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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

20-Gb/s QPSK transmission over 10-km-long holey fiber using a wavelength tunable quantum dot light source in O-band

Akihiro Murano, Fumiya Yagi, Naokatsu Yamamoto, Atsushi Kanno, Tetsuya Kawanishi, Hideyuki Sotobayashi

Published Feb 7, 2015• 2 citations
Publisher

Abstract

We successfully demonstrate 20-Gb/s quadrature phase-shift keying (QPSK) signal transmission. The transmission was carried out over endlessly single-mode holey fiber in the waveband of 1276.02–1304.26 nm, whose bandwidth of 5.09- THz is broader than the traditional C-band. A wavelength-tunable quantum dot (QD) laser with broad wavelength tunability helps realize bandwidth availability. A single InAs/InGaAs QD optical gainchip was grown using a sandwiched sub-nanometer separator technique in the wavelength of 1.3-μm band. Using this gain chip, the QD light source has good wavelength stability, compactness and wavelength tunablility. The measured transmission results show bit error rates within a forward error correction limit of 2×10–3 using intradyne coherent detection with offline digital signal processing. In this study, it is expected that abundant frequency resources such as the O-band are coherently enhanced by the use of a large number of wavelength channels by effectively using the QD- laser.

DOI
10.1117/12.2078118

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•May 7, 2021•1 citation
We successfully demonstrate 20-Gb/s quadrature phase-shift keying (QPSK) signal transmission. The transmission was carried out over endlessly single-mode holey fiber in the waveband of 1276.02–1304.26 nm, whose bandwidth of 5.09- THz is broader than the traditional C-band. A wavelength-tunable quantum dot (QD) laser with broad wavelength tunability helps realize bandwidth availability. A single InAs/InGaAs QD optical gainchip was grown using a sandwiched sub-nanometer separator technique in the wavelength of 1.3-μm band. Using this gain chip, the QD light source has good wavelength stability, compactness and wavelength tunablility. The measured transmission results show bit error rates within a forward error correction limit of 2×10–3 using intradyne coherent detection with offline digital signal processing. In this study, it is expected that abundant frequency resources such as the O-band are coherently enhanced by the use of a large number of wavelength channels by effectively using the QD- laser.
We successfully demonstrate 10-Gbbaud quadrature phase-shift keying signal transmission over a 4-km-long endlessly single-mode holey fiber using a wavelength-tunable semiconductor quantum dot (QD) laser. The transmission is carried out in the waveband of 1040.72–1070.02 nm with a bandwidth of 7.89 THz, which is broader than conventional wavebands such as the C and L bands. In the study, a QD laser notable for its wavelength stability and tunability is employed for its broad bandwidth availability on the transmission. Observed bit error rates are within a forward error correction limit of 2×10–3 under homodyne coherent detection configuration with offline digital signal processing. The abundant frequency resources in this T band (1000–1260 nm) help increase the capacity of a transmission link using a large number of wavelength channels by the QD laser.
Future quantum networks will rely on the ability to coherently transfer optically encoded quantum information between different wavelength bands. Bragg-scattering four-wave mixing in optical fiber is a promising route to achieving this, but requires fibers with precise dispersion control and broadband transmission at signal, target, and pump wavelengths. Here, we introduce a photonic crystal fiber with a germanium-doped core featuring group velocity matching at 1550 nm, the telecoms C-band, and 920 nm, within the emission range of efficient single photon sources based on InAs quantum dots. With low chromatic walk-off and good optical guidance even at long wavelengths, large lengths of this fiber are used to achieve nanometer-scale frequency shifts between wavelengths around 920 nm with up to 79.4% internal conversion efficiency, allowing dissimilar InAs dots to be interfaced. We also show how cascading this frequency conversion can be used to generate a frequency comb away from telecoms wavelengths. Finally, we use the fiber to demonstrate tunable frequency conversion of weak classical signals around 918 nm to the telecoms C-band.
Quantum-dot single-photon sources are promising for quantum communication. Yet, the most advanced devices operate near 900 nm, where standard single-mode fibers experience significant losses. We address this by employing a hollow-core fiber engineered for low-loss transmission at quantum-dot wavelengths, with measured loss of 0.65 dB/km and potentially as low as 0.13 dB/km near 934 nm. The same fiber also supports strong classical signals at 1550 nm without introducing detectable Raman noise. Using this platform, we transmit all four BB84 polarization states from an InAs quantum dot over 340 m with a quantum bit error rate of 0.1% while preserving single-photon purity and indistinguishability even in the presence of a strong classical signal. These results establish an integrated transmission platform that combines a custom-engineered anti-resonant hollow-core fiber with co-existence of classical and quantum channels, enabling practical quantum-dot-based quantum key distribution beyond the conventional telecom bands.
Entangled light sources are considered as core technology for multiple quantum network architectures. Of particular interest are sources that are based on a single quantum system as these offer intrinsic security due to the sub-Poissonian nature of the photon emission process. This is important for applications in quantum communication where multi-pair emission generally compromises performance. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks, mainly due to lack of compatibility with telecommunication wavelengths. In this regard, single semiconductor quantum dots are highly promising photon pair sources as they can be engineered for direct emission at telecom wavelengths. In this work we demonstrate the feasibility of this approach. We report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23 km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.
For quantum light sources in everyday telecommunication networks, quantum science needs to be fully transformed into quantum technology. The first necessary step to move outside a well-controlled lab environment requires the use of quantum light sources operating in the technologically relevant telecom O- and C-band. This can be provided using epitaxial quantum dots as deterministic sources of quantum light. Particularly intriguing is that emitters operating at telecom wavelengths are rapidly catching up with their short wavelength counterparts in terms of performances. Here, we make a decisive step forward in the development of quantum communication networks: a state-of-the-art source of quantum light, a semiconductor quantum dot (QD), with record coincidence rate for entangled photon emission in the telecom C-band, is operated inside an optimized rack-based setup. This setup includes a tunable pulsed laser for the QD excitation (from quasi- to fully resonant excitation), all optics for the excitation filtering, and QD signal coupling into single-mode fibers. Overall, the setup allows for above 50%50\%50% transmission for both exciton and biexciton photons. These results show that quantum dots-based telecom light sources can now be transported and integrated into existing fiber infrastructures, an important step to demonstrate the feasibility of the upcoming quantum internet.
Deterministic solid-state quantum light sources are key building blocks in photonic quantum technologies. While several proof-of-principle experiments of quantum communication using such sources have been realized, all of them required bulky setups. Here, we evaluate for the first time the performance of a compact and stand-alone fiber-coupled single-photon source emitting in the telecom O-band (1321 1321\,1321nm) for its application in quantum key distribution (QKD). For this purpose, we developed a compact 19'' rack module including a deterministically fiber-coupled quantum dot single-photon source integrated into a Stirling cryocooler, a pulsed diode laser for driving the quantum dot, and a fiber-based spectral filter. We further employed this compact quantum light source in a QKD testbed designed for polarization coding via the BB84 protocol resulting in g(2)(0)=0.10±0.01g^{(2)}(0) = 0.10\pm0.01g(2)(0)=0.10±0.01 and a raw key rate of up to (4.72±0.13) (4.72\pm0.13)\,(4.72±0.13)kHz using an external laser for excitation. In this setting we investigate the achievable performance expected in full implementations of QKD. Using 2D temporal filtering on receiver side, we evaluate optimal parameter settings for different QKD transmission scenarios taking also finite key size effects into account. Using optimized parameter sets for the temporal acceptance time window, we predict a maximal tolerable loss of 23.19 23.19\,23.19dB. Finally, we compare our results to previous QKD systems using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.