Fiber-Pigtailed Single-Photon Source

Updated 12 December 2025

Fiber-pigtailed single-photon sources are quantum-optical devices that integrate engineered quantum emitters with nanophotonic structures to channel single photons directly into optical fibers with high purity.
They employ advanced photonic engineering techniques such as nanobeam, mesa, and photonic crystal cavities to optimize mode-matching and maximize fiber coupling efficiency.
Their robust design and turnkey operation make them ideal for applications in quantum key distribution, long-haul quantum networking, and photonic quantum computing.

A fiber-pigtailed single-photon source is a quantum-optical device specifically engineered to emit single photons of well-defined quantum statistics directly into a single-mode optical fiber. The integration of the emitter, photonic nanostructure, and fiber pigtail enables stable, alignment-free, and efficient interfacing with complex fiber-based quantum networks or distributed photonic systems. These devices are central to long-distance quantum key distribution, quantum communication, and photonic quantum computing, with strict requirements on photon purity, spectral properties, brightness, and mechanical robustness.

1. Fundamental Design Principles

A fiber-pigtailed single-photon source comprises a quantum emitter—typically a semiconductor quantum dot (QD) or a point defect—deterministically positioned within a nanophotonic structure that spatially and spectrally shapes its spontaneous emission. The emission is funneled or mode-matched into the guided mode of a single-mode optical fiber via precise optical coupling elements.

Critical parameters in the design are:

Active region and stack: Structures utilize, for example, GaAs substrates with InGaAs QDs in a GaAs cavity, or InAs/InP QDs in an InP photonic-crystal cavity.
Photonic engineering: These include micro-mesas, nanobeams, circular Bragg gratings, or photonic crystal cavities to control the local density of states, Purcell enhancement, and far-field emission profile.
Fiber-chip coupling: Custom single-mode fibers (core diameters 2.5–9 μm, NA ≈ 0.4–0.42) are directly terminated onto the photonic structures with sub-100 nm alignment tolerances, often glued using low CTE epoxies for cryogenic compatibility (Musial et al., 2019, Margaria et al., 10 Oct 2024, Mikulicz et al., 25 Nov 2024).
Thermal and mechanical stability: Integration with compact cryocoolers and robust housing permits long-term turnkey operation with sub-2% output fluctuation over >10 hours (Musial et al., 2019, Margaria et al., 10 Oct 2024).

2. Optical Coupling Mechanisms and Efficiency

Fiber coupling involves maximizing the overlap between the emitter’s engineered far field and the fiber’s fundamental mode:

Nanophotonic approach: Mesa or waveguide structures act as truncated waveguides or lenses. Finite-element modeling, frequently using Bayesian global optimization, yields optimal mesa dimensions and fiber standoff distances (Musial et al., 2019).
Mode-matching: Metrics are defined as the ratio η_fiber = R_coupled / R_emitted, with R_coupled being the detected single-photon rate in the fiber. Experimentally achieved coupling efficiencies vary: η_fiber ≈ 2–27% for QD-mesa-fiber sources, up to 53% for hCBG devices, and up to 75% simulated with advanced overlap engineering (Margaria et al., 10 Oct 2024, Rickert et al., 13 Sep 2024, Mikulicz et al., 25 Nov 2024).
Comparative methods:
- Custom fibers end-glued to mesa tops (Musial et al., 2019)
- Adiabatic nanobeam-to-fiber tapers (Lee et al., 2019)
- 3D-printed micro-objectives and fiber holders for optimal mode overlap (Bremer et al., 2020)
- Micro-transfer-printing of nanophotonic cavities onto fiber facets (Mikulicz et al., 25 Nov 2024)

Key loss mechanisms include reflection at the nanobeam–fiber interface, scattering from adhesive or oxide layers, and mismatch in mode field diameter or NA.

Device/Class	Fiber-Coupling Efficiency (η)	Reference
QD-mesa (O-band)	2–9% (measured), few% inferred	(Musial et al., 2019)
Nanobeam-fiber	1.5% (measured), 78% (theory)	(Lee et al., 2019)
Micro-objective	26%	(Bremer et al., 2020)
hCBG cavity	53%	(Rickert et al., 13 Sep 2024)
PhC cavity on fiber	27% (sim), 10–15% (measured)	(Mikulicz et al., 25 Nov 2024)

3. Single-Photon Performance Metrics

Primary metrics include emission rate, photon statistics, and spectral purity:

Photon flux: Rates into the fiber span from 73 kHz (O-band QD under 80 MHz driving) (Musial et al., 2019) to 1.5 MHz (visible QD at saturation) (Bremer et al., 2020). Higher brightness (e.g., 1.2 Mcps, 53% per pulse) is achieved in GHz-clocked cavity-enhanced designs (Rickert et al., 13 Sep 2024).
Second-order correlation: The zero-delay autocorrelation function,

$g^{(2)}(0) = \frac{ \langle I(0) I(\tau) \rangle }{ \langle I(0) \rangle^2 }$

quantifies single-photon purity. Values reported include g^{{(2)}(0) = 0.15±0.05} (O-band) (Musial et al., 2019), g^{{(2)}(0) = 0.13} (3D-printed, pulsed operation) (Bremer et al., 2020), and <0.01 (cw operation, micro-objective). hCBG sources consistently reach g^{{(2)}(0) < 1%} (Rickert et al., 13 Sep 2024).

