Change to expectation calculation to accept hamiltonian object

src/qibotn/backends/cutensornet.py

@@ -1,9 +1,10 @@
 import numpy as np
 from qibo.backends import NumpyBackend
 from qibo.config import raise_error
-from qibo.result import QuantumState
+from qibotn.result import TensorNetworkResult
 
 from qibotn.backends.abstract import QibotnBackend
+from qibo import hamiltonians
 
 CUDA_TYPES = {}
 
@@ -28,15 +29,17 @@ class CuTensorNet(QibotnBackend, NumpyBackend):  # pragma: no cover
             expectation_enabled_value = runcard.get("expectation_enabled")
             if expectation_enabled_value is True:
                 self.expectation_enabled = True
-                self.pauli_string_pattern = "XXXZ"
+                self.observable = None
             elif expectation_enabled_value is False:
                 self.expectation_enabled = False
             elif isinstance(expectation_enabled_value, dict):
                 self.expectation_enabled = True
-                expectation_enabled_dict = runcard.get("expectation_enabled", {})
+                self.observable = runcard.get("expectation_enabled", {})
-                self.pauli_string_pattern = expectation_enabled_dict.get(
+            elif isinstance(
-                    "pauli_string_pattern", None
+                expectation_enabled_value, hamiltonians.SymbolicHamiltonian
-                )
+            ):
+                self.expectation_enabled = True
+                self.observable = expectation_enabled_value
             else:
                 raise TypeError("expectation_enabled has an unexpected type")
 
@@ -158,17 +161,15 @@ class CuTensorNet(QibotnBackend, NumpyBackend):  # pragma: no cover
             and self.NCCL_enabled == False
             and self.expectation_enabled == True
         ):
-            state = eval.expectation_pauli_tn(
+            state = eval.expectation_tn(circuit, self.dtype, self.observable)
-                circuit, self.dtype, self.pauli_string_pattern
-            )
         elif (
             self.MPI_enabled == True
             and self.MPS_enabled == False
             and self.NCCL_enabled == False
             and self.expectation_enabled == True
         ):
-            state, rank = eval.expectation_pauli_tn_MPI(
+            state, rank = eval.expectation_tn_MPI(
-                circuit, self.dtype, self.pauli_string_pattern, 32
+                circuit, self.dtype, self.observable, 32
             )
             if rank > 0:
                 state = np.array(0)
@@ -178,15 +179,27 @@ class CuTensorNet(QibotnBackend, NumpyBackend):  # pragma: no cover
             and self.NCCL_enabled == True
             and self.expectation_enabled == True
         ):
-            state, rank = eval.expectation_pauli_tn_nccl(
+            state, rank = eval.expectation_tn_nccl(
-                circuit, self.dtype, self.pauli_string_pattern, 32
+                circuit, self.dtype, self.observable, 32
             )
             if rank > 0:
                 state = np.array(0)
         else:
             raise_error(NotImplementedError, "Compute type not supported.")
 
-        if return_array:
+        if self.expectation_enabled:
-            return state.flatten()
+            return state.flatten().real
         else:
-            return QuantumState(state.flatten())
+            if return_array:
+                statevector = state.flatten()
+            else:
+                statevector = state
+
+            return TensorNetworkResult(
+                nqubits=circuit.nqubits,
+                backend=self,
+                measures=None,
+                measured_probabilities=None,
+                prob_type=None,
+                statevector=statevector,
+            )

src/qibotn/circuit_convertor.py

@@ -195,12 +195,12 @@ class QiboCircuitToEinsum:
             gates.append((operand, (qubit,)))
         return gates
 
-    def expectation_operands(self, pauli_string):
+    def expectation_operands(self, ham_gates):
         """Create the operands for pauli string expectation computation in the
         interleave format.
 
         Parameters:
-            pauli_string: A string representating the list of pauli gates.
+            ham_gates: A list of gates derived from Qibo hamiltonian object.
 
