BooleanHamiltonianGate implementation

cirq-core/cirq/__init__.py

@@ -184,6 +184,7 @@
     bit_flip,
     BitFlipChannel,
     BooleanHamiltonian,
+    BooleanHamiltonianGate,
     CCX,
     CCXPowGate,
     CCZ,

cirq-core/cirq/json_resolver_cache.py

@@ -56,6 +56,7 @@ def _parallel_gate_op(gate, qubits):
         'BitFlipChannel': cirq.BitFlipChannel,
         'BitstringAccumulator': cirq.work.BitstringAccumulator,
         'BooleanHamiltonian': cirq.BooleanHamiltonian,
+        'BooleanHamiltonianGate': cirq.BooleanHamiltonianGate,
         'ProductState': cirq.ProductState,
         'CCNotPowGate': cirq.CCNotPowGate,
         'CCXPowGate': cirq.CCXPowGate,

cirq-core/cirq/ops/__init__.py

@@ -31,6 +31,7 @@
 
 from cirq.ops.boolean_hamiltonian import (
     BooleanHamiltonian,
+    BooleanHamiltonianGate,
 )
 
 from cirq.ops.common_channels import (

cirq-core/cirq/ops/boolean_hamiltonian.py

@@ -31,7 +31,7 @@
 
 import cirq
 from cirq import value
-from cirq.ops import raw_types
+from cirq.ops import gate_operation, raw_types
 from cirq.ops.linear_combinations import PauliSum, PauliString
 
 
@@ -64,7 +64,12 @@ def __init__(
             boolean_strs: The list of Sympy-parsable Boolean expressions.
             qubit_map: map of string (boolean variable name) to qubit.
             theta: The evolution time (angle) for the Hamiltonian
+
+        Raises:
+            ValueError: If the any qubits are not 2D.
         """
+        if any(q.dimension != 2 for q in qubit_map.values()):
+            raise ValueError('All qubits must be 2-dimensional.')
         self._qubit_map: Dict[str, 'cirq.Qid'] = qubit_map
         self._boolean_strs: Sequence[str] = boolean_strs
         self._theta: float = theta
@@ -115,6 +120,94 @@ def _decompose_(self):
             hamiltonian_polynomial_list, self._qubit_map, self._theta
         )
 
+    def _has_unitary_(self):
+        return True
+
+    def __repr__(self):
+        return (
+            f'cirq.BooleanHamiltonian('
+            f'qubit_map={self._qubit_map!r}, '
+            f'boolean_strs={self._boolean_strs!r}, '
+            f'theta={self._theta!r})'
+        )
+
+
+@value.value_equality
+class BooleanHamiltonianGate(raw_types.Gate):
+    """A gate that represents a Hamiltonian from a set of Boolean functions."""
+
+    def __init__(
+        self,
+        parameter_names: Sequence[str],
+        boolean_strs: Sequence[str],
+        theta: float,
+    ):
+        """Builds a BooleanHamiltonianGate.
+
+        For each element of a sequence of Boolean expressions, the code first transforms it into a
+        polynomial of Pauli Zs that represent that particular expression. Then, we sum all the
+        polynomials, thus making a function that goes from a series to Boolean inputs to an integer
+        that is the number of Boolean expressions that are true.
+
+        For example, if we were using this gate for the unweighted max-cut problem that is typically
+        used to demonstrate the QAOA algorithm, there would be one Boolean expression per edge. Each
+        Boolean expression would be true iff the vertices on that are in different cuts (i.e. it's)
+        an XOR.
+
+        Then, we compute exp(-j * theta * polynomial), which is unitary because the polynomial is
+        Hermitian.
+
+        Args:
+            parameter_names: The names of the inputs to the expressions.
+            boolean_strs: The list of Sympy-parsable Boolean expressions.
+            theta: The evolution time (angle) for the Hamiltonian
+        """
+        self._parameter_names: Sequence[str] = parameter_names
+        self._boolean_strs: Sequence[str] = boolean_strs
+        self._theta: float = theta
+
+    def _qid_shape_(self):
+        return (2,) * len(self._parameter_names)
+
+    def on(self, *qubits) -> 'cirq.Operation':
+        return gate_operation.GateOperation(self, qubits)
+
+    def _value_equality_values_(self):
+        return self._parameter_names, self._boolean_strs, self._theta
+
+    def _json_dict_(self) -> Dict[str, Any]:
+        return {
+            'cirq_type': self.__class__.__name__,
+            'parameter_names': self._parameter_names,
+            'boolean_strs': self._boolean_strs,
+            'theta': self._theta,
+        }
+
+    @classmethod
+    def _from_json_dict_(cls, parameter_names, boolean_strs, theta, **kwargs):
+        return cls(parameter_names, boolean_strs, theta)
+
+    def _decompose_(self, qubits: Sequence['cirq.Qid']) -> 'cirq.OP_TREE':
+        qubit_map = dict(zip(self._parameter_names, qubits))
+        boolean_exprs = [sympy_parser.parse_expr(boolean_str) for boolean_str in self._boolean_strs]
+        hamiltonian_polynomial_list = [
+            PauliSum.from_boolean_expression(boolean_expr, qubit_map)
+            for boolean_expr in boolean_exprs
+        ]
+
+        return _get_gates_from_hamiltonians(hamiltonian_polynomial_list, qubit_map, self._theta)
+
+    def _has_unitary_(self):
+        return True
+
+    def __repr__(self):
+        return (
+            f'cirq.BooleanHamiltonianGate('
+            f'parameter_names={self._parameter_names!r}, '
+            f'boolean_strs={self._boolean_strs!r}, '
+            f'theta={self._theta!r})'
+        )
+
 
 def _gray_code_comparator(k1: Tuple[int, ...], k2: Tuple[int, ...], flip: bool = False) -> int:
     """Compares two Gray-encoded binary numbers.

