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arXiv:1409.3025 · Scientific Reports

Efficient generation of twin photons at telecom wavelengths with 2.5 GHz repetition-rate-tunable comb laser

Rui-Bo Jin, Ryosuke Shimizu, Isao Morohashi, Kentaro Wakui, Masahiro Takeoka, Shuro Izumi, Takahide Sakamoto, Mikio Fujiwara, Taro Yamashita, Shigehito Miki, Hirotaka Terai, Zhen Wang, Masahide Sasaki

Published Dec 19, 2014• 47 citations

Abstract

Efficient generation and detection of indistinguishable twin photons are at the core of quantum information and communications technology (Q-ICT). These photons are conventionally generated by spontaneous parametric down conversion (SPDC), which is a probabilistic process, and hence occurs at a limited rate, which restricts wider applications of Q-ICT. To increase the rate, one had to excite SPDC by higher pump power, while it inevitably produced more unwanted multi-photon components, harmfully degrading quantum interference visibility. Here we solve this problem by using recently developed 10 GHz repetition-rate-tunable comb laser, combined with a group-velocity-matched nonlinear crystal, and superconducting nanowire single photon detectors. They operate at telecom wavelengths more efficiently with less noises than conventional schemes, those typically operate at visible and near infrared wavelengths generated by a 76 MHz Ti Sapphire laser and detected by Si detectors. We could show high interference visibilities, which are free from the pump-power induced degradation. Our laser, nonlinear crystal, and detectors constitute a powerful tool box, which will pave a way to implementing quantum photonics circuits with variety of good and low-cost telecom components, and will eventually realize scalable Q-ICT in optical infra-structures.

DOI: 10.1038/srep07468

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Entangled-photon sources are a key element for most quantum-information applications, ranging from quantum communication systems to quantum simulators and full photonic quantum computers. Spontaneous parametric downconversion (SPDC), requiring a pump laser and a nonlinear medium, is up-to-date still the preferred technique to generate such photonic entanglement. To scale the transmission rates or gate operations of the aforementioned applications, high clock rates of the pump lasers are needed. Moreover, photonic quantum-processing algorithms require photons with not only high entanglement visibility but also high spectral indistinguishability and purity. This can either be achieved by narrow spectral filtering, costly in rate, or by suitable choice of the pump pulse length (typically in the ps range). Attempts have been made to increase the rate of conventional fs-lasers by adding multiple spatial-paths [1] but this method cannot be used to go beyond GHz repetition rates. This results in the quest to produce pump lasers with very high repetition rate, ps-pulse width and high enough power to drive the weak SPDC process.

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Multi-photon state generation is of great interest for near-future quantum simulation and quantum computation experiments. To-date spontaneous parametric down-conversion is still the most promising process, even though two major impediments still exist: accidental photon noise (caused by the probabilistic non-linear process) and imperfect single-photon purity (arising from spectral entanglement between the photon pairs). In this work, we overcome both of these difficulties by (1) exploiting a passive temporal multiplexing scheme and (2) carefully optimizing the spectral properties of the down-converted photons using periodically-poled KTP crystals. We construct two down-conversion sources in the telecom wavelength regime, finding spectral purities of > 91%, while maintaining high four-photon count rates. We use single-photon grating spectrometers together with superconducting nanowire single-photon detectors to perform a detailed characterization of our multi-photon source. Our methods provide practical solutions to produce high-quality multi-photon states, which are in demand for many quantum photonics applications.