Narrowband Biphotons: Generation, Manipulation, and Applications

Entangled paired photons (biphotons) have become a benchmark tool not only for understanding fundamental quantum physics but also for realizing practical quantum applications. In this talk, I will review our recently developed narrow-band biphoton source from spontaneous four-wave mixing in a cold atomic ensemble. The long temporal coherence time (up to 2000 s) of these photons allows modulating their quantum waveform in time domain directly. These photons can be further hyperentangled in time-frequency, polarization, and space-momentum. For the first time, using modulated single photons, we observed single-photon optical precursors and verified the information speed limit of a single photon. We demonstrated two-photon quantum interference with nondegenerate paired photons, the efficient photon-atom quantum interface with a single-photon storage efficiency of 50%, the coherent control of single-photon absorption and reemission in atomic ensemble, the application in differential-phase-shift quantum key distribution, and loading a single photon into a single-sided Fabry-Perot cavity with an efficiency of 87%. We developed a quantum-state tomography for measuring the biphoton temporal wave function. Most recently, we produced biphotons with polarization-frequency-coupled hyperentanglement, and demonstrated biphoton shaping with parallel-to-series conversion. The works were supported by the Hong Kong Research Grants Council (Project Nos. 601113, 16301214, and HKU8/CRF/11G).