Fig. 4: Delegated computation between two NV centre nodes using QNodeOS.

a, DQC circuit (effective computation: single-qubit rotation R_Z(α); Methods). The DQC application consists of k circuit repetitions (varying measurement bases for tomography on |ψ⟩) realized by two programs: the DQC-client program (client node, repeating the sequence ‘quantum block (C1, orange)–classical block (computing δ)’ k times) and the DQC-server program (server node, repeating ‘quantum block (S1, blue)–classical block (receiving δ)–quantum block (S2, purple)’ k times). Client and server produce entanglement |Φ⁺⟩ = (|00⟩ + |11⟩)/√2 (S1 and first part of C1). The client performs gates and a measurement, resulting in outcome bit m_c (rest of C1). Client computes δ from m_c and DQC parameters α ∈ [0, 2π) and θ ∈ [0, 2π) and sends δ. Meanwhile, the server keeps its qubit coherent. On receiving δ, the server applies gates depending on δ, resulting in single-qubit state |ψ⟩ (S2) depending only on α and θ. b, Experimental results executing DQC for six different sets of (α, θ) parameters (k = 1,200, that is, 7,200 executions of circuit Fig. 4a). Fidelity to |ψ⟩ estimated using single-qubit tomography (1,200 measurement results per data point) and corrected for known tomography errors (SSRO, blue), post-selected for charge-resonance (CR) check validation (purple) and post-selected for latencies (orange) (Methods). c, Sequence diagram including the interaction CNPU–QNPU–QDevice for one execution of the DQC circuit (repeated k = 1,200 times in each experiment) (time flows to the right; not to scale). CNPUs prepare NetQASM subroutines (C1, S1, S2) and send them to their respective QNPUs. CNPUs perform classical computation (message δ). QNPUs execute subroutines, sending physical instructions to their QDevices. Entanglement is generated by QDevices performing a batch of attempts (triggered by a ‘create EPR’ physical instuction), resulting in the heralding of a two-qubit entangled state rotated to |Φ⁺⟩ by the server. d, Processing times and latencies while server qubit is live (time frame red line in c, averaged over all 7,200 circuit executions except executions with latency spikes; see Methods), including CNPU–QNPU communication latencies, CNPU processing on both nodes and client–server communication latency (CC) (average total of about 4.8(±0.8) ms; error bars (standard deviation) for the sum of individual segments (per segment: Supplementary Information section 4.6)).