Figure 3

Weak measurement protocol. (a) Full pulse sequence of the ancilla measurement algorithm used in the BLGI experiment. The measurement consists of a variable-amplitude Y rotation by an angle ϕ, which controls the strength of the measurement. This is followed by a CZ gate that entangles the ancilla qubit with the target qubit. Finally, the ancilla is rotated by an angle −π/2, bringing it into the desired measurement basis. Two cases are compared: that of the target qubit in the ground (blue) or excited (red, π rotation) state. (b) Bloch sphere representation of the ancilla qubit during the weak measurement protocol when the target qubit is in either the ground (blue) or excited (red) state. The Z averages of the ancilla and target qubit are correlated such that ${〈Z〉}_{\alpha }=\sin \left(\varphi \right){〈Z〉}_{\mathit{\tau }}$, where a full projective measurement corresponds to ϕ=π/2 and no measurement corresponds to ϕ=0. (c) Ancilla measurement of prepared target state before and after calibrating for measurement strength. We calibrate both curves by the scaling factor required to normalise the average 0 state curve. This is almost equivalent to dividing by sin (ϕ) but bounds the calibrated mean by ±1. In the calibrated case, the measured mean remains unchanged while the measured variance increases as ϕ decreases. The gold-shaded region denotes angles at which weak measurement data can violate the BLGI while still being reliably calibrated.