Multiphoton probability: Calculated as P_{n≥2} ≈ g^{(2)}(0)/2 for pulsed sources.
Spectral characteristics: Linewidths are typically 0.4–0.5 nm (O-band), with central wavelengths designed for optimal fiber transmission and minimal chromatic dispersion (e.g., λ_0 = 1294.7 nm) (Musial et al., 2019).

4. Device Packaging, Integration, and Stability

Robust packaging is essential for field deployment:

Cryogenic housing: Integration into 19″ rack-mount, 4U enclosures with compact Stirling or closed-cycle cryocoolers, enabling operation at 15–40 K for QDs (Musial et al., 2019, Mikulicz et al., 25 Nov 2024).
Turnkey operation: All excitation, filtering, and coupling are fiber-based, obviating free-space optical alignment. The device achieves long-term stability with count-rate fluctuation σ/μ <2% over tens of hours of continuous operation (Musial et al., 2019, Margaria et al., 10 Oct 2024, Mikulicz et al., 25 Nov 2024).
Mechanical interfaces: Fiber mounting uses low thermal expansion epoxies (C_T = 14 ppm/°C), fiber chucks printed with sub-micron tolerances, and strain-relief clamping to ensure alignment integrity over repeated thermal cycling (Bremer et al., 2020, Rickert et al., 13 Sep 2024, Margaria et al., 10 Oct 2024).
Spectral filtering: Integrated fiber-based dichroics and bandpass filters reject the pump (>62 dB attenuation out of band), reduce spectral noise windows to 0.6 nm FWHM, and ensure channel selection for telecom compatibility (Musial et al., 2019).

5. Applications and Relevance in Quantum Technologies

The practical realization of fiber-pigtailed single-photon sources in the telecom O-band (1260–1360 nm) aligns the emission with the minimum of silica fiber attenuation (≈0.35 dB/km) and the region of zero chromatic dispersion (Musial et al., 2019). This makes them ideally suited for:

Quantum key distribution: Sources meet or exceed requirements for error probability and flux necessary for >100 km low-noise quantum links, as demonstrated in turnkey devices and fiber-based QKD testbeds (Gao et al., 2021).
Long-haul quantum networking: Fiber-pigtailed designs are compatible with standard single-mode telecom infrastructure, permitting secure data exchange over deployed metropolitan or intercity links.
Photonic quantum computing: Deterministic sources with integrated fiber outputs facilitate large-scale multi-photon experiments.
Quantum repeaters and nodes: Compact, robust packaging with telecom compatibility allows integration into repeater stations or distributed quantum processing units.

6. Comparative Assessment and Future Prospects

Comparison to alternative platforms: Fiber-pigtailed sources offer higher ruggedness and integration than free-space-coupled or bulk microcavity sources, albeit with typically lower instantaneous brightness due to in-fiber losses. The performance gap is narrowing with advanced mode-matching (e.g., hCBG at >50%), but state-of-the-art open microcavity sources still lead purely in absolute system efficiency (>57%) (Tomm et al., 2020).
Ongoing challenges: Major limiting factors include loss at fiber–chip interfaces, spectral diffusion at elevated temperatures, and the need for cryogenic operation for direct bandgap QDs. Mode conversion losses and imperfect optical matching remain critical barriers. There is a trend toward wafer-scale fabrication, advanced 3D-printed optics, and on-chip filtering to boost scalability and performance (Bremer et al., 2020, Margaria et al., 10 Oct 2024).
Outlook: Future directions include deterministic QD positioning within photonic structures, higher-Q/low-V_m cavity designs for Purcell enhancement, and push toward room-temperature operation in wide-bandgap materials. The development of robust plug-and-play architectures is expected to enable deployment in real-world telecom environments for quantum communication and distributed computation.

7. Tabulated Device Characteristics

Device Type	Emission λ (nm)	η_fiber (%)	g^{(2)}(0)	Max Flux (cps/MHz)	T (K)	Reference
QD-mesa O-band	1294.7	~2–9	0.15	73k / 80	40	(Musial et al., 2019)
Nanobeam-on-fiber	1.3×10³	~1.5	0.14	84k / 76	4	(Lee et al., 2019)
3D-printed micro-optic	~916	26	0.13	1.5M / 80	4	(Bremer et al., 2020)
hCBG-QD (GHz)	~930	53	<0.01	1.2M / 80–1280	5	(Rickert et al., 13 Sep 2024)
PhC cavity-on-fiber	1532	10–15	0.14	~3k / 40	15	(Mikulicz et al., 25 Nov 2024)

All devices above are compatible with single-mode fiber interconnection and designed for turnkey operation or field deployment.

References:

(Musial et al., 2019) Plug&play fibre-coupled 73 kHz single-photon source operating in the telecom O-band
(Bremer et al., 2020) Quantum dot single-photon emission coupled into single-mode fibers with 3D printed micro-objectives
(Lee et al., 2019) A fiber-integrated single photon source emitting at telecom wavelengths
(Margaria et al., 10 Oct 2024) Efficient fiber-pigtailed source of indistinguishable single photons
(Rickert et al., 13 Sep 2024) A Fiber-pigtailed Quantum Dot Device Generating Indistinguishable Photons at GHz Clock-rates
(Mikulicz et al., 25 Nov 2024) InAs/InP quantum dot based C-Band all-fiber plug-and-play triggered single-photon source integrated using micro-transfer printing
(Gao et al., 2021) A Quantum Key Distribution Testbed using a Plug&Play Telecom-wavelength Single-Photon Source
(Tomm et al., 2020) A bright and fast source of coherent single photons