add inputs and state for stabilizercircuit

Author: refraction-ray

docs/source/api/stabilizercircuit.rst

@@ -0,0 +1,7 @@
+tensorcircuit.stabilizercircuit
+================================================================================
+.. automodule:: tensorcircuit.stabilizercircuit
+    :members:
+    :undoc-members:
+    :show-inheritance:
+    :inherited-members:

tensorcircuit/__init__.py

@@ -34,6 +34,8 @@
 
 try:
     from .stabilizercircuit import StabilizerCircuit
+
+    CliffordCircuit = StabilizerCircuit
 except ModuleNotFoundError:
     pass
 

tensorcircuit/stabilizercircuit.py

@@ -31,14 +31,24 @@ class StabilizerCircuit(AbstractCircuit):
         "sd": "S_DAG",
     }
 
-    def __init__(self, nqubits: int) -> None:
+    def __init__(self, nqubits: int, inputs: Tensor = None) -> None:
+        """
+        ``StabilizerCircuit`` class based on stim package
+
+        :param nqubits: Number of qubits
+        :type nqubits: int
+        :param inputs: initial state by stabilizers, defaults to None
+        :type inputs: Tensor, optional
+        """
         self._nqubits = nqubits
         self._stim_circuit = stim.Circuit()
         self._qir: List[Dict[str, Any]] = []
         self.is_dm = False
         self.inputs = None
         self._extra_qir: List[Dict[str, Any]] = []
         self.current_sim = stim.TableauSimulator()
+        if inputs:
+            self.current_sim.set_state_from_stabilizers(inputs)
 
     def apply_general_gate(
         self,
@@ -89,6 +99,14 @@ def apply_general_gate(
 
     apply = apply_general_gate
 
+    def state(self) -> Tensor:
+        """
+        Return the wavefunction of the circuit.
+        Note that the state can have smaller qubit count if no gate is applied on later qubits
+        """
+        tab = self.current_tableau()
+        return tab.to_state_vector(endian="big")
+
     def random_gate(self, *index: int, recorded: bool = False) -> None:
         """
         Apply a random Clifford gate to the circuit.
@@ -296,6 +314,20 @@ def mid_measurement(self, index: int, keep: int = 0) -> Tensor:
     post_select = mid_measurement
     post_selection = mid_measurement
 
+    def depolarizing(self, *index: int, p: float) -> None:
+        """
+        Apply depolarizing noise to a qubit.
+
+        :param index: Index of the qubit to apply noise to
+        :type index: int
+        :param p: Noise parameter (probability of depolarizing)
+        :type p: float
+        """
+        self._stim_circuit.append_from_stim_program_text(
+            f"DEPOLARIZE1({p}) {' '.join(map(str, index))}"
+        )
+        self.current_sim.depolarize1(*index, p=p)
+
     def current_simulator(self) -> stim.TableauSimulator:
         """
         Return the current simulator of the circuit.

tests/test_stabilizer.py

@@ -10,6 +10,9 @@
 sys.path.insert(0, modulepath)
 import tensorcircuit as tc
 
+# Skip all tests if stim is not installed
+stim = pytest.importorskip("stim")
+
 
 def test_basic_gates():
     c = tc.StabilizerCircuit(2)
@@ -155,3 +158,30 @@ def test_random_gates():
     c.random_gate(1, 2)
     print(c.entanglement_entropy(list(range(2))))
     print(len(c.current_circuit()))
+
+
+def test_circuit_state():
+    c = tc.StabilizerCircuit(2)
+    c.h(1)
+    c1 = tc.Circuit(2)
+    c1.h(1)
+    np.testing.assert_allclose(c.state(), c1.state(), atol=1e-5)
+
+
+def test_circuit_inputs():
+    c = tc.StabilizerCircuit(2, inputs=[stim.PauliString("XX"), stim.PauliString("ZZ")])
+    c.cnot(0, 1)
+    c.h(0)
+    np.testing.assert_allclose(c.expectation_ps(z=[0]), 1, atol=1e-6)
+    np.testing.assert_allclose(c.expectation_ps(z=[1]), 1, atol=1e-6)
+
+
+def test_depolarize():
+    r = []
+    for _ in range(20):
+        c = tc.StabilizerCircuit(2)
+        c.h(0)
+        c.depolarizing(0, 1, p=0.2)
+        c.h(0)
+        r.append(c.expectation_ps(z=[0]))
+    assert 10 < np.sum(r) < 20