
Dancing with Qubits
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When plotting in three dimensions, we need either three Cartesian coordinates (x0, y0, z0) or a magnitude r and two angles φ and θ, as shown in Figure 4.26. coordinates$Cartesian R3
The magnitude is
φ is the angle from the positive x-axis to the dotted line from (0, 0) to the projection (x0, y0) of P into the xy-plane.
θ is the angle from the positive z-axis to the line segment 0P.
That’s a lot to absorb, but it builds up systematically from what we saw in R2. When r = 1, we get the unit sphere in R3. It’s the set of all points (x0, y0, z0) in R3 where x02 + y02 + z02 = 1. unit$sphere unit$ball
The unit ball is the set of all points where x02 + y02 + z02 ≤ 1.
We frequently return to the graphic...