Add barrier labels and display in 3 circuit drawers

qiskit/circuit/barrier.py

@@ -25,9 +25,18 @@ class Barrier(Instruction):
 
     _directive = True
 
-    def __init__(self, num_qubits):
-        """Create new barrier instruction."""
-        super().__init__("barrier", num_qubits, 0, [])
+    def __init__(self, num_qubits, label=None):
+        """Create new barrier instruction.
+
+        Args:
+            num_qubits (int): the number of qubits for the barrier type [Default: 0].
+            label (str): the barrier label
+
+        Raises:
+            TypeError: if barrier label is invalid.
+        """
+        self._label = label
+        super().__init__("barrier", num_qubits, 0, [], label=label)
 
     def inverse(self):
         """Special case. Return self."""

qiskit/circuit/quantumcircuit.py

@@ -2863,10 +2863,14 @@ def _rebind_definition(
                             inner.operation.params[idx] = param.bind({parameter: value})
                         self._rebind_definition(inner.operation, parameter, value)
 
-    def barrier(self, *qargs: QubitSpecifier) -> InstructionSet:
+    def barrier(self, *qargs: QubitSpecifier, label=None) -> InstructionSet:
         """Apply :class:`~qiskit.circuit.Barrier`. If qargs is empty, applies to all qubits in the
         circuit.
 
+        Args:
+            qargs (QubitSpecifier): Specification for one or more qubit arguments.
+            label (str): The string label of the barrier.
+
         Returns:
             qiskit.circuit.InstructionSet: handle to the added instructions.
         """
@@ -2889,7 +2893,7 @@ def barrier(self, *qargs: QubitSpecifier) -> InstructionSet:
             else:
                 qubits.append(qarg)
 
-        return self.append(Barrier(len(qubits)), qubits, [])
+        return self.append(Barrier(len(qubits), label=label), qubits, [])
 
     def delay(
         self,

qiskit/extensions/simulator/snapshot.py

@@ -60,26 +60,6 @@ def snapshot_type(self):
         """Return snapshot type"""
         return self._snapshot_type
 
-    @property
-    def label(self):
-        """Return snapshot label"""
-        return self._label
-
-    @label.setter
-    def label(self, name):
-        """Set snapshot label to name
-
-        Args:
-            name (str or None): label to assign unitary
-
-        Raises:
-            TypeError: name is not string or None.
-        """
-        if isinstance(name, str):
-            self._label = name
-        else:
-            raise TypeError("label expects a string")
-
     def c_if(self, classical, val):
         raise QiskitError("Snapshots are simulator directives and cannot be conditional.")
 

qiskit/visualization/latex.py

@@ -603,7 +603,10 @@ def _build_barrier(self, node, col):
                     first = last = index
             pos = self._wire_map[self._qubits[first]]
             self._latex[pos][col - 1] += " \\barrier[0em]{" + str(last - first) + "}"
-            self._latex[pos][col] = "\\qw"
+            if node.op.label is not None:
+                pos = indexes[0]
+                label = node.op.label.replace(" ", "\\,")
+                self._latex[pos][col] = "\\cds{0}{^{\\mathrm{%s}}}" % label
 
     def _add_controls(self, wire_list, ctrlqargs, ctrl_state, col):
         """Add one or more controls to a gate"""

qiskit/visualization/matplotlib.py

@@ -414,7 +414,9 @@ def _get_layer_widths(self):
                 self._data[node] = {}
                 self._data[node]["width"] = WID
                 num_ctrl_qubits = 0 if not hasattr(op, "num_ctrl_qubits") else op.num_ctrl_qubits
-                if getattr(op, "_directive", False) or isinstance(op, Measure):
+                if (
+                    getattr(op, "_directive", False) and (not op.label or not self._plot_barriers)
+                ) or isinstance(op, Measure):
                     self._data[node]["raw_gate_text"] = op.name
                     continue
 