         Returns:
             Operands for the contraction in the interleave format.
@@ -208,8 +208,6 @@ class QiboCircuitToEinsum:
         input_bitstring = "0" * self.circuit.nqubits
 
         input_operands = self._get_bitstring_tensors(input_bitstring)
-        pauli_string = dict(zip(range(self.circuit.nqubits), pauli_string))
-        pauli_map = pauli_string
 
         (
             mode_labels,
@@ -228,11 +226,7 @@ class QiboCircuitToEinsum:
 
         next_frontier = max(qubits_frontier.values()) + 1
 
-        pauli_gates = self.get_pauli_gates(
+        gates_inverse = ham_gates + self.gate_tensors_inverse
-            pauli_map, dtype=self.dtype, backend=self.backend
-        )
-
-        gates_inverse = pauli_gates + self.gate_tensors_inverse
 
         (
             gate_mode_labels_inverse,

src/qibotn/eval.py

@@ -9,6 +9,154 @@ from qibotn.circuit_convertor import QiboCircuitToEinsum
 from qibotn.circuit_to_mps import QiboCircuitToMPS
 from qibotn.mps_contraction_helper import MPSContractionHelper
 
+import cuquantum.cutensornet as cutn
+from cuquantum import Network
+from mpi4py import MPI
+from cupy.cuda import nccl
+from qibo import hamiltonians
+from qibo.symbols import X, Y, Z, I
+
+
+def check_observable(observable, circuit_nqubit):
+    """Checks the type of observable and returns the appropriate Hamiltonian."""
+    if observable is None:
+        return build_observable(circuit_nqubit)
+    elif isinstance(observable, dict):
+        return create_hamiltonian_from_dict(observable, circuit_nqubit)
+    elif isinstance(observable, hamiltonians.SymbolicHamiltonian):
+        # TODO: check if the observable is compatible with the circuit
+        return observable
+    else:
+        raise TypeError("Invalid observable type.")
+
+
+def build_observable(circuit_nqubit):
+    """Helper function to construct a target observable."""
+    hamiltonian_form = 0
+    for i in range(circuit_nqubit):
+        hamiltonian_form += 0.5 * X(i % circuit_nqubit) * Z((i + 1) % circuit_nqubit)
+
+    print("Default hamiltonian: ", hamiltonian_form)
+    hamiltonian = hamiltonians.SymbolicHamiltonian(form=hamiltonian_form)
+    return hamiltonian
+
+
+def create_hamiltonian_from_dict(data, circuit_nqubit):
+    """Create a Qibo SymbolicHamiltonian from a dictionary representation.
+
+    Ensures that each Hamiltonian term explicitly acts on all circuit qubits
+    by adding identity (`I`) gates where needed.
+
+    Args:
+        data (dict): Dictionary containing Hamiltonian terms.
+        circuit_nqubit (int): Total number of qubits in the quantum circuit.
+
+    Returns:
+        hamiltonians.SymbolicHamiltonian: The constructed Hamiltonian.
+    """
+    PAULI_GATES = {"X": X, "Y": Y, "Z": Z}
+
+    terms = []
+
+    for term in data["terms"]:
+        coeff = term["coefficient"]
+        operators = term["operators"]  # List of tuples like [("Z", 0), ("X", 1)]
+
+        # Convert the operator list into a dictionary {qubit_index: gate}
+        operator_dict = {q: PAULI_GATES[g] for g, q in operators}
+
+        # Build the full term ensuring all qubits are covered
+        full_term_expr = [
+            operator_dict[q](q) if q in operator_dict else I(q)
+            for q in range(circuit_nqubit)
+        ]
+
+        # Multiply all operators together to form a single term
+        term_expr = full_term_expr[0]
+        for op in full_term_expr[1:]:
+            term_expr *= op
+
+        # Scale by the coefficient
+        final_term = coeff * term_expr
+        # print(f"Adding term: {final_term}")  # Debugging output
+        terms.append(final_term)
+
+    if not terms:
+        raise ValueError("No valid Hamiltonian terms were added.")
+
+    # Combine all terms
+    hamiltonian_form = sum(terms)
+    # print(f"Hamiltonian Form After Summation: {hamiltonian_form}")
+
+    return hamiltonians.SymbolicHamiltonian(hamiltonian_form)
+
+
+def get_ham_gates(pauli_map, dtype="complex128", backend=cp):
+    """Populate the gates for all pauli operators.
+
+    Parameters:
+        pauli_map: A dictionary mapping qubits to pauli operators.
+        dtype: Data type for the tensor operands.
+        backend: The package the tensor operands belong to.
+
+    Returns:
+        A sequence of pauli gates.
+    """
+    asarray = backend.asarray
+    pauli_i = asarray([[1, 0], [0, 1]], dtype=dtype)
+    pauli_x = asarray([[0, 1], [1, 0]], dtype=dtype)
+    pauli_y = asarray([[0, -1j], [1j, 0]], dtype=dtype)
+    pauli_z = asarray([[1, 0], [0, -1]], dtype=dtype)
+
+    operand_map = {"I": pauli_i, "X": pauli_x, "Y": pauli_y, "Z": pauli_z}
+    gates = []
+    for qubit, pauli_char, coeff in pauli_map:
+        operand = operand_map.get(pauli_char)
+        if operand is None:
+            raise ValueError("pauli string character must be one of I/X/Y/Z")
+        operand = coeff * operand
+        gates.append((operand, (qubit,)))
+    return gates
+
+
+def extract_gates_and_qubits(hamiltonian):
+    """
+    Extracts the gates and their corresponding qubits from a Qibo Hamiltonian.
+
+    Parameters:
+        hamiltonian (qibo.hamiltonians.Hamiltonian or qibo.hamiltonians.SymbolicHamiltonian):
+            A Qibo Hamiltonian object.
+
+    Returns:
+        list of tuples: [(coefficient, [(gate, qubit), ...]), ...]
+            - coefficient: The prefactor of the term.
+            - list of (gate, qubit): Each term's gates and the qubits they act on.
+    """
+    extracted_terms = []
+
+    if isinstance(hamiltonian, hamiltonians.SymbolicHamiltonian):
+        for term in hamiltonian.terms:
+            coeff = term.coefficient  # Extract coefficient
+            gate_qubit_list = []
+
+            # Extract gate and qubit information
+            for factor in term.factors:
+                gate_name = str(factor)[
+                    0
+                ]  # Extract the gate type (X, Y, Z) from 'X0', 'Z1'
+                qubit = int(str(factor)[1:])  # Extract the qubit index
+                gate_qubit_list.append((qubit, gate_name, coeff))
+                coeff = 1.0
+
+            extracted_terms.append(gate_qubit_list)
+
+    else:
+        raise ValueError(
+            "Unsupported Hamiltonian type. Must be SymbolicHamiltonian or Hamiltonian."
+        )
+
+    return extracted_terms
+
 
