Operator(circuit) does not return the correct unitary

[rabaniten]

Information

-Qiskit Terra version: 0.8.1
-Python version: 3.7.2
-Operating system: MacOS Mojave

What is the current behavior?

The data stored in Operator(circuit).data (with Operator imported from qiskit.quantum_info
) seems not to correspond to the right unitary for certain circuits.

Steps to reproduce the problem

Choose the following circuit:

qasm = 'OPENQASM 2.0;'
qasm += 'include "qelib1.inc";'
qasm += 'qreg q[3];'
qasm += 'creg c[3];'
qasm += 'rx(3.141592653589793) q[2];'
qasm += 'ry(0.294524311274043) q[2];'
qasm += 'cx q[0],q[2];'
qasm += 'ry(-0.0981747704246810) q[2];'
qasm += 'cx q[1],q[2];'
qasm += 'ry(0.0981747704246810) q[2];'
qasm += 'cx q[0],q[2];'
qasm += 'ry(-3.141592653589793) q[2];'
circuit = QuantumCircuit.from_qasm_str(qasm)

One finds a different result [than the one found by(Operator(circuit).data] using the unitary_simulator:

simulator = BasicAer.get_backend('unitary_simulator')
result = execute(circuit, simulator).result()
unitary = result.get_unitary(circuit)
matrix_equal(Operator(circuit).data, unitary, ignore_phase=True)

The last line returns False.

[nonhermitian]

This does indeed look like a bug in the operator class. The numerical values appear to be correct, but the indices are not. So it seems that there is some array manipulations that are not correct.

[nonhermitian]

To make reproducing easier, here is the circuit:

q = QuantumRegister(3, 'q')
c = ClassicalRegister(3, 'c')
qc = QuantumCircuit(q, c)
qc.rx(3.141592653589793, q[2])
qc.ry(0.294524311274043, q[2])
qc.cx(q[0], q[2])
qc.ry(-0.0981747704246810, q[2])
qc.cx(q[1], q[2])
qc.ry(0.0981747704246810, q[2])
qc.cx(q[0], q[2])
qc.ry(-3.141592653589793, q[2])

[chriseclectic]

Looks like the single qubit gate is getting applied to the wrong index, but only for rx, ry, rz.
If I run the following circuit with some debugging prints added the last three are getting applied to q[0] instead of q[2].

q = QuantumRegister(3, 'q')
qc = QuantumCircuit(q)
qc.iden(q[2])
qc.x(q[2])
qc.y(q[2])
qc.z(q[2])
qc.h(q[2])
qc.s(q[2])
qc.sdg(q[2])
qc.t(q[2])
qc.tdg(q[2])
qc.u1(0.2, q[2])
qc.u2(0.1, 0.2, q[2])
qc.u3(0.1, 0.2, 0.3, q[2])
qc.rz(0.294524311274043, q[2])
qc.ry(0.294524311274043, q[2])
qc.rz(0.294524311274043, q[2])

My guess is there is a bug in how Operator unrolls gates without a matrix def. I'll look into a fix.