From a979d978918a3303bc8a76f5bca5307e0daf9b93 Mon Sep 17 00:00:00 2001
From: MatteoRobbiati <robbiati.matteo@gmail.com>
Date: Mon, 27 Jan 2025 12:28:19 +0100
Subject: [PATCH] feat: drafting qmatchatea backend

---
 src/qibotn/backends/qmatchatea.py | 116 ++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)
 create mode 100644 src/qibotn/backends/qmatchatea.py

diff --git a/src/qibotn/backends/qmatchatea.py b/src/qibotn/backends/qmatchatea.py
new file mode 100644
index 0000000..ba40475
--- /dev/null
+++ b/src/qibotn/backends/qmatchatea.py
@@ -0,0 +1,116 @@
+"""Implementation of Quantum Matcha Tea backend"""
+
+from dataclasses import dataclass
+from qiskit import QuantumCircuit
+
+from qibotn.backends.abstract import QibotnBackend
+from qibo.config import raise_error
+
+@dataclass
+class QMatchaTeaBackend(QibotnBackend):
+    
+    def __init__(self):   
+        super().__init__()
+        import qmatchatea  # pylint: disable=import-error
+        import qiskit   # pylint: disable=import-error
+        import qtealeaves    # pylint: disable=import-error
+
+        self.qmatchatea = qmatchatea
+        self.qiskit = qiskit
+        self.qtleaves = qtealeaves
+
+
+        # Set default configurations
+        self.configure_tn_simulation()
+        # TODO: update this function whenever ``set_device`` and ``set_precision``
+        # are set (?)
+        self._setup_qmatchatea_backend()
+
+
+    def execute_circuit(
+            self, circuit, initial_state=None, nshots=None, return_array=False
+    ):
+        
+        # TODO: verify if the QCIO mechanism of matcha is supported by Fortran only
+        # as written in the docstrings or by Python too (see ``io_info`` argument of 
+        # ``qmatchatea.interface.run_simulation`` function)
+        if initial_state is not None:
+            raise_error(
+                NotImplementedError,
+                f"Backend {self.name}-{self.platform} currently does not support initial state."
+            )
+
+        # TODO: do we want to keep it like this or we aim to implement a different 
+        # idea of "shots" here? 
+        nshots = None
+        
+        circuit = self._qibocirc_to_qiskitcirc(circuit)
+
+        run_qk_params = self.qmatchatea.preprocessing.qk_transpilation_params(False)
+
+        results = self.qmatchatea.run_simulation(
+            circ = circuit,
+            convergence_parameters = self.convergence_params,
+            transpilation_parameters = run_qk_params,
+            backend = self.qmatchatea_backend,
+        )
+
+        # TODO: construct a proper TNResult object?
+        # It does not make sense to reconstruct QuantumState here!
+        return results.measure_probabilities
+
+
+    def configure_tn_simulation(
+            self, 
+            convergence_params = None,
+            ansatz: str = "MPS",
+        ):
+        """
+        Configure TN simulation given Quantum Matcha Tea interface.
+        
+        Args:
+            ansatz (str): tensor network ansatz. It can be  tree tensor network "TTN"
+            or Matrix Product States "MPS" (default).
+            convergence_params (qmatchatea.utils.QCConvergenceParameters): 
+                convergence parameters class adapted to the quantum computing 
+                execution. See https://baltig.infn.it/quantum_matcha_tea/py_api_quantum_matcha_tea/-/blob/master/qmatchatea/utils/utils.py?ref_type=heads#L540
+                for more instructions. If not passed, the default values proposed 
+                by Quantum Matcha Tea's authors are set.
+        """
+
+        # Set configurationsor defaults
+        self.convergence_params = convergence_params or self.qmatchatea.QCConvergenceParameters()
+        self.ansatz = ansatz
+
+        # Initializing the TNObservables according to qmatchatea 
+        self.observables = self.qtleaves.observables.TNObservables()
+
+
+    def _setup_qmatchatea_backend(self):
+        """Configure qmatchatea QCBackend object."""
+
+        self.qmatchatea_device = "cpu" if "CPU" in self.device else "gpu" if "GPU" in self.device else None
+        self.qmatchatea_precision = "C" if self.precision == "single" else "Z" if self.precision == "double" else "A"
+
+        # TODO: once MPI is available for Python, integrate it here
+        self.qmatchatea_backend = self.qmatchatea.QCBackend(
+            backend = "PY",  # The only alternative is Fortran, but we use Python here
+            precision = self.qmatchatea_precision,
+            device = self.qmatchatea_device,
+            ansatz = self.ansatz,
+        )
+
+    def _qibocirc_to_qiskitcirc(self, qibo_circuit) -> QuantumCircuit:
+        """Convert a Qibo Circuit into a Qiskit Circuit."""
+        # Convert the circuit to QASM 2.0 to qiskit
+        qasm_circuit = qibo_circuit.to_qasm()
+        qiskit_circuit = QuantumCircuit.from_qasm_str(qasm_circuit)
+
+        # Transpile the circuit to adapt it to the linear structure of the MPS, 
+        # with the constraint of having only the gates basis_gates
+        qiskit_circuit = self.qmatchatea.preprocessing.preprocess(
+            qiskit_circuit, 
+            qk_params=self.qmatchatea.preprocessing.qk_transpilation_params()
+        )
+
+        return qiskit_circuit
\ No newline at end of file