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Long-distance entanglement distribution is an important task for quantum communication, but difficult to achieve due to the loss of photons in optical fiber transmission. Quantum repeater is a scheme to solve this problem. In this scheme, the long distance of entanglement distribution is divided into several small parts, the entanglement is established first at both ends of each part, then, the entanglement distance is extended through the entanglement exchange of adjacent interval parts, in order to achieve the long distance entanglement distribution. Of them, the Duan-Lukin-Cirac-Zoller (DLCZ) protocol based on the cold atom ensemble and the linear optics which can generate and store entanglement, is regarded as one of the most potential schemes. In the process of DLCZ, retrieval efficiency is an important index of the quantum repeater, because it will influence each entanglement exchange operation between adjacent quantum repeater nodes. Generally, the retrieval efficiency is improved by optimizing the reading pulse, increasing the optical depth (OD) of the atomic ensemble and the cavity enhancement. The ring cavity constrains the light field to increase the intensity of the interaction between light and atoms, and effectively improve the retrieval efficiency of the quantum memory. In this work, atomic ensembles are placed in a ring cavity. The cavity length is 3.3 m and the fineness is 13.5. The optical loss of all ring cavity is 21%, mainly including 15% loss of other optical elements and 6% loss of the cell. In order to increase the retrieval efficiency, we need to ensure the mode resonance of read-out photon, write-out photon and locking. The cavity needs two input beams of light: one comes from the path of read-out photon and the other from the path of write-out photon in the reverse direction. The two beams are locked at the same frequency as the write-out photon and the read-out photon respectively. The cavity length is adjusted by moving the cavity mirrors’ positions through translating the frame, to make two light modes resonate. The acousto-optic modulator (AOM) is inserted into the path of the locking to control the frequency of the locking. By adjusting the AOM to change the frequency of the locking, the locking can be coincident with the write-out and read-out cavity modes. Then, the three-mode resonance can be achieved When the cavity mode resonates with the atomic line, it will lead the atomic formants to split. thereby affecting the enhancement effect of retrieval efficiency. In the experiment, the detuning of the read light will affect the frequency of the read-out photon, and further affect the detuning of the cavity mode with the resonance line of the atom. Thus, by increasing the detuning between the reading light and the atomic transition line, the frequency splitting between the two modes can be reduced, then enhance the retrieval efficiency. We study the relation between the enhancement factor of the retrieval efficiency and the detuning amount of the reading light relative to the atomic resonance line. The results show that when the detuning amount of reading light is 80 MHz, the intrinsic readout efficiency is 45%, and the readout efficiency is enhanced by 1.68 times. -
Keywords:
- quantum memory /
- retrieval efficiency /
- optical ring cavity
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图 1 (a)
$ {}^{87}{\text{Rb}} $ 原子能级. 其中左图为写过程,$ {\sigma ^+}\left( {{\sigma ^-}} \right) $ 分别代表左(右)旋圆偏振的斯托克斯光, W代表写光光场. 右图为读过程,$ {\sigma ^+}\left( {{\sigma ^-}} \right) $ 代表左(右)旋圆偏振的反斯托克斯光, R代表读光光场;$\varDelta$ 代表读光和写光相对于原子共振跃迁线的失谐; (b) 实验时序图, 图中Cleaning为态制备过程, Writing代表写过程, Reading代表读过程, Locking表示腔锁定时序, MOT代表冷原子俘获过程Fig. 1. (a) Relevant
$ {}^{87}{\text{Rb}} $ atomic levels. The left is writing process,$ {\sigma ^+}\left( {{\sigma ^-}} \right) $ represents left (right) polarization of Stokes, W represents writing field. The right is reading process,$ {\sigma ^+}\left( {{\sigma ^-}} \right) $ represents left (right) polarization of anti-Stokes, R represents reading field;$\varDelta$ denotes the detuning of the reading and writing laser relative to the resonance transition; (b) time sequence of experimental cycle, Cleaning: the state cleaning process, Write: the writing process, Reading: the writing process, Locking: the locking cavity process, MOT: the cold atom preparation process.
