This award, "One-way quantum computing in the optical frequency comb," supports the Quantum Fields and Quantum Information (QFQI) group in the Physics Department of the University of Virginia (UVa). Quantum computing (QC) is a fascinating endeavor whose most important promise is the exponential speedup of particular mathematical problems such as quantum system simulation and integer factoring, the latter being key to breaking cyber encryption. The physical challenges posed to the experimental implementation of QC are daunting and can be summarized by the requirement of exquisitely controlling thousands of single quantum systems (e.g. atoms) at the individual level. Because light is easy to control efficiently, as opposed to atoms, this research project is based on a novel proposal for a quantum computer, by the PI and his collaborators, using laser-like beams of light at different frequencies. A large amount of prior theoretical results has shown that this proposal offers exciting perspectives, in particular a spectacular increase of the size of prototype quantum computers.
Broader impacts of the proposed work comprise an active research contribution to the UVa Physics graduate and undergraduate programs, in the form of the continuous advising of 3 Ph.D. students per year and 2 undergraduate students per summer (typically supported by NSF REU supplements to the grant). Also stemming from this research, departmental seminars have been given several times a year by different QFQI members and an advanced graduate course "Quantum Optics and Quantum Information" (Phys 888) is now taught by the PI at UVa on a regular basis. On the interdisciplinary front, this research is spawning collaborative efforts between the QFQI group and Prof. Arthur Lichtenberger, director of the UVa Microfabrication Laboratory at the UVa School of Engineering, in particular for microphotolithography applied to the fabrication of photonic materials. Last but not least, there exists the potential for this research to usher in fundamental tests of quantum mechanics by exploring large-scale quantum behavior in a regime where theoretical predictions are untested and even difficult to make. This would therefore represent a scientific endeavor on a broader scale than the field of Atomic, Molecular, and Optical Physics.
'' was to demonstrate the implementation of scalable quantum entanglement in the frequency spectrum of a single optical parametric oscillator. This was successfully achieved, at a world-record level. Quantum entanglement is a term coined in 1935 by Schrödinger to describe the particularly nonintuitive measurement correlations that can occur in well-separated physical systems, as first predicted by Einstein, Podolsky, and Rosen that same year. Entangled systems can be used to ask a number of philosophical questions about quantum physics, but they also appear to be at the heart of the exponential speedup that quantum computers are predicted to claim over classical ones for certain calculations of relevance to encryption or to the simulation of complicated quantum systems, such as chemical or biological molecules. There is therefore a worldwide, all-out experimental effort, encompassing many different fields of physics and a variety of physical systems (atoms or ions, light, superconducting circuits, nanofabricated semiconductors), toward realizing large-scale entanglement of quantum systems in order to build the first practical quantum computer, which could revolutionize physics, mathematics, computer science, as well as reshape the landscape of secure communications. The research of Prof. Pfister's team at the University of Virginia is focused on using the quantum properties of light to implement quantum entanglement and quantum computing at previously unattained scales. Use was made of an exotic laser called an optical parametric oscillator (OPO), in which the quantum particles of light, the photons, are emitted in pairs rather than one by one (albeit very rapidly) in a conventional laser. This resulted, after sophisticated optical engineering, in an OPO that emitted light in a spectrum of a multitude of entangled OPO beams, the "quantum optical frequency comb," well-defined by their colors and their polarization. In this implementation, each beam corresponds to the quantum bit (qubit) of a quantum computer. In the first experimental demonstration of this proposal, Pfister's team generated 15 independent sets of 4 entangled beams each, which constitute as many quantum computing registers. While the number of beams entangled together (4) was not a record in itself, the number of simultaneously entangled beams (60) was the largest ever achieved and was only limited by the particular verification technology that was used. From a careful analysis of the spectral linewidth of their OPO's photon-pair emission, Pfister and his team estimated that up to 600 beams were probably involved in the quadripartite entanglement generation. This research was featured in the September 2011 issue of Physics Today, pp. 21-24.
