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Expanding two rotation operators

Quantum Computing Asked by Oilobobolus on April 24, 2021

I have the following operators acting on two qubits, denoted as $(1)$ and $(2)$:

$$T_1=displaystyleexpleft(-ifrac{pi}{4}Zotimes Zright)cdot R_z^{(1)}left(frac{pi}{2}right)cdot R_z^{(2)}left(frac{pi}{2}right),quad T_2=R_y^{(2)}left(-frac{pi}{2}right)cdot C_Zcdot R_y^{(2)}left(frac{pi}{2}right).$$

I should prove that up to global phases $T_1=C_Z$ and $T_2=C_X$ (the controlled $Z$ and $X$ gates).

For convenience, I omit the labels for the qubits, which should be clear by the ordering. Using the usual expansion for rotations, I ended up finding

$$T_1=frac{1}{2sqrt 2}(mathbb Iotimesmathbb{I}-iZotimes Z)(mathbb Iotimes mathbb I-iZotimesmathbb I)(mathbb Iotimes mathbb I-imathbb I otimes Z)=frac{1+i}{sqrt 2}begin{pmatrix} -Z & mathbb Iend{pmatrix}, T_2=frac{1}{2}(mathbb Iotimes mathbb I+imathbb Iotimes Y)C_Z(mathbb Iotimes mathbb I-imathbb Iotimes Y)=begin{pmatrix} mathbb I & & Xend{pmatrix}.$$
So, I’ve checked the calculations a few times and it looks like there is something wrong with the first operator. Here’s the catch: there was a mistake in the original $T_2$ I was given that I had to change in order to find the correct sequence to obtain a $C_X$, and there might be a similar problem with $T_1$ (unless I am of course making some mistake somewhere). Does anyone see what the issue may be?

(As a reference, $T_1$ comes from the interaction shift in NMR computing, while $T_2$ from Wineland’s experiment with trapped ions).

One Answer

I agree with your calculation for $T_1$ and $T_2$ starting from your expansion of the operations (the second set of equations). Since there's a couple of possible different conventions for the definition of, for example $R_z(theta)$, I cannot be certain that the expansion itself is correct. Indeed, it requires relatively minor modification to make things work. For example, if you replace both of the $pi/2$ rotation angles in the statement for $T_1$ with $-pi/2$ rotation angles, it all works happily.

It may assist you to know the way that I do this calculation. For $T_1$, you've got 3 matrix exponentials, all of which are using diagonal operators. This means they all commute and hence can be grouped together as a single exponential. $$ T_1=e^{-ipi Z_1Z_2/4}e^{ipi Z_1/4}e^{ipi Z_2/4}=e^{ipi(Z_1+Z_2-Z_1Z_2)/4} $$ So I can just concentrate on the diagonal elements of these, $$ Z_1+Z_2-Z_1Z_2=text{diag}(1,1,1,-3) $$ I can add (or subtract) identity matrices as this only contributes a global phase $$ Z_1+Z_2-Z_1Z_2-I=text{diag}(0,0,0,-4). $$ So when I multiply by $pi/4$, I'll get a $pi$ for the final element which, exponentiated, gives the -1 I need.

Correct answer by DaftWullie on April 24, 2021

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