Minimal Determination of a Pure State through Adaptive Tomography

Abstract: Finding the least measurement settings to determine an arbitrary pure state has been long known as the Pauli problem. In the fixed measurement scheme four orthonormal bases are required even though there are far less parameters in a pure state. Peres conjectured that two unbiased bases suffice to determine a pure state up to some finite ambiguities. Here we shall at first prove Peres conjecture in the case of $d=3,4$, namely, two unbiased measurements determine a pure state up to to 6 and 16 candidates for a qutrit and ququad, respectively. And then, taking Peres' conjecture for established, we propose an adaptive 3-measurement scheme involving the minimal number of measurements, based on the observation that the ambiguities can be removed by an adaptive two-outcome projective measurement. With the help of this observation, we simplify a recent five-basis protocol $[Phys. Rev. Lett. 115, 090401 (2015)]$ to a three-basis one at the cost of the extra dichotomic measurement.