【作者】 许金时；

【导师】 郭光灿； 李传锋；

【作者基本信息】 中国科学技术大学 ， 光学， 2009， 博士

【摘要】 量子纠缠是一种奇异的非局域关联,在量子物理中起着基础性的作用。它不仅是理解量子力学非局域性的着眼点而且是量子信息处理的重要资源。另一方面,由于环境噪声不可避免的干扰,纠缠会很容易地被破坏。而且人们发现纠缠在演化过程中也会出现一些特殊的性质。比如两粒子间的纠缠在独立的热库中演化时会在有限的演化时间内完全消失,这与单粒子相干性指数衰减完全不同。因此本人将纠缠态的制备、应用和演化作为博士论文的研究课题。同时,我们还利用量子信息的方法对一些量子力学的基本问题进行实验探讨。本论文所取得的成果主要有:1、我们实验上利用单次自发参量下转换过程直接制备得到高可见度的四光子纠缠态,并且应用这个四光子纠缠态来完成一个四体量子通讯复杂度方案。由于我们的实验方案没有用到干涉装置并且下转换光子的走离效应可以完全补偿,因此四光子纠缠态的可见度可以高达(95.53±0.45)%。在利用此纠缠态来降低四体通讯复杂度的方案中,我们得到的成功概率为(81.54±1.38)%远远超过了经典极限50%。这个四光子纠缠态还可以用来实现无消相干子空间的量子信息处理及其他量子信息方案。2、我们实验实现了1→2最优量子普适克隆和无辅助比特的1→3最优量子相位协变克隆。在我们的实验中这两种克隆过程可以方便地进行转换并且所用到的Hong-Ou-Mandel型干涉装置也可以用到其他量子信息处理过程。3、我们实验上通过对在消相干环境中演化的光子偏振态进行测量,得到量子态相干性的恢复。测量导致量子相干性恢复的实验结果加深了我们对量子测量的理解。这种方法可以推广到其他两能级体系。实验中所采用的测量装置还可以用来实现量子纠缠的恢复和Leggett-Garg型不等式的违背。4、我们实验实现了Leggett-Garg型不等式的违背,并利用它来区分量子演化过程和经典演化过程。我们还在实验上比较了两类Leggett-Garg型不等式的违背情况。违背Leggett-Garg型不等式排除了经典实在理论对量了系统的描述,从另一个角度支持量子力学的描述。当这些Leggett-Garg型不等式被运用到在噪声信道中演化的量子体系时,它们可以作为区分量子演化过程和经典演化过程的判据。通过改变入射态,我们比较了两类Leggett-Garg型不等式,找到较严格的一类。我们所使用的方法可以推广到更大的体系,这对宏观量子干涉现象的观测有着重要的启发意义。5、我们实验研究了纠缠在非马尔科夫环境中演化时的塌缩和恢复现象。在初始入射态为部分纠缠态的情况下,我们甚至观测到纠缠在突然死亡后的恢复现象。利用光学的spin-echo技术,我们实现控制纠缠在突然死亡后的恢复时间。最后我们实验实现利用最大纠缠态刻画不同量子信道中纠缠的动力学演化情况,验证了纠缠演化的因式分解公式。与不可逆的马尔科夫环境相比,纠缠在有记忆作用的非马尔科夫环境下的演化更加基本。我们所观测到的纠缠塌缩和恢复现象(甚至是突然死亡后的恢复)有助于更好地理解纠缠的动力学演化。并且这一恢复现象能够延长纠缠的使用时间,在量子信息处理过程中有着实际的应用。我们所使用的光学spin-echo技术能够有效地控制纠缠的恢复时间,将在构造量子网络方面发挥重要作用。纠缠在量子信道中演化的因式分解公式简化了纠缠演化的描述,有利于设计更多抗消相干基于纠缠的量子信息方案。更多还原

【Abstract】 Quantum entanglement,a kind of counterintuitive nonlocal correlation,is fundamental in quantum physics both for its essential role in understanding the nonlocality of quantum mechanics and its practical application in quantum information processing. At the same time,entanglement will become degraded due to the unavoidable interaction with the environment.Moreover,the evolution of entanglement may possess some distinct properties.It has been shown that entanglement between two particles evolved in independent reservoirs may disappear completely at a finite time in spite of the asymptotical coherence decay of single particle.This thesis mainly concerns the preparation,application and evolution of entangled states.We also make discussions on some fundamental problems of quantum mechanics with the technology of quantum information.The main results of the dissertation are as follow:1.We prepare a four-photon polarization-entangled state with high visibility directly from a single down-conversion source and finish a four-party quantum communication complexity scenario by using the obtained entangled state.Due to the interference-free experimental setup and the complete compensation of the walk-off effect in the birefringent crystals,the visibility of the prepared four photons state can reach as high as(95.53±0.45)%.The success probability for us to get the correct result in the four-party communication complexity scenario is(81.54±1.38)%, which greatly surpass the classical limit of 50%.This four-photon state can also be used to fulfill decoherence-free quantum information processing and other advanced quantum communication schemes.2.We experimentally realize the 1→2 optimal universal quantum cloning and the ancilla-free 1→3 optimal phase-covariant quantum cloning.In our experiment,these two kinds of cloning can be exchanged readily and the two Hong-Ou-Mandel-type interference setups may also be used to implement other quantum information processing.3.We experimentally demonstrate that measurement can recover the quantum coherence of a single photon polarization state evolved in a dephasing environment.The experimental result that measurement can induced quantum coherence recovery gives us a deep understanding of quantum measurement.This method can be extended to other two-level quantum systems.The measurement setup is also useful to demonstrate entanglement recovery and the violation of the Leggett-Garg inequality.4.We experimentally realize the violation of Leggett-Garg-type inequalities which are also used to distinguish the quantum evolution process and classical evolution process.We also experimentally compare the conditions of violation of two kinds of Leggett-Garg-type inequalities.The experimental maximal violation of Leggett-Garg-type inequalities excludes the classical reality description of quantum systems and support the quantum description in a different way.When these Leggett-Garg-type inequalities are applied to quantum systems evolved in noise environment,they can be used as the criterion to distinguish the quantum evolution process and classical evolution process.By changing input states,we compare two Leggett-Garg-type inequalities and find the tighter one. The method we used can be extend to other lager quantum systems and is important in the realization of macroscopic quantum coherence.5.We experimentally investigate the collapse and revival of entanglement in a non-Markovian environment.We even observe the revival of entanglement after it suffers from sudden death when the input state is the partially entangled state. With the application of a spin-echo like technology,we can readily control the time when the revival from sudden death occurs.Finally,we experimentally characterize the entanglement dynamics in different quantum channels by using maximally entangled states and verify the factorization law of entanglement dynamics.Different from irreversible evolution in a Markovian environment,entanglement dynamics in a non-Markovian noise with memory effect appears to be more fundamental. The observed entanglement collapse and revival(even revival from sudden death) gives us a deep understand of entanglement.The revival phenomenon has potential application in quantum information processing since it extends the usage time of entanglement. What is more,by using the spin-echo technology,we can readily control the time when the entanglement revival occurs,which will play an important role on the construction of the quantum network.The factorization law greatly simplify the description of the entanglement dynamics and will lead to the discovery of more robust entanglement-based quantum information processing protocols.更多还原