Fig. 1: Architecture, performance and stability of Ascella.

a, Sketch of the overall architecture of the six single-photon quantum computer. A quantum-dot single-photon source (SPS) device at 5 K is operated at a repetition rate of 80 MHz. An active demultiplexer followed by fibred delays converts the train of single photons into six photons arriving simultaneously at the universal 12-mode photonic chip. Photons are detected at the chip output by superconducting nanowire single-photon detectors (SNSPD) and detection times are processed by a correlator. A full software stack controls the unitary matrix U implemented on the chip through the voltages \({\overrightarrow{V}}\) applied on 126 thermal phase shifters, yielding phase shifts \({\overrightarrow{\phi }}\), and the photonic input state according to the job requested. It also recalibrates hourly and readjusts all hardware control knobs for optimal performance. The single photons are sent into a photonic chip featuring a universal interferometer scheme capable of implementing any 12 × 12 unitary matrix. b, Detected N-photon coincidence rates for N-photon inputs as a function of time, with the photonic circuit configured to implement the identity matrix. The rates are integrated for 50 s. The grey areas correspond to maintenance and upgrade periods. In the right figure, we also monitor the on-chip photon indistinguishability and single-photon purity, as quantified by the Hong–Ou–Mandel (HOM) visibility V_HOM and 1 − g⁽²⁾(0), respectively, where g⁽²⁾ is the normalized second-order correlation function. HOM kT is V_HOM for delays k × ΔT between emitted photons where ΔT = 180 ns. Each data point corresponds to a correlation histogram integrated over 10 s. c, Job execution flowchart on Ascella. Perceval users may send jobs consisting in photonic circuits, or a gate-based circuit (GB) or a unitary matrix (U), along with the desired input state to the Quandela Cloud. The job is first processed by a CPU, which computes the necessary phase shifts \({\overrightarrow{\phi }}\) to apply, and subsequently the voltages \({\overrightarrow{V}}\) for the on-chip phase shifters from our compilation and transpilation process. Finally, the user receives the collected single-photon and coincidence counts after the computation on the quantum processing unit (QPU).