 def initialize_mpi():
     """Initialize MPI communication and device selection."""
@@ -166,7 +314,7 @@ def dense_vector_tn(qibo_circ, datatype):
     return contract(*myconvertor.state_vector_operands())
 
 
-def expectation_pauli_tn_nccl(qibo_circ, datatype, pauli_string_pattern, n_samples=8):
+def expectation_tn_nccl(qibo_circ, datatype, observable, n_samples=8):
     """Convert qibo circuit to tensornet (TN) format and perform contraction to
     expectation of given Pauli string using multi node and multi GPU through
     NCCL.
@@ -194,12 +342,19 @@ def expectation_pauli_tn_nccl(qibo_circ, datatype, pauli_string_pattern, n_sampl
 
     comm_nccl = initialize_nccl(comm_mpi, rank, size)
 
-    # Perform circuit conversion
+    observable = check_observable(observable, qibo_circ.nqubits)
+
+    ham_gate_map = extract_gates_and_qubits(observable)
+
     if rank == 0:
         myconvertor = QiboCircuitToEinsum(qibo_circ, dtype=datatype)
-        operands = myconvertor.expectation_operands(
+
-            pauli_string_gen(qibo_circ.nqubits, pauli_string_pattern)
+    exp = 0
-        )
+    for each_ham in ham_gate_map:
+        ham_gates = get_ham_gates(each_ham)
+        # Perform circuit conversion
+        if rank == 0:
+            operands = myconvertor.expectation_operands(ham_gates)
         else:
             operands = None
 
