cirq.Simulator doesn't support inverse gates that use sympy arguments for gates that don't define __pow__

[augustehirth]

Simulating the inverse of a gate that uses sympy Symbols as arguments causes a "Simulator doesn't support" Type Error, if the gate inherits its pow magic method from cirq.Gate. Of particular note is the cirq.PhasedFSimGate, which decomposes into the cirq.FSimGate and some cirq.Z gates. The cirq.FSimGate has it's own pow method implemented, and can correctly be inverted and simulated when using sympy arguments. It seems likely that somewhere in the pipeline, the _InverseCompositeGate produced by cirq.Gate's pow is being rejected as an unsupported gate instead of being decomposed, due to it's use of sympy arguments. @tanujkhattar, can you provide more insight on this?

Replication: Create a circuit with an inverse PhasedFSimGate that uses sympy arguments and attempt to simulate it. This issue doesn't occur with FSimGate, which is the primary component of PhasedFSimGate's decomposition.

import cirq
import sympy
simulator = cirq.Simulator()
qubit0, qubit1 = cirq.GridQubit(0,0), cirq.GridQubit(0,1)
theta, phi = sympy.Symbol('theta'), sympy.Symbol('phi')
for gate in [cirq.FSimGate(theta, phi), cirq.PhasedFSimGate(theta, phi)]: 
    for exponent in [1, -1]: 
        circuit = cirq.Circuit()
        circuit.append(pow(gate, exponent).on(qubit0, qubit1))
        results = simulator.simulate(circuit, param_resolver={'theta': 0, 'phi': 0})
        print(results)

Output log

measurements: (no measurements)
output vector: |00⟩
measurements: (no measurements)
output vector: |00⟩
measurements: (no measurements)
output vector: |00⟩
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/cirq/sim/simulator_base.py in _core_iterator(self, circuit, sim_state, all_measurements_are_terminal)
    220                     # Simulate the operation
--> 221                     protocols.act_on(op, sim_state)
    222                 except TypeError:

11 frames
TypeError: Can't simulate operations that don't implement SupportsUnitary, SupportsConsistentApplyUnitary, SupportsMixture or is a measurement: (cirq.Z**sympy.Mul(sympy.Float('0.5', precision=53), sympy.Pow(sympy.pi, sympy.Integer(-1)), sympy.Symbol('phi'))).on(cirq.GridQubit(0, 1))

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/cirq/sim/simulator_base.py in _core_iterator(self, circuit, sim_state, all_measurements_are_terminal)
    221                     protocols.act_on(op, sim_state)
    222                 except TypeError:
--> 223                     raise TypeError(f"{self.__class__.__name__} doesn't support {op!r}")
    224 
    225             step_result = self._create_step_result(sim_state)

TypeError: Simulator doesn't support (cirq.PhasedFSimGate(theta=sympy.Symbol('theta'), zeta=sympy.Symbol('phi'), chi=0.0, gamma=0.0, phi=0.0)**-1).on(cirq.GridQubit(0, 0), cirq.GridQubit(0, 1))

Cirq version
0.14.0.dev20211110190326

[tanujkhattar]

Sent a PR to fix the bug.