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.
