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Quantum key distributions, which could make legitimate communication parties Alice and Bob achieve the same random key with unconditional security, will have broad applications in defense, commerce, and communication. The protocol of the continuous variable quantum key distribution (CVQKD) has many advantages, such as easy preparation of the light source, high detector efficiency, and good compatibility with the classic fiber-optic communication systems. In recent years, great progress in the research of CVQKD has been made both theoretically and experimentally. In the protocol, the quadratures of the optical field with Gaussian or Non-Gaussian modulation are employed as the carriers of the key.The quadratures of the pulsed optical quantum states in CVQKD can be detected with a time-domain pulsed homodyne detector. The performance of the detector has great influences on the excess noises and the safe key rate of the quantum communication system. The measurement accuracy, which depends crucially on the common mode rejection ratio and the long-term stability, is the key performance of the detector. In order to improve the accuracy of measurement and avoid saturating the detector, we propose and demonstrate a technique to balance the two output beams of a 50/50 fiber coupler of the homodyne detector automatically. The auto-balance technique, which improves the long-term stability and high common mode rejection ratio, is described in the following.Firstly, the relation between the balance degree and the measurement accuracy is theoretically analyzed in detail. The result shows that a balance degree larger than 10-4 should be reached to ensure a high precision measurement when the intensity of the local oscillator pulse is 108 photons per pulse. Secondly, a fiber-based variable attenuator based on computer-controlled linear stepper motor is designed. The linear stepper motor that is used to drive the fiber coils has a small dimension of 20 cm20 cm28 cm and a minimum step size of 78 nm, and is controlled through the I/O port of a multifunction data acquisition card connected to a computer. The attenuations of the fiber coils of different radii are detected. The precision of attenuation is estimated to be on the order of 10-6 per 100 nm.The principle of the feedback control is described. A method of changing step-size which depends on the balance degree is proposed to fulfill a fast auto-balance process. Using the auto-feedback-control system, a balance degree of about 1.5610-6 can be achieved. The procedure of auto-balance takes about 1 s, and the evolution curves that represent the transformation process from various unbalanced states to the balanced state are presented.The auto-balance apparatus can ensure that the time-domain pulsed homodyne detector run stably in a longterm with a high common mode rejection ratio. The nonlinear and saturation effects due to the drift of the balance point are eliminated. The presented auto-balance time-domain pulsed homodyne detector can be well integrated into the continuous variable quantum key distribution system, and is expected to play an important role in improving the measurement accuracy and reducing the excess noises of the system. We believe that it could also be found to have potential applications in other areas.
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Keywords:
- continuous variable quantum key distribution /
- time-domain homodyne detector /
- auto-balancing control
[1] Lo H K, Curty M, Tamaki K 2014 Nat. Photonics 8 595
[2] Gisin N, Ribordy G, Tittle W, Zbinden H 2002 Rev. Mod. Phys. 74 145
[3] Scarani V, Pasquinucci H B, Cerf N J, Dusek M, Lutkenhaus N, Peev Momtchil 2009 Rev. Mod. Phys. 81 1301
[4] Weedbrook C, Pirandola S, Patron R G, Cerf N J, Ralph T C, Shapiro J H, Lloyd S 2012 Rev. Mod. Phys. 84 621
[5] Grosshans F, Grangier P 2002 Phys. Rev. Lett. 88 057902
[6] Grosshans F, Grangier P 2002 Proceeding of the 6th International Conference on Quantum Communication, Measurement and Computing MIT, Cambridge, MA, July 22-26, 2002 p351
[7] Grosshans F, Assche G V, Wenger J, Brouri R, Cerf N J, Grangier P 2003 Nature 421 238
[8] Lodewyck J, Debuisschert T, Brouri R T, Grangier P 2005 Phys. Rev. A 72 050303
