[Feature] Implement constructor function for alternating layer ansatz (…

…#565)

qadence/constructors/__init__.py

@@ -5,7 +5,7 @@
     exp_fourier_feature_map,
 )
 
-from .ansatze import hea
+from .ansatze import hea, alt
 
 from .iia import identity_initialized_ansatz
 
@@ -29,6 +29,7 @@
     "feature_map",
     "exp_fourier_feature_map",
     "hea",
+    "alt",
     "identity_initialized_ansatz",
     "hamiltonian_factory",
     "ising_hamiltonian",

qadence/constructors/ansatze.py

@@ -320,3 +320,179 @@ def hea_bDAQC(*args: Any, **kwargs: Any) -> Any:
 
 def hea_analog(*args: Any, **kwargs: Any) -> Any:
     raise NotImplementedError
+
+
+def alt(
+    n_qubits: int,
+    m_block_qubits: int,
+    depth: int = 1,
+    param_prefix: str = "theta",
+    support: tuple[int, ...] = None,
+    strategy: Strategy = Strategy.DIGITAL,
+    **strategy_args: Any,
+) -> AbstractBlock:
+    """
+    Factory function for the alternating layer ansatz (alt).
+
+    Args:
+        n_qubits: number of qubits in the block
+        m_block_qubits: number of qubits in the local entangling block
+        depth: number of layers of the alt
+        param_prefix: the base name of the variational parameters
+        support: qubit indexes where the alt is applied
+        strategy: Strategy.Digital or Strategy.DigitalAnalog
+        **strategy_args: see below
+
+    Keyword Arguments:
+        operations (list): list of operations to cycle through in the
+            digital single-qubit rotations of each layer. Valid for
+            Digital .
+        entangler (AbstractBlock):
+            - Digital: 2-qubit entangling operation. Supports CNOT, CZ,
+            CRX, CRY, CRZ, CPHASE. Controlled rotations will have variational
+            parameters on the rotation angles.
+    """
+
+    if support is None:
+        support = tuple(range(n_qubits))
+
+    alt_func_dict = {
+        Strategy.DIGITAL: alt_digital,
+        Strategy.SDAQC: alt_sDAQC,
+        Strategy.BDAQC: alt_bDAQC,
+        Strategy.ANALOG: alt_analog,
+    }
+
+    try:
+        alt_func = alt_func_dict[strategy]
+    except KeyError:
+        raise KeyError(f"Strategy {strategy} not recognized.")
+
+    hea_block: AbstractBlock = alt_func(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        param_prefix=param_prefix,
+        support=support,
+        **strategy_args,
+    )  # type: ignore
+
+    return hea_block
+
+
+#################
+## DIGITAL ALT ##
+#################
+
+
+def _entanglers_block_digital(
+    n_qubits: int,
+    m_block_qubits: int,
+    depth: int,
+    param_prefix: str = "theta",
+    support: tuple[int, ...] = None,
+    entangler: Type[DigitalEntanglers] = CNOT,
+) -> list[AbstractBlock]:
+    if support is None:
+        support = tuple(range(n_qubits))
+    iterator = itertools.count()
+    ent_list: list[AbstractBlock] = []
+
+    for d in range(depth):
+        start_i = 0 if not d % 2 else -m_block_qubits // 2
+        ents = [
+            kron(
+                _entangler(
+                    control=support[i + j],
+                    target=support[i + j + 1],
+                    param_str=param_prefix + f"_ent_{next(iterator)}",
+                    op=entangler,
+                )
+                for j in range(start_j, m_block_qubits, 2)
+                for i in range(start_i, n_qubits, m_block_qubits)
+                if i + j + 1 < n_qubits and j + 1 < m_block_qubits and i + j >= 0
+            )
+            for start_j in [i for i in range(2) if m_block_qubits > 2 or i == 0]
+        ]
+
+        ent_list.append(chain(*ents))
+    return ent_list
+
+
+def alt_digital(
+    n_qubits: int,
+    m_block_qubits: int,
+    depth: int = 1,
+    support: tuple[int, ...] = None,
+    param_prefix: str = "theta",
+    operations: list[type[AbstractBlock]] = [RX, RY],
+    entangler: Type[DigitalEntanglers] = CNOT,
+) -> AbstractBlock:
+    """
+    Construct the digital alternating layer ansatz (ALT).
+
+    Args:
+        n_qubits (int): number of qubits in the ansatz.
+        m_block_qubits (int): number of qubits in the local entangling block.
+        depth (int): number of layers of the ALT.
+        param_prefix (str): the base name of the variational parameters
+        operations (list): list of operations to cycle through in the
+            digital single-qubit rotations of each layer.
+        support (tuple): qubit indexes where the ALT is applied.
+        entangler (AbstractBlock): 2-qubit entangling operation.
+            Supports CNOT, CZ, CRX, CRY, CRZ. Controlld rotations
+            will have variational parameters on the rotation angles.
+    """
+
+    try:
+        if entangler not in [CNOT, CZ, CRX, CRY, CRZ, CPHASE]:
+            raise ValueError(
+                "Please provide a valid two-qubit entangler operation for digital ALT."
+            )
+    except TypeError:
+        raise ValueError("Please provide a valid two-qubit entangler operation for digital ALT.")
+
+    rot_list = _rotations_digital(
+        n_qubits=n_qubits,
+        depth=depth,
+        support=support,
+        param_prefix=param_prefix,
+        operations=operations,
+    )
+
+    ent_list = _entanglers_block_digital(
+        n_qubits,
+        m_block_qubits,
+        param_prefix=param_prefix + "_ent",
+        depth=depth,
+        support=support,
+        entangler=entangler,
+    )
+
+    layers = []
+    for d in range(depth):
+        layers.append(rot_list[d])
+        layers.append(ent_list[d])
+
+    return tag(chain(*layers), "ALT")
+
+
+#################
+## sdaqc ALT ##
+#################
+def alt_sDAQC(*args: Any, **kwargs: Any) -> Any:
+    raise NotImplementedError
+
+
+#################
+## bdaqc ALT ##
+#################
+def alt_bDAQC(*args: Any, **kwargs: Any) -> Any:
+    raise NotImplementedError
+
+
+#################
+## analog ALT ##
+#################
+def alt_analog(*args: Any, **kwargs: Any) -> Any:
+    raise NotImplementedError

