Add prefix argument to state_to_latex (fixes #8460)

qiskit/visualization/state_visualization.py

@@ -1155,7 +1155,7 @@ def mod(v):
     return colors
 
 
-def state_to_latex(state, dims=None, convention="ket", **args):
+def state_to_latex(state, dims=None, convention="ket", prefix="", **args):
     """Return a Latex representation of a state. Wrapper function
     for `qiskit.visualization.array_to_latex` for convention 'vector'.
     Adds dims if necessary.
@@ -1177,10 +1177,9 @@ def state_to_latex(state, dims=None, convention="ket", **args):
         else:
             dims = True
 
-    prefix = ""
     suffix = ""
     if dims:
-        prefix = "\\begin{align}\n"
+        prefix = "\\begin{align}\n" + prefix
         dims_str = state._op_shape.dims_l()
         suffix = f"\\\\\n\\text{{dims={dims_str}}}\n\\end{{align}}"
 

test/python/visualization/test_state_latex_drawer.py

@@ -47,6 +47,21 @@ def test_state_max_size(self):
         )
         self.assertEqual(output, expected_output)
 
+    def test_ket_prefix(self):
+        """Test `prefix` parameter for latex ket notation."""
+
+        sv = Statevector.from_label("+-rl")
+        output = state_drawer(sv, "latex_source", prefix=r"|\psi\rangle=")
+        expected_output = (
+                "|\\psi\\rangle=\\frac{1}{4} |0000\\rangle - \\frac{i}{4} "
+                "|0001\\rangle +\\frac{i}{4} |0010\\rangle +\\frac{1}{4} "
+                "|0011\\rangle - \\frac{1}{4} |0100\\rangle +\\frac{i}{4} "
+                "|0101\\rangle + \\ldots +\\frac{1}{4} |1011\\rangle - "
+                "\\frac{1}{4} |1100\\rangle +\\frac{i}{4} |1101\\rangle - "
+                "\\frac{i}{4} |1110\\rangle - \\frac{1}{4} |1111\\rangle"
+        )
+        self.assertEqual(output, expected_output)
+
     def test_state_to_latex_for_none(self):
         """
         Test for `\rangleNone` output in latex representation