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QuantumGraph.py - Changes Summary

Imports and Library Adjustments

Qiskit Version Comptability

  • Previous: from qiskit import QuantumCircuit, execute, Aer
  • Corrected: from qiskit import transpile, QuantumCircuit from qiskit_aer import AerSimulator

Explanation: Updated to match Qiskit 2.0.2, replacing deprecated imports with the current supported simulator (AerSimulator).

Decomposition Libraries

  • Previous from qiskit.quantum_info.synthesis import OneQubitEulerDecomposer from qiskit.quantum_info.synthesis.two_qubit_decompose import two_qubit_cnot_decompose
  • Corrected from qiskit.synthesis import OneQubitEulerDecomposer from qiskit.synthesis import TwoQubitBasisDecomposer from qiskit.circuit.library import CXGate

Explanation: Adjusted imports to use TwoQubitBasisDecomposer and CXGate for compatibility.

Tomography Changes

  • Previous: from pairwise_tomography.pairwise_state_tomography_circuits import pairwise_state_tomography_circuits from pairwise_tomography.pairwise_fitter import PairwiseStateTomographyFitter
  • Corrected: from qiskit_experiments.library.tomography import StateTomography

Explanation: qiskit-ignis dependencies replaced with qiskit-experiments library to align with newer Qiskit versions.

Backend Handling

  • Previous: self.backend = Aer.get_backend('qasm_simulator')
  • Corrected: self.backend = AerSimulator()

Explanation: Updated backend initialization to reflect the new method in qiskit-aer.

Execution Method Updates

  • Previous: Used execute() method.
  • Corrected: Directly used backend’s .run() method (recommended practice for newer Qiskit versions).

State Tomography

  • Previous: Custom pairwise tomography implementation.
  • Corrected: Utilized built-in StateTomography from qiskit-experiments.

Gate Implementation Adjustments

  • Previous: Used deprecated u3 gate: self.qc.u3(the, phi, lam, qubit)
  • Corrected: Replaced with universal gate (u): self.qc.u(the, phi, lam, qubit)

Two-Qubit Decomposition

  • Previous: two_qubit_cnot_decompose(U)
  • Corrected: TwoQubitBasisDecomposer(CXGate())

Explanation: Updated method to use the new decomposer explicitly specifying CXGate.

Result Handling

  • Added: Custom FakeResult class used as a workaround for qiskit-aer issue.

Miscellaneous Changes

  • Added extensive debug print statements to monitor the internal state of computations.

Overall Summary

The changes enhance compatibility with Qiskit 2.0.2 and improve stability, clarity, and maintainability of the quantum circuit implementations, especially in tomography and gate decomposition areas.