CircuitError: 'inverse() not implemented for reset.'

Question

Initially, this code was functioning correctly, but I am currently encountering an error. I am attempting to create a circuit using a unitary vector and applying inversion.

desire_vector = [0.  0.5 0.  0.5 0.  0.5 0.  0.5]
qc = QuantumCircuit(qr, cr)
qc.initialize(desired_vector,qr)
backend = BasicAer.get_backend("qasm_simulator")
qc = transpile(qc,backend).inverse()

The Error:

Can we inverse a quantum ciruit that is initialized using a statevector?

Answer 1

Solution -1:

The initialize() method in Qiskit is designed to prepare a specific quantum state, and it does this by first resetting the qubits to the |0⟩ state before applying a series of gates to achieve the desired state. This reset operation is integral to the initialize() method and cannot be removed or stopped. The important thing is the reset operation is not reversible.

There is another way to achieve this by using 'prepare_state'. The prepare_state method does not add resets before state preparation, unlike initialize which resets all qubits to the |0⟩ state before state preparation.

from qiskit import QuantumCircuit
from qiskit.quantum_info import Statevector

vector = [0, 0.5, 0, 0.5, 0, 0.5, 0, 0.5]
np_vector = np.array(vector)
state = Statevector(np_vector)
qc = QuantumCircuit(3)
qc.prepare_state(state, [0, 1, 2])
circuit =transpile(qc, backend)
inverse_qc = circuit.inverse()
print(inverse_qc)

Output:

Solution -2:

In Qiskit, the "initialize" method automatically applies a "reset" operation to the qubits before initializing them. This is because the "initialize" method is designed to set the qubits to a specific state, and applying a "reset" operation ensures that the qubits are in a known state before initialization.

However, if we want to prepare a state without the "reset" operation being automatically applied, we can use the "StatePreparation" class introduced in Qiskit 0.35. This class allows we to prepare a state in the same fashion as "initialize" without the "reset" being automatically applied.

from qiskit import QuantumCircuit
from qiskit.quantum_info import Statevector
from qiskit.circuit.library import StatePreparation


state_vector = [1/np.sqrt(2), 0, 0, 1/np.sqrt(2)]

qc = QuantumCircuit(2)

state_prep = StatePreparation(Statevector(state_vector))
qc.append(state_prep, [0, 1])
qc= qc.inverse(); // or qc.power(-1)

In this example, the "StatePreparation" method is used to apply the transformation represented by the state vector to the quantum circuit. Please note that the "StatePreparation" method does not return a gate and therefore cannot be converted into a gate.