CN104330938B - 基于光学超晶格和波导光路的量子光源芯片 - Google Patents

基于光学超晶格和波导光路的量子光源芯片 Download PDF

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CN104330938B
CN104330938B CN201410551173.2A CN201410551173A CN104330938B CN 104330938 B CN104330938 B CN 104330938B CN 201410551173 A CN201410551173 A CN 201410551173A CN 104330938 B CN104330938 B CN 104330938B
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徐平
金华
祝世宁
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Nanjing University
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    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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Abstract

一种基于光学超晶格波导的量子光源芯片的设置方法，采用波导光路和光学超晶格、电光调制器的集成，波导光路通过波导分束器将进入的经典抽运激光分束，分束后的激光进入光学超晶格区域进行频率下转到得到纠缠光子对，纠缠光子对随后继续进入干涉仪进行量子干涉；干涉仪的相位由芯片上内置的电光调制器来控制，通过电压调节得到几种不同的量子态。

Description

基于光学超晶格和波导光路的量子光源芯片
技术领域
本发明涉及量子信息技术、光电子技术和非线性光学领域，尤其是用集成光学技术和思路来实现芯片化的量子光源。
背景技术
纠缠是量子通信、量子计算等量子信息技术中的核心资源。所以如何制备纠缠光源，特别是可调控、高效、稳定、便携的纠缠光源一直是量子信息领域的研究难点和热点。历史上产生纠缠光子对的方法有：(1)原子级联跃迁[1]；(2)原子系统中的四波混频过程[2]；(3)硅基[3,4]或光纤中[5,6]的四波混频过程；(4)二阶非线性晶体中的光学参量下转换过程[7,8]。其中原子级联跃迁过程在量子光学实验初期被采用，后来因为其产生的态不够理想而被放弃。现在人们多采用后面三种方案，而其中基于非线性晶体的二阶参量过程由于其产率高、装置简单成当前产生纠缠光子对最普遍的方法。在二阶非线性晶体中，一个高频抽运光子会劈裂为一对低频的下转换光子，分别称为信号光子和闲置光子，也称纠缠光子对。该过程需要满足能量守恒和动量守恒条件。按照动量守恒的实现条件来分，产生纠缠光子对的非线性晶体分为两类，一类是均匀的双折射晶体，另一种是光学超晶格。光学超晶格就是铌酸锂、钽酸锂等畴结构受到调制的人工非线性晶体，因为可以利用它的较大非线性系数，所以它可以产生高亮度的纠缠光子对[9-12]。特别是，加工成波导之后可以进一步提高产生效率[13,14]。
但是无论是双折射晶体、光学超晶格体块材料还是波导结构，一块晶体往往只能产生一种纠缠光源，而且在非线性晶体前后往往需要很多其他的光学元件来实现对泵浦光以及产生光子对的处理和收集等，导致纠缠光源产生光路一般都较为复杂、庞大、不易稳定、不具有很好的功能扩展能力。
参考资料:
1.A.Aspect,P.Grangier,and G.Roger,Phys.Rev.Let.49,91(1982).
2.V.D.A.Braje,P.Kolchin,G.Y.Yin,and S.E.Harris,Phys.Rev.Lett.94,183601(2005).
3.H.Takesue,Y.Tokura,H.Fukuda,T.Tsuchizawa,T.Watanabe,K.Yamada,andS.Itabashi,Appl.Phys.Lett.91,201108,(2007).
4 J.E.Sharping,K.F.Lee,M.A.Foster,A.C.Turner,B.S.Schmidt,M.Lipson,A.L.Gaeta,and P.Kumar,Opt.Exp.14,12388,(2006).
5.J.E.Sharping,M.Fiorentino,and P.Kumar,Opt.Lett.26,367–369(2001).
6.X.Li,P.L.Voss,J.E.Sharping,and P.Kumar,Phys.Rev.Lett.94,53601(2005).
7.Y.H.Shih and C.O.Alley,Phys.Rev.Lett.61,2921(1988).
