摘要
纠缠光子是实现许多量子信息过程和完成量子光学实验很重要的工具。目前为止,广泛用来获得纠缠光子源的方法是非线性晶体的自发参量下转换过程(SPDC)。量子通信的其中重要一环是要有量子中继器,也就是光子能被贮存在中继器中,而原子系综则被认为是一个好的量子中继器的备选者。为了实现原子与光子之间有效的耦合的必需使光子具有与原子自然线宽相比拟的带宽。但是普遍普的SPDC光子因为其极大的带宽而无法与原子相互作用,所有就必需压窄它的线宽。近来,有不少这个方面的一些简单实验被报导,在这些实验中Rb或Cs的热原子系综或它们的冷原子云被做为贮存器,而光子被做为飞行比特。这个论文的工作就是备制这样的光子。
整个论文分为六个部分,前面三个是关于如何备制窄线宽的非经典光子对的实验;然后的二个是实验上所测的关于这类光子的一些性质。最后一部分关于锁腔和激光稳频的一些介绍。
1)获得390 nm紫外光的外腔式倍频。这个实验第一个需求是要有一个紫外光源。虽然固体激光器已经被很好的研究过,但在紫外光谱范围内的半导体激光器还是非常的难找,几乎没有。在我们实验中需求的光波长为390纳米,它的倍频正好是Rb原子的D2线的吸收峰。因此,最好的办法是通过二次谐波倍频(SHG)的方法来获得光源。SHG的理论早已被很好的建立,同时由于激光稳频和锁腔技术的发展,新型的非线性晶体被开发出来,许多关于这方面的实验被报道。但据我们所知还没有在390纳米或以下的频段有过外腔式倍频的报道。我们知道,这个频段光的应用还是比较广的,如生物医药,高分辨率光谱,原子冷却,量子光学等等。我们在实验中所使用的晶体是PPKTP,一种新型的具有高光学非线性效率,高功率损失阈值的晶体。我们实验的转化效率为9.5%,泵浦功率为73mW。
2)用PPKTP晶体产生的超亮的Ⅰ型双光子源。SPDC作为一种易操作、高效率、高信噪比的产生纠缠光子的方法已经被广泛的应用于量子信息过程和量子光学实验。人们对SPDC光子已经研究了几十年。不少物理问题通过它被搞清楚,像Bell不等式的测量,量子隐形传态。它也被应用于量子密码,量子计算等等。这里,我们使有Ⅰ型的PPKTP晶体来备制双光子。这个实验的目的是通过测量来了解一些晶体的性质,如它温度的带宽,双光子的产生效率,这些数据对下一步的实验有不少帮助。
3)频率对应于Rb原子D2线多模双光子的时间关联函数的获得。相比普通的SPDC光子,窄带的SPDC光子有更多的优点。它的线宽要小上6个量级,因此它具有很长的相干时间和距离。更重要的是由于它的窄线宽,它有望与原子发生高效的相互作用,并被贮存起来。光子的一个缺点是它总是在飞,所以不容易被局域化或是贮存。基于这个原因它所携带的信息在光子消失后也随之消失。不过根据电磁诱导透明理论(EIT),如果一个窄带光子的频率与一种原子的吸收峰对应,那它有可能被这种原子的系综所贮存,信息也会被贮在原子系综里。因此有了非经典窄带光子以后,人们就可以让信息在光子与原子之间相互传递。这是实现量子网络与量子贮存器的关键一步。我们实验得到是光子带宽为21MHz。
4)多模双光子对在Michelson干涉仪中的量子干涉现象。量子干涉是量子力学中既重要又神秘的现象之一。光子的性质可以由干涉现象来表现出来。一般来说,有两种类型的干涉常被关注:一价干涉与二价干涉。人们设计了许多干涉仪去实现两种干涉,像Michelson干涉仪,Sagnac干涉仪等。日本的Goto等人利用一个不等臂干涉仪观察了多模光子的干涉条纹。我们用Michelson干涉仪做了一个相似的实验,但我们不仅观察了不等臂的情况而且也看了等臂的情况。即使在不等臂清况时,我们的两臂长度也与Goto等人的不同,当然干涉图象也是不同的。通过实验我们发现这种类型的光子对干涉仪的臂差和位相是比较敏感的。
5)实验上测量多模光子的单光子相干长度。在这部分我们测量了直接从远低阈值的OPO腔内出来的多模光子的单光子相干长度。虽然光子的带宽是由它所出来的OPO腔的线宽决定的,但是由于光子丰富的纵模数,它还是只有一个很短的相干长度。一个类似的实验曾在Phys.Rev.Lett.90,043602上报道过,但是他们使用的是单次通过的结构,相比之下我们使用的是从OPO腔内出来的光子。实验的主要想法是当一个光子进入干涉仪后,如果干涉仪的臂差大于光子的相干长度,那么就不会有干涉现象。反之,臂差小于相干长度,就会有干涉现象出现,而且条纹对一个波长之内的长度改变极其敏感。实验的结果显示我们实验中的光子的单光子相干长度为90μm,对应于大概0.47 nm的带宽,然而实验中的OPO腔的线宽为8 MHz。它说明了如果没有进一步的滤波,这种光子是无法与原子发生有效的相互作用的。
6)因为激光稳频与光学腔的锁定在现代光学实验中显出越来越重要的作用,了解和明白一些关于这方面的技术是十分有必要的。在这一部分,我翻译了一些这方面很好的入门文章,希望有所帮助。
Sources for creating entangled photon pairs are essential parts of many quantum information protocols and quantum optical experiments.To date,the most widely used way of obtaining the entangled photon pairs is spontaneous parametric downconversion (SPDC) in a nonlinear crystal.An important step of quantum communication is quantum reperter which can store the photons.The atomic system is regard as a good candidate.To realize the efficient coupling between the atom and the photon is how to get a photon that has a comparable bandwidth with the natural bandwidth of the atom. But the SPDC photons can't interact with atoms efficiently because of its ultrawide bandwidht.So,its bandwidth should be compressed.Recently,there have been many primitive experiments in this field,in which an atomic system such as a hot atomic vapor cell or a cold atomic cloud of Rb or Cs is used as a quantum memory and a photonis used as a flying bit.The research aim of this subject is to prepare such photons.
