QnTM: Tools for Distributed Quantum Information Processing

量子信息处理(QIP)由于提出的量子算法(如大数因式分解、改进数据库搜索等)的前景而受到广泛的研究。量子信息处理领域还可能提供新的方法来表征、理解和控制从量子世界到我们生活的经典世界的过渡。最近开发了基于线性光学的QIP协议，其中普通光学元件与高效率的单光子探测器相结合，如果系统的整体量子效率能够保持近乎完美，并且可以制备合适的纠缠量子态，则显示出可扩展的潜力。到目前为止，这种基于线性光学的QIP已经使用自由空间光学元件和通过自发参数下转换产生的纠缠态在块状非线性光学晶体中实现。然而，在基于QIP的线性光学的自由空间实现中，量子比特的传输、随后的量子比特的相干叠加以及来自不同光学时空模式的量子比特的检测都由于使用了体光学非线性晶体而缺乏可扩展性。这也是因为光学衍射，这限制了在自由空间中传输量子比特的能力。为了避免衍射，人们通常将块状晶体产生的纠缠量子比特注入光纤中。这种将非线性晶体产生的纠缠耦合到用于传输的光纤在技术上具有挑战性，限制了整体量子效率，从而限制了可实现的可扩展性。另一方面，通过在光纤中直接实现QIP功能，解决了模式纯度的问题，因为所涉及的所有光学模式都具有光纤的非常纯粹的导模。西北大学最近的工作展示了在标准光纤的电信频段中产生关联光子对，产生偏振纠缠，并将这种光纤产生的纠缠传输到长达50公里的距离。有了这样的纠缠源，广域QIP可以被设想为在分布式QIP环境中，电信频带中的低光纤损耗可以允许量子比特从一个节点传输到另一个节点长达50-100公里。本项目的目的是演示在实现分布式QIP方面取得进展的原型工具和功能。具有上述优点的光纤量子逻辑门将被开发出来。此外，还将展示一种全光单光子开关，这有望成为基于线性光学的量子处理器中控制量子比特流的关键技术。这种全光交换有望消除电光交换固有的低效。它将允许检测到一个光子，以导致将其孪生光子存储在光纤环路中，或对存储的光子进行非破坏性读出以进行进一步处理。总之，这些QIP功能将显著增加传输、存储、检测和以其他方式控制基于线性光学的量子处理器的能力。拟议活动的智力价值：拟议活动将解决发展基于光学的QIP的一个基本障碍，即需要基于光纤的设备来操纵和控制光学量子比特。理论和实践需要齐头并进，才能取得成功。PI的研究小组带来了理论和实验专业知识和技能的独特组合来解决问题。拟议活动的广泛影响：学生将参与到跨学科的工作中。除了课程中常见的光纤光学和光子学之外，他们还将接受新兴的量子信息领域的培训。将通过西北大学光子通信和计算中心开展外展活动，以便对我们整个社区产生更广泛的影响。将通过大学现有的所有资源作出全面努力，使代表人数不足的学生参与这一项目。

Quantum information processing (QIP) is being intensively studied due to the promise of proposed quantum algorithms such as those for factoring large numbers, improving database searches, etc. The field of QIP is also likely to provide new ways of characterizing, understanding, and controlling the transition from the quantum world to the classical one we live in. Linear-optics based QIP protocols have been recently developed, in which ordinary optical elements are used in conjunction with high-efficiency single-photon detectors, that show the potential for scalability if the overall quantum efficiency of the system can be maintained nearly perfect and suitably entangled quantum states can be prepared. To date, such linear-optics based QIP has been implemented using free-space optical elements in conjunction with entangled states generated via spontaneous parametric down-conversion in bulk nonlinear-optical crystals. In free-space implementations of linear-optics based QIP, however, the transportation of quantum bits (qubits), subsequent coherent superposition of qubits, and the detection of qubits from different optical spatio-temporal modes, all suffer from a lack of scalability due to the use of bulk-optic nonlinear crystals. This is also because of optical diffraction, which limits the ability to transport qubits in free space. To avoid diffraction, one usually injects bulk-crystal generated entangled qubits into optical fibers. Such coupling of nonlinear-crystal generated entanglement to optical fibers for transportation is technically challenging, limiting the overall quantum efficiency and thus the achievable scalability. On the other hand, by implementing QIP functions directly in optical fiber, the problem of modal purity is solved as all involved optical modes have the very pure guided mode of an optical fiber. Recent work at Northwestern University has demonstrated the production of correlated-photon pairs in the telecom band of standard optical fibers, creation of polarization entanglement, and transmission of such fiber-generated entanglement for distances of up to 50 km. With such a source of entanglement, wide-area QIP can be envisioned as the low fiber loss in the telecom band can permit transmission of qubits for up to 50-100 km from one node to another in a distributed QIP environment. This aim of this project is to demonstrate prototype tools and functionalities that make progress towards realizing distributed QIP. A fiber-based quantum logic gate with advantages discussed above will be developed. Additionally, an all-optical single-photon switch will be demonstrated, which is expected to be a crucial technology for controlling the flow of qubits in a linear-optics based quantum processor. Such all-optical switching promises to eliminate the inefficiencies inherent in electro-optical switching. It will allow the detection of one photon to cause the storage of its twin in a fiber loop, or the nondestructive readout of a stored photon to occur for further processing. Together, these QIP functions will add significantly to the ability to transmit, store, detect, and otherwise control a linear-optics based quantum processor. Intellectual Merit of the Proposed Activity: The proposed activity will address a fundamental roadblock in the development of optics-based QIP, namely the need for optical-fiber based devices for manipulation and control of optical qubits. Theory and practice will need to go hand in hand for a successful outcome. The PI's research group brings a unique combination of theoretical and experimental expertise and skills to bear upon the problem.Broader impacts of the proposed activity: The students will get involved in cross-disciplinary work. They will be trained in the emerging field of quantum information in addition to the usual fiber optics and photonics in the curriculum. Outreach activities through the Center for Photonic Communication and Computing at Northwestern will be undertaken to have a much wider impact on our community at large. Full scale efforts will be made through all resources available at the University to engage underrepresented students in this project.