add quoperator method for circuit

Author: refraction-ray

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Added
+
+- Add `quoperator` method to get `QuOperator` representation of the circuit unitary
+
 ## 0.1.0
 
 ### Added

tensorcircuit/circuit.py

@@ -1216,6 +1216,23 @@ def _copy_state_tensor(
 
     state = wavefunction
 
+    def get_quoperator(self) -> QuOperator:
+        """
+        Get the ``QuOperator`` MPO like representation of the circuit unitary without contraction.
+
+        :return: ``QuOperator`` object for the circuit unitary (open indices for the input state)
+        :rtype: QuOperator
+        """
+        mps = identity([2 for _ in range(self._nqubits)])
+        c = Circuit(self._nqubits)
+        ns, es = self._copy()
+        c._nodes = ns
+        c._front = es
+        c.replace_mps_inputs(mps)
+        return QuOperator(c._front[: self._nqubits], c._front[self._nqubits :])
+
+    quoperator = get_quoperator
+
     def matrix(self) -> Tensor:
         """
         Get the unitary matrix for the circuit irrespective with the circuit input state.

tests/test_circuit.py

@@ -815,6 +815,20 @@ def test_apply_multicontrol_gate():
     np.testing.assert_allclose(c.expectation([tc.gates.z(), [3]]), -1, atol=1e-5)
 
 
+@pytest.mark.parametrize("backend", [lf("npb"), lf("tfb"), lf("jaxb")])
+def test_circuit_quoperator(backend):
+    c = tc.Circuit(3)
+    c.x(0)
+    c.cnot(0, 1)
+    c.cz(1, 2)
+    c.y(2)
+    c.exp1(0, 2, theta=1.0, unitary=tc.gates._xx_matrix)
+    c.H(1)
+    c.multicontrol(0, 2, 1, ctrl=[1, 0], unitary=tc.gates.z())
+    qo = c.quoperator()
+    np.testing.assert_allclose(qo.eval_matrix(), c.matrix(), atol=1e-5)
+
+
 @pytest.mark.parametrize("backend", [lf("npb"), lf("tfb"), lf("jaxb")])
 def test_qir2qiskit(backend):
     try: