where formula_45 and formula_46 is the single-spin operator in the formula_47-direction. Here formula_48 will correspond to the population difference in the two level system, i.e. for an equal superposition of the up and down state formula_49. The formula_50−formula_51 plane represents the phase difference between the two states. This is also known as the Bloch sphere picture. We can then define uncertainty relations such as formula_52. For a coherent (unentangled) state, formula_53. Squeezing is here considered the redistribution of uncertainty from one variable (typically formula_48) to another (typically formula_51). If we consider a state pointing in the formula_50 direction, we can define the Wineland criterion for squeezing, or the metrological enhancement of the squeezed state as