Update NoiseAwarePlacement::cost_placement to cost errors appropria…

…tely (#944)

* only add errors if error rate is reasonable

* Bump

pytket/conanfile.py

@@ -32,7 +32,7 @@ def package(self):
         cmake.install()
 
     def requirements(self):
-        self.requires("tket/1.2.27@tket/stable")
+        self.requires("tket/1.2.28@tket/stable")
         self.requires("tklog/0.3.3@tket/stable")
         self.requires("tkrng/0.3.3@tket/stable")
         self.requires("tkassert/0.3.3@tket/stable")

pytket/tests/placement_test.py

@@ -245,6 +245,27 @@ def test_big_placement() -> None:
     assert DefaultMappingPass(arc).apply(c)
 
 
+def test_large_error_rate_noise_aware() -> None:
+    nodes = [Node(i) for i in range(3)]
+    arc = Architecture([(nodes[0], nodes[1]), (nodes[1], nodes[2])])
+    nap = NoiseAwarePlacement(
+        arc,
+        {},
+        {
+            (nodes[0], nodes[1]): 0.2,
+            (nodes[1], nodes[0]): 0.5,
+            (nodes[1], nodes[2]): 2,
+            (nodes[2], nodes[1]): 1,
+        },
+    )
+
+    c = Circuit(2).CX(0, 1)
+
+    placement_map = nap.get_placement_map(c)
+    assert placement_map[Qubit(0)] == Node(0)
+    assert placement_map[Qubit(1)] == Node(1)
+
+
 if __name__ == "__main__":
     test_placements()
     test_placements_serialization()
@@ -253,3 +274,4 @@ def test_big_placement() -> None:
     test_big_placement()
     test_place_fully_connected()
     test_placement_config()
+    test_large_error_rate_noise_aware()

tket/conanfile.py

@@ -23,7 +23,7 @@
 
 class TketConan(ConanFile):
     name = "tket"
-    version = "1.2.27"
+    version = "1.2.28"
     package_type = "library"
     license = "Apache 2"
     homepage = "https://github.com/CQCL/tket"

tket/src/Placement/NoiseAwarePlacement.cpp

@@ -77,8 +77,11 @@ double NoiseAwarePlacement::cost_placement(
           this->characterisation_.get_error({node, neighbour});
       gate_error_t bck_error =
           this->characterisation_.get_error({neighbour, node});
-      edge_sum += fwd_edge_weighting * (1.0 - fwd_error);
-      edge_sum += bck_edge_weighting * (1.0 - bck_error);
+
+      if (fwd_error < 1.0 && bck_error < 1.0) {
+        edge_sum += fwd_edge_weighting * (1.0 - fwd_error);
+        edge_sum += bck_edge_weighting * (1.0 - bck_error);
+      }
     }
     // bigger edge sum -> smaller cost
     cost += 1.0 / (edge_sum);