Fix PauliString exponentiation (#2989)

Fixes the bug pointed out in #2710 .
quantumlib · May 11, 2020 · e5af689 · e5af689
1 parent f532a86
commit e5af689
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Showing 2 changed files with 12 additions and 7 deletions.
diff --git a/cirq/ops/pauli_string.py b/cirq/ops/pauli_string.py
@@ -606,7 +606,7 @@ def __rpow__(self, base):
                     'Exponentiated to a non-Hermitian PauliString '
                     f'<{base}**{self}>. Coefficient must be imaginary.')
 
-            half_turns = math.log(base) * (-self.coefficient.imag / math.pi)
+            half_turns = 2 * math.log(base) * (-self.coefficient.imag / math.pi)
 
             if len(self) == 1:
                 q, p = next(iter(self.items()))
@@ -621,8 +621,8 @@ def __rpow__(self, base):
             from cirq.ops import pauli_string_phasor
             return pauli_string_phasor.PauliStringPhasor(
                 PauliString(qubit_pauli_map=self._qubit_pauli_map),
-                exponent_neg=+half_turns / 2,
-                exponent_pos=-half_turns / 2)
+                exponent_neg=+half_turns / 4,
+                exponent_pos=-half_turns / 4)
         return NotImplemented
 
     def map_qubits(self, qubit_map: Dict[raw_types.Qid, raw_types.Qid]

diff --git a/cirq/ops/pauli_string_test.py b/cirq/ops/pauli_string_test.py
@@ -148,16 +148,16 @@ def test_exponentiation_as_exponent():
 def test_exponentiate_single_value_as_exponent():
     q = cirq.LineQubit(0)
 
-    assert cirq.approx_eq(math.e**(-0.25j * math.pi * cirq.X(q)),
+    assert cirq.approx_eq(math.e**(-0.125j * math.pi * cirq.X(q)),
                           cirq.rx(0.25 * math.pi).on(q))
 
-    assert cirq.approx_eq(math.e**(-0.25j * math.pi * cirq.Y(q)),
+    assert cirq.approx_eq(math.e**(-0.125j * math.pi * cirq.Y(q)),
                           cirq.ry(0.25 * math.pi).on(q))
 
-    assert cirq.approx_eq(math.e**(-0.25j * math.pi * cirq.Z(q)),
+    assert cirq.approx_eq(math.e**(-0.125j * math.pi * cirq.Z(q)),
                           cirq.rz(0.25 * math.pi).on(q))
 
-    assert cirq.approx_eq(np.exp(-0.3j * math.pi * cirq.X(q)),
+    assert cirq.approx_eq(np.exp(-0.15j * math.pi * cirq.X(q)),
                           cirq.rx(0.3 * math.pi).on(q))
 
     assert cirq.approx_eq(cirq.X(q)**0.5, cirq.XPowGate(exponent=0.5).on(q))
@@ -202,6 +202,11 @@ def test_exponentiation_as_base():
         np.exp(0.5j * math.pi * p),
         cirq.PauliStringPhasor(p, exponent_neg=-0.25, exponent_pos=0.25))
 
+    assert cirq.approx_eq(
+        cirq.unitary(np.exp(0.5j * math.pi * cirq.Z(a))),
+        np.diag([np.exp(0.5j * math.pi),
+                 np.exp(-0.5j * math.pi)]))
+
 
 @pytest.mark.parametrize('pauli', (cirq.X, cirq.Y, cirq.Z))
 def test_list_op_constructor_matches_mapping(pauli):