tests/constructors/test_ansatz.py

@@ -14,6 +14,7 @@
     QuantumModel,
     VariationalParameter,
     Z,
+    alt,
     chain,
     hamiltonian_factory,
     hea,
@@ -169,3 +170,72 @@ def test_iia_sDAQC(n_qubits: int, depth: int, hamiltonian: str) -> None:
         assert not has_duplicate_vparams(iia)
     if hamiltonian == "parametric_local":
         assert has_duplicate_vparams(iia)
+
+
+@pytest.mark.parametrize("n_qubits", [10, 11])
+@pytest.mark.parametrize("m_block_qubits", [2, 3, 4])
+@pytest.mark.parametrize("depth", [2, 3])
+@pytest.mark.parametrize("entangler", [CNOT, CRX])
+def test_alt_duplicate_params(
+    n_qubits: int, m_block_qubits: int, depth: int, entangler: AbstractBlock
+) -> None:
+    """Tests that ALTs are initialized with correct parameter namings."""
+    common_params = {
+        "n_qubits": n_qubits,
+        "m_block_qubits": m_block_qubits,
+        "depth": depth,
+        "operations": [RZ, RX, RZ],
+        "entangler": entangler,
+    }
+    alt1 = alt(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        operations=[RZ, RX, RZ],
+        entangler=entangler,
+    )
+    alt2 = alt(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        operations=[RZ, RX, RZ],
+        entangler=entangler,
+    )
+    block1 = chain(alt1, alt2)
+    assert has_duplicate_vparams(block1)
+    alt1 = alt(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        operations=[RZ, RX, RZ],
+        entangler=entangler,
+        param_prefix="0",
+    )
+    alt2 = alt(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        operations=[RZ, RX, RZ],
+        entangler=entangler,
+        param_prefix="1",
+    )
+    block2 = chain(alt1, alt2)
+    assert not has_duplicate_vparams(block2)
+
+
+@pytest.mark.parametrize("n_qubits", [10, 11])
+@pytest.mark.parametrize("m_block_qubits", [2, 3, 4])
+@pytest.mark.parametrize("depth", [2, 3])
+def test_alt_forward(n_qubits: int, m_block_qubits: int, depth: int) -> None:
+    alt1 = alt(
+        n_qubits=n_qubits,
+        m_block_qubits=m_block_qubits,
+        depth=depth,
+        operations=[RZ, RX, RZ],
+        param_prefix="0",
+    )
+    circuit = QuantumCircuit(n_qubits, alt1)
+    model = QuantumModel(circuit)
+
+    wf = model.run({})
+    assert wf.shape == Size([1, 2**n_qubits])