There are six parts in this thesis:the first three are the three steps to prepare narrow bandwidth nonclassical photon pairs;the next two are the investigations of some properties of mode locked photon pairs;the last one is the introduction of cavity locking and laser stabilization.
1) Efficient cw violet-light generation in a ring cavity with a periodically poled KTP.The first requirement of this experiment is blue-UV laser source.Although the compact solid-state laser sources have been investigated thoroughly,the blue-UV spectral region laser diodes are still very difficult to make.The wavelength in our experiment is 390 nm,and its double frequency corresponds to the Rb atomic D2 line transition.So the best way to get the blue-UV source is through SHG.The theory of SHG is well established,and thanks to the development of cavity locking technique and production of new type nonlinear crystal,many realization of such sources have been reported.But according to our knowledge,no SHG experiment at 390 nm or below has been reported.As we known,the application of this spectral region is powerful,such as biomedical,high-resolution spectroscopy,atom cooling,quantum optics and so on. The crystal in our SHG experiment is PPKTP crystal,a new type crystal has larger optical nonlinearity and a good power-handing capability in the UV and visible spectra, and the conversion efficiency is about 9.5%with 73 mW pumping power.
2) An ultra-bright two-photon source with a type-Ⅰbulk periodically poled potas- sium titanyl phosphate.SPDC as an easy manipulation,high signal-to-noise ratios and high efficiency way of preparing entangled photon pairs is widely used in quantum information protocols and quantum optics.The SPDC photons have been investigated for several decades.Many physical questions have been cleared through SPDC experiments, like Bell inequalities,teleportation.It's also applied to quantum cryptography, quantum computer and others.Here,we use type-ⅠPPKTP crystal to prepare biphotons. The purpose of this experiment is just to know the property of the crystal,like the width of crystal temperature,the efficiency of producing biphotons,which will be used in an OPO cavity in the next step.
3) Observation of time correlation function of multimode two-photon pairs on a rubidium D2 line.Comparing with the normal SPDC photons,the narrow bandwidth SPDC photons have more advantages.Its bandwidth is about 6 orders of magnitudes less than the normal one.So it has very long coherence time and distance,and more importantly thanks to the narrow bandwidth,it could be expected to interact with atoms efficiently then it could be stored in atomic system.One of the disadvantages of the photon is that it always fly,so it is hardly to be located or to store.For this reason,the information inside the photons will be gone with the photons.But according to electromagnetically induced transparency(EIT) theory,if a narrow bandwidth photon with its frequency corresponding to a transition of a kind of atom,it can be stored in atomic system and its information could be preserved.So one could transfer the quantum information from the SPDC photons to atomic system and the converse works as well. This is the key point to realize quantum memory and quantum net.In our experiment, 21 MHz bandwidth biphotons are observed.
4) Quantum interference of multimode two-photon pairs with a Michelson interferometer. Quantum interference is one of important and mystery things in quantum mechanics.The properties of a kind of photons can be obtained vis interferences.Commonly, there are two classes of it often being investigated,one is first-order interference and the other is second-order interference.One design many interferometers to realize the two classes interferences,like Michelson interferometer,Sagnac interferometer, and so on.Goto et.al investigate the mode locked photon pairs vis an unbalanced interferometer. We do a similar experiment in which a Michelson interferometer is used and we investigate the balance and unbalance cases.Even on the unbalance case,the ann difference is different from theirs and the interference figures are different too.We find the shape of interference fringes is sensitive to the arm difference and the phase.
5) Experimentally measuring the coherence length of the single photon generated via a degenerated optical parametric oscillator far below threshold.At this part we measure the single photon coherence length of the mode locked photons directly from the OPO cavity.Though the bandwidth of the photons is determined by the linewidth of OPO cavity,the photon still have very short coherence length due to its abundant longitudinal modes.A similar experiment has been reported in Phys.Rev.Lett.90, 043602,but the photons in theirs is generated via a single-pass configuration in contrast with ours coming from an OPO of far below threshold.The basic idea of this work is when a photon is input a Michelson interferometer,if the path difference between two interfering beams is larger than the coherence length of the photon,there is no interference. If the path difference is less than the coherence length,then there is interference and the single count rate depends on the fine path difference of the order of the single photon wavelength.The result in our experiment is that the single photon coherence length is only about 90μm,corresponding to about 0.47 nm bandwidth,however the linewidth of OPO cavity is about 8 MHz.It indicates that if the further frequency filter absents,the mode locked photons couldn't interact with atoms effectively.
6) For the laser stabilization and optical cavity locking are more and more important in modern optical experiments,understanding and knowing some skills about these is necessary.At this part,Ⅰtranslate some good articles about teaching basic knowledge of cavity and laser stabilization for beginners.These articles are all very good beginner's guides.I hope it will take some helps.
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