qiskit/releasenotes/notes/0.24/fix-deprecated-bit-qpy-roun...

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---
fixes:
- |
The deprecated :class:`~qiskit.circuit.Qubit` and :class:`.Clbit` properties :attr:`~.Qubit.register` and
:attr:`~qiskit.circuit.Qubit.index` will now be correctly round-tripped by QPY (:mod:`qiskit.qpy`) in all
valid usages of :class:`.QuantumRegister` and :class:`.ClassicalRegister`. In earlier releases
in the Terra 0.23 series, this information would be lost. In versions before 0.23.0, this
information was partially reconstructed but could be incorrect or produce invalid circuits for
certain register configurations.
The correct way to retrieve the index of a bit within a circuit, and any registers in that
circuit the bit is contained within is to call :meth:`.QuantumCircuit.find_bit`. This method
will return the correct information in all versions of Terra since its addition in version 0.19.