From afdc0a1e91df2274552c8f29e1219c422c58bbb6 Mon Sep 17 00:00:00 2001
From: ciaranra <ciaranra@gmail.com>
Date: Fri, 18 Sep 2026 11:42:12 -0600
Subject: [PATCH] Normalize two-qubit noise-rate inputs to signed principal
 angles

---
 crates/pecos-engines/src/noise/general.rs | 58 +++++++++++++++++++++++
 exp/pecos-neo/src/noise/two_qubit.rs      | 26 +++++++++-
 2 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/crates/pecos-engines/src/noise/general.rs b/crates/pecos-engines/src/noise/general.rs
index cafa18574..5152da777 100644
--- a/crates/pecos-engines/src/noise/general.rs
+++ b/crates/pecos-engines/src/noise/general.rs
@@ -1432,8 +1432,21 @@ impl GeneralNoiseModel {
     /// Calculate the two-qubit gate error rate based on the rotation angle
     ///
     /// with additional support for asymmetric scaling and power-law scaling
+    ///
+    /// Angles are reduced modulo a full turn into `(-pi, pi]` before selecting
+    /// the signed coefficients. Thus an unsigned angle such as `7*pi/4` uses
+    /// the negative branch at `-pi/4`; both half-turn endpoints use `+pi`.
     #[must_use]
     pub fn p2_angle_error_rate(&self, angle: f64) -> f64 {
+        let angle = angle % std::f64::consts::TAU;
+        let angle = if angle > std::f64::consts::PI {
+            angle - std::f64::consts::TAU
+        } else if angle <= -std::f64::consts::PI {
+            angle + std::f64::consts::TAU
+        } else {
+            angle
+        };
+
         // Normalize angle by π - convert to a value in [0, 1] range
         let theta = angle.abs() / std::f64::consts::PI;
 
@@ -3762,6 +3775,51 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_rzz_error_rate_signed_periodicity() {
+        use std::f64::consts::{FRAC_PI_2, FRAC_PI_4, PI, TAU};
+
+        let mut model = GeneralNoiseModel::builder()
+            .with_p2(0.01)
+            .with_p2_angle_params(2.0, 0.1, 3.0, 0.2)
+            .with_p2_angle_power(2.0)
+            .build();
+        let noise = model
+            .as_any_mut()
+            .downcast_mut::<GeneralNoiseModel>()
+            .unwrap();
+
+        // Distinct coefficients expose selection of the wrong sign branch.
+        for (angle, scaling) in [
+            (0.0, 0.15),
+            (FRAC_PI_4, 0.3875),
+            (-FRAC_PI_4, 0.225),
+            (FRAC_PI_2, 0.95),
+            (-FRAC_PI_2, 0.6),
+            (PI, 3.2),
+            (-PI, 3.2),
+        ] {
+            for turns in [-4.0, -1.0, 0.0, 1.0, 4.0] {
+                let input = angle + turns * TAU;
+                let actual = noise.p2_angle_error_rate(input);
+                let expected = 0.01 * scaling;
+                assert!(
+                    (actual - expected).abs() < 1e-12,
+                    "angle {input}: expected {expected}, got {actual}"
+                );
+            }
+        }
+
+        // Avoid adding pi before reducing: that can erase tiny signed inputs.
+        assert!((noise.p2_angle_error_rate(-1e-20) - 0.001).abs() < 1e-12);
+        assert!((noise.p2_angle_error_rate(1e-20) - 0.002).abs() < 1e-12);
+        // The positive half-turn endpoint must not absorb neighboring angles.
+        assert!((noise.p2_angle_error_rate(PI.next_up()) - 0.021).abs() < 1e-12);
+        assert!((noise.p2_angle_error_rate(PI.next_down()) - 0.032).abs() < 1e-12);
+        assert!((noise.p2_angle_error_rate((-PI).next_up()) - 0.021).abs() < 1e-12);
+        assert!((noise.p2_angle_error_rate((-PI).next_down()) - 0.032).abs() < 1e-12);
+    }
+
     #[test]
     fn test_noiseless_gates() {
         // Create a noise model and mark RZ as a noiseless gate
diff --git a/exp/pecos-neo/src/noise/two_qubit.rs b/exp/pecos-neo/src/noise/two_qubit.rs
index 2b0f21894..2646acef5 100644
--- a/exp/pecos-neo/src/noise/two_qubit.rs
+++ b/exp/pecos-neo/src/noise/two_qubit.rs
@@ -224,7 +224,8 @@ impl AngleScaling {
 
     /// Calculate the scaling factor for a given angle.
     ///
-    /// Uses signed radians in [-pi, pi] for asymmetric scaling.
+    /// Uses signed radians in `(-pi, pi]` for asymmetric scaling. Unsigned
+    /// angles above pi use the negative branch; a half turn uses positive pi.
     /// The magnitude is normalized by pi before applying power.
     ///
     /// Formula: `offset + linear*|θ/π| + scale*|θ/π|^power`
@@ -812,6 +813,29 @@ mod tests {
         assert!((scaling.scale(Angle64::HALF_TURN) - 1.2).abs() < 1e-10);
     }
 
+    #[test]
+    fn test_asymmetric_angle_scaling_wrapped_boundaries() {
+        use std::f64::consts::{FRAC_PI_4, PI, TAU};
+
+        let scaling = AngleScaling::asymmetric(0.1, 0.0, 2.0, 0.2, 0.0, 3.0, 2.0);
+        for (angle, expected) in [
+            (0.0, 0.15),
+            (FRAC_PI_4, 0.3875),
+            (-FRAC_PI_4, 0.225),
+            (PI, 3.2),
+            (-PI, 3.2),
+        ] {
+            for turns in [-4.0, -1.0, 0.0, 1.0, 4.0] {
+                let input = angle + turns * TAU;
+                let actual = scaling.scale(Angle64::from_radians(input));
+                assert!(
+                    (actual - expected).abs() < 1e-12,
+                    "angle {input}: expected {expected}, got {actual}"
+                );
+            }
+        }
+    }
+
     #[test]
     fn test_polynomial_angle_scaling() {
         // Test the full polynomial formula: a + b*|θ/π| + c*|θ/π|^d