8.Z.Y.Ou and L.Mandel,Phys.Rev.Lett.61,50(1988).
9.C.E.Kuklewicz,M.Fiorentino,G.Messin,F.N.C.Wong,and J.H.Shapiro,Phys.Rev.A 69,13807(2004).
10.V.Giovannetti,L.Maccone,J.H.Shapiro,and F.N.C.Wong,Phys.Rev.Lett.88,183602(2002).
11.M.C.Booth,M.Atatüre,G.Di Giuseppe,B.E.A.Saleh,A.V.Sergienko,andM.C.Teich,Phys.Rev.A 66,023815(2002).
12.X.Q.Yu,P.Xu,Z.D.Xie,J.F.Wang,H.Y.Leng,J.S.Zhao,S.N.Zhu,N.B.Ming,Phys Rev.Lett.101,233601(2008).
13.S.Tanzilli,H.De Riedmatten,H.Tittel,H.Zbinden,P.Baldi,M.DeMicheli,D.B.Ostrowsky,and N.Gisin,Electron.Lett.37,26(2001).
14.K.Sanaka,K.Kawahara,and T.Kuga,Phys.Rev.Lett.86,5620(2001).
15.A.Kanno,T.Sakamoto,A.Chiba,T.Kawanishi,K.Higuma,M.Sudou,andJ.Ichikawa,IEICE Electron.Express 7,817(2010).
发明内容
本发明目的是解决上面所提到的问题，提供一种基于光学超晶格波导的量子光源芯片的设置方法及提供一种芯片化的纠缠光源，利用集成光学的思路和技术来提升纠缠光源的扩展性、集成度、稳定性、便携性等。
本发明的技术方案是：一种基于光学超晶格波导的量子光源芯片的设置方法，采用波导光路和光学超晶格、电光调制器的集成，波导光路通过波导分束器将进入的激光进行分束，分束后的激光进入光学超晶格区域进行频率下转到得到纠缠光子对，纠缠光子对随后继续进入干涉仪进行量子干涉。干涉仪的相位由芯片上内置的电光调制器来控制，通过电压调节得到几种不同的量子态。
本发明的主要内容是以铌酸锂等铁电材料为基质材料进行波导加工并对部分区域进行极化，使得该芯片同时实现纠缠光子的产生和干涉，将输入的经典光光转化为可调控的量子态，不同的量子态通过集成在芯片上的电光调制器完成；整个芯片依次分为三个区域，区域I是对经典激光的处理，主要是对抽运光(经典激光)的分束和相位调制，区域II是非线性区，上下两路中的光学超晶格结构将抽运光转化为简并的纠缠光子对；光子对从上路或下路产生，组成一个路径聚束态区域III对纠缠光子的处理，在波导分束器上实现纠缠光子的Hong-Ou-Mandel(HOM)干涉；如果II区域产生的路径聚束态中上下两路相位相同，那么该聚束态经由HOM干涉后得到分离态，即对于具有相位差φ的一般路径聚束态，干涉后得到分离态和聚束态的叠加，即III区域还包含对抽运光的滤波单元，纠缠光子通过倏逝波耦合到更外侧的两根波导，抽运光保留到原来波导中。II区域产生的光子聚束态中的相位是通过电光效应来控制，电光效应可以施加在芯片内其他等效区域。电压施加采用推挽结构，在一路上增加相位的同时另一路等量减小。
本发明的关键之处是波导光路中设置光学超晶格区域，所述光学超晶格包含一维周期结构、非周期结构、啁啾结构等所有一维极化序列，其基质材料包括可以进行极化的铌酸锂材料、钽酸锂、磷酸钛氧钾等铁电材料。光学超晶格区域发生的参量下转换过程包括简并和非简并两种情形，产生纠缠光子；波导光路中设置干涉仪，能够对产生的纠缠光子进行进一步处理，通过干涉等方法得到多种量子态，波导光路干涉仪设计包括Hong-Ou-Mandel干涉仪、Mach-Zehnder干涉仪、Michelson干涉仪、Franson干涉仪等以及由这些基本干涉仪组成的复杂光路。进一步的，加工波导光路的方法包括质子交换方法、钛扩散方法以及机械加工方法。
本发明依上述方法得到芯片光源，本发明芯片设计以铌酸锂等铁电材料为基质材料，在基质材料上加工出波导光路并对部分区域进行极化，使得抽运激光耦入芯片后经过极化区域后转化为纠缠光子对然后进行再量子干涉，最终输出可调控的量子光源。芯片内含电光调制器，通过施加电压改变光子相位，使得量子干涉给出不同的输出结果，得到不同的量子态，如光子路径分离态、聚束态以及这两种量子态的任意比例叠加。设计通过调节抽运激光波长、温度、超晶格周期、波导结构参数、波导光路等可以改变输出光子波长，得到可见光和近红外等多种波段的量子光源。
