Quantum Optics seminar by Jeremy Adcock, University of Bristol


Quantum computers promise a paradigm shift in humanity's information processing capability. Measurement-based quantum computing - built on graph states - is the prevailing architecture for large-scale quantum computation [2]. Meanwhile, silicon quantum photonics is a high-performance, scalable quantum technology platform, boasting circuits of unparalleled size [3]. However, integrated quantum photonics has so far been constrained to two on-chip generated photons. Here, we present the first device to wield four-photon entanglement, and measure state-of-the-art on-chip quantum interference.

On our silicon chip (Fig. 1a), four sources of spontaneous four-wave mixing generate two Bell pairs in four dual-rail qubits. These are entangled using a two-qubit gate, programmably generating either star- or line-type graph states|a rst in optics. Then, recon gurable single-qubit gates access the remaining four-qubit graphstates and implement projective measurements. Finally, the photons are routed on-chip to superconducting nanowire single photon detectors.

Our star and line graph states have delities 0:780:01 and 0:680:02 respectively. Further, we verify our photons' purity via high visibiltiy on-chip Hong-Ou-Mandel interference: VHOM = 0:800:01 (Fig. 1b). Finally, we bound dominant sources of error with Bayesian parameter estimation, boosting scalability (Fig. 1c). Our device breaks the multiphoton barrier for integrated quantum photonics and pioneers programmable entanglement generation with photons, expediting progress towards large-scale quantum computing.