Fig. 1: Details of the quantum circuit.
From: Experimental characterization of the energetics of quantum logic gates
a The element we inspect is a two-qubit controlled operation. b This can be decomposed as a series of three operations: a single-qubit gate uθ, a two-qubit control-σz gate, and a second gate uθ. This relies on the fact that uθuθ = 1, that is, the identity, while uθσzuθ = u2θ. c The circuit is implemented in a photonic platform with polarization encoding. The two qubits are represented by the polarization of photons from a standard down conversion source (not shown). The basic element of the gate is a partially polarizing beam splitter (PPBS) with expected transmittivities TH = 1, TV = 1/3 for the two polarizations. This is embedded in a polarization Mach–Zehnder to mitigate the effects of the deviation of TH from 1 (we have TH ≃ 0.985 in our experiment)—for the sake of phase stability this was implemented as a Sagnac loop. Further, PPBSs are needed in order to equalize losses on the two polarizations. Half-wave plates (HWPs) set at an angle θ/2 are used to implement uθ. Polarization measurements in the logic basis are implemented using polarizing beam splitters (PBSs) and single-photon counting modules (SPCMs).
