Fig. 1: Experimental implementation of optimal collective measurements using quantum computers.
From: Approaching optimal entangling collective measurements on quantum computing platforms
a,b, Probe states are sent to the quantum computers (QC) individually for the single-copy measurement (a) and in pairs for the two-copy measurement (b). c,d, The qubit probes experience rotations, θx and θy, about the x and y axes of the Bloch sphere (c) before undergoing decoherence that has the effect of shrinking the Bloch vector (d). This rotation can be thought of as being caused by an external magnetic field that we wish to sense. e,f, The QCs then implement quantum circuits corresponding to the optimal single-copy (e) and two-copy (f) measurements. Two optimal single-copy circuits are shown, one for estimating θx and one for θy. g, Finally, error mitigation is used to improve the accuracy of the estimated angle. We create a model (green line) for how the noisy estimate of θ, \({\hat{\theta }}_{{{{\rm{noisy}}}}}\) (black dots), is related to the true value (red line). The model is then used to correct \({\hat{\theta }}_{{{{\rm{noisy}}}}}\) to produce the final estimate \(\hat{\theta }\). Sample data from the F-IBM QS1 device downsampled by a factor of three are shown in g. Error bars are smaller than the markers.