@@ -1013,11 +1015,16 @@ def _measure(self, node):
 
     def _barrier(self, node):
         """Draw a barrier"""
-        for xy in self._data[node]["q_xy"]:
+        for i, xy in enumerate(self._data[node]["q_xy"]):
             xpos, ypos = xy
+            # For the topmost barrier, reduce the rectangle if there's a label to allow for the text.
+            if i == 0 and node.op.label is not None:
+                ypos_adj = -0.35
+            else:
+                ypos_adj = 0.0
             self._ax.plot(
                 [xpos, xpos],
-                [ypos + 0.5, ypos - 0.5],
+                [ypos + 0.5 + ypos_adj, ypos - 0.5],
                 linewidth=self._lwidth1,
                 linestyle="dashed",
                 color=self._style["lc"],
@@ -1026,7 +1033,7 @@ def _barrier(self, node):
             box = self._patches_mod.Rectangle(
                 xy=(xpos - (0.3 * WID), ypos - 0.5),
                 width=0.6 * WID,
-                height=1,
+                height=1.0 + ypos_adj,
                 fc=self._style["bc"],
                 ec=None,
                 alpha=0.6,
@@ -1035,6 +1042,21 @@ def _barrier(self, node):
             )
             self._ax.add_patch(box)
 
+            # display the barrier label at the top if there is one
+            if i == 0 and node.op.label is not None:
+                dir_ypos = ypos + 0.65 * HIG
+                self._ax.text(
+                    xpos,
+                    dir_ypos,
+                    node.op.label,
+                    ha="center",
+                    va="top",
+                    fontsize=self._fs,
+                    color=self._data[node]["tc"],
+                    clip_on=True,
+                    zorder=PORDER_TEXT,
+                )
+
     def _gate(self, node, xy=None):
         """Draw a 1-qubit gate"""
         if xy is None:

qiskit/visualization/text.py

@@ -376,18 +376,18 @@ def __init__(self, label=""):
 
 
 class Barrier(DirectOnQuWire):
-    """Draws a barrier.
+    """Draws a barrier with a label at the top if there is one.
 
     ::
 
-        top:  ░     ░
+        top:  ░   label
         mid: ─░─ ───░───
         bot:  ░     ░
     """
 
     def __init__(self, label=""):
         super().__init__("░")
-        self.top_connect = "░"
+        self.top_connect = label if label else "░"
         self.bot_connect = "░"
         self.top_connector = {}
         self.bot_connector = {}
@@ -1089,9 +1089,10 @@ def add_connected_gate(node, gates, layer, current_cons):
             if not self.plotbarriers:
                 return layer, current_cons, current_cons_cond, connection_label
 
-            for qubit in node.qargs:
+            for i, qubit in enumerate(node.qargs):
                 if qubit in self.qubits:
-                    layer.set_qubit(qubit, Barrier())
+                    label = op.label if i == 0 else ""
+                    layer.set_qubit(qubit, Barrier(label))
 
         elif isinstance(op, SwapGate):
             # swap

releasenotes/notes/add-barrier-label-8e677979cb37461e.yaml

@@ -0,0 +1,18 @@
+---
+features:
+  - |
+    Added a ``label`` parameter to the :class:`.Barrier` which now allows
+    a user to enter a label for the ``barrier`` directive and the label
+    will be printed at the top of the ``barrier`` in the `mpl`, `latex`,
+    and `text` circuit drawers. Printing of existing ``snapshot`` labels
+    to the 3 circuit drawers was also added.
+
+    .. code-block:: python
+
+      from qiskit import QuantumCircuit
+
+      circuit = QuantumCircuit(2)
+      circuit.h(0)
+      circuit.h(1)
+      circuit.barrier(label="After H")
+      circuit.draw('mpl')

test/ipynb/mpl/circuit/test_circuit_matplotlib_drawer.py

@@ -920,6 +920,17 @@ def test_wire_order(self):
             filename="wire_order.png",
         )
 
+    def test_barrier_label(self):
+        """Test the barrier label"""
+        circuit = QuantumCircuit(2)
+        circuit.x(0)
+        circuit.y(1)
+        circuit.barrier()
+        circuit.y(0)
+        circuit.x(1)
+        circuit.barrier(label="End Y/X")
+        self.circuit_drawer(circuit, filename="barrier_label.png")
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=1)