本发明的有益效果，主要基于铌酸锂材料的几个特点：(1)铌酸锂材料具有较高的二阶非线性光学系数，且非线性系数可以进行人工调控，从而实现高效的准相位匹配光学频率转换如光学参量下转换过程，得到明亮的纠缠光子对，而且纠缠光子对的波长可以通过改变结构周期灵活设计；(2)铌酸锂材料具有较大的电光系数，可以实现快速准确的光子相位调控，调制速率一般为40GHz左右(实验室演示可达到100GHz[15])；(3)铌酸锂可以通过质子交换或者钛扩散等方法加工成波导，这样可以进一步提高纠缠光子的产生效率，通过设计波导光路还可以实现线性量子计算，使得铌酸锂成为一个有潜力的量子信息处理平台。本发明正是基于铌酸锂的以上优点，设计了一种基于铌酸锂材料的基于光学超晶格波导的量子光源芯片，提供一种芯片化的、稳定的、便携的纠缠光源。
附图说明
图1是本发明的结构示意图；
图2是本发明中电光调制器的截面图；
图3是实施例1给出的纠缠态随电压的切换；
图4是实施例1给出的纠缠光子波长随极化周期的变化。
具体实施方式
下面是芯片结构的详细说明。以铌酸锂材料为例，设置了一种基于铌酸锂光学超晶格波导的量子光源芯片。z-切铌酸锂基片上分为波导光路、周期极化区域和纠缠光子干涉区三个区域。区域I是对经典抽运光的处理，主要是对抽运光的分束和相位调制。区域II是非线性区，上下两路中的周期极化铌酸锂可以将经典抽运光转化为简并的纠缠光子对。光子对从上路或下路产生，组成一个路径聚束态区域III是对纠缠光子的处理，主要功能是在波导分束器上实现纠缠光子的Hong-Ou-Mandel(HOM)干涉。如果II区域产生的路径聚束态中上下两路相位相同，那么该聚束态经由HOM干涉后得到分离态，即对于具有相位差φ的一般路径聚束态，干涉后得到分离态和聚束态的叠加，即III区域还包含对泵浦光的滤波单元，纠缠光子通过倏逝波耦合到更外侧的两根波导，泵浦光保留到原来波导中。II区域产生的光子聚束态中的相位是通过电光效应来控制，电压施加采用推挽结构，在一路上增加相位的同时另一路等量减小。
图1芯片中各个单元进行标号。在z-切铌酸锂基片上制备波导光路、周期极化区域和纠缠光子干涉区。铌酸锂芯片和输入光线2以及输出光纤26、27相连接。经典抽运激光1从光纤2输入，纠缠光子对从光纤26、27输出。铌酸锂基片28为z-切，主要包含波导光路20、21和周期极化区域15。当抽运激光1经由光纤2输入后进入输入波导3。然后被波导Y-分束器一分为二，分别进入波导4和5。波导3、4、5都是针对抽运激光波长的单模波导。4、5波导的夹角小于1°。过渡波导11、12分别将抽运光波导6、7转换到参量光波长的单模波导13、14。在6、7波导上有8、9、10三个电极，用来调节两路泵浦之间的相位差。波导13、14所在区域进行了周期极化，将抽运光转化为纠缠光子对。之后光子对到达波导2×2分束器进行干涉，波导分束器中平行波导16、17的间隔和长度设计为半个耦合长度(一个耦合长度是指能量全部从13(14)耦合到波导33(32)的长度)。干涉后的路径纠缠光子对在32或者33波导中。这两段波导后面分别级联了一个波导滤波器，滤波器中的平行波导区18和22使得参量光子从32转移拐弯波导34进而到直波导24中，平行波导19和23使得参量光子从33转移到拐弯波导35进而到直波导25中。而抽运光能量除了在前面波导拐弯时的损耗，大多数能量都保留在18、19波导中，起到滤波作用。图1中所有波导拐弯实际上都应该用具有一定曲率的弯曲波导实现，这里简化为钝角拐弯作示意。纠缠光子从26、27输出后需要光纤Bragg光栅或者干涉滤波片进一步衰减抽运光提取参量光。整个铌酸锂芯片上方覆盖一层二氧化硅缓冲层用来保护波导光路，减少表面划伤，降低损耗。
图2是图1中相位调制器的截面图。电极8接地，在波导6上方。电极9、10接直流电压。电极10在波导7上方。29，30，31是二氧化硅缓冲层，电极对之间的缓冲层36、37被腐蚀掉以降低直流漂移。电光调制器也可以加在13、14波导中周期极化区域15之后的部分，这样直流电压直接调制参量光的相位。整个铌酸锂芯片约50mm。
实施例1：抽运光为780nm，简并光子对为1560nm。当电极8、9、10设置为8mm，7V电压可以使得两个抽运光相对相位变化2π，从而使得芯片的输出光子态经历聚束态到分离态再到聚束态一个完整的变化周期。图3是测量输出光纤26、27之间光子符合计数随着施加电压的变化图。图中符合计数呈周期性变化，周期为7V，极大和极小值分别表示分离态和聚束态。第一个分离态对应电压2.3V(理论上应为0V)，我们称为偏移电压。
周期极化区域15的设计如下。设计其周期满足波导中自发参量下转换的准相位匹配条件βp-βs-βi＝2π/Λ，其中βp,βs,βi分别是泵浦、信号和闲置光的基模传播常数，由铌酸锂材料的色散以及波导加工工艺共同决定。对于质子交换波导，只有e光可以传播，可以利用最大的非线性系数d33。选取合适的波导制备工艺，室温下产生1560nm光子对的极化周期为15.32微米。根据我们的工艺条件，改变极化周期将对应不同波长的简并光子对输出，我们的测试结果如图4。图中1-9通道的极化周期为14.36微米-16.28微米，以0.24微米为单位依次递增，图中我们测试了25.5℃倍频工作波长，其反过程就是纠缠光子产生过程，对应抽运波长是图中该倍频过程基波波长的一半，产生简并纠缠光子的波长就是倍频中基波波长。

