FIG. 3. Experimental results. We evaluate the speed of a two-qubit system in the state ρp = p|φ+〉〈φ+| + (1 − p)|φ−〉〈φ−|, for unitary evolutions UτρpU †τ ,Uτ = e−iH2τ , H2 = σx,y,z A ⊗ IB + IA ⊗ σx,y,z B, over an interval τ = π/6. In (a)–(c), the blue continuous line is the theoretical speed function Sπ/6(ρp,H2), which we aim at reconstructing, while the blue points are the experimental values extracted from purity and overlap measurements, for p = 0,0.1,0.2, . . . ,0.9,1. The error bars are determined by Monte Carlo simulation with Poisson-distributed error (1000 samples for each point). For comparison, the two green dashed lines depict the speed function computed from the reconstructed states of copy 1,2 (the density matrices are reported in the main text), respectively. Superlinear scaling due to entanglement is detected for values above the horizontal, black dotted line.
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