Room-Temperature Single-Photon Source for Secure Quantum Communication

Throughout Justin Winkler’s time as a NASA Space Technology Research Fellow, he has researched a variety of approaches to generating single photons for applications in quantum cryptography. This research has included characterization of fluorescence of nanocrystal quantum dots in liquid crystal microcavities, color centers in nanocrystals with plasmonic nanoantennas, correlated photon pairs from hot atomic vapor, and whispering gallery mode resonators. He has also performed research on techniques for characterizing the quantum state of light. Here is a brief description of his research accomplishments: • Polarized single-photon source based on nanocrystal quantum dot fluorescence in a chiral photonic bandgap glassy liquid crystal host The earliest project during funding through NSTRF was on the fabrication and characterization of a single-photon source based on nanocrystal quantum dots in a chiral photonic bandgap glassy cholesteric liquid crystal microcavity. The usage of cholesteric liquid crystal allows for the photons emitted from the source to have a definite circular polarization. This is significant as it would double the efficiency of any quantum communication system based on this source relative to using an unpolarized source. The fluorescence of quantum dot emitters was studied using a confocal fluorescence microscope. The fluorescence was collected by the microscope where it could then be sent to a free-space spectrometer, which was used to characterize the emitters’ spectra and check for polarization. A Hanbury Brown and Twiss interferometer was used to check for single-photon behavior (photon antibunching). The work on this project contributed to Optics Letters and Molecular Crystals and Liquid Crystals journal publications. • Plasmonic bowtie nanoantenna for enhanced single-photon emission In collaboration with Professors Boyd and Shi, we studied gold plasmonic nanoantennas patterned as bowties. The purpose of this research was to create an optical antenna to increase single photon emission rates through the application of plasmonic resonances for light-field enhancement. Arrays of gold bowtie nanoantennas were prepared using high-resolution electron-beam lithography at the Cornell NanoScale Science and Technology Facility and the University of Rochester’s Integrated Nanosystems Center. We characterized the structure of these antennas using both an atomic force microscope and a scanning electron microscope. We used a confocal fluorescence microscope to check for polarization dependent photoluminescence to search for plasmonic resonances caused by these antennas. To utilize these antennas to enhance the emission rate of single-photon emitters, we studied using an atomic force microscope to deterministically place a nanocrystal, with the goal of placing a nanocrystal with a single color center within one of these bowtie nanostructures. We successfully created a protocol that could be used to push nanocrystals and thereby move these nanocrystals to locations of our choice. These results have been published in invited paper of SPIE Proceedings (Quantum-Physics-based Information Security). • Four-wave mixing with atomic rubidium vapor for a heralded single-photon source In collaboration with Professor John Howell of University of Rochester, Justin Winkler worked on the creation of a narrowband heralded single-photon source through the usage of four-wave mixing within hot atomic rubidium vapor. This source would be compatible with rubidium-based quantum memory and therefore might be applied for long-haul quantum cryptography. The setup was built using pulsed pump and probe beams that were generated through the usage of a tapered amplifier and acousto-optic modulators. The two beams were combined in a cell of rubidium vapor. By choosing appropriate frequencies and modulating the pump beam, the probe beam could be stored in the electron state of the rubidium vapor through electromagnetically induced transparency. Upon release, the stored pulse would generate a correlated idler pulse through the process of four-wave mixing. Extended to the single-photon level, this process generates correlated photon pairs and can therefore potentially be used as a heralded photon source. Justin Winkler built and tested this setup and was able to confirm four-wave mixing and the production of correlated idler pulses. To extend this research to the single-photon level and prove that time-correlated photon pairs were being produced, he built an apparatus to characterize time-correlations based on avalanche photodiodes and worked to remove sources of noise from the setup. • Wavefront sensing for photon state characterization through weak measurements Over the past year with Professor Howell’s group, Justin Winkler has been studying weak measurement techniques for high-resolution characterization of the spatial wavefunction of light. This research can be applied to the characterization of single-photon states and is also applicable to most classical experiments that rely on wavefront sensing. This research was started towards the end of Justin’s work under NSTRF and is currently on-going. A paper based on this research is expected by spring of 2016. • Research on Whispering Gallery Mode Resonators at Jet Propulsion Laboratory (NASA onsite experience) Each summer under NSTRF, Justin Winkler performed research under the supervision of NASA collaborator Dr. Nan Yu at the Quantum Science and Technology Group at JPL. His research at JPL focused on various applications of crystalline whispering gallery mode resonators, such as mid-IR ring-down spectroscopy and the usage as an optical parametric oscillator for the production of heralded single-photons. His most recent research at JPL this past summer was with Dr. Ivan Grudinin and Dr. Nan Yu and focused on studying the quality factor of single-mode whispering gallery mode resonators, a particular morphology of resonators that has applications in the creation of Kerr frequency combs. Publication of the work from this research experience is planning before the end of the year.