test/python/visualization/references/test_latex_barrier_label.tex

@@ -0,0 +1,12 @@
+\documentclass[border=2px]{standalone}
+
+\usepackage[braket, qm]{qcircuit}
+\usepackage{graphicx}
+
+\begin{document}
+\scalebox{1.0}{
+\Qcircuit @C=1.0em @R=0.2em @!R { \\
+	 	\nghost{{q}_{0} :  } & \lstick{{q}_{0} :  } & \gate{\mathrm{X}} \barrier[0em]{1} & \qw & \gate{\mathrm{Y}} \barrier[0em]{1} & \cds{0}{^{\mathrm{End\,Y/X}}} & \qw & \qw\\
+	 	\nghost{{q}_{1} :  } & \lstick{{q}_{1} :  } & \gate{\mathrm{Y}} & \qw & \gate{\mathrm{X}} & \qw & \qw & \qw\\
+\\ }}
+\end{document}

test/python/visualization/references/test_latex_plot_barriers_true.tex

@@ -6,7 +6,7 @@
 \begin{document}
 \scalebox{1.0}{
 \Qcircuit @C=1.0em @R=0.2em @!R { \\
-	 	\nghost{{q}_{0} :  } & \lstick{{q}_{0} :  } & \gate{\mathrm{H}} \barrier[0em]{1} & \qw & \qw \barrier[0em]{1} & \qw & \qw & \qw\\
+	 	\nghost{{q}_{0} :  } & \lstick{{q}_{0} :  } & \gate{\mathrm{H}} \barrier[0em]{1} & \qw & \qw \barrier[0em]{1} & \cds{0}{^{\mathrm{sn\,1}}} & \qw & \qw\\
 	 	\nghost{{q}_{1} :  } & \lstick{{q}_{1} :  } & \qw & \qw & \gate{\mathrm{H}} & \qw & \qw & \qw\\
 	 	\nghost{\mathrm{{c} :  }} & \lstick{\mathrm{{c} :  }} & \lstick{/_{_{2}}} \cw & \cw & \cw & \cw & \cw & \cw\\
 \\ }}

test/python/visualization/test_circuit_latex.py

@@ -241,7 +241,7 @@ def test_plot_barriers(self):
         # this import appears to be unused, but is actually needed to get snapshot instruction
         import qiskit.extensions.simulator  # pylint: disable=unused-import
 
-        circuit.snapshot("1")
+        circuit.snapshot("sn 1")
 
         # check the barriers plot properly when plot_barriers= True
         circuit_drawer(circuit, filename=filename1, output="latex_source", plot_barriers=True)
@@ -265,6 +265,22 @@ def test_no_barriers_false(self):
 
         self.assertEqualToReference(filename)
 
+    def test_barrier_label(self):
+        """Test the barrier label"""
+        filename = self._get_resource_path("test_latex_barrier_label.tex")
+        qr = QuantumRegister(2, "q")
+        circuit = QuantumCircuit(qr)
+        circuit.x(0)
+        circuit.y(1)
+        circuit.barrier()
+        circuit.y(0)
+        circuit.x(1)
+        circuit.barrier(label="End Y/X")
+
+        circuit_drawer(circuit, filename=filename, output="latex_source")
+
+        self.assertEqualToReference(filename)
+
     def test_big_gates(self):
         """Test large gates with params"""
         filename = self._get_resource_path("test_latex_big_gates.tex")

test/python/visualization/test_circuit_text_drawer.py

@@ -1052,6 +1052,28 @@ def test_text_justify_right_barrier(self):
         circuit.h(qr1[1])
         self.assertEqual(str(_text_circuit_drawer(circuit, justify="right")), expected)
 
+    def test_text_barrier_label(self):
+        """Show barrier label"""
+        expected = "\n".join(
+            [
+                "        ┌───┐ ░ ┌───┐ End Y/X ",
+                "q_0: |0>┤ X ├─░─┤ Y ├────░────",
+                "        ├───┤ ░ ├───┤    ░    ",
+                "q_1: |0>┤ Y ├─░─┤ X ├────░────",
+                "        └───┘ ░ └───┘    ░    ",
+            ]
+        )
+
+        qr = QuantumRegister(2, "q")
+        circuit = QuantumCircuit(qr)
+        circuit.x(0)
+        circuit.y(1)
+        circuit.barrier()
+        circuit.y(0)
+        circuit.x(1)
+        circuit.barrier(label="End Y/X")
+        self.assertEqual(str(_text_circuit_drawer(circuit)), expected)
+
     def test_text_overlap_cx(self):
         """Overlapping CX gates are drawn not overlapping"""
         expected = "\n".join(