Generation and application of narrow-band biphotons

Abstract

Manipulating photons at quantum level lays the foundation of quantum information and quantum optics field. Within spontaneous four-wave mixing (SFWM) process in cold atom ensembles, we could produce narrow-band Stokes (ω_s) and anti-Stokes (ω_as) paired photons (also called biphotons). As detecting a Stokes photon heralds the generation of its paired anti-Stokes photon, the biphoton source is also used as a good heralded single photon source. In this PhD thesis research work, we use two types of cold ⁸⁵Rb atom ensembles: 3-dimensional magneto-optical trap and 2-dimensional dark-line magneto-optical trap (MOT). With the 3-dimensional MOT, narrow-band frequency-tunable entangled biphotons are produced. We observe coalescence interference for both degenerate and nondegenerate photons, and find that the path-exchange symmetry plays a more important role in the Hong-Ou-Mandel interference than the temporal or frequency indistinguishability. As a basic method for entangling independent photons, two-photon interference with different colors is of crucial importance for quantum information and quantum computing. Our results show the potential applications in linear optical quantum information processing involving photons with different colors. The long coherence time (∼ 1μs) of heralded single photons generated from 2-dimensional dark-line MOT allows us to directly modulate its waveform with arbitrary phase pattern. Based on this technique, we demonstrate the first proof of principle differential phase shift quantum key distribution using heralded single photons. We obtain a quantum bit error rate as low as 3.06%, which meets the unconditional security requirement and indicates that narrow-band single photons maybe a promising source for the DPS-QKD protocol. When single-photon waveform is modulated as an exponential growth, we also experimentally demonstrate that a single photon can be almost completely loaded into a Fabry-Perot cavity. Due to a destructive interference between the reflected packet and the transmitted packet after each round trip inside the cavity, a loading efficiency of (87.2±2.3)% is observed with optimized waveform parameters. Our result and approach may enable promising applications in realizing large-scale quantum networks.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology