Hello... How can I get the actual quantum state when I only know the expectation value of that particular state?

How do I create a unitary parameterized gate with ParameterVector and specifying its matrix? I tried to build it in the same way than circuit.u is built, but it raises an error when I use my gate via circuit.unitary(): "ParameterExpression with unbound parameters (dict_keys([ParameterVectorElement(θ[0]), ParameterVectorElement(θ[1])])) cannot be cast to a complex." I saw that a general rotation (circuit.u) in qiskit is permormed by 3 rotations Rz·Ry·Rz, but I'm trying to build a general rotation with the following operator: U_xyz = exp[-i theta (nx sigmaX + ny sigmaY+ nz sigmaZ) ] , where the parameters would be theta, nx, ny and nz, in principle. My question is, why it raises that error in my case and not in circuit.u? I also checked that circuit.u matrix is complex, indeed. On the other hand, I tried to build my gate via circuit.append(), but I'm not sure how to do it. Thanks

What does the circuit (qc) actually do? It would be helpful to know this to understand the result

@horse_chestnut2359

2 ай бұрын

The X gate creates a flip of the qubit such that the result you get from setting an initial state of |0> will now be |1>. You can see this in the cell below the one the person is writing in, that the probability for getting |1> is 1.0. However once you add the depolarising error, you get a nonzero probability of getting |0> again, not just |1>.