Claims (6)

1.一种基于光学超晶格波导的量子光源芯片的设置方法，其特征是采用波导光路和光学超晶格、电光调制器的集成，波导光路通过波导分束器将进入的经典抽运激光分束，分束后的激光进入光学超晶格区域进行频率下转到得到纠缠光子对，纠缠光子对随后继续进入干涉仪进行量子干涉；干涉仪的相位由芯片上内置的电光调制器来控制，通过电压调节得到几种不同的量子态；
在铌酸锂铁电材料为基质材料的单片晶体上加工出波导干涉光路并对部分区域进行畴反转；整个芯片依次分为三个区域，区域I是对经典激光的处理，对抽运光的分束和相位调制，区域II是畴反转区，也称非线性区，可以将上下两路抽运光转化为简并的纠缠光子对，光子对从上光路或下光路产生，组成一个路径聚束态区域III是对纠缠光子的处理，在波导分束器上实现纠缠光子的Hong-Ou-Mandel(HOM)干涉；如果II区域产生的路径聚束态中上下两光路相位相同，那么该聚束态经由HOM干涉后得到分离态，即对于具有相位差φ的一般路径聚束态，干涉后得到分离态和聚束态的叠加，即III区域还包含对泵浦光的滤波单元，纠缠光子通过倏逝波耦合到更外侧的两根波导，泵浦光保留到原来波导中。
2.根据权利要求1所述的基于光学超晶格波导的量子光源芯片的设置方法，其特征是在z-切铌酸锂基片上制备波导光路、非线性区的周期极化区域和对纠缠光子的处理干涉区；设有光纤(2)将抽运激光输入波导光路的第一波导(3)；然后经波导Y-分束器分别输入第二与第三波导(4、5)；第一至第三波导都是针对光的过渡波导；第二第三波导的夹角小于1°；设有第一和第二过渡波导(11、12)分别将第一和第二抽运光波导(6、7)转换到参量光波长的第一与第二单模波导(13、14)；第一与第二抽运光的波导(6、7)分别接第一与第二波导(2、3)；在第一与第二抽运光的波导(6、7)上有三个电极(8、9、10)，用来调节两路泵浦光之间的相位差；第一与第二单模波导(13、14)所在区域是周期极化区域(15)，将抽运光转化为纠缠光子对；之后光子对到达波导分束器进行干涉，波导分束器中第一与第二平行波导(16、17)的间隔和长度设计为半个耦合长度；干涉后的路径纠缠光子对分别接第六与第七波导(32、33)；第六与第七波导分别连接拐弯波导和直波导，即第六与第七波导后面分别级联了一个波导滤波器，滤波器中的第一与第二平行波导区(18和22)使得参量光子从第六波导(32)转移第一拐弯波导(34)进而到第一直波导(24)中；第三与第四平行平行波导(19和23)使得参量光子从第七波导(33)转移到第二拐弯波导(35)进而到第二直波导(25)中。
3.根据权利要求1或2所述的基于光学超晶格波导的量子光源芯片的设置方法，其特征是在波导光路中设置光学超晶格区域，所述光学超晶格包含一维周期结构、非周期结构、啁啾结构的所有一维极化序列，光学超晶格区域发生的参量下转换过程包括简并和非简并两种情形，产生纠缠光子；波导干涉仪能够对产生的纠缠光子进行进一步处理，通过干涉方法得到多种量子态，波导干涉仪包括Hong-Ou-Mandel干涉仪、Mach-Zehnder干涉仪、Michelson干涉仪或Franson干涉仪。
4.根据权利要求1或2所述的基于光学超晶格波导的量子光源芯片的设置方法，其特征是加工波导光路的方法包括质子交换方法、钛扩散方法以及机械加工方法。
5.根据权利要求1或2所述的基于光学超晶格波导的量子光源芯片的设置方法，其特征是基于光学超晶格波导的量子光源芯片上通过电光效应来控制光子相位，电光效应能施加在芯片内其他等效区域；电压施加采用推挽结构，在一路上增加相位的同时另一路等量减小。
6.根据权利要求1或2所述的基于光学超晶格波导的量子光源芯片的设置方法，其特征是波导材料包括能进行极化的铌酸锂材料、钽酸锂、磷酸钛氧钾铁电材料。
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