Adding tests for single qubit simulation, and working on support

pytket/extensions/cutensornet/structured_state/mps.py

@@ -102,8 +102,6 @@ def __init__(
         n_tensors = len(qubits)
         if n_tensors == 0:  # There's no initialisation to be done
             pass
-        elif n_tensors == 1:
-            raise ValueError("Please, provide at least two qubits.")
         else:
             self.qubit_position = {q: i for i, q in enumerate(qubits)}
 
@@ -238,6 +236,8 @@ def apply_qubit_relabelling(self, qubit_map: dict[Qubit, Qubit]) -> MPS:
         Raises:
             ValueError: If any of the keys in ``qubit_map`` are not qubits in the state.
         """
+        # TODO: Is this properly supported in MPSxMPO?
+
         new_qubit_position = dict()
         for q_orig, q_new in qubit_map.items():
             # Check the qubit is in the state
@@ -268,6 +268,8 @@ def add_qubit(self, new_qubit: Qubit, position: int, state: int = 0) -> MPS:
             ValueError: If ``position`` is negative or larger than ``len(self)``.
             ValueError: If ``state`` is not ``0`` or ``1``.
         """
+        # TODO: Is this properly supported in MPSxMPO?
+
         options = {"handle": self._lib.handle, "device_id": self._lib.device_id}
 
         if new_qubit in self.qubit_position.keys():
@@ -575,6 +577,7 @@ def measure(self, qubits: set[Qubit], destructive: bool = True) -> dict[Qubit, i
         Raises:
             ValueError: If an element in ``qubits`` is not a qubit in the state.
         """
+        self._flush()
         result = dict()
 
         # Obtain the positions that need to be measured and build the reverse dict
@@ -658,6 +661,8 @@ def postselect(self, qubit_outcomes: dict[Qubit, int]) -> float:
             ValueError: If all of the qubits in the MPS are being postselected. Instead,
                 you may wish to use ``get_amplitude()``.
         """
+        self._flush()
+
         for q, v in qubit_outcomes.items():
             if q not in self.qubit_position:
                 raise ValueError(f"Qubit {q} is not a qubit in the MPS.")
@@ -756,6 +761,8 @@ def expectation_value(self, pauli_string: QubitPauliString) -> float:
         Raises:
             ValueError: If a key in ``pauli_string`` is not a qubit in the MPS.
         """
+        self._flush()
+
         for q in pauli_string.map.keys():
             if q not in self.qubit_position:
                 raise ValueError(f"Qubit {q} is not a qubit in the MPS.")
@@ -795,6 +802,7 @@ def expectation_value(self, pauli_string: QubitPauliString) -> float:
 
     def get_fidelity(self) -> float:
         """Returns the current fidelity of the state."""
+        self._flush()
         return self.fidelity
 
     def get_statevector(self) -> np.ndarray:
@@ -803,6 +811,8 @@ def get_statevector(self) -> np.ndarray:
         Raises:
             ValueError: If there are no qubits left in the MPS.
         """
+        self._flush()
+
         if len(self) == 0:
             raise ValueError("There are no qubits left in this MPS.")
 
@@ -860,6 +870,7 @@ def get_amplitude(self, state: int) -> complex:
         Returns:
             The amplitude of the computational state in the MPS.
         """
+        self._flush()
 
         # Auxiliar dictionary of physical bonds to qubit IDs
         qubit_id = {location: qubit for qubit, location in self.qubit_position.items()}

pytket/extensions/cutensornet/structured_state/mps_mpo.py

@@ -118,6 +118,11 @@ def _apply_1q_unitary(self, unitary: cp.ndarray, qubit: Qubit) -> MPSxMPO:
         # Apply the gate to the MPS with eager approximation
         self._aux_mps._apply_1q_unitary(unitary, qubit)
 
