單光子製造與偵測是量子資訊領域的重要研究課題。我們的實驗是利用室溫銣原子蒸氣室(vapor cell)，產生有時間關聯（time-correlated）的一對單光子（biphoton）。室溫原子的系統比冷原子系統簡單許多，在實際應用上，可行性比較高，且我們的系統使用連續波（continuous-wave）雷射，可以不中斷地產生單光子，而這些單光子可以攜帶量子資訊（quantum information），得廣泛應用於量子通訊及量子計算。
我們以四波混頻（Four wave mixing）的非線性光學機制來產生一對單光子，實驗上我們打兩道強光，分別稱作幫浦場(pump field)及耦合場(coupling field)，原子以自發輻射的方式產生兩顆單光子，分別稱作訊號(signal)光子及探測(probe)光子。由於幫浦場、耦合場、訊號光子及探測光子在空間中重合，且行進方向相同，兩道強光的功率約為每秒2×〖10〗^16顆光子，從這兩道強光中取得微弱的單光子訊號是一件困難的事情，因此降低背景雜訊是非常重要的工作。訊號光子及探測光子經損耗即消滅，降低產生率。不損耗單光子且可以去除強光造成的背景雜訊是此論文的研究課題。我們利用雙折射率晶體的偏振器(birefringent crystal polarizer)和濾波器(etalon)來達到最佳訊雜比，目前實驗系統可使得背景雜訊降低到每秒一萬顆光子左右，目標是每秒1,000顆光子以下。

Generating and detecting single photons are important research topics in the field of quantum information. We carry out the experiment in an atomic vapor cell at the room temperature, filled with rubidium atoms consisting of only one stable isotope Rb-87. Our goal is to produce a pair of time-correlated single photons, called biphotons, for the experiments of quantum information manipulation in our lab. Atomic systems at room temperature are much simpler than those at low temperature. In practical applications, these systems have relatively high feasibility. Furthermore, our system adopts a continuous wave (continuous-wave) laser, which can produce single photons continuously. These single photons can carry quantum information. And this technique is widely used in quantum communication and quantum computation.
We use four-wave mixing, a nonlinear optical mechanism, to generate a single photon. Experimentally, two strong lights are shined into the rubidium vapor chamber, which are called pump field and coupling field. At the same time, the system spontaneously radiates two single photons, which are called signal and probe fields. The four lights mentioned above are overlapped in space and travel in the same direction. Due to the high powers of the pump field and the coupling field (both are about 2×〖10〗^16count/s), extracting the signal of single photons is extremely difficult. Therefore, reducing the background (pump and coupling) noises is a very important work. If signal and probe photons loss that is eliminated, reducing the production rate. This thesis aims to discuss how to get rid of the large noises of the strong lights without eliminating the single photons. We use birefringent crystal polarizers and etalon filters to achieve the best signal-noise ratio. Currently, we can reduce the background noise to about ten thousand photons per second. Our goal is to lower this value to around one thousand photons per second.

[1] Shu, Chi, et al. "Subnatural-linewidth biphotons from a Doppler-broadened
hot atomic vapor cell." arXiv preprint arXiv:1602.08561 (2016).
[2] Y.-q. Li, and M. Xiao, “Enhancement of nondegenerate four-wave mixing based on electromagnetically induced transparency in rubidium atoms,” Optics Letters, vol. 21, no. 14, pp. 1064-1066, 1996/07/15, 1996
[3] Lee, M. J., Chen, Y. H., Wang, I. C., & Ite, A. Y. (2012). EIT-based all-optical switching and cross-phase modulation under the influence of four-wave mixing.Optics express, 20(10), 11057-11063.
[4] P. Palittapongarnpim, A. MacRae, and A. I. Lvovsky, “Note: A monolithic filter cavity for experiments in quantum optics,” Review of Scientific Instruments, vol. 83, no. 6, pp. 066101, 2012.
[5] 陳應誠, and 余怡德, “光速減慢至每秒600公尺－原子的電磁波引發透明效應,” 物理雙月刊, vol. 23, no. 5, pp. 572-578, 2001.
[6]http://www.toptica.com/products/laser_diodes/fabry_perot_laser_diodes.html
[7] Eugene Hecht, “Interference” in Optics 4th ed.(Addison-Wesley, 2002), pp.385-438.
[8] F. R. Connor, Wave Transmission, pp.52-53, London: Edward Arnold 1971 ISBN 0-7131-3278-7
[9] U.S. Navy-Marine Corps Military Auxiliary Radio System (MARS), NAVMARCORMARS Operator Course, Chapter 1, Waveguide Theory and Application, Figure 1-38.—Various modes of operation for rectangular and circular waveguides.