Naive crosstalk (#175)

* Basic prototype. I think there's a major issue, I have not tested yet.

* Now discarding outcomes from crosstalk projections

* Changed nomenclature from initialization to preparation

* I forgot to change initialized_qubits to prepared_qubits in default.rs

* Adding with_average methods for crosstalk

Author: PabloAndresCQ

crates/pecos-engines/src/noise/general.rs

@@ -92,7 +92,7 @@ use pecos_core::QubitId;
 use pecos_core::errors::PecosError;
 use rand_chacha::ChaCha8Rng;
 use std::any::Any;
-use std::collections::HashSet;
+use std::collections::{BTreeSet, HashSet};
 
 /// General noise model implementation that includes parameterized error channels for various quantum operations
 ///
@@ -172,9 +172,9 @@ pub struct GeneralNoiseModel {
     /// qubit states after initialization. Ranges from 0 to 1.
     p_prep_leak_ratio: f64,
 
-    /// Probability of crosstalk during initialization operations
+    /// Probability of crosstalk during preparation operations
     ///
-    /// Models the probability that an initialization operation on one qubit affects nearby qubits.
+    /// Models the probability that a preparation operation on one qubit affects nearby qubits.
     /// In ion trap systems, this could represent scattered light during optical pumping affecting
     /// neighboring ions.
     p_prep_crosstalk: f64,
@@ -326,10 +326,23 @@ pub struct GeneralNoiseModel {
     /// Random number generator for stochastic noise processes
     rng: NoiseRng<ChaCha8Rng>,
 
+    /// Set of qubits that have been prepared at any point in the program.
+    ///
+    /// This is so that we know which qubits exists and we can apply crosstalk
+    /// to them. Qubits that are measured / discarded are not removed from here, since
+    /// PECOS does not assume measurements are destructive. This should not cause a
+    /// problem, since inactive qubits suffering error have no effect on the state,
+    /// and active qubits should always suffer errors under this naive crosstalk model.
+    ///
+    /// Using a `BTreeSet` because we will iterate over the qubits and we want determinism.
+    prepared_qubits: BTreeSet<usize>,
+
     /// Track which qubits are being measured in the current batch and their gate types
-    /// This is needed to properly handle leakage during measurements
-    /// Each entry is (`qubit_id`, `is_measure_leaked`)
-    measured_qubits: Vec<(usize, bool)>,
+    /// This is needed to properly handle leakage during measurements as well
+    /// as crosstalk.
+    /// TODO: manage this via result tags.
+    /// Each entry is (`qubit_id`, `is_measure_leaked`, `is_crosstalk`)
+    measured_qubits: Vec<(usize, bool, bool)>,
 }
 
 impl ControlEngine for GeneralNoiseModel {
@@ -503,21 +516,24 @@ impl GeneralNoiseModel {
                     );
                 }
                 GateType::Prep => {
+                    for &q in &gate.qubits {
+                        self.prepared_qubits.insert(usize::from(q));
+                    }
                     self.apply_prep_faults(&gate, &mut builder);
-
-                    // TODO: Implement prep crosstalk when needed
+                    self.apply_crosstalk_faults(&gate, self.p_prep_crosstalk, &mut builder);
                 }
                 GateType::Measure | GateType::MeasureLeaked => {
                     // Track which qubits are being measured for leakage handling
                     let is_measure_leaked = gate.gate_type == GateType::MeasureLeaked;
                     self.measured_qubits.extend(
                         gate.qubits
                             .iter()
-                            .map(|q| (usize::from(*q), is_measure_leaked)),
+                            .map(|q| (usize::from(*q), is_measure_leaked, false)),
                     );
                     // Measurement noise is handled in apply_noise_on_continue_processing
                     // We still need to add the original gate here
                     builder.add_gate_command(&gate);
+                    self.apply_crosstalk_faults(&gate, self.p_meas_crosstalk, &mut builder);
                 }
                 GateType::I => {
                     let err_msg = format!(
@@ -589,15 +605,26 @@ impl GeneralNoiseModel {
 
         // Check if we have leaked qubits that were measured
         let has_leakage = !self.leaked_qubits.is_empty()
-            && self.measured_qubits.iter().any(|(q, _)| self.is_leaked(*q));
+            && self
+                .measured_qubits
+                .iter()
+                .any(|(q, _, _)| self.is_leaked(*q));
 
         for (idx, outcome) in measurement_outcomes.into_iter().enumerate() {
             let mut val = outcome;
 
