我們展示了一套高效率的頻率轉換系統，該系統基於冷卻的銣原子雲，並能在單光子等級下以弱相干光操作。此系統採用鑽石型能階結構，其中一個微弱的探測光場與兩個強光場（耦合光場與驅動光場）共同驅動四波混頻過程，將795 奈米的可見光探測光子轉換為1367 奈米的通訊波段訊號光子。所有光場皆精確重疊並共線排列，在此配置下，我們並未觀察到任何可測量的相位不匹配現象。

為了抑制錐形放大器產生的受激自發輻射光，並提升偵測路徑中的頻譜濾波效能，我們優化了光學濾波系統，使耦合光場與驅動光場的阻擋比分別達到約137 dB 與超過147 dB。

由於我們的理論模型假設輸入為單模連續波，因此無法直接套用至實驗中使用的100奈秒探測脈衝。為解決此差異，我們將輸入脈衝分解為連續波頻率分量的線性疊加，並根據各分量的光譜振幅進行加權計算，重建整體脈衝響應。此方法使得理論預測與實驗結果之間能夠精確比對。

在最終的量測中，我們在暗場自發力光學阱配置下進行實驗，並透過最小平方法進行分析，量測到77% 的轉換效率。本研究成果代表向實用化單光子頻率轉換邁出的重要一步，並顯示其在量子通訊與光子資訊處理等應用上具有高度潛力。

We demonstrate a high-efficiency frequency conversion system based on a cold rubidium atomic ensemble, operating at the single-photon level using weak coherent light. The system employs a diamond-type energy level configuration, in which a weak probe field and two strong fields, specifically a coupling field and a driving field, drive a four-wave mixing process that converts 795 nm visible probe photons into 1367 nm signal photons in the telecommunication band. All optical fields are precisely overlapped and aligned collinearly, resulting in no measurable phase mismatch in our configuration.

To suppress the amplified spontaneous emission generated by the tapered amplifier and to enhance the spectral filtering performance in the detection path, we optimized the optical filtering system, achieving blocking ratios of approximately 137 dB for the coupling field and over 147 dB for the driving field.

Since our theoretical model assumes a single-mode continuous-wave input, it cannot be directly applied to the 100 ns probe pulses used in the experiment. To address this discrepancy, we decomposed the input pulse into a linear superposition of continuous-wave frequency components, calculated the conversion efficiency for each component using the single-mode model, and reconstructed the expected pulse response by summing the weighted contributions. This approach allows for a quantitatively accurate comparison between theoretical predictions and experimental results.

In the final measurement, performed under a dark spontaneous-force optical trap configuration and analyzed using a least-squares fitting method, we measured a conversion efficiency of 77%. This result represents an important step toward practical single-photon frequency conversion and indicates strong potential for applications in quantum communication and photonic information processing.

