[Feature] Projector blocks (#208)

pyproject.toml

@@ -40,7 +40,7 @@ dependencies = [
     "jsonschema",
     "nevergrad",
     "scipy",
-    "pyqtorch==1.0.1",
+    "pyqtorch==1.0.3",
     "matplotlib"
 ]
 

qadence/backends/pyqtorch/convert_ops.py

@@ -49,6 +49,7 @@
     block_to_diagonal,
     block_to_tensor,
 )
+from qadence.blocks.primitive import ProjectorBlock
 from qadence.operations import (
     OpName,
     U,
@@ -127,7 +128,14 @@ def convert_block(
             # which would be wrong.
             return [pyq.QuantumCircuit(n_qubits, ops)]
     elif isinstance(block, tuple(non_unitary_gateset)):
-        return [getattr(pyq, block.name)(qubit_support[0])]
+        if isinstance(block, ProjectorBlock):
+            projector = getattr(pyq, block.name)
+            if block.name == OpName.N:
+                return [projector(target=qubit_support)]
+            else:
+                return [projector(qubit_support=qubit_support, ket=block.ket, bra=block.bra)]
+        else:
+            return [getattr(pyq, block.name)(qubit_support[0])]
     elif isinstance(block, tuple(single_qubit_gateset)):
         pyq_cls = getattr(pyq, block.name)
         if isinstance(block, ParametricBlock):

qadence/blocks/block_to_tensor.py

@@ -15,8 +15,11 @@
     PrimitiveBlock,
     ScaleBlock,
 )
+from qadence.blocks.primitive import ProjectorBlock
 from qadence.blocks.utils import chain, kron, uuid_to_expression
 from qadence.parameters import evaluate, stringify
+
+# from qadence.states import product_state
 from qadence.types import Endianness, TensorType, TNumber
 
 J = torch.tensor(1j)
@@ -463,6 +466,14 @@ def _block_to_tensor_embedded(
         # add missing identities on unused qubits
         mat = _fill_identities(block_mat, block.qubit_support, qubit_support, endianness=endianness)
 
+    elif isinstance(block, ProjectorBlock):
+        from qadence.states import product_state
+
+        bra = product_state(block.bra)
+        ket = product_state(block.ket)
+
+        mat = torch.kron(ket, bra.T)
+
     else:
         raise TypeError(f"Conversion for block type {type(block)} not supported.")
 

qadence/blocks/primitive.py

@@ -454,3 +454,41 @@ def _block_title(self) -> str:
 
         s += rf" \[params: {params_str}]"
         return s if self.tag is None else (s + rf" \[tag: {self.tag}]")
+
+
+class ProjectorBlock(PrimitiveBlock):
+    """The abstract ProjectorBlock."""
+
+    name = "ProjectorBlock"
+
+    def __init__(
+        self,
+        ket: str,
+        bra: str,
+        qubit_support: int | tuple[int, ...],
+    ) -> None:
+        """
+        Arguments:
+
+            ket (str): The ket given as a bitstring.
+            bra (str): The bra given as a bitstring.
+            qubit_support (int | tuple[int]): The qubit_support of the block.
+        """
+        if isinstance(qubit_support, int):
+            qubit_support = (qubit_support,)
+        if len(bra) != len(ket):
+            raise ValueError(
+                "Bra and ket must be bitstrings of same length in the 'Projector' definition."
+            )
+        elif len(bra) != len(qubit_support):
+            raise ValueError("Bra or ket must be of same length as the 'qubit_support'")
+        for wf in [bra, ket]:
+            if not all(int(item) == 0 or int(item) == 1 for item in wf):
+                raise ValueError(
+                    "All qubits must be either in the '0' or '1' state"
+                    " in the 'ProjectorBlock' definition."
+                )
+
+        self.ket = ket
+        self.bra = bra
+        super().__init__(qubit_support)

qadence/operations.py

@@ -31,6 +31,7 @@
     WaitBlock,
 )
 from qadence.blocks.block_to_tensor import block_to_tensor
+from qadence.blocks.primitive import ProjectorBlock
 from qadence.blocks.utils import (
     add,  # noqa
     block_is_commuting_hamiltonian,
@@ -169,13 +170,35 @@ def dagger(self) -> Z:
         return self
 
 
-class N(PrimitiveBlock):
+class Projector(ProjectorBlock):
+    """The projector operator."""
+
+    name = OpName.PROJ
+
+    def __init__(
+        self,
+        ket: str,
+        bra: str,
+        qubit_support: int | tuple[int, ...],
+    ):
+        super().__init__(ket=ket, bra=bra, qubit_support=qubit_support)
+
+    @property
+    def generator(self) -> None:
+        raise ValueError("Property `generator` not available for non-unitary operator.")
+
+    @property
+    def eigenvalues_generator(self) -> None:
+        raise ValueError("Property `eigenvalues_generator` not available for non-unitary operator.")
+
+
+class N(Projector):
     """The N = (1/2)(I-Z) operator."""
 
