Quantum Computing and Quantum Simulation in the Optical Frequency Comb

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

The quest for fully functional, universal quantum computing is an important scientific and societal goal. A working quantum computer would bring about revolutionary advances by enabling quantum calculations at currently unfathomable scales, as first proposed by Richard Feynman. An example would be that of large biological molecules, which could empower drug discovery in an unprecedented manner. Another important application for the quantum computer is provided by Shor's algorithm for factoring integers exponentially faster than a classical computer, which would provide a way to defeat the current standard encryption methods (such as RSA) and is hence of relevance to national security. The realization of quantum computing is an inordinately difficult task for which the ideal experimental platform is not yet known. The two daunting challenges that stand in the way of the realization of a practical quantum computer of nontrivial size are overcoming decoherence, i.e., making reliable quantum bits 'qubits' and achieving scalability, i.e., producing large numbers of individually addressable qubits. Competing approaches on a worldwide scale involve ions in electromagnetic traps, atoms in optical traps, superconducting circuits, artificial atoms such as quantum dots or engineered dopant-vacancy defects in diamond, and pure light. The last approach has been successfully developed, with NSF support, by the Quantum Fields and Quantum Information (QFQI) group at the University of Virginia. It builds on exploiting the density of spectral encoding available to braodband emitting lasers and, more precisely, optical parametric oscillators (OPO). This project addresses a unique, scalable implementation of quantum information and quantum computing in an ultracompact physical system: the quantum optical frequency comb defined by the resonant modes (qumodes) of a single OPO. With NSF support, the QFQI group initiated the idea and pioneered its implementation in the laboratory, demonstrating record-levels of multipartite entanglement (60 qumodes, the optical field analogs of qubits) and obtaining several theoretical results in collaboration with Nick Menicucci at the U. of Sydney. The project will expand this widely successful frequency-domain entanglement approach to the time domain, and use hybrid frequency-time entanglement in order to implement a universal quantum computer in a single OPO. This will require the first ever realization of a fully scalable two-dimensional square-grid-lattice cluster state, which will still take place in a single OPO, by combining frequency-domain and time-domain entanglement --- as frequency and time will effectively constitute each dimension of the square-grid lattice. Such a realization includes the possibility of quantum error encoding using the Gottesman-Kitaev-Preskill scheme, for which Menicucci recently proved the existence of a fault tolerance threshold.