Innovative nanophotonics: Integrating quantum light sources with nanofibers for quantum internet applications

"I had the idea for 'nanofiber quantum photonics' about 14 years ago," says Kohzo Hakuta, Director of the Center for Photonic Innovations at the University of Electro-Communications (UEC). "I want to integrate quantum light sources e.g. single quantum dot / single atom, into specially designed nanofibers. This 'fiber in-line technology' holds the potential to revolutionize distributed quantum networks for secure, ultra-high speed communication. Namely, the birth of the 'quantum internet. We are supported by Japan Science and Technology Agency through Strategic Innovation Program." Fiber in-line technology is advantageous for integrating these sources to the conventional fiber-based communication network. Now, Hakuta and his group at the Center for Photonic Innovations are addressing the following issues to develop fiber in-line technology to integrate quantum light sources into optical nanofibers. Fabrication of high efficiency tapered glass nanofibers; development of reproducible methods for integrating single quantum dots with nanofibers; integration of cavity structures with nanofibers; and experimental demonstration of cavity quantum electrodynamics (QED) with nanofibers. The work is carried out by an international group of researchers from countries including India, Vietnam, China and New Zealand. Composite photonic crystal cavity formed by combining an optical nanofiber and a nanofabricated grating Composite photonic crystal cavity formed by combining an optical nanofiber and a nanofabricated grating. The PL intensity spectra of single quantum dots show strong enhancement at the cavity resonance demonstrating the cavity QED effect. "We have been working with our industrial partner Ishihara Sangyo Inc. to develop equipment for producing tapered nanofibers," explains Hakuta. "The resulting 400 nm diameter tapered fibers have 99% light transmission." A critical technology is to pick up single quantum dots from colloidal solution and deposit it on the nanofiber. This is accomplished using a computer controlled pico-liter liquid dispenser combined with an inverted microscope and precision translation stages. Photon counting experiments show the realization of single quantum dot deposition with spatial accuracy better than 3µm, and importantly, the maximum photon channelling efficiency is measured to be 22.0% as predicted from the theory. Furthermore, Hakuta and colleagues have developed a novel method to enhance this photon channelling efficiency by incorporating cavity structures. They are developing two methods. "On one hand, we can produce photonic crystal nanofibers with an array of thousands of highly ordered nano-craters using femto second lasers" explains Hakuta. "We were surprised to find highly periodic craters produced on the shadow sides of the nanofibers. Promptly we understood, it is due to the lensing effect of the nanofiber itself. On the other hand we are developing composite nanofiber cavities with external nano-grating structures". Using these composite nanofiber cavities they have demonstrated cavity QED with single quantum dots. This research has the potential of being a new paradigm in cavity QED, and forms the basis for quantum internet and other applications. Furthermore, femto-second laser fabricated photonic crystal nanofiber cavities coupled with cold atoms can realize various manipulation methods of single photons which offer the basic tools for the next generation of internet communications. References R. R. Yalla, M. Sadgrove, K. P. Nayak, and K. Hakuta. "Cavity quantum electrodynamics on a nanofiber using a composite photonic crystal cavity," . 113, 143601 (2014). K. P. Nayak, P. Zhang, and K. Hakuta, "Optical nanofiber based photonic crystal cavity," 39, 232 (2014). M. Sadgrove, R. R. Yalla, K. P. Nayak, and K. Hakuta. "Photonic crystal nanofiber using an external grating," 38, 2542 (2013). K. P. Nayak and K. Hakuta, "Photonic crystal formation on optical nanofibers using femtosecond laser ablation technique," 21, 2480 (2013). R. R. Yalla, Fam Le Kien, M. Morinaga, and K. Hakuta, "Efficient channeling of fluorescence photons from single quantum dots into guided modes of optical nanofiber," 109, 063602 (2012). R. R. Yalla, K. P. Nayak, and K. Hakuta, "Fluorescence photon measurements from single quantum dots on an optical nanofiber," 20, 2932 (2012).

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