Massively Scalable Quantum Entanglement and Quantum Processing in the Optical Frequency Comb

量子计算承诺，在处理特定但重要的任务（如数据加密和量子物理模拟）时，量子计算将比经典计算有指数级的速度提升。然而，量子计算的实际实现面临着艰巨的挑战：需要“Qbit”（这里是“Qmode”）寄存器的可扩展性，需要规避退相干。该项目由弗吉尼亚大学（UVa）的Pfister教授主持，旨在实现光学参量振荡器（OPO）周期性发射光谱中的大规模纠缠，即“量子光学频率梳”（QOFC）。它是基于Pfister的高质量纠缠组最近实现的一个OPO的QOFC的60个特征模（“q模”）的世界纪录，分为15组4个q模，每个q模处于方形簇态。这项成功的实验是该项目的核心，该项目获得了美国国家科学基金会（NSF）“光频率梳中的单向量子计算”奖的支持。当前项目的目标是在这一成功的基础上，沿着两条努力方向，向高度可扩展的量子信息迈进。一方面，我们寻求生成记录大小的线性、方网格和立方晶格簇态，从而实现通用量子计算。另一方面，该小组正在努力实现量子ofc中量子处理所需的量子技术。这包括:(i)开发低损耗、高色散的光学元件来分离q模，（ii）使用集成光学实现高效PIN光电二极管的平衡同差检测网络，以及（iii）通过光子数分辨检测的方式进行高效的非高斯测量，这是最近在UVa的Pfister小组与NIST的Sae Woo Nam和Albion学院的Aaron Miller合作实现的。由美国国家科学基金会核磁共振奖资助，题为“开发用于通用量子计算的光子数分辨探测器系统”。这个雄心勃勃的计划相当于在连续变量上创建一个真正的量子计算机，并在此背景下研究量子信息。这项工作的广泛影响包括对弗吉尼亚大学物理研究生和本科课程的积极研究贡献。最近的一名本科生是前戈德华特学者，刚刚加入哈佛大学物理学研究生项目，是2011年美国物理学会LeRoy Apker奖的决赛选手，基于他与Pfister教授发表的一篇论文。同样源于这项研究，PI现在定期教授高级研究生课程“量子光学和量子信息”。在跨学科方面，这项研究已经催生了全球范围内的合作努力。最后，必须指出的是，量子计算研究对基础物理学以及国防和国家安全都有利害关系：肖尔以指数速度分解整数的算法将击败广泛使用的RSA加密协议。基本尺寸的通用量子处理器的另一个直接应用将是化学、材料科学和凝聚态物理中目前棘手的量子问题的建模。最后，可扩展量子寄存器的实现为在介观纠缠和Schrödinger猫的制度下进行量子力学的基本测试提供了可能性，在这些制度下，理论预测变得难以解决。随着量子信息的成熟，人们可以期待在自然科学的所有领域都有意义深远的科学发现。

Quantum computing promises exponential speedups over classical computing for specific but important tasks, such as data encryption and the simulation of quantum physics. However, the practical implementation of quantum computing faces daunting challenges: the need for scalability of "Qbit" (here, "Qmode") registers and the need to circumvent decoherence. This project from Prof. Pfister at the University of Virginia (UVa) aims at implementing large-scale entanglement in the periodic emission spectrum of an optical parametric oscillator (OPO), a.k.a. the "quantum optical frequency comb" (QOFC). It is based on the recent realization by Pfister's group of high-quality entanglement in a world-record 60 eigenmodes ("Qmodes") of the QOFC of a single OPO, into 15 sets of 4 Qmodes, each set being in a square cluster state. This successful experiment was the core of the project supported by an NSF award entitled "One-way quantum computing in the optical frequency comb." The objective of the current project is to build on this success and forge ahead toward highly scalable quantum information, along two lines of effort. On the one hand, we seek to generate record-size linear, square-grid, and cubic-lattice cluster states, which enable universal quantum computation. On the other hand, the group is striving to implement the quantum technologies needed for quantum processing in the QOFC. This includes: (i) developing low-loss, highly dispersive optical elements to separate Qmodes, (ii) implementing a network of balanced homodyne detection with high-efficiency PIN photodiodes using integrated optics, and (iii) performing high-efficiency nonGaussian measurements by way of photon-number-resolved detection, which has recently been implemented in Pfister's group at UVa thanks to a collaboration with Sae Woo Nam at NIST and Aaron Miller at Albion College, funded by an NSF MRI award entitled "Development of a photon-number-resolving detector system for universal quantum computing." This ambitious program is tantamount to creating a bona fide quantum computer over continuous variables, and studying quantum information in this context.The broader impacts of this work comprise an active research contribution to the UVa physics graduate and undergraduate programs. One recent undergraduate student was a former Goldwater Scholar who just joined the physics graduate program at Harvard, was a finalist of the 2011 LeRoy Apker Award of the American Physical Society, based on a paper he published with Prof. Pfister. Also stemming from this research, an advanced graduate course "Quantum Optics and Quantum Information" is now taught by the PI on a regular basis. On the interdisciplinary front, this research has spawned worldwide collaborative efforts. Finally, it is important to point out that quantum computing research has stakes in fundamental physics, as well as Defense and National Security: Shor's algorithm for factoring integers exponentially faster would defeat the widely-used RSA encryption protocol. Another direct application of a universal quantum processor of elementary size would be the modeling of presently intractable quantum problems in chemistry, materials science, and condensed-matter physics. Finally, the realization of a scalable quantum register offers possibilities for fundamental tests of quantum mechanics in the regime of mesoscopic entanglement and Schrödinger cats, where theoretical predictions become intractable. As quantum information comes of age, one can thus expect deeply significant scientific discoveries in all fields of the natural sciences.