     name = OpName.N
 
-    def __init__(self, target: int):
-        super().__init__((target,))
+    def __init__(self, target: int, state: str = "1"):
+        super().__init__(ket=state, bra=state, qubit_support=(target,))
 
     @property
     def generator(self) -> None:
@@ -668,7 +691,7 @@ class CNOT(ControlBlock):
     name = OpName.CNOT
 
     def __init__(self, control: int, target: int) -> None:
-        self.generator = kron((I(control) - Z(control)) * 0.5, X(target) - I(target))
+        self.generator = kron(N(control), X(target) - I(target))
         super().__init__((control,), X(target))
 
     @property
@@ -699,9 +722,7 @@ class MCZ(ControlBlock):
     name = OpName.MCZ
 
     def __init__(self, control: tuple[int, ...], target: int) -> None:
-        self.generator = kron(
-            *[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], Z(target) - I(target)
-        )
+        self.generator = kron(*[N(qubit) for qubit in control], Z(target) - I(target))
         super().__init__(control, Z(target))
 
     @property
@@ -749,7 +770,7 @@ def __init__(
         target: int,
         parameter: Parameter | TNumber | sympy.Expr | str,
     ) -> None:
-        self.generator = kron(*[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], X(target))
+        self.generator = kron(*[N(qubit) for qubit in control], X(target))
         super().__init__(control, RX(target, parameter))
 
     @classmethod
@@ -792,7 +813,7 @@ def __init__(
         target: int,
         parameter: Parameter | TNumber | sympy.Expr | str,
     ) -> None:
-        self.generator = kron(*[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], Y(target))
+        self.generator = kron(*[N(qubit) for qubit in control], Y(target))
         super().__init__(control, RY(target, parameter))
 
     @classmethod
@@ -835,7 +856,7 @@ def __init__(
         target: int,
         parameter: Parameter | TNumber | sympy.Expr | str,
     ) -> None:
-        self.generator = kron(*[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], Z(target))
+        self.generator = kron(*[N(qubit) for qubit in control], Z(target))
         super().__init__(control, RZ(target, parameter))
 
     @classmethod
@@ -878,10 +899,10 @@ def __init__(self, control: int | tuple[int, ...], target1: int, target2: int) -
         if isinstance(control, tuple):
             control = control[0]
 
-        a00m = 0.5 * (Z(control) - I(control))
-        a00p = -0.5 * (Z(control) + I(control))
-        a11 = 0.5 * (Z(target1) - I(target1))
-        a22 = -0.5 * (Z(target2) + I(target2))
+        a00m = -N(target=control)
+        a00p = -N(target=control, state="0")
+        a11 = -N(target=target1)
+        a22 = -N(target=target2, state="0")
         a12 = 0.5 * (chain(X(target1), Z(target1)) + X(target1))
         a21 = 0.5 * (chain(Z(target2), X(target2)) + X(target2))
         no_effect = kron(a00m, I(target1), I(target2))
@@ -1031,9 +1052,7 @@ def __init__(
         target: int,
         parameter: Parameter | TNumber | sympy.Expr | str,
     ) -> None:
-        self.generator = kron(
-            *[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], Z(target) - I(target)
-        )
+        self.generator = kron(*[N(qubit) for qubit in control], Z(target) - I(target))
         super().__init__(control, PHASE(target, parameter))
 
     @classmethod
@@ -1082,9 +1101,7 @@ class Toffoli(ControlBlock):
     name = OpName.TOFFOLI
 
     def __init__(self, control: tuple[int, ...], target: int) -> None:
-        self.generator = kron(
-            *[(I(qubit) - Z(qubit)) * 0.5 for qubit in control], X(target) - I(target)
-        )
+        self.generator = kron(*[N(qubit) for qubit in control], X(target) - I(target))
         super().__init__(control, X(target))
 
     def dagger(self) -> Toffoli:
@@ -1288,4 +1305,4 @@ def AnalogRZ(
     entangle,
     wait,
 ]
-non_unitary_gateset = [Zero, N]
+non_unitary_gateset = [Zero, N, Projector]

qadence/types.py

@@ -368,3 +368,5 @@ class OpName(StrEnum):
     """The entanglement operation."""
     WAIT = "wait"
     """The wait operation."""
+    PROJ = "Projector"
+    """The projector operation."""