The Bloch Vector

Brian P. Anderson

Gen Kimura

Paweł Kurzyński

An ability to describe quantum states directly by average values of measurement outcomes is provided by the Bloch vector. For an informationally complete set of measurements one can construct unique Bloch vector for any quantum state. However, not every Bloch vector corresponds to a quantum state. It seems that only for two-dimensional quantum systems it is easy to distinguish proper Bloch vectors from improper ones, i.e. the ones corresponding to quantum states from the other ones. I propose an alternative approach to the problem in which more than one vector is used. In particular, I show that a state of the qutrit can be described by the three qubit-like Bloch vectors.

Charles P. Poole, Horácio A. Farach

Joseph V. Pulé

Taro Yoshino, Koichi Tojo, Kei Hagihara +2 more

An unbiased qubit positive operator-valued measure (POVM) can be uniquely expressed in terms of the Bloch vector. We prove that for POVMs with coplanar Bloch vectors, they are jointly measurable if and only if the perimeter of the convex closure of Bloch vectors is less than or equal to 4. Moreover, as a corollary, we give an affirmative answer to the conjecture suggested by [Andrejic-Kunjwal 2020] on the existence of a joint device for POVMs whose Bloch vectors lie on the same circle.