-            // Check if this measurement corresponds to a leaked qubit
-            if has_leakage && idx < self.measured_qubits.len() {
-                let (qubit, is_measure_leaked) = self.measured_qubits[idx];
-                if self.is_leaked(qubit) {
+            // Check if this measurement corresponds to a leaked qubit or comes from
+            // crosstalk
+            if idx < self.measured_qubits.len() {
+                let (qubit, is_measure_leaked, is_crosstalk) = self.measured_qubits[idx];
+
+                // Check if this measurement comes from crosstalk noise. If so, ignore it.
+                if is_crosstalk {
+                    trace!("Qubit {qubit} was measured by crosstalk; outcome is ignored.");
+                    continue; // Skip this iteration
+                }
+
+                if has_leakage && self.is_leaked(qubit) {
                     if is_measure_leaked {
                         trace!("Qubit {qubit} is leaked, MeasureLeaked returns 2");
                         // For MeasureLeaked, return 2 for leaked qubits
@@ -783,6 +810,41 @@ impl GeneralNoiseModel {
         }
     }
 
+    /// Apply crosstalk noise
+    ///
+    /// Naive crosstalk noise model:
+    /// 1. All qubits in the trap but the ones in the `gate` are subject to crosstalk
+    //     error. The `gate` should be either qubit measurement or preparation.
+    //  2. *Each* qubit not in `gate` has the given `probability` to suffer an error.
+    /// 3. Affected qubits are collapsed into the computational basis (Z measurement).
+    ///
+    /// In ion trap systems, this could represent scattered light during optical pumping
+    /// affecting neighboring ions.
+    pub fn apply_crosstalk_faults(
+        &mut self,
+        gate: &Gate,
+        probability: f64,
+        builder: &mut ByteMessageBuilder,
+    ) {
+        let mut affected_qubits = Vec::new();
+        let gate_qubits: Vec<usize> = gate.qubits.iter().map(|q| usize::from(*q)).collect();
+
+        for q in self.prepared_qubits.clone() {
+            if !gate_qubits.contains(&q) && self.rng.occurs(probability) {
+                affected_qubits.push(q);
+                trace!("Qubit {q} affected by crosstalk error");
+            }
+        }
+
+        builder.add_measurements(&affected_qubits);
+        // We need to mark these measurements as being introduced by crosstalk rather
+        // than the user's program so that we can discard the results in
+        // apply_noise_on_continue_processing.
+        self.measured_qubits.extend(
+            affected_qubits.iter().map(|&q| (q, false, true)), // (qubit, is_measure_leaked, is_crosstalk)
+        );
+    }
+
     /// Apply single-qubit gate noise faults
     ///
     /// Models errors that occur during single-qubit gate operations:
@@ -2225,6 +2287,132 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_prep_crosstalk() {
+        use crate::byte_message::ByteMessageBuilder;
+
+        let mut model = GeneralNoiseModel::builder()
+            .with_p_prep_crosstalk(1.0)
+            .with_seed(42)
+            .build();
+        let noise = model
+            .as_any_mut()
+            .downcast_mut::<GeneralNoiseModel>()
+            .unwrap();
+
+        let mut builder = ByteMessageBuilder::new();
+        let _ = builder.for_quantum_operations();
+        // Prepare a bunch of |0> states
+        builder.add_prep(&[0, 1, 2, 3, 4]);
+        // Apply mid-circuit measurement and reset
+        builder.add_measurements(&[2]);
+        builder.add_prep(&[2]);
+        let _cmd = noise.apply_noise_on_start(&builder.build()).unwrap();
+
+        assert_eq!(
+            noise.measured_qubits.len(),
+            5,
+            "There should be 5 measured qubits: one from MCMR and the others from
+            crosstalk got: {:?}",
+            noise.measured_qubits
+        );
+
+        let (q, _, is_crosstalk) = noise.measured_qubits[0];
+        assert_eq!(q, 2, "The first measurement should be the MCMR on qubit 2");
+        assert!(!is_crosstalk, "The first measurement should come from MCMR");
+
+        for (_, _, is_crosstalk) in &noise.measured_qubits[1..] {
+            assert!(
+                is_crosstalk,
+                "The other measurements should come from crosstalk"
+            );
+        }
+
+        // All results are 0
+        let mut outcome_builder = ByteMessageBuilder::new();
+        let _ = outcome_builder.for_outcomes();
+        outcome_builder.add_outcomes(&[0, 0, 0, 0, 0]);
+
+        let mcmr = noise
+            .apply_noise_on_continue_processing(outcome_builder.build())
+            .unwrap();
+        let results = mcmr.outcomes().unwrap();
+
+        assert_eq!(
+            noise.measured_qubits.len(),
+            0,
+            "The list of measured_qubits should have been cleared."
+        );
+        assert_eq!(
+            results.len(),
+            1,
+            "There should only be one outcome: that of the mid-circ measurement"
+        );
+    }
+
+    #[test]
+    fn test_meas_crosstalk() {
+        use crate::byte_message::ByteMessageBuilder;
+
+        let mut model = GeneralNoiseModel::builder()
+            .with_p_meas_crosstalk(1.0)
+            .with_seed(42)
+            .build();
+        let noise = model
+            .as_any_mut()
+            .downcast_mut::<GeneralNoiseModel>()
+            .unwrap();
+
+        let mut builder = ByteMessageBuilder::new();
+        let _ = builder.for_quantum_operations();
+        // Prepare a bunch of |0> states
+        builder.add_prep(&[0, 1, 2, 3, 4]);
+        // Apply mid-circuit measurement and reset
+        builder.add_measurements(&[2]);
+        builder.add_prep(&[2]);
+        let _cmd = noise.apply_noise_on_start(&builder.build()).unwrap();
+
+        assert_eq!(
+            noise.measured_qubits.len(),
+            5,
+            "There should be 5 measured qubits: one from MCMR and the others from
+            crosstalk got: {:?}",
+            noise.measured_qubits
+        );
+
+        let (q, _, is_crosstalk) = noise.measured_qubits[0];
+        assert_eq!(q, 2, "The first measurement should be the MCMR on qubit 2");
+        assert!(!is_crosstalk, "The first measurement should come from MCMR");
+
+        for (_, _, is_crosstalk) in &noise.measured_qubits[1..] {
+            assert!(
+                is_crosstalk,
+                "The other measurements should come from crosstalk"
+            );
+        }
+
+        // All results are 0
+        let mut outcome_builder = ByteMessageBuilder::new();
+        let _ = outcome_builder.for_outcomes();
+        outcome_builder.add_outcomes(&[0, 0, 0, 0, 0]);
+
+        let mcmr = noise
+            .apply_noise_on_continue_processing(outcome_builder.build())
+            .unwrap();
+        let results = mcmr.outcomes().unwrap();
+
+        assert_eq!(
+            noise.measured_qubits.len(),
+            0,
+            "The list of measured_qubits should have been cleared."
+        );
+        assert_eq!(
+            results.len(),
+            1,
+            "There should only be one outcome: that of the mid-circ measurement"
+        );
+    }
+
     #[test]
     fn test_parameter_scaling() {
         // Test that scaling factors are applied correctly - use builder pattern

