High-dimensional entanglement-based quantum key distribution

Abstract

Conventional quantum key distribution (QKD) uses a discrete two-dimensional Hilbert space for key encoding, such as the polarization state of a single photon. In contrast, high-dimensional QKD allows encoding onto a larger state space, such as multiple levels of a continuous variable of a single photon, thus enabling the system to achieve higher photon information efficiency (bits/photon) and potentially higher key rate (bits/second). However, its deployment requires high-performance source, detector, and routing technologies tailored to the specific large-alphabet encoding scheme. One such high-dimensional QKD system of interest is based on time-energy entanglement, in which keys are derived from the arrival times of photon pairs generated from continuous-wave (CW) spontaneous parametric downconversion (SPDC). This thesis focuses on the implementation of a time-energy entanglement-based QKD system, with the development of several enabling technologies including an efficient single-spatial-mode source of time-energy entangled photons based on a periodically-poled KTiOPO4 (PPKTP) waveguide, GHz self-differencing InGaAs singlephoton avalanche diodes (SPADs), and the first demonstration of non-locally dispersion-canceled Franson quantum interferometry achieving 99.6% visibility. We then utilize these technologies to perform two full QKD protocols. The first protocol uses SPDCgenerated entangled photons for both key extraction and Franson interferometry, yielding a secure key rate -90 kbits/s with up to 4 bits/photon after error-correction and privacy amplification. The second protocol deploys two different photon sources: an amplified spontaneous emission (ASE) source is pulse-position modulated to perform random key generation, and a CW-SPDC source is for Franson security check. In this latter case, we have achieved a secure key rate 7.3 Mbits/s with 2.9 bits/photon, which represents the state-of-the-art in today's QKD technology.