[1] Richard P. Feynman. Simulating physics with computers. International Journal of Theoretical Physics, 21(6):467–488, 1982.
[2] David Deutsch and Richard Jozsa. Rapid solution of problems by quantum computation. Proceedings of the Royal Society of London A, 439(1907):553–558, 1992.
[3] Charles H. Bennett and Gilles Brassard. Quantum cryptography: Public key distribution and coin tossing. Theoretical Computer Science, 560:7–11, 2014. Theoretical Aspects of Quantum Cryptography–celebrating 30 years of BB84.
[4] Artur K. Ekert. Quantum cryptography based on bell’s theorem. Phys. Rev. Lett., 67:661–663, Aug 1991.
[5] Charles Ci Wen Lim, Christopher Portmann, Marco Tomamichel, Renato Renner, and Nicolas Gisin. Device-independent quantum key distribution with local bell test. Phys. Rev. X, 3:031006, Jul 2013.
[6] Morten Kjaergaard, Mollie E. Schwartz, Jochen Braumüller, Philip Krantz, Joel I.-J. Wang, Simon Gustavsson, and William D. Oliver. Superconducting qubits: Current state of play. Annual Review of Condensed Matter Physics, 11:369–395, 2020.
[7] Fulvio Flamini, Nicolò Spagnolo, and Fabio Sciarrino. Photonic quantum information processing: a review. Reports on Progress in Physics, 82(1):016001, nov 2018.
[8] Thomas Walker, Koichiro Miyanishi, Rikizo Ikuta, Hiroki Takahashi, Samir Vartabi Kashanian, Yoshiaki Tsujimoto, Kazuhiro Hayasaka, Takashi Yamamoto, Nobuyuki Imoto, and Matthias Keller. Long-distance single photon transmission from a trapped ion via quantum frequency conversion. Phys. Rev. Lett., 120:203601, May 2018.
[9] Simeon I. Bogdanov, Alexandra Boltasseva, and Vladimir M. Shalaev. Overcoming quantum decoherence with plasmonics. Science, 364(6440):532–533, 2019.
[10] M. Körber, O. Morin, S. Langenfeld, A. Neuzner, S. Ritter, and G. Rempe. Decoherence-protected memory for a single-photon qubit. Nature Photonics, 12(1):18–21, 2018.
[11] Andreas Reiserer, Norbert Kalb, Gerhard Rempe, and Stephan Ritter. A quantum gate between a flying optical photon and a single trapped atom. Nature, 508(7495):237–240, 2014.
[12] Thomas Stolz, Hendrik Hegels, Maximilian Winter, Bianca Röhr, Ya-Fen Hsiao, Lukas Husel, Gerhard Rempe, and Stephan Dürr. Quantum-logic gate between two optical photons with an average efficiency above 40%. Phys. Rev. X, 12:021035, May 2022.
[13] Meng Li, Chu Li, Yang Chen, Lan-Tian Feng, Linyu Yan, Qian Zhang, Jueming Bao, Bi-Heng Liu, Xi-Feng Ren, Jianwei Wang, Shufeng Wang, Yunan Gao, Xiaoyong Hu, Qihuang Gong, and Yan Li. On-chip path encoded photonic quantum toffoli gate. Photon. Res., 10(7):1533–1542, Jul 2022.
[14] S. Tanzilli, W. Tittel, M. Halder, O. Alibart, P. Baldi, N. Gisin, and H. Zbinden. A photonic quantum information interface. Nature, 437(7055):116–120, 2005.
[15] Nicolas Maring, Dario Lago-Rivera, Andreas Lenhard, Georg Heinze, and Hugues de Riedmatten. Quantum frequency conversion of memory-compatible single photons from 606 nm to the telecom c-band. Optica, 5(5):507–513, 2018.
[16] Helge Rütz, Kai-Hong Luo, Hubertus Suche, and Christine Silberhorn. Quantum frequency conversion between infrared and ultraviolet. Phys. Rev. Appl., 7:024021, Feb2017.
[17] H. J. McGuinness, M. G. Raymer, C. J. McKinstrie, and S. Radic. Quantum frequency translation of single-photon states in a photonic crystal fiber. Phys. Rev. Lett., 105:093604, Aug 2010.
[18] Po-Han Tseng, Ling-Chun Chen, Jiun-Shiuan Shiu, and Yong-Fan Chen. Quantum interface for telecom frequency conversion based on diamond-type atomic ensembles. Phys. Rev. A, 109:043716, Apr 2024.
[19] R. T. Willis, F. E. Becerra, L. A. Orozco, and S. L. Rolston. Four-wave mixing in the diamond configuration in an atomic vapor. Phys. Rev. A, 79:033814, Mar 2009.
[20] A. G. Radnaev, Y. O. Dudin, R. Zhao, H. H. Jen, S. D. Jenkins, A. Kuzmich, and T. A. B. Kennedy. A quantum memory with telecom-wavelength conversion. Nature Physics, 6(11):894–899, 2010.
[21] Ling-Chun Chen, Meng-Yi Lin, Jiun-Shiuan Shiu, Xuan-Qing Zhong, Po-Han Tseng, and Yong-Fan Chen. High-efficiency telecom frequency conversion via a diamond-type atomic ensemble. Phys. Rev. A, 112:013709, Jul 2025.
[22] M. S. Safronova and U. I. Safronova. Critically evaluated theoretical energies, lifetimes, hyperfine constants, and multipole polarizabilities in 87Rb. Phys. Rev. A, 83:052508, May 2011.
[23] Daniel A. Steck. Rubidium 87 D Line Data. http://steck.us/alkalidata, May 2024.