cirq-core/cirq/ops/boolean_hamiltonian_test.py

@@ -87,6 +87,69 @@ def test_circuit(boolean_str):
     np.testing.assert_array_equal(actual, expected)
 
 
+@pytest.mark.parametrize(
+    'boolean_str',
+    [
+        'x0',
+        '~x0',
+        'x0 ^ x1',
+        'x0 & x1',
+        'x0 | x1',
+        'x0 & x1 & x2',
+        'x0 & x1 & ~x2',
+        'x0 & ~x1 & x2',
+        'x0 & ~x1 & ~x2',
+        '~x0 & x1 & x2',
+        '~x0 & x1 & ~x2',
+        '~x0 & ~x1 & x2',
+        '~x0 & ~x1 & ~x2',
+        'x0 ^ x1 ^ x2',
+        'x0 | (x1 & x2)',
+        'x0 & (x1 | x2)',
+        '(x0 ^ x1 ^ x2) | (x2 ^ x3 ^ x4)',
+        '(x0 ^ x2 ^ x4) | (x1 ^ x2 ^ x3)',
+        'x0 & x1 & (x2 | x3)',
+        'x0 & ~x2',
+        '~x0 & x2',
+        'x2 & ~x0',
+        '~x2 & x0',
+        '(x2 | x1) ^ x0',
+    ],
+)
+def test_gate_circuit(boolean_str):
+    boolean_expr = sympy_parser.parse_expr(boolean_str)
+    var_names = cirq.parameter_names(boolean_expr)
+    qubits = [cirq.NamedQubit(name) for name in var_names]
+
+    # We use Sympy to evaluate the expression:
+    n = len(var_names)
+
+    expected = []
+    for binary_inputs in itertools.product([0, 1], repeat=n):
+        subed_expr = boolean_expr
+        for var_name, binary_input in zip(var_names, binary_inputs):
+            subed_expr = subed_expr.subs(var_name, binary_input)
+        expected.append(bool(subed_expr))
+
+    # We build a circuit and look at its output state vector:
+    circuit = cirq.Circuit()
+    circuit.append(cirq.H.on_each(*qubits))
+
+    hamiltonian_gate = cirq.BooleanHamiltonianGate(
+        [q.name for q in qubits], [boolean_str], 0.1 * math.pi
+    ).on(*qubits)
+
+    assert cirq.qid_shape(hamiltonian_gate) == cirq.qid_shape(qubits)
+
+    circuit.append(hamiltonian_gate)
+
+    phi = cirq.Simulator().simulate(circuit, qubit_order=qubits, initial_state=0).state_vector()
+    actual = np.arctan2(phi.real, phi.imag) - math.pi / 2.0 > 0.0
+
+    # Compare the two:
+    np.testing.assert_array_equal(actual, expected)
+
+
 def test_with_custom_names():
     q0, q1, q2, q3 = cirq.LineQubit.range(4)
     original_op = cirq.BooleanHamiltonian(
@@ -102,6 +165,48 @@ def test_with_custom_names():
     with pytest.raises(ValueError, match='Length of replacement qubits must be the same'):
         original_op.with_qubits(q2)
 
+    with pytest.raises(ValueError, match='All qubits must be 2-dimensional'):
+        original_op.with_qubits(q0, cirq.LineQid(1, 3))
+
+
+def test_gate_with_custom_names():
+    q0, q1, q2, q3 = cirq.LineQubit.range(4)
+    gate = cirq.BooleanHamiltonianGate(
+        ['a', 'b'],
+        ['a'],
+        0.1,
+    )
+    assert cirq.decompose(gate.on(q0, q1)) == [cirq.Rz(rads=-0.05).on(q0)]
+    assert cirq.decompose_once_with_qubits(gate, (q0, q1)) == [cirq.Rz(rads=-0.05).on(q0)]
+    assert cirq.decompose(gate.on(q2, q3)) == [cirq.Rz(rads=-0.05).on(q2)]
+    assert cirq.decompose_once_with_qubits(gate, (q2, q3)) == [cirq.Rz(rads=-0.05).on(q2)]
+
+    with pytest.raises(ValueError, match='Wrong number of qubits'):
+        gate.on(q2)
+    with pytest.raises(ValueError, match='Wrong shape of qids'):
+        gate.on(q0, cirq.LineQid(1, 3))
+
+
+def test_consistent():
+    q0, q1 = cirq.LineQubit.range(2)
+    op = cirq.BooleanHamiltonian(
+        {'a': q0, 'b': q1},
+        ['a'],
+        0.1,
+    )
+    cirq.testing.assert_implements_consistent_protocols(op)
+
+
+def test_gate_consistent():
+    gate = cirq.BooleanHamiltonianGate(
+        ['a', 'b'],
+        ['a'],
+        0.1,
+    )
+    op = gate.on(*cirq.LineQubit.range(2))
+    cirq.testing.assert_implements_consistent_protocols(gate)
+    cirq.testing.assert_implements_consistent_protocols(op)
+
 
 @pytest.mark.parametrize(
     'n_bits,expected_hs',

cirq-core/cirq/protocols/json_test_data/BooleanHamiltonianGate.json

@@ -0,0 +1,8 @@
+[
+  {
+    "cirq_type": "BooleanHamiltonianGate",
+    "parameter_names": ["q0", "q1"],
+    "boolean_strs": ["q0"],
+    "theta": 0.20160913
+  }
+]

cirq-core/cirq/protocols/json_test_data/BooleanHamiltonianGate.repr

@@ -0,0 +1 @@
+[cirq.BooleanHamiltonianGate(parameter_names=['q0', 'q1'], boolean_strs=['q0'], theta=0.20160913)]