@@ -223,10 +378,12 @@ def expectation_pauli_tn_nccl(qibo_circ, datatype, pauli_string_pattern, n_sampl
         # Sum the partial contribution from each process on root.
         result = reduce_result(result, comm_nccl, method="NCCL", root=0)
 
-    return result, rank
+        exp += result
+
+    return exp, rank
 
 
-def expectation_pauli_tn_MPI(qibo_circ, datatype, pauli_string_pattern, n_samples=8):
+def expectation_tn_MPI(qibo_circ, datatype, observable, n_samples=8):
     """Convert qibo circuit to tensornet (TN) format and perform contraction to
     expectation of given Pauli string using multi node and multi GPU through
     MPI.
@@ -252,13 +409,19 @@ def expectation_pauli_tn_MPI(qibo_circ, datatype, pauli_string_pattern, n_sample
     # Initialize MPI and device
     comm, rank, size, device_id = initialize_mpi()
 
-    # Perform circuit conversion
+    observable = check_observable(observable, qibo_circ.nqubits)
+
+    ham_gate_map = extract_gates_and_qubits(observable)
+
     if rank == 0:
         myconvertor = QiboCircuitToEinsum(qibo_circ, dtype=datatype)
-
+    exp = 0
-        operands = myconvertor.expectation_operands(
+    for each_ham in ham_gate_map:
-            pauli_string_gen(qibo_circ.nqubits, pauli_string_pattern)
+        ham_gates = get_ham_gates(each_ham)
-        )
+        # Perform circuit conversion
+        # Perform circuit conversion
+        if rank == 0:
+            operands = myconvertor.expectation_operands(ham_gates)
         else:
             operands = None
 
@@ -284,10 +447,13 @@ def expectation_pauli_tn_MPI(qibo_circ, datatype, pauli_string_pattern, n_sample
         # Sum the partial contribution from each process on root.
         result = reduce_result(result, comm, method="MPI", root=0)
 
-    return result, rank
+        if rank == 0:
+            exp += result
+
+    return exp, rank
 
 
-def expectation_pauli_tn(qibo_circ, datatype, pauli_string_pattern):
+def expectation_tn(qibo_circ, datatype, observable):
     """Convert qibo circuit to tensornet (TN) format and perform contraction to
     expectation of given Pauli string.
 
@@ -300,11 +466,16 @@ def expectation_pauli_tn(qibo_circ, datatype, pauli_string_pattern):
         Expectation of quantum circuit due to pauli string.
     """
     myconvertor = QiboCircuitToEinsum(qibo_circ, dtype=datatype)
-    return contract(
+
-        *myconvertor.expectation_operands(
+    observable = check_observable(observable, qibo_circ.nqubits)
-            pauli_string_gen(qibo_circ.nqubits, pauli_string_pattern)
+
-        )
+    ham_gate_map = extract_gates_and_qubits(observable)
-    )
+    exp = 0
+    for each_ham in ham_gate_map:
+        ham_gates = get_ham_gates(each_ham)
+        expectation_operands = myconvertor.expectation_operands(ham_gates)
+        exp += contract(*expectation_operands)
+    return exp
 
 
 def dense_vector_mps(qibo_circ, gate_algo, datatype):
@@ -325,27 +496,3 @@ def dense_vector_mps(qibo_circ, gate_algo, datatype):
     return mps_helper.contract_state_vector(
         myconvertor.mps_tensors, {"handle": myconvertor.handle}
     )
-
-
-def pauli_string_gen(nqubits, pauli_string_pattern):
-    """Used internally to generate the string based on given pattern and number
-    of qubit.
-
-    Parameters:
-        nqubits(int): Number of qubits of Quantum Circuit
-        pauli_string_pattern(str): Strings representing sequence of pauli gates.
-
-    Returns:
-        String representation of the actual pauli string from the pattern.
-
-    Example: pattern: "XZ", number of qubit: 7, output = XZXZXZX
-    """
-    if nqubits <= 0:
-        return "Invalid input. N should be a positive integer."
-
-    result = ""
-
-    for i in range(nqubits):
-        char_to_add = pauli_string_pattern[i % len(pauli_string_pattern)]
-        result += char_to_add
-    return result