crates/pecos-engines/src/noise/general/builder.rs

@@ -412,6 +412,15 @@ impl GeneralNoiseModelBuilder {
         self
     }
 
+    // TODO: See if we should put a average scaling...
+    /// Set the average prep crosstalk
+    #[must_use]
+    pub fn with_average_p_prep_crosstalk(mut self, prob: f64) -> Self {
+        let prob: f64 = prob * 18.0 / 5.0;
+        self.p_prep_crosstalk = Some(prob);
+        self
+    }
+
     /// Set the scaling factor for initialization errors
     ///
     /// Multiplier for preparation error probabilities. Allows adjustment of the relative
@@ -642,6 +651,15 @@ impl GeneralNoiseModelBuilder {
         self
     }
 
+    // TODO: See if we should put a average scaling...
+    /// Set the average measurement crosstalk
+    #[must_use]
+    pub fn with_average_p_meas_crosstalk(mut self, prob: f64) -> Self {
+        let prob: f64 = prob * 18.0 / 5.0;
+        self.p_meas_crosstalk = Some(prob);
+        self
+    }
+
     /// Set the scaling factor for measurement faults
     ///
     /// Multiplier for measurement error probabilities. Allows adjustment of the relative

crates/pecos-engines/src/noise/general/default.rs

@@ -1,7 +1,7 @@
 use crate::noise::{
     GeneralNoiseModel, NoiseRng, SingleQubitWeightedSampler, TwoQubitWeightedSampler,
 };
-use std::collections::{BTreeMap, HashSet};
+use std::collections::{BTreeMap, BTreeSet, HashSet};
 
 impl Default for GeneralNoiseModel {
     /// Create a new noise model with default error parameters
@@ -100,6 +100,7 @@ impl Default for GeneralNoiseModel {
             p2_idle: 0.0,
             leaked_qubits: HashSet::new(),
             rng: NoiseRng::default(),
+            prepared_qubits: BTreeSet::new(),
             measured_qubits: Vec::new(),
             p_meas_crosstalk: 0.0,
             p_prep_crosstalk: 0.0,