Violation of Heisenberg’s Uncertainty Principle

Recently, violation of Heisenberg’s uncertainty relation in spin measurements is discussed [J. Erhart et al., Nature Physics 8, 185 (2012)] and [G. Sulyok et al., Phys. Rev. A 88, 022110 (2013)]. We derive the optimal limitation of Heisenberg’s uncertainty principle in a specific two-level system (e.g., electron spin, photon polarizations, and so on). Some physical situation is that we would measure σ_x and σ_y, simultaneously. The optimality is certified by the Bloch sphere. We show that a violation of Heisenberg’s uncertainty principle means a violation of the Bloch sphere in the specific case. Thus, the above experiments show a violation of the Bloch sphere when we use ±1 as measurement outcome. This conclusion agrees with recent researches [K. Nagata, Int. J. Theor. Phys. 48, 3532 (2009)] and [K. Nagata et al., Int. J. Theor. Phys. 49, 162 (2010)].