404 stim compatibility (#5)

* Update stim_code_circuit.py

* Update stim_tools.py

* Update stim_tools.py

* more linting

* remove decoder dependence on css_logical

* final linting (hopefully)

src/qiskit_qec/circuits/__init__.py

@@ -34,3 +34,4 @@
 from .repetition_code import RepetitionCodeCircuit, ArcCircuit
 from .surface_code import SurfaceCodeCircuit
 from .css_code import CSSCodeCircuit
+from .stim_code_circuit import StimCodeCircuit

src/qiskit_qec/circuits/code_circuit.py

@@ -52,6 +52,14 @@ def string2nodes(self, string, **kwargs):
         """
         pass
 
+    @abstractmethod
+    def measured_logicals(self):
+        """
+        Returns a list of logical operators, each expressed as a list of qubits for which
+        the parity of the final readouts corresponds to the raw logical readout.
+        """
+        pass
+
     @abstractmethod
     def check_nodes(self, nodes, ignore_extra_boundary=False, minimal=False):
         """

src/qiskit_qec/circuits/css_code.py

@@ -186,6 +186,13 @@ def _get_code_properties(self):
         self.css_x_logical = self.logical_x
         self.css_z_logical = self.logical_z
 
+    def measured_logicals(self):
+        if self.basis == "x":
+            measured_logicals = self.logical_x
+        else:
+            measured_logicals = self.logical_z
+        return measured_logicals
+
     def _prepare_initial_state(self, qc, qregs, state):
         if state[0] == "1":
             if self.basis == "z":

src/qiskit_qec/circuits/repetition_code.py

@@ -241,6 +241,9 @@ def readout(self):
             self.circuit[log].add_register(self.code_bit)
             self.circuit[log].measure(self.code_qubit, self.code_bit)
 
+    def measured_logicals(self):
+        return [[0]]
+
     def _process_string(self, string):
         # logical readout taken from
         measured_log = string[0] + " " + string[self.d - 1]
@@ -349,7 +352,7 @@ def string2raw_logicals(self, string):
         Returns:
             list: Raw values for logical operators that correspond to nodes.
         """
-        return _separate_string(self._process_string(string))[0]
+        return string.split(" ", maxsplit=1)[0][-1]
 
     def check_nodes(self, nodes, ignore_extra_boundary=False, minimal=False):
         """
@@ -964,6 +967,9 @@ def _readout(self):
             qc.add_register(self.code_bit)
             qc.measure(self.code_qubit, self.code_bit)
 
+    def measured_logicals(self):
+        return [[self.z_logicals[0]]]
+
     def _process_string(self, string):
         # logical readout taken from assigned qubits
         measured_log = ""

src/qiskit_qec/circuits/stim_code_circuit.py

@@ -12,8 +12,9 @@
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
 
-# pylint: disable=invalid-name, disable=no-name-in-module
+# pylint: disable=invalid-name, disable=no-name-in-module, disable=no-member
 
+"""Generates CodeCircuits from stim circuits"""
 import warnings
 
 from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
@@ -112,9 +113,7 @@ def _helper(stim_circuit: StimCircuit, reps: int):
                         rep_block_count += 1
                     elif isinstance(instruction, CircuitInstruction):
                         inst_name = instruction.name
-                        if inst_name == "QUBIT_COORDS":
-                            m = 1
-                        elif inst_name in single_qubit_gate_dict:
+                        if inst_name in single_qubit_gate_dict:
                             qubits = [target.value for target in instruction.targets_copy()]
                             for q in qubits:
                                 self.qc.append(single_qubit_gate_dict[inst_name], qargs=[q])
@@ -211,7 +210,7 @@ def _helper(stim_circuit: StimCircuit, reps: int):
         # further code parameters
         try:
             self.d = len(self.stim_circuit.shortest_graphlike_error())  # code distance
-        except:
+        except ValueError:
             self.d = 0
         self.n = stim_circuit.num_qubits
         # the number of rounds is not necessarily well-defined (Floquet codes etc.)

