Abstract
Optical qubits are made of single photons. Due to photons’ excellent fidelity, we can expect that optical qubits or single photons will play a more important role in future quantum communication. As single photons are generated one by one, they appear randomly and it is difficult to utilize qubits in random timing. Therefore, the practical operation method is to generate a pair of single photons, i.e., biphoton. The first photon of a pair heralds the coming of the second photon of the same pair, which is the heralded single photon for a subsequent quantum operation. We employed a hot atomic vapor to generate biphotons. For the first time, the all-copropagation scheme was utilized in our system, resulting in an excellent phase match. The phase-match scheme enables us to make the linewidth of the single-photon wave packets as narrow as 290 kHz. This is the world record for the narrowest linewidth of single-mode single photons generated from room-temperature or hot media. We increased the spectral brightness of biphotons to 3.8×105 pairs/s/MHz. This spectral brightness is the best result to date among all kinds of media such as hot or cold atomic vapors, solid crystals, and integrated photonic chips. A biphoton source of a higher generation rate can produce information carriers faster for a higher bandwidth of information transmission. A quantum operation utilizing biphotons of a narrower linewidth can achieve a better efficiency. Hence, the generation rate per linewidth, i.e., spectral brightness, is an important figure of merit of a biphoton source, and strongly influences the success rate of quantum information transmission. This source can become a key component in the future quantum network.
This work was supported by Grants Nos. 111-2639-M-007-001-ASP, 112-2119-M-007-007, and 112-2112- M-007 -020 -MY3 of the National Science and Technology Council, Taiwan.