Fang-Xiang Wang

Entanglement based quantum communication offers an increased level of security in practical secret shared key distribution. One of the fundamental principles enabling this security -- the fact that interfering with one photon will destroy entanglement and thus be detectable -- is also the greatest obstacle. Random encounters of traveling photons, losses and technical imperfections make noise an inevitable part of any quantum communication scheme, severely limiting distance, key rate and environmental conditions in which QKD can be employed. Using photons entangled in their spatial degree of freedom, we show that the increased noise resistance of high-dimensional entanglement, can indeed be harnessed for practical key distribution schemes. We perform quantum key distribution in eight entangled paths at various levels of environmental noise and show key rates that, even after error correction and privacy amplification, still exceed 1 bit per photon pair and furthermore certify a secure key at noise levels that would prohibit comparable qubit based schemes from working.

Hong-Ou-Mandel (HOM) interference is the foundation of quantum optics to test the degree of indistinguishability of two incoming photons, playing a key role in quantum communication, sensing, and photonic quantum computing. Realizing high-visibility HOM interference with massively parallel optical channels is challenging due to the lack of available natural optical references for aligning independent arrayed laser pairs. Here, we demonstrate 50 parallel comb-teeth pairs of continuous-wave weak coherent photons HOM interference using two independently frequency post-aligned soliton microcombs (SMCs), achieving an average fringe visibility over 46%. The frequencies of all comb-teeth pairs are long-term aligned by developing two sets of fully frequency-stabilized SMCs with independent reference and adjusting free spectral range beyond 100 kilohertz through perturbations in soliton state. The verification experiment proves the feasibility of constructing massively quantum information channels by coopting classical wavelength division multiplexing measurement-device-independent quantum key distribution (MDI-QKD), which paves the way for practical large-scale quantum communication systems.