Add ISWAP and its inverse to FSIM_GATESET (#3743)

- Adding ISWAP to FSIM_GATESET as an allowed gate
for experimental usage.

This allows serialization of ISWAP and ISWAP ** -1 using
this gateset.  Note that this gate may not be available on
all processors and may not have good/any performance.

cirq/google/common_serializers.py

@@ -486,8 +486,8 @@ def _convert_physical_z(op: ops.Operation, proto: v2.program_pb2.Operation):
 
 #
 # FSim serializer
-# Only allows sqrt_iswap, its inverse, identity, and sycamore
-#
+# Only allows iswap, sqrt_iswap and their inverses, iswap, CZ, identity, and sycamore
+# Note that not all combinations may not be available on all processors
 def _can_serialize_limited_fsim(theta: float, phi: float):
     # Symbols for LIMITED_FSIM are allowed, but may fail server-side
     # if an incorrect run context is specified
@@ -503,6 +503,12 @@ def _can_serialize_limited_fsim(theta: float, phi: float):
         # inverse sqrt ISWAP
         if _near_mod_2pi(theta, np.pi / 4):
             return True
+        # ISWAP
+        if _near_mod_2pi(theta, -np.pi / 2):
+            return True
+        # Inverse ISWAP
+        if _near_mod_2pi(theta, np.pi / 2):
+            return True
     # Sycamore
     if (
         (_near_mod_2pi(theta, np.pi / 2) or isinstance(theta, sympy.Symbol))
@@ -531,6 +537,12 @@ def _can_serialize_limited_iswap(exponent: float):
     # Inverse Sqrt ISWAP
     if _near_mod_n(exponent, -0.5, 4):
         return True
+    # ISWAP
+    if _near_mod_n(exponent, -1.0, 4):
+        return True
+    # Inverse ISWAP
+    if _near_mod_n(exponent, 1.0, 4):
+        return True
     # Identity
     if _near_mod_n(exponent, 0.0, 4):
         return True

cirq/google/common_serializers_test.py

@@ -501,6 +501,8 @@ def test_wait_gate_multi_qubit():
     [
         (cirq.ISWAP ** 0.5, -np.pi / 4, 0),
         (cirq.ISWAP ** -0.5, np.pi / 4, 0),
+        (cirq.ISWAP ** 1.0, -np.pi / 2, 0),
+        (cirq.ISWAP ** -1.0, np.pi / 2, 0),
         (cirq.ISWAP ** 0.0, 0, 0),
         (cirq.CZ, 0, np.pi),
         (cirq.CZ ** -1.0, 0, np.pi),
@@ -509,6 +511,8 @@ def test_wait_gate_multi_qubit():
         (cirq.FSimGate(theta=np.pi / 4, phi=0), np.pi / 4, 0),
         (cirq.FSimGate(theta=7 * np.pi / 4, phi=0), -np.pi / 4, 0),
         (cirq.FSimGate(theta=-np.pi / 4, phi=0), -np.pi / 4, 0),
+        (cirq.FSimGate(theta=np.pi / 2, phi=0), np.pi / 2, 0),
+        (cirq.FSimGate(theta=-np.pi / 2, phi=0), -np.pi / 2, 0),
         (cirq.FSimGate(theta=-7 * np.pi / 4, phi=0), np.pi / 4, 0),
         (cirq.google.SYC, np.pi / 2, np.pi / 6),
         (cirq.FSimGate(theta=np.pi / 2, phi=np.pi / 6), np.pi / 2, np.pi / 6),
@@ -596,7 +600,7 @@ def test_serialize_deserialize_iswap_symbols():
     'gate',
     [
         cirq.ISWAP ** 0.25,
-        cirq.ISWAP,
+        cirq.ISWAP ** 0.75,
         cirq.FSimGate(theta=0.1, phi=0),
         cirq.FSimGate(theta=0, phi=0.1),
         cirq.FSimGate(theta=sympy.Symbol('t'), phi=0.1),