图 2 实验装置示意图. 其中PZT代表压电陶瓷; BS为耦合镜; SPD1(SPD2)表示读接收(写接收)单光子探测器; Locking为锁腔光; Flipper为可折叠式镜架;
$ {\lambda / 2} $ 和$ {\lambda / 4} $ 分别为半玻片和四分之一玻片Fig. 2. Experimental setup. PZT represents the piezoelectric ceramic transducer; BS, coupling mirror; SPD1(SPD2), read receive (write receive) single photon detector; Locking, the lock cavity light;
$ {\lambda / 2} $ and$ {\lambda / 4} $ , half wave plate and quarter wave plate.
图 3 读出效率的增强倍数和读出效率随着读光失谐量的变化
Fig. 3. The variation of enhancement factor of retrieval efficiency and retrieval efficiency with the detuning of reading laser.
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[1] Sangouard N, Simon C, de Riedmatten H, Gisin N 2011 Rev. Mod. Phys. 83 33
Google Scholar
[2] Simon C 2017 Nat. Photonics 11 678
Google Scholar
[3] Bussières F, Sangouard N, Afzelius M, de Riedmatten H, Simon C, Tittel W 2013 J. Mod. Opt. 60 1519
Google Scholar
[4] Inagaki T, Matsuda N, Tadanaga O, Asobe M, Takesue H 2013 Opt. Express 21 23241
Google Scholar
[5] Korzh B, Lim C C W, Houlmann R, Gisin N, Li M J, Nolan D, Sanguinetti B, Thew R, Zbinden H 2015 Nat. Photonics 9 163
Google Scholar
[6] Chen G H, Wang H C, Chen Z F 2015 Front. Phys. 10 1
Google Scholar
[7] Chrapkiewicz R, Wasilewski W 2012 Opt. Express 20 29540
Google Scholar
[8] Briegel H J, Dur W, Cirac J I, Zoller P 1998 Phys. Rev. Lett 81 5932
Google Scholar
[9] Gisin N 2015 Front. Phys. 10 100307
Google Scholar
[10] Reiserer A, Rempe G 2015 Rev. Mod. Phys. 87 1379
Google Scholar
[11] Volz J, Weber M, Schlenk D, Rosenfeld W, Vrana J, Saucke K, Kurtsiefer C, Weinfurter H 2006 Phys. Rev. Lett. 96 030404
Google Scholar
[12] Duan L M, Monroe C 2010 Rev. Mod. Phys. 82 1209
Google Scholar
[13] Gao W B, Imamoglu A, Bernien H, Hanson R 2015 Nat. Photonics 9 363
Google Scholar
[14] Clausen C, Usmani I, Bussieres F, Sangouard N, Afzelius M, de Riedmatten H, Gisin N 2011 Nature 469 508
Google Scholar
[15] Saglamyurek E, Sinclair N, Jin J, Slater J A, Oblak D, Bussieres F, George M, Ricken R, Sohler W, Tittel W 2011 Nature 469 512
Google Scholar
[16] Lo Piparo N, Razavi M 2013 Phys. Rev. A 88 012332
Google Scholar
[17] Duan L M, Lukin M D, Cirac J I, Zoller P 2001 Nature 414 413
Google Scholar
[18] Novikova I, Phillips N B, Gorshkov A V 2008 Phys. Rev. A 78 021802(R)
[19] Bao X H, Reingruber A, Dietrich P, Rui J, Dück A, Strassel T, Li L, Liu N L, Zhao B, Pan J W 2012 Nat. Phys. 8 517
Google Scholar
[20] Zhang S, Chen J F, Liu C, Zhou S, Loy M M, Wong G K, Du S 2012 Rev. Sci. Instrum. 83 073102
Google Scholar
[21] Yang S J, Wang X J, Li J, Rui J, Bao X H, Pan J W 2015 Phys. Rev. Lett. 114 210501
Google Scholar
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