[9] Lodewyck J, Bloch M, Patron R G, Fossier S, Karpov E, Diamanti E, Debuisschert T, Cerf R G, Brouri R T, Mclaughlin S W, Grangier P 2007 Phys. Rev. A 76 042305
[10] Qi B, Huang L L, Qian L, Lo H K 2007 Phys. Rev. A 76 052323
[11] Fossier S, Diamanti E, Debuisschert, Villing A, Brouri R T, Grangier P 2009 New J. Phys. 11 045023
[12] Xuan Q D, Zhang Z S, Voss P L 2009 Opt. Express 17 24244
[13] Wang X Y, Bai Z L, Wang S F, Li Y M, Peng K C 2013 Chin. Phys. Lett. 30 010305
[14] Jouguet P, Jacques S K, Leverrier A, Grangier P, Diamanti E 2013 Nat. Photonics 7 379
[15] Wang S F, Wang X Y, Bai Z L, Li Y M 2014 Acta Sin. Quan. Opt. 2 167 (in Chinese) [王少锋, 王旭阳, 白增亮, 李永民 2014 量子光学学报 2 167]
[16] Smithey D T, Beck M, Raymer M G 1993 Phys. Rev. Lett. 70 1244
[17] Breitenbach G, Schiller S, Mlynek J 1997 Nature 387 471
[18] Zavatta A, Viciani S, Bellini M 2006 Laser Phys. Lett. 3 3
[19] Lvovsky A I, Raymer M G 2009 Rev. Mod. Phys. 81 299
[20] Hansen H, Aichele T, Hettich C, Lodahl P, Lvovsky A I, Mlynek J, Schiller S 2001 Opt. Lett. 26 1714
[21] Chi Y M, Qi B, Zhu W, Qian L, Lo H K, Youn S H, Lvovsky A I, Tian L 2011 New J. Phys. 13 013003
[22] Wang X Y, Bai Z L, Du P Y, Li Y M, Peng K C 2012 Chin. Phys. Lett. 29 124202
[23] Jin X L, Su J, Zheng Y H, Chen C Y, Wang W Z, Peng K C 2015 Opt. Express 23 023859
[24] Wang X Y 2013 Ph. D. Dissertation (Taiyuan: Shanxi University) (in Chinese) [王旭阳 2013 博士学位论文(太原: 山西大学)]
[25] Ma R L 2006 Quantum Cryptography Communication (Beijing: Science Press) p125 (in Chinese) [马瑞霖 2006 量子密码通信(北京: 科学出版社) 第125页]
[26] Chen J J, Hang Z P, Zhao Y B, Gui Y Z, Guo G C 2007 Acta Phys. Sin. 56 0005 (in Chinese) [陈进建, 韩正甫, 赵义博, 桂有珍, 郭光灿 2007 56 0005]
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[1] Lo H K, Curty M, Tamaki K 2014 Nat. Photonics 8 595
[2] Gisin N, Ribordy G, Tittle W, Zbinden H 2002 Rev. Mod. Phys. 74 145
[3] Scarani V, Pasquinucci H B, Cerf N J, Dusek M, Lutkenhaus N, Peev Momtchil 2009 Rev. Mod. Phys. 81 1301
[4] Weedbrook C, Pirandola S, Patron R G, Cerf N J, Ralph T C, Shapiro J H, Lloyd S 2012 Rev. Mod. Phys. 84 621
[5] Grosshans F, Grangier P 2002 Phys. Rev. Lett. 88 057902
[6] Grosshans F, Grangier P 2002 Proceeding of the 6th International Conference on Quantum Communication, Measurement and Computing MIT, Cambridge, MA, July 22-26, 2002 p351
[7] Grosshans F, Assche G V, Wenger J, Brouri R, Cerf N J, Grangier P 2003 Nature 421 238
[8] Lodewyck J, Debuisschert T, Brouri R T, Grangier P 2005 Phys. Rev. A 72 050303
[9] Lodewyck J, Bloch M, Patron R G, Fossier S, Karpov E, Diamanti E, Debuisschert T, Cerf R G, Brouri R T, Mclaughlin S W, Grangier P 2007 Phys. Rev. A 76 042305
[10] Qi B, Huang L L, Qian L, Lo H K 2007 Phys. Rev. A 76 052323
[11] Fossier S, Diamanti E, Debuisschert, Villing A, Brouri R T, Grangier P 2009 New J. Phys. 11 045023
[12] Xuan Q D, Zhang Z S, Voss P L 2009 Opt. Express 17 24244
[13] Wang X Y, Bai Z L, Wang S F, Li Y M, Peng K C 2013 Chin. Phys. Lett. 30 010305
[14] Jouguet P, Jacques S K, Leverrier A, Grangier P, Diamanti E 2013 Nat. Photonics 7 379
[15] Wang S F, Wang X Y, Bai Z L, Li Y M 2014 Acta Sin. Quan. Opt. 2 167 (in Chinese) [王少锋, 王旭阳, 白增亮, 李永民 2014 量子光学学报 2 167]
[16] Smithey D T, Beck M, Raymer M G 1993 Phys. Rev. Lett. 70 1244
[17] Breitenbach G, Schiller S, Mlynek J 1997 Nature 387 471
[18] Zavatta A, Viciani S, Bellini M 2006 Laser Phys. Lett. 3 3
[19] Lvovsky A I, Raymer M G 2009 Rev. Mod. Phys. 81 299
[20] Hansen H, Aichele T, Hettich C, Lodahl P, Lvovsky A I, Mlynek J, Schiller S 2001 Opt. Lett. 26 1714
[21] Chi Y M, Qi B, Zhu W, Qian L, Lo H K, Youn S H, Lvovsky A I, Tian L 2011 New J. Phys. 13 013003
[22] Wang X Y, Bai Z L, Du P Y, Li Y M, Peng K C 2012 Chin. Phys. Lett. 29 124202
[23] Jin X L, Su J, Zheng Y H, Chen C Y, Wang W Z, Peng K C 2015 Opt. Express 23 023859
[24] Wang X Y 2013 Ph. D. Dissertation (Taiyuan: Shanxi University) (in Chinese) [王旭阳 2013 博士学位论文(太原: 山西大学)]
[25] Ma R L 2006 Quantum Cryptography Communication (Beijing: Science Press) p125 (in Chinese) [马瑞霖 2006 量子密码通信(北京: 科学出版社) 第125页]
[26] Chen J J, Hang Z P, Zhao Y B, Gui Y Z, Guo G C 2007 Acta Phys. Sin. 56 0005 (in Chinese) [陈进建, 韩正甫, 赵义博, 桂有珍, 郭光灿 2007 56 0005]
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