Collaborating researchers from Asia, Australia, and Europe developed a prototype of a graph-based quantum photonic device. The researchers based their architecture on the very large-scale integration (VLSI) of silicon quantum photonics to devise what they said is the largest-scale integrated quantum photonic device to date. VLSI quantum photonics refers to the integration of quantum photonic devices on a very large scale. The development of a reconfigurable, graph-based quantum device on a VLSI photonic chip — and the integration of quantum photonic devices on a very large scale — will enable the development of quantum circuits that can perform many diverse operations. Moreover, it will offer the potential for large-scale production of such devices, the researchers said. To date, scientists have found that mathematical models, including graphs that are based on 2D lattices of nonlinear optical crystals and linear optical circuits, can be used to design quantum devices and systems. The use of graphs allows characterizations of quantum correlations and investigations of quantum networks. However, building these graph-based quantum devices using traditional methods presents a formidable challenge. This is because global quantum coherence over the full device is necessary to impose genuine multiprocess quantum interference, the researchers said. The developers called their device Boya. It comprises nonlinear optical sources and linear optical circuits, and was fabricated on a 200-mm silicon-on-insulator wafer using advanced CMOS process technology. Each wafer contains 30 dies, each die contains four devices with slightly different designs, and each device integrates 2446 components in a 12- × 15-mm footprint. The device is optically and electrically packaged and accessed by 100 optical inputs/outputs and 432 electronic inputs. The completed device integrates about 2500 components to form a synthetic, 2D, 4 × 4 lattice consisting of an array of spontaneous, four-wave mixing, integrated photon-pair sources and a network of programmable, linear optical waveguide circuits. Using different topologies and tasks associated with the properties of graphs, the researchers reconfigured the device’s integrated lattice structure to generate and process complex graphs. The device directly enabled complex-weighted, undirected graphs with eight vertices, with each pathway of single photons, traveling from one source to one detector, representing a vertex. Each photon-pair source connects two separate pathways and represents an edge. The connections between vertices can be altered by reconfiguring the optical waveguide circuits. The amplitudes and phases of the edges can be individually controlled through an array of switches. In the bulk optical scheme, pairs of single photons that are generated in different crystals and routed along different pathways to the same detectors are no longer distinguishable. These single-photon pairs undergo quantum interference of identical processes. At each crystal, pump photons must synchronously meet with the incoming single photons from the previous crystal. Pump beams must simultaneously reach the crystals positioned in the same column. The pumps and single photons have different colors, and typically propagate noncollinearly in bulk optics. As a result, retaining global coherence of the device requires precise control of many-photon wavefunctions in the temporal, spatial, and spectral domains. The researchers retained quantum coherence over the entire device by ensuring that all the processes contributing to multiphoton correlations were quantum-mechanically indistinguishable. In tests, the researchers achieved strong reconfigurability by altering the links between crystals and rerouting single photons in linear optical circuits. The reconfigurability of the device enables arbitrary changes to the device structure and the topologies of graphs, they said. The researcher said that the realization of the graph quantum device using an integrated optics approach offers several advantages compared to the bulk optical scheme. The integrated optics can be tailored to perfectly match the optical length of paths for all the photons routing along lithographically defined circuits, ensuring good temporal mode matching. Further, all the sources and circuits are monolithically integrated, ensuring that graphs are reliably processed. Instead of pinning sources to the 4 × 4 grid, the researchers flattened the 2D grid into a 1D array, transposing the device structure into a braiding of waveguide circuits and forming a synthetic graph lattice. This allowed the researchers to overcome issues like loss accumulation and simplified the de-multiplexing and re-multiplexing of photons with different colors. To demonstrate the device’s capabilities, the researchers reconfigured the device to generate, manipulate, and verify genuine multiphoton, multidimensional quantum entanglement and demonstrated different entanglement structures. In the future, this capability could be a key resource for universal, multidimensional quantum computing and information processing. The reconfigurable nature of the device could enable algorithmic optimization for generating multiphoton, multidimensional Greenberger-Horne-Zeilinger states and implementing gates, the researchers said. The team also demonstrated quantum measurements and read out results from multiphoton coincidence patterns, probability distribution measurements of modulus-squared permanent and hafnian matrix functions for various graphs, mapping of abstract general graphs onto the physical hardware, and quantum measurements to estimate the graphs’ perfect matchings. The researchers said that the theoretical, graph-based quantum device can be arbitrarily reprogrammed to implement diverse tasks in quantum information processing. With the high-level visualization capabilities of graphs and its powerful mathematical machinery, the device could provide a versatile hardware platform on which to engineer complex quantum entanglement, design new quantum gates and resource states, learn complex quantum systems, and train future quantum processors. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01187-z).
