Evaluation of Thin-film Lithium-niobate Devices for Quantum Networks

Elkus, Bradley Scott

doi:https://doi.org/10.21985/n2-8fxd-df80

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Evaluation of Thin-film Lithium-niobate Devices for Quantum Networks

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Future large-scale quantum networks will likely require more efficient on-chip devices for quantum key distribution (QKD) systems that can exploit CMOS manufacturing techniques. QKD systems on chip have been produced in a number of material platforms including silicon, indium phosphide, and a range of hybrid structures. Advanced industrial silicon-electronics manufacturing can be exploited to create a higher density of devices per chip in thin-film lithium niobate (TFLN) bonded to Si/SiO2 chips. These TFLN devices see superior modulation performance compared to lithium niobate devices made from traditional proton-exchange manufacturing techniques, and state-of-the art silicon devices. Furthermore, the enhanced nonlinear conversion efficiency in quasi-phase matched TFLN structures promise better quantum frequency conversion interfaces for quantum memories, and higher generation rates of entangled photons through spontaneous parametric down-conversion. This thesis will present some of our progress in evaluating periodically-poled TFLN devices as sources of entangled photon-pairs for future QKD systems. We look at quantum-correlated photon-pair generation rates, and the noise process at play for differing lengths of periodically-poled TFLN structures that show promise in future QKD systems. This new platform of periodically-poled lithium niobate (PPLN) waveguides has larger small-signal normalized conversion efficiency (>4000%/Wcm^2) than standard state-of-the art reverse proton exchange buried PPLN waveguides (150%/Wcm^2).Utilizing various short lengths of these periodically-poled TFLN devices from the Fathpour group at the Center for Research and Education in Optics and Lasers (CREOL), we can investigate quantum-correlated photon-pair generation rates from spontaneous parametric down conversion (SPDC) by constructing and running test setups composed of off-the-shelf photonic components and high-efficiency superconducting nanowire single photon detectors (SNSPDs). The large phase-matching bandwidth in our shortest devices (300 microns long) are used to generate quantum-photon correlations between channels separated by up to 140 nm --the system had high coincidence-to accidental (CAR) count ratios of orders 10^3. Such a structure requires under a microwatt of pump power, but was pumped directly at the second harmonic wavelength(772.2 nm). Long-distance telecom systems are optimized primarily in the optical C-band where most telecom equipment is designed to operate. Thus pumping away from 775 nm, and into the optical C-band at 1550 nm would be more preferential. We can use an optical C- or L-band pump to seed a cascaded interaction in a single waveguide with higher %/W conversion efficiency that can be found in longer waveguides. In a cascaded interaction, the pump for the SPDC process is generated internally within the same waveguide by means of second harmonic generation (SHG); this is demonstrated in the longer 4- and 8-mm long TFLN devices. In these longer devices, we measure photon-pair correlations in signal-idler channels separated by 40 nm, but find lower CARs between 10 and 40. The CAR is found to be limited by Stokes/anti-Stokes Raman scattering generated primarily in the waveguide. A Raman peak of photon counts at 250 cm^{-1} from the fundamental-pump wavenumber suggest this noise originate from the lithium niobate. Whether or not there is a significant Raman contribution from the SiO2 is unclear from our measurements. This noise can be circumvented by changes in both the device design and system measurement. This detected noise is not anticipated to hinder future entanglement measurements of quantum-correlated photon pairs.

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