From 9439c8fb188163985c1d5a8a3a57649d976ed172 Mon Sep 17 00:00:00 2001
From: Luciano Bello <bel@zurich.ibm.com>
Date: Tue, 19 Dec 2023 15:53:39 +0100
Subject: [PATCH] Remove Bit.register and Bit.index (#10996)

* Remove Bit.register and Bit.index

* remove impor

* black

* reno

* Fixup release note

---------

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
---
 qiskit/circuit/bit.py                         | 47 -------------------
 .../notes/fixes_10744-83e5f33f5db74a22.yaml   |  9 ++++
 test/python/circuit/test_bit.py               | 14 +-----
 3 files changed, 10 insertions(+), 60 deletions(-)
 create mode 100644 releasenotes/notes/fixes_10744-83e5f33f5db74a22.yaml

diff --git a/qiskit/circuit/bit.py b/qiskit/circuit/bit.py
index d51e82c8462..fd5ce54cc46 100644
--- a/qiskit/circuit/bit.py
+++ b/qiskit/circuit/bit.py
@@ -16,7 +16,6 @@
 import copy
 
 from qiskit.circuit.exceptions import CircuitError
-from qiskit.utils.deprecation import deprecate_func
 
 
 class Bit:
@@ -59,52 +58,6 @@ def __init__(self, register=None, index=None):
             self._hash = hash((self._register, self._index))
             self._repr = f"{self.__class__.__name__}({self._register}, {self._index})"
 
-    @property
-    @deprecate_func(
-        is_property=True,
-        since="0.17",
-        package_name="qiskit-terra",
-        additional_msg=(
-            "Instead, use :meth:`~qiskit.circuit.quantumcircuit.QuantumCircuit.find_bit` to find "
-            "all the containing registers within a circuit and the index of the bit within the "
-            "circuit."
-        ),
-    )
-    def register(self):  # pylint: disable=bad-docstring-quotes
-        """Get the register of an old-style bit.
-
-        In modern Qiskit Terra (version 0.17+), bits are the fundamental object and registers are
-        aliases to collections of bits.  A bit can be in many registers depending on the circuit, so
-        a single containing register is no longer a property of a bit.  It is an error to access
-        this attribute on bits that were not constructed as "owned" by a register."""
-        if (self._register, self._index) == (None, None):
-            raise CircuitError("Attempt to query register of a new-style Bit.")
-
-        return self._register
-
-    @property
-    @deprecate_func(
-        is_property=True,
-        since="0.17",
-        package_name="qiskit-terra",
-        additional_msg=(
-            "Instead, use :meth:`~qiskit.circuit.quantumcircuit.QuantumCircuit.find_bit` to find "
-            "all the containing registers within a circuit and the index of the bit within the "
-            "circuit."
-        ),
-    )
-    def index(self):  # pylint: disable=bad-docstring-quotes
-        """Get the index of an old-style bit in the register that owns it.
-
-        In modern Qiskit Terra (version 0.17+), bits are the fundamental object and registers are
-        aliases to collections of bits.  A bit can be in many registers depending on the circuit, so
-        a single containing register is no longer a property of a bit.  It is an error to access
-        this attribute on bits that were not constructed as "owned" by a register."""
-        if (self._register, self._index) == (None, None):
-            raise CircuitError("Attempt to query index of a new-style Bit.")
-
-        return self._index
-
     def __repr__(self):
         """Return the official string representing the bit."""
         if (self._register, self._index) == (None, None):
diff --git a/releasenotes/notes/fixes_10744-83e5f33f5db74a22.yaml b/releasenotes/notes/fixes_10744-83e5f33f5db74a22.yaml
new file mode 100644
index 00000000000..22af9ea1bac
--- /dev/null
+++ b/releasenotes/notes/fixes_10744-83e5f33f5db74a22.yaml
@@ -0,0 +1,9 @@
+---
+upgrade:
+  - |
+    The properties ``Bit.register`` and ``Bit.index`` are removed. They were deprecated in
+    Qiskit 0.25 (released in April, 2021). The qubits and bits now live only in the context of
+    a :class:`.QuantumCircuit`. The alternative to the properties is
+    to use :meth:`.QuantumCircuit.find_bit` to find all the containing
+    registers within a circuit and the index of the bit within the 
+    circuit.
diff --git a/test/python/circuit/test_bit.py b/test/python/circuit/test_bit.py
index a02e123ce20..cbacba159ab 100644
--- a/test/python/circuit/test_bit.py
+++ b/test/python/circuit/test_bit.py
@@ -17,7 +17,7 @@
 from unittest import mock
 
 from qiskit.test import QiskitTestCase
-from qiskit.circuit import bit, QuantumRegister
+from qiskit.circuit import bit
 
 
 class TestBitClass(QiskitTestCase):
@@ -106,15 +106,3 @@ def test_new_style_bit_equality(self):
         self.assertEqual(bit1, bit1)
         self.assertNotEqual(bit1, bit2)
         self.assertNotEqual(bit1, 3.14)
-
-    def test_bit_register_backreferences_deprecated(self):
-        """Verify we raise a deprecation warning for register back-references."""
-
-        qr = QuantumRegister(3, "test_qr")
-        qubit = qr[0]
-
-        with self.assertWarnsRegex(DeprecationWarning, "deprecated"):
-            _ = qubit.index
-
-        with self.assertWarnsRegex(DeprecationWarning, "deprecated"):
-            _ = qubit.register