Adds 2q prep for iswap instead of sqrt_iswap (#6314)

Author: Dripto

cirq-core/cirq/__init__.py

@@ -372,6 +372,7 @@
     optimize_for_target_gateset,
     parameterized_2q_op_to_sqrt_iswap_operations,
     prepare_two_qubit_state_using_cz,
+    prepare_two_qubit_state_using_iswap,
     prepare_two_qubit_state_using_sqrt_iswap,
     quantum_shannon_decomposition,
     RouteCQC,

cirq-core/cirq/transformers/__init__.py

@@ -24,6 +24,7 @@
     is_negligible_turn,
     parameterized_2q_op_to_sqrt_iswap_operations,
     prepare_two_qubit_state_using_cz,
+    prepare_two_qubit_state_using_iswap,
     prepare_two_qubit_state_using_sqrt_iswap,
     quantum_shannon_decomposition,
     single_qubit_matrix_to_gates,

cirq-core/cirq/transformers/analytical_decompositions/__init__.py

@@ -66,6 +66,7 @@
 from cirq.transformers.analytical_decompositions.two_qubit_state_preparation import (
     prepare_two_qubit_state_using_cz,
     prepare_two_qubit_state_using_sqrt_iswap,
+    prepare_two_qubit_state_using_iswap,
 )
 
 from cirq.transformers.analytical_decompositions.single_to_two_qubit_isometry import (

cirq-core/cirq/transformers/analytical_decompositions/two_qubit_state_preparation.py

@@ -106,3 +106,41 @@ def prepare_two_qubit_state_using_cz(
     return op_list + _1q_matrices_to_ops(
         np.dot(u, np.linalg.inv(u_CZ)), np.dot(vh.T, np.linalg.inv(vh_CZ.T)), q0, q1
     )
+
+
+def prepare_two_qubit_state_using_iswap(
+    q0: 'cirq.Qid', q1: 'cirq.Qid', state: 'cirq.STATE_VECTOR_LIKE', use_iswap_inv: bool = False
+) -> List['cirq.Operation']:
+    """Prepares the given 2q state from |00> using at-most 1 ISWAP gate + single qubit rotations.
+
+    Entangled states are prepared using exactly 1 ISWAP gate while product states are prepared
+    using only single qubit rotations (0 ISWAP gates)
+
+    Args:
+        q0: The first qubit being operated on.
+        q1: The other qubit being operated on.
+        state: 4x1 matrix representing two qubit state vector, ordered as 00, 01, 10, 11.
+        use_iswap_inv: If True, uses `cirq.ISWAP_INV` instead of `cirq.ISWAP`.
+
+    Returns:
+        List of operations (at-most 1 ISWAP + single qubit rotations) preparing state from |00>.
+    """
+    state_vector = qis.to_valid_state_vector(state, num_qubits=2)
+    state_vector = state_vector / np.linalg.norm(state_vector)
+    u, s, vh = np.linalg.svd(state_vector.reshape(2, 2))
+    if np.isclose(s[0], 1):
+        # Product state can be prepare with just single qubit unitaries.
+        return _1q_matrices_to_ops(u, vh.T, q0, q1, True)
+    alpha = np.arccos(np.clip(s[0], 0, 1))
+    op_list = [
+        ops.ry(2 * alpha).on(q0),
+        ops.H.on(q1),
+        ops.ISWAP_INV.on(q0, q1) if use_iswap_inv else ops.ISWAP.on(q0, q1),
+    ]
+    intermediate_state = circuits.Circuit(op_list).final_state_vector(
+        ignore_terminal_measurements=False, dtype=np.complex64
+    )
+    u_CZ, _, vh_CZ = np.linalg.svd(intermediate_state.reshape(2, 2))
+    return op_list + _1q_matrices_to_ops(
+        np.dot(u, np.linalg.inv(u_CZ)), np.dot(vh.T, np.linalg.inv(vh_CZ.T)), q0, q1
+    )

cirq-core/cirq/transformers/analytical_decompositions/two_qubit_state_preparation_test.py

@@ -71,6 +71,24 @@ def test_prepare_two_qubit_state_using_cz(state):
     )
 
 
+@pytest.mark.parametrize("state", STATES_TO_PREPARE)
+@pytest.mark.parametrize("use_iswap_inv", [True, False])
+def test_prepare_two_qubit_state_using_iswap(state, use_iswap_inv):
+    state = cirq.to_valid_state_vector(state, num_qubits=2)
+    q = cirq.LineQubit.range(2)
+    circuit = cirq.Circuit(
+        cirq.prepare_two_qubit_state_using_iswap(*q, state, use_iswap_inv=use_iswap_inv)
+    )
+    iswap_gate = cirq.ISWAP_INV if use_iswap_inv else cirq.ISWAP
+    ops_iswap = [*circuit.findall_operations(lambda op: op.gate == iswap_gate)]
+    ops_2q = [*circuit.findall_operations(lambda op: cirq.num_qubits(op) > 1)]
+    assert ops_iswap == ops_2q
+    assert len(ops_iswap) <= 1
+    assert cirq.allclose_up_to_global_phase(
+        circuit.final_state_vector(ignore_terminal_measurements=False, dtype=np.complex64), state
+    )
+
+
 @pytest.mark.parametrize("state", STATES_TO_PREPARE)
 @pytest.mark.parametrize("use_sqrt_iswap_inv", [True, False])
 def test_prepare_two_qubit_state_using_sqrt_iswap(state, use_sqrt_iswap_inv):