Rotation angles of unitary operator

Given a complex unitary $$2*2$$ matrix $$A$$ that represents some quantum gate on a single qubit.

What is the formula to extract to $$\theta_X, \theta_Y, \theta_Z$$ rotations around each one of the axes in the Bloch Sphere?

I will mention that I know that :

$$U_3(\theta, \phi, \lambda) = \begin{pmatrix}\cos(\theta/2) & -e^{i\lambda}\sin(\theta/2) \\ e^{i\phi}\sin(\theta/2) & e^{i\lambda + i\phi}\cos(\theta/2)\end{pmatrix}$$

But while trying to use it, I struggled with $$cos()=complex-value$$ that gave me complex angles

• – glS
Commented May 29, 2022 at 14:27

The Euler angles that you obtain depend on the decomposition that you implement over your 1 qubit gate. i.e it is possible to decompose any 1 qubit gate $$U$$ as:

$$U = e^{i\gamma}R_z(\theta)R_y(\phi)R_z(\lambda)$$

or

$$U = e^{i\gamma}R_z(\theta)R_x(\phi)R_z(\lambda)$$

In those cases, you'd obtain 2 $$\theta_Z$$ angles and 1 $$\theta_Y$$ or 2 $$\theta_Z$$ angles and 1 $$\theta_X$$ respectively. You can fin the methodology for some of the decompositions in this page:

https://purva-thakre.github.io/purva-blog/gsoc/qutip/single-qubit-example/

Qiskit has a class to implement several types of decompositions:

https://qiskit.org/documentation/stubs/qiskit.quantum_info.OneQubitEulerDecomposer.html

You can see the source code to know how they work:

https://qiskit.org/documentation/_modules/qiskit/quantum_info/synthesis/one_qubit_decompose.html#OneQubitEulerDecomposer

• Thabk you so much! Commented May 29, 2022 at 15:30