On-chip generation and coherent control of d-level entangled photon states through integrated frequency combs

Optical quantum states based on systems of complex entangled photons play a central role in quantum information science. Recently, integrated photonics has become a leading platform for the compact, cost-efficient, and stable... [ view full abstract ]

Optical quantum states based on systems of complex entangled photons play a central role in quantum information science. Recently, integrated photonics has become a leading platform for the compact, cost-efficient, and stable generation and processing of such states. The realization of d-level quantum states (qudits, with d>2) and their coherent control are, however, necessary for improving robustness and the key rate of quantum communications protocols, and for achieving more efficient and error-tolerant quantum computation. To date, on-chip high-dimensional entangled states, as well as multi-photon states, have either been inaccessible, lacked scalability, or were difficult to manipulate, using elaborate control approaches.

We show that using integrated frequency combs (on-chip light sources with a broad spectrum of equally-spaced frequency modes) based on high-Q nonlinear microring resonators provides a solution for achieving sources of scalable complex quantum states. We exploit time-bin and frequency-bin entanglement, which are fully scalable and promising for quantum information processing, as time and frequency modes can be robustly transmitted in a single spatial mode. By using spontaneous four-wave mixing within the microring, we demonstrate the on-chip generation and coherent control of two- and multi-photon entangled states of qubits, as well as two-photon d-level entangled states. Remarkably, these are generated in a coherent superposition of pure frequency modes, demonstrating the first realization of maximally-entangled d-level optical quantum states on integrated devices (see attachment). We confirm the realization of a quantum system with at least one hundred dimensions. Furthermore, using off-the-shelf telecommunications components, we introduce a platform for the coherent manipulation and control of frequency-entangled qudit states. We characterize the dimensionality of the states by means of Schmidt mode decomposition, and their entanglement and nonlocal properties through full quantum state tomography and quantum interference, demonstrating Bell inequality violation. Our results suggest that the generation of entangled photons in microcavities and their coherent control through accessible telecommunications infrastructures introduce a powerful and scalable platform for quantum information science.