src/qiskit_qec/circuits/surface_code.py

@@ -316,6 +316,13 @@ def _string2changes(self, string):
 
         return syndrome_changes
 
+    def measured_logicals(self):
+        if self.basis == "x":
+            measured_logicals = self.css_x_logical
+        else:
+            measured_logicals = self.css_z_logical
+        return measured_logicals
+
     def string2raw_logicals(self, string):
         """
         Extracts raw logicals from output string.

src/qiskit_qec/decoders/hdrg_decoders.py

@@ -40,13 +40,8 @@ def __init__(
     ):
         self.code = code_circuit
 
-        if hasattr(self.code, "_xbasis"):
-            if self.code._xbasis:
-                self.measured_logicals = self.code.css_x_logical
-            else:
-                self.measured_logicals = self.code.css_z_logical
-        else:
-            self.measured_logicals = self.code.css_z_logical
+        self.measured_logicals = self.code.measured_logicals()
+
         if hasattr(self.code, "code_index"):
             self.code_index = self.code.code_index
         else:

src/qiskit_qec/utils/stim_tools.py

@@ -16,7 +16,7 @@
 
 """Tools to use functionality from Stim."""
 from typing import Union, List, Dict, Callable
-from math import log
+from math import log as loga
 from stim import Circuit as StimCircuit
 from stim import DetectorErrorModel as StimDetectorErrorModel
 from stim import DemInstruction as StimDemInstruction
@@ -404,7 +404,7 @@ def handle_error(p: float, dets: List[int], frame_changes: List[int], hyperedge:
             )
             edge = DecodingGraphEdge(
                 qubits=qubits,
-                weight=log((1 - p) / p),
+                weight=loga((1 - p) / p),
                 properties={"fault_ids": set(frame_changes), "error_probability": p},
             )
             g.add_edge(dets[0], dets[1], edge)
@@ -457,10 +457,17 @@ def string2nodes_with_detectors(
     `DecodingGraph`.
     Args:
         string (string): Results string to convert.
-        detectors:
-        logicals:
-        clbits:
-        det_ref_values:
+        detectors: A list of measurement comparisons. A measurement comparison
+                (detector) is either a list of measurements given by a the name and index
+                of the classical bit or a list of dictionaries, with a mandatory clbits
+                key containing the classical bits. A dictionary can contain keys like
+                'qubits', 'time', 'basis' etc.
+        logicals: A list of logical measurements. A logical measurement is a
+                list of classical bits whose total parity is the logical eigenvalue.
+                Again it can be a list of dictionaries.
+        clbits: classical bits of the qiskit circuit, needed to identify
+                measurements in the output string
+        det_ref_values: Reference value for the detector outcomes, 0 by default
 
         kwargs (dict): Any additional keyword arguments.
             logical (str): Logical value whose results are used ('0' as default).

test/code_circuits/test_rep_codes.py

@@ -538,7 +538,7 @@ def clustering_decoder_test(
         # now run them all and check it works
         for c, code in enumerate(codes):
             decoding_graph = DecodingGraph(code)
-            if c == 3 and Decoder is UnionFindDecoder:
+            if c >= 0 and Decoder is UnionFindDecoder:
                 decoder = Decoder(code, decoding_graph=decoding_graph, use_peeling=False)
             else:
                 decoder = Decoder(code, decoding_graph=decoding_graph)
@@ -555,7 +555,7 @@ def clustering_decoder_test(
                 for j, z_logical in enumerate(decoder.measured_logicals):
                     error = corrected_z_logicals[j] != 1
                     if error:
-                        error_num = string.count("0")
+                        error_num = string.split(" ", maxsplit=1)[0].count("0")
                         if error_num < min_error_num:
                             min_error_num = error_num
                             min_error_string = string