Initializing the zero state and applying a circuit returns a different statevector than applying a circuit without initializing the zero state. #1977
Description
Informations
- Qiskit Aer version: 0.13.0
- Python version: 3.9.16
- Operating system: MacOS
What is the current behavior?
When using method="statevector" with AerSimulator, a circuit with an initialize of |00> (i.e. [1,0,0,0]) returns a different statevector than the same circuit without explicitly initializing the |00> state.
Steps to reproduce the problem
import qiskit
from qiskit_aer import AerSimulator
circuits = dict()
circuits["zero"] = qiskit.QuantumCircuit(2)
circuits["zero"].initialize([1., 0., 0., 0.])
circuits["X"] = qiskit.QuantumCircuit(2)
circuits["X"].x(0)
circuits["zero+X"] = circuits["zero"].compose(circuits["X"])
def transpile_run_and_return_statevector(circ):
aersim = AerSimulator(method="statevector")
transpiled_circ = qiskit.transpile(circ, aersim)
transpiled_circ.save_statevector()
sim_results = aersim.run(transpiled_circ).result()
print(f"Circuit:\n{transpiled_circ}")
print(f"Statevector:\n{sim_results.get_statevector(transpiled_circ)}")
for name, circ in circuits.items():
print(f"For circuit {name}\n")
transpile_run_and_return_statevector(circ)
print("\n\n")results in
For circuit zero
Circuit:
┌──────────────────────┐ statevector
q_0: ┤0 ├──────░──────
│ Initialize(1,0,0,0) │ ░
q_1: ┤1 ├──────░──────
└──────────────────────┘ ░
Statevector:
Statevector([1.+0.j, 0.+0.j, 0.+0.j, 0.+0.j],
dims=(2, 2))
For circuit X
Circuit:
global phase: π/2
┌────────────────┐ statevector
q_0: ┤ U3(π,-π/2,π/2) ├──────░──────
└────────────────┘ ░
q_1: ────────────────────────░──────
░
Statevector:
Statevector([3.74939946e-33+6.123234e-17j, 1.00000000e+00+0.000000e+00j,
0.00000000e+00+0.000000e+00j, 0.00000000e+00+0.000000e+00j],
dims=(2, 2))
For circuit zero+X
Circuit:
global phase: π/2
┌──────────────────────┐┌────────────────┐ statevector
q_0: ┤0 ├┤ U3(π,-π/2,π/2) ├──────░──────
│ Initialize(1,0,0,0) │└────────────────┘ ░
q_1: ┤1 ├────────────────────────░──────
└──────────────────────┘ ░
Statevector:
Statevector([6.123234e-17+0.j, 6.123234e-17-1.j, 0.000000e+00+0.j,
0.000000e+00+0.j],
dims=(2, 2))
What is the expected behavior?
Circuits "X" and "zero+X" should return the same statevector (i.e. [0, 1, 0, 0]) but "zero+X" returns [0,-1j,0,0].
Suggested solutions
I don't know the code well enough to determine where this bug is coming from. But it is new as of 0.13.0