Alias BaseOperator.__mul__ to BaseOperator._multiply in quantum_info (#…

…8007)

* add left multiplication

* add tests

* add releasenote

* fix incorrectly mixed module

* change method of __mul__

* fix lint

* Reword release note

Co-authored-by: Jake Lishman <[email protected]>
Co-authored-by: Ikko Hamamura <[email protected]>

qiskit/quantum_info/operators/mixins/multiply.py

@@ -35,6 +35,9 @@ class MultiplyMixin(ABC):
     def __rmul__(self, other):
         return self._multiply(other)
 
+    def __mul__(self, other):
+        return self._multiply(other)
+
     def __truediv__(self, other):
         return self._multiply(1 / other)
 

releasenotes/notes/scaler-multiplication-left-side-7bea0d73f9afabe2.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Classes in the :mod:`.quantum_info` module that support scalar multiplication
+    can now be multiplied by a scalar from either the left or the right.
+    Previously, they would only accept scalar multipliers from the left.

test/python/quantum_info/operators/channel/test_chi.py

@@ -363,6 +363,8 @@ def test_multiply(self):
         targ = Chi(val * self.chiI)
         self.assertEqual(chan._multiply(val), targ)
         self.assertEqual(val * chan, targ)
+        targ = Chi(self.chiI * val)
+        self.assertEqual(chan * val, targ)
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/channel/test_choi.py

@@ -450,6 +450,8 @@ def test_multiply(self):
         targ = Choi(val * self.choiI)
         self.assertEqual(chan._multiply(val), targ)
         self.assertEqual(val * chan, targ)
+        targ = Choi(self.choiI * val)
+        self.assertEqual(chan * val, targ)
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/channel/test_kraus.py

@@ -419,6 +419,9 @@ def test_multiply(self):
         self.assertEqual(rho & chan, targ)
         chan = val * chan1
         self.assertEqual(rho & chan, targ)
+        targ = (rho & chan1) * val
+        chan = chan1 * val
+        self.assertEqual(rho & chan, targ)
 
         # Double Kraus set
         chan2 = Kraus((kraus1, kraus2))

test/python/quantum_info/operators/channel/test_ptm.py

@@ -356,6 +356,8 @@ def test_multiply(self):
         targ = PTM(val * self.ptmI)
         self.assertEqual(chan._multiply(val), targ)
         self.assertEqual(val * chan, targ)
+        targ = PTM(self.ptmI * val)
+        self.assertEqual(chan * val, targ)
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/channel/test_stinespring.py

@@ -411,6 +411,9 @@ def test_multiply(self):
         self.assertEqual(rho_init.evolve(chan), rho_targ)
         chan = val * chan1
         self.assertEqual(rho_init.evolve(chan), rho_targ)
+        rho_targ = (rho_init & chan1) * val
+        chan = chan1 * val
+        self.assertEqual(rho_init.evolve(chan), rho_targ)
 
         # Double Stinespring set
         chan2 = Stinespring((stine1, stine2), input_dims=2, output_dims=4)

test/python/quantum_info/operators/channel/test_superop.py

@@ -647,6 +647,8 @@ def test_multiply(self):
         targ = SuperOp(val * self.sopI)
         self.assertEqual(chan._multiply(val), targ)
         self.assertEqual(val * chan, targ)
+        targ = SuperOp(self.sopI * val)
+        self.assertEqual(chan * val, targ)
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/symplectic/test_pauli.py

@@ -336,6 +336,9 @@ def test_multiply(self, val):
         op = val * Pauli(([True, True], [False, False], 0))
         phase = (-1j) ** op.phase
         self.assertEqual(phase, val)
+        op = Pauli(([True, True], [False, False], 0)) * val
+        phase = (-1j) ** op.phase
+        self.assertEqual(phase, val)
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py

@@ -450,8 +450,13 @@ def test_mul(self, num_qubits, value):
         spp_op = self.random_spp_op(num_qubits, 2**num_qubits)
         target = value * Operator(spp_op)
         op = value * spp_op
-        value = op.to_operator()
-        self.assertEqual(value, target)
+        value_mat = op.to_operator()
+        self.assertEqual(value_mat, target)
+        np.testing.assert_array_equal(op.paulis.phase, np.zeros(op.size))
+        target = Operator(spp_op) * value
+        op = spp_op * value
+        value_mat = op.to_operator()
+        self.assertEqual(value_mat, target)
         np.testing.assert_array_equal(op.paulis.phase, np.zeros(op.size))
 
     @combine(num_qubits=[1, 2, 3], value=[1, 1j, -3 + 4.4j])

test/python/quantum_info/operators/test_operator.py

@@ -635,6 +635,7 @@ def test_multiply(self):
         op = Operator(mat)
         self.assertEqual(op._multiply(val), Operator(val * mat))
         self.assertEqual(val * op, Operator(val * mat))
+        self.assertEqual(op * val, Operator(mat * val))
 
     def test_multiply_except(self):
         """Test multiply method raises exceptions."""

test/python/quantum_info/operators/test_scalar_op.py

@@ -152,6 +152,9 @@ def test_multiply(self, coeff1, coeff2):
         val = coeff2 * ScalarOp(dims, coeff=coeff1)
         target = coeff1 * coeff2
         self.assertScalarOp(val, dims, target)
+        val = ScalarOp(dims, coeff=coeff1) * coeff2
+        target = coeff2 * coeff1
+        self.assertScalarOp(val, dims, target)
 
     @combine(coeff1=[0, 1, -3.1, 1 + 3j], coeff2=[-1, -5.1 - 2j])
     def test_add(self, coeff1, coeff2):