Merge pull request #27 from upsideon/add-power-wrapper-for-gates

[unitaryhack] Adding a power wrapper for Gates.

src/orquestra/quantum/circuits/_gates.py

@@ -89,6 +89,12 @@ def controlled(self, num_control_qubits: int) -> "Gate":
     def dagger(self) -> "Gate":
         raise NotImplementedError()
 
+    def power(self, exponent: float) -> "Gate":
+        """Gate representing the underlying matrix raised to the power
+        of the given exponent.
+        """
+        raise NotImplementedError()
+
     def bind(self, symbols_map: Dict[sympy.Symbol, Parameter]) -> "Gate":
         raise NotImplementedError()
 
@@ -208,6 +214,9 @@ def controlled(self, num_controlled_qubits: int) -> Gate:
     def dagger(self) -> Union["MatrixFactoryGate", Gate]:
         return self if self.is_hermitian else Dagger(self)
 
+    def power(self, exponent: float) -> "Gate":
+        return Power(self, exponent)
+
     def __str__(self):
         return (
             f"{self.name}({', '.join(map(str,self.params))})"
@@ -289,6 +298,12 @@ def dagger(self) -> "ControlledGate":
             num_control_qubits=self.num_control_qubits,
         )
 
+    def power(self, exponent: float) -> "Gate":
+        return ControlledGate(
+            wrapped_gate=self.wrapped_gate.power(exponent),
+            num_control_qubits=self.num_control_qubits,
+        )
+
     def bind(self, symbols_map) -> "Gate":
         return self.wrapped_gate.bind(symbols_map).controlled(self.num_control_qubits)
 
@@ -334,6 +349,60 @@ def replace_params(self, new_params: Tuple[Parameter, ...]) -> "Gate":
     def dagger(self) -> "Gate":
         return self.wrapped_gate
 
+    def power(self, exponent: float) -> "Gate":
+        return Power(self, exponent)
+
+
+POWER_GATE_SYMBOL = "^"
+
+
+@dataclass(frozen=True)
+class Power(Gate):
+    wrapped_gate: Gate
+    exponent: float
+
+    def __post_init__(self):
+        if len(self.wrapped_gate.free_symbols) > 0:
+            raise ValueError("Gates with free symbols cannot be exponentiated")
+
+    @property
+    def name(self) -> str:
+        return f"{self.wrapped_gate.name}{POWER_GATE_SYMBOL}{self.exponent}"
+
+    @property
+    def params(self) -> Tuple[Parameter, ...]:
+        return self.wrapped_gate.params
+
+    @property
+    def free_symbols(self) -> Iterable[sympy.Symbol]:
+        return get_free_symbols(self.params)
+
+    @property
+    def num_qubits(self) -> int:
+        return self.wrapped_gate.num_qubits
+
+    @property
+    def matrix(self) -> sympy.Matrix:
+        return self.wrapped_gate.matrix**self.exponent
+
+    def controlled(self, num_control_qubits: int) -> "Gate":
+        return self.wrapped_gate.controlled(num_control_qubits).power(self.exponent)
+
+    @property
+    def dagger(self) -> "Gate":
+        return self.wrapped_gate.dagger.power(self.exponent)
+
+    def power(self, exponent: float) -> "Gate":
+        return Power(self, exponent)
+
+    def bind(self, symbols_map: Dict[sympy.Symbol, Parameter]) -> "Gate":
+        raise NotImplementedError(
+            "Gates raised to a power do not possess free symbols to bind",
+        )
+
+    def replace_params(self, new_params: Tuple[Parameter, ...]) -> "Gate":
+        return self.wrapped_gate.replace_params(new_params).power(self.exponent)
+
 
 def _n_qubits(matrix):
     n_qubits = math.floor(math.log2(matrix.shape[0]))

src/orquestra/quantum/circuits/_serde.py

@@ -165,6 +165,15 @@ def _dagger_gate_to_dict(gate: _gates.Dagger):
     }
 
 
+@to_dict.register
+def _power_gate_to_dict(gate: _gates.Power):
+    return {
+        "name": gate.name,
+        "wrapped_gate": to_dict(gate.wrapped_gate),
+        "exponent": gate.exponent,
+    }
+
+
 # ---------- deserialization ----------
 
 
@@ -232,6 +241,10 @@ def _special_gate_from_dict(dict_, custom_gate_defs) -> _gates.Gate:
         wrapped_gate = _gate_from_dict(dict_["wrapped_gate"], custom_gate_defs)
         return _gates.Dagger(wrapped_gate)
 
+    elif _gates.POWER_GATE_SYMBOL in dict_["name"]:
+        wrapped_gate = _gate_from_dict(dict_["wrapped_gate"], custom_gate_defs)
+        return _gates.Power(wrapped_gate, dict_["exponent"])
+
     else:
         raise KeyError()
 

tests/orquestra/quantum/circuits/_gates_test.py

@@ -8,8 +8,8 @@
 import pytest
 import sympy
 
-from orquestra.quantum.circuits import _builtin_gates
-from orquestra.quantum.circuits._gates import GateOperation, MatrixFactoryGate
+from orquestra.quantum.circuits import _builtin_gates, _gates
+from orquestra.quantum.circuits._gates import GateOperation, MatrixFactoryGate, Power
 
 GATES_REPRESENTATIVES = [
     _builtin_gates.X,
@@ -33,6 +33,8 @@
     _builtin_gates.CPHASE(1.5),
 ]
 
+POWER_GATE_EXPONENTS = [-2.0, 0, 0.5, 1.0]
+
 
 def example_one_qubit_matrix_factory(a, b):
     return sympy.Matrix([[a, b], [b, a]])
@@ -122,6 +124,10 @@ def test_dagger_of_hermitian_gate_is_the_same_gate(self):
         )
         assert gate.dagger is gate
 
+    def test_power_of_dagger_is_dagger_wrapped_by_power(self):
+        gate = MatrixFactoryGate("V", example_one_qubit_matrix_factory, (1, 0), 1)
+        assert gate.dagger.power(0.5) == Power(gate.dagger, 0.5)
+
     def test_binding_gates_in_dagger_is_propagated_to_wrapped_gate(self):
         theta = sympy.Symbol("theta")
         gate = MatrixFactoryGate("V", example_one_qubit_matrix_factory, (theta, 0), 1)
@@ -193,6 +199,11 @@ def test_dagger_of_controlled_gate_is_controlled_gate_wrapping_dagger(self, gate
 
         assert controlled_gate.dagger == gate.dagger.controlled(4)
 
+    def test_power_of_controlled_gate_is_controlled_gate_wrapping_power(self, gate):
+        if len(gate.free_symbols) == 0:
+            controlled_gate = gate.controlled(2)
+            assert controlled_gate.power(0.5) == gate.power(0.5).controlled(2)
+
     def test_binding_parameters_in_control_gate_is_propagated_to_wrapped_gate(
         self, gate
     ):
@@ -213,6 +224,70 @@ def test_constructing_controlled_gate_with_zero_control_raises_error(self, gate)
             gate.controlled(0)
 
 
+@pytest.mark.parametrize("gate", GATES_REPRESENTATIVES)
+@pytest.mark.parametrize("exponent", POWER_GATE_EXPONENTS)
+class TestPowerGate:
+    def test_constructing_a_power_gate_with_free_symbols_raises_error(
+        self, gate, exponent
+    ):
+        if len(gate.free_symbols) > 0:
+            with pytest.raises(ValueError):
+                gate.power(exponent)
+
+    def test_power_gate_naming_scheme(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            power_gate = gate.power(exponent)
+            assert power_gate.name == f"{gate.name}{_gates.POWER_GATE_SYMBOL}{exponent}"
+
+    def test_has_same_parameters_as_wrapped_gate(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            assert gate.power(exponent).params == gate.params
+
+    def test_has_same_free_symbols_as_wrapped_gate(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            assert gate.power(exponent).free_symbols == gate.free_symbols
+
+    def test_has_same_number_of_qubits_as_wrapped_gate(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            assert gate.power(exponent).num_qubits == gate.num_qubits
+
+    def test_has_matrix_equal_to_wrapped_gate_matrix_exponentiated(
+        self, gate, exponent
+    ):
+        if len(gate.free_symbols) == 0:
+            wrapped_gate_matrix_exponentiated = gate.matrix**exponent
+            assert gate.power(exponent).matrix == wrapped_gate_matrix_exponentiated
+
+    def test_dagger_of_power_gate_power_gate_of_dagger(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            power_gate = gate.power(exponent)
+            assert power_gate.dagger == gate.dagger.power(exponent)
+
+    def test_creating_power_gate_from_power_gate(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            power_gate = gate.power(exponent)
+            powered_power_gate = power_gate.power(exponent)
+            assert powered_power_gate.matrix == power_gate.matrix**exponent
+
+    def test_parameter_binding_not_implemented_for_power_gates(self, gate, exponent):
+        if len(gate.free_symbols) == 0:
+            power_gate = gate.power(exponent)
+            symbols_map = {sympy.Symbol("theta"): 0.5, sympy.Symbol("x"): 3}
+            with pytest.raises(NotImplementedError):
+                power_gate.bind(symbols_map)
+
+    def test_constructing_power_gate_and_replacing_parameters_commute(
+        self, gate, exponent
+    ):
+        if len(gate.free_symbols) == 0:
+            power_gate = gate.power(exponent)
+            new_params = tuple(3 * param for param in power_gate.params)
+
+            assert power_gate.replace_params(new_params) == gate.replace_params(
+                new_params
+            ).power(exponent)
+
+
 @pytest.mark.parametrize("gate", GATES_REPRESENTATIVES)
 class TestGateOperation:
     def test_bound_symbols_are_not_present_in_gate_parameters(self, gate):

tests/orquestra/quantum/circuits/_serde_test.py

@@ -128,6 +128,12 @@
             _builtin_gates.RY(PARAMETER_VECTOR[0] * PARAMETER_VECTOR[1])(1),
         ]
     ),
+    _circuit.Circuit(
+        [
+            _builtin_gates.Z.power(0.25)(0),
+            _builtin_gates.SWAP.power(0.5)(0, 1),
+        ]
+    ),
 ]