Detectors now preserve total probability

adgt · Mar 23, 2019 · 25849af · 25849af
1 parent b40b950
commit 25849af
Show file tree

Hide file tree

Showing 3 changed files with 46 additions and 2 deletions.
diff --git a/src/gates/Detector.js b/src/gates/Detector.js
@@ -100,7 +100,12 @@ let detectorShader = makePseudoShaderWithInputsAndOutputAndCode(
             float detectChance = read_detection_weight(0.0) / read_total_weight(0.0);
             float detection_type = float(rnd < detectChance);
             float own_type = read_classification(k);
-            return read_ket(k) * float(detection_type == own_type);
+            if (detection_type == own_type) {
+                float matchChance = detectChance * own_type + (1.0 - own_type) * (1.0 - detectChance);
+                return read_ket(k) / sqrt(matchChance);
+            } else {
+                return vec2(0.0, 0.0);
+            }
         }
     `);
 

diff --git a/test/circuit/CircuitStats.test.js b/test/circuit/CircuitStats.test.js
@@ -16,7 +16,6 @@ import {Suite, assertThat, assertTrue} from "test/TestUtil.js"
 import {CircuitStats} from "src/circuit/CircuitStats.js"
 
 import {CircuitDefinition} from "src/circuit/CircuitDefinition.js"
-import {Complex} from "src/math/Complex.js"
 import {GateColumn} from "src/circuit/GateColumn.js"
 import {Gates} from "src/gates/AllGates.js"
 import {Matrix} from "src/math/Matrix.js"
@@ -367,3 +366,25 @@ suite.testUsingWebGL("initial_states", () => {
     assertThat(stats.qubitDensityMatrix(9, 4).qubitDensityMatrixToBlochVector()).isApproximatelyEqualTo([0, +1, 0]);
     assertThat(stats.qubitDensityMatrix(9, 5).qubitDensityMatrixToBlochVector()).isApproximatelyEqualTo([0, -1, 0]);
 });
+
+suite.testUsingWebGL("distillation", () => {
+    let c = circuit(
+        `
+        -X-X--X-X--X-X--X-X-------X-X--X-X-------X-X------------HTH-0-
+        -X-X--X-X--X-X-------X-X--X-X-------X-X-------X-X-------HTH-0-
+        -X-X--X-X-------X-X--X-X-------X-X--X-X------------X-X--HTH-0-
+        -X-X-------X-X--X-X--X-X-----------------X-X--X-X--X-X--HTH-0-
+        -X-X----------------------X-X--X-X--X-X--X-X--X-X--X-X--------
+        -#T]--#T]--#T]--#T]--#T]--#T]--#T]--#T]--#T]--#T]--#T]--------
+        `,
+        [']', Gates.Detectors.XDetectControlClear],
+        ['0', Gates.PostSelectionGates.PostSelectOff],
+        ['#', Gates.Controls.XControl],
+        ['T', Gates.OtherZ.Z4]);
+    for (let i = 0; i < 5; i++) {
+        let stats = CircuitStats.fromCircuitAtTime(c, 0);
+        assertThat(stats.qubitDensityMatrix(Infinity, 4).qubitDensityMatrixToBlochVector()).isApproximatelyEqualTo(
+            [0, Math.sqrt(0.5), -Math.sqrt(0.5)]);
+        assertThat(stats.survivalRate(Infinity)).isApproximatelyEqualTo(1, 0.001);
+    }
+});
diff --git a/test/gates/Detector.test.js b/test/gates/Detector.test.js
@@ -88,3 +88,21 @@ suite.testUsingWebGL("collapsed-control-clicks", () => {
         }
     }
 });
+
+suite.testUsingWebGL("renormalizes", () => {
+    // Doesn't decrease survival probability.
+    let c = circuit(
+        '-]-D-]-D-]-D-]-',
+        [']', Gates.Detectors.XDetector],
+        ['0', Gates.PostSelectionGates.PostSelectOff]);
+    let stats = CircuitStats.fromCircuitAtTime(c, 0);
+    assertThat(stats.survivalRate(Infinity)).isApproximatelyEqualTo(1, 0.001);
+
+    // Renormalization doesn't increase survival probability.
+    let c2 = circuit(
+        '-]-0-D-]-D-]-D-]-',
+        [']', Gates.Detectors.XDetector],
+        ['0', Gates.PostSelectionGates.PostSelectOff]);
+    let stats2 = CircuitStats.fromCircuitAtTime(c2, 0);
+    assertThat(stats2.survivalRate(Infinity)).isApproximatelyEqualTo(0.5, 0.001);
+});