By exciting a nonlinear optical resonant element with a coherent set of multiple laser pulses in which the pulse separation is much larger than the photon lifetime in the resonator, researchers from the Institut National de la Recherche Scientifique (INRS-EMT; Varennes, QC, Canada) have prepared and coherently manipulated discrete d-level multipartite quantum systems based on frequency-time hyper-entangled two photon states. To do so, they use integrated photonic chips and fiber-optic telecommunications components. The demonstration is the first experimental realization and characterization of so-called qudit cluster states that are equivalent to the realization of a quantum computer, as well as the first proof-of-concept demonstration of high-dimensional one-way quantum processing.
The photon pairs (signal and idler) are simultaneously generated in a four-wave mixing process as a superposition of several time modes (d = 3, given by the number of pulses) generating a time-based d-level two-photon state. One pulse exciting the nonlinear resonant medium (microring resonator) additionally generates a frequency-based d-level photon state. Together, this creates a d-level hyperentangled multipartite state. Next, frequency-to-time mapping is accomplished using a fiber Bragg grating (FBG) array in a phase-stable loop configuration, transforming the hyperentangled state into a three-level four-partite cluster state using an appropriate phase pattern. These d-level cluster states can tolerate up to 66.6% of incoherent noise compared to only 50% for four- and six-qubit cluster states. Furthermore, the team transformed these cluster states into different orthogonal bi-partite states and confirmed, through quantum interference measurements, that the states are mutually orthogonal and entangled, enabling powerful, noise-tolerant quantum operations using standard optical components. Reference: C. Reimer et al., Nat. Phys. Lett.; https://www.nature.com/articles/s41567-018-0347-x (Dec. 3, 2018).