+        if len(self) == 1:
+            # If there is only one qubit, there is no benefit in using an MPO, so
+            # simply copy from the standard MPS
+            self.tensors[0] = self._aux_mps.tensors[0].copy()
+
         # Glossary of bond IDs
         # i -> input to the MPO tensor
         # o -> output of the MPO tensor
@@ -337,6 +342,10 @@ def _flush(self) -> None:
         The method applies variational optimisation of the MPS until it
         converges. Based on https://arxiv.org/abs/2207.05612.
         """
+        if self._mpo_bond_counter == 0:
+            # MPO is empty, there is nothing to flush
+            return None
+
         self._logger.info("Applying variational optimisation.")
         self._logger.info(f"Fidelity before optimisation={self._aux_mps.fidelity}")
 

tests/conftest.py

@@ -51,6 +51,12 @@ def quantum_volume_circuit(n_qubits: int) -> Circuit:
     return c
 
 
+@pytest.fixture
+def q1_empty() -> Circuit:
+    circuit = Circuit(1)
+    return circuit
+
+
 @pytest.fixture
 def q5_empty() -> Circuit:
     circuit = Circuit(5)
@@ -63,6 +69,13 @@ def q8_empty() -> Circuit:
     return circuit
 
 
+@pytest.fixture
+def q1_h0rz() -> Circuit:
+    circuit = Circuit(1)
+    circuit.H(0).Rz(0.3, 0)
+    return circuit
+
+
 @pytest.fixture
 def q2_x0() -> Circuit:
     circuit = Circuit(2)

tests/test_structured_state.py

@@ -248,8 +248,10 @@ def test_canonicalise_ttn(center: Union[RootPath, Qubit]) -> None:
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_empty"),  # type: ignore
         pytest.lazy_fixture("q5_empty"),  # type: ignore
         pytest.lazy_fixture("q8_empty"),  # type: ignore
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_x0"),  # type: ignore
         pytest.lazy_fixture("q2_x1"),  # type: ignore
         pytest.lazy_fixture("q2_v0"),  # type: ignore
@@ -308,7 +310,9 @@ def test_exact_circ_sim(circuit: Circuit, algorithm: SimulationAlgorithm) -> Non
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_empty"),  # type: ignore
         pytest.lazy_fixture("q5_empty"),  # type: ignore
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_lcu1"),  # type: ignore
         pytest.lazy_fixture("q2_lcu2"),  # type: ignore
         pytest.lazy_fixture("q2_lcu3"),  # type: ignore
@@ -363,8 +367,10 @@ def test_prepare_circuit_mps(circuit: Circuit, algorithm: SimulationAlgorithm) -
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_empty"),  # type: ignore
         pytest.lazy_fixture("q5_empty"),  # type: ignore
         pytest.lazy_fixture("q8_empty"),  # type: ignore
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_x0"),  # type: ignore
         pytest.lazy_fixture("q2_x1"),  # type: ignore
         pytest.lazy_fixture("q2_v0"),  # type: ignore
@@ -409,8 +415,10 @@ def test_approx_circ_sim_gate_fid(
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_empty"),  # type: ignore
         pytest.lazy_fixture("q5_empty"),  # type: ignore
         pytest.lazy_fixture("q8_empty"),  # type: ignore
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_x0"),  # type: ignore
         pytest.lazy_fixture("q2_x1"),  # type: ignore
         pytest.lazy_fixture("q2_v0"),  # type: ignore
@@ -453,7 +461,9 @@ def test_approx_circ_sim_chi(circuit: Circuit, algorithm: SimulationAlgorithm) -
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_empty"),  # type: ignore
         pytest.lazy_fixture("q5_empty"),  # type: ignore
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_x0cx01cx10"),  # type: ignore
         pytest.lazy_fixture("q2_lcu2"),  # type: ignore
         pytest.lazy_fixture("q3_cx01cz12x1rx0"),  # type: ignore
@@ -715,6 +725,7 @@ def test_expectation_value(circuit: Circuit, observable: QubitPauliString) -> No
 @pytest.mark.parametrize(
     "circuit",
     [
+        pytest.lazy_fixture("q1_h0rz"),  # type: ignore
         pytest.lazy_fixture("q2_v0cx01cx10"),  # type: ignore
         pytest.lazy_fixture("q2_hadamard_test"),  # type: ignore
         pytest.lazy_fixture("q2_lcu2"),  # type: ignore