Add loss policