## 3.5 Universality, again

Although this may all seem tediously abstract, it is surprisingly useful. Take another look at the single qubit interference circuit

and the corresponding sequence of unitary operations \begin{aligned} H \left( e^{-i\frac{\varphi}{2}Z} \right) H &= e^{-i\frac{\varphi}{2}X} \\&= \begin{bmatrix} \cos\varphi/2 & -i\sin\varphi/2 \\-i\sin\varphi/2 & \cos\varphi/2 \end{bmatrix} \end{aligned}

The single qubit interference circuit has a simple geometrical meaning: it shows how a rotation about the z-axis, induced by the phase gate P_\varphi, is turned, by the two Hadamard gates, into a rotation about the x-axis.

Now, take a look at this circuit:

What does it represent? The central part is a rotation by \varphi about the x-axis, but is it is sandwiched between two rotations about the z-axis. Now we have to appeal to your knowledge of classical mechanics: you may recall that any rotation in the Euclidean space can be performed as a sequence of three rotations: one about z-axis, one about x-axis, and one more about the z-axis. In this context, this implies that any unitary U, up to a global phase factor, can be written as \begin{aligned} U(\alpha, \beta, \varphi) &= e^{-i\frac{\beta}{2}Z} e^{-i\frac{\varphi}{2}X} e^{-i\frac{\alpha}{2}Z} \\&= \begin{bmatrix} e^{-i\left(\frac{\alpha+\beta}{2}\right)}\cos\frac{\varphi}{2} & ie^{i\left(\frac{\alpha-\beta}{2}\right)}\sin\frac\varphi{2} \\ie^{-i\left(\frac{\alpha-\beta}{2}\right)}\sin\frac\varphi{2} & e^{i\left(\frac{\alpha+\beta}{2}\right)}\cos\frac\varphi{2} \end{bmatrix}. \end{aligned}

Thus once you are given a couple of Hadamard gates and an infinite supply of phase gates, so that you can choose the three phases you need, you can construct an arbitrary unitary operation on a single qubit. Needless to say, the two axes in question, z and x, do not have any special status, geometrically speaking — if we have rotations about any two orthogonal58 axes then we can create any one-qubit unitary that we want.

Consider the following circuit:

where both A and B are unitary operations. We claim that any unitary U can be represented in this form.

Again, we can prove this geometrically. The circuit represents two rotations by 180^\circ about two axes obtained by rotating the z-axis with unitaries A and B, respectively. Any rotation in the three-dimensional space is the composition of two rotations by 180^\circ, as shown in Figure 3.9. The resulting axis of rotation is perpendicular to the two axes about which rotations by 180^\circ are performed, and the angle of the composed rotation is twice the angle between the two axes.

1. In fact, even this condition isn’t necessary! See Figure 3.8↩︎