, and operation at >1 GHz—place this platform among the most advanced solid‑state single‑photon emitters reported to date.

Future work should address operation at higher temperatures (e.g., 10 K or above) to reduce cooling overhead, extensive mechanical‑vibration testing for field deployment, and integration with on‑chip photonic circuits or quantum network nodes. Nonetheless, the demonstrated performance strongly supports the suitability of fiber‑pigtailed hCBG quantum dot devices for scalable quantum communication, distributed quantum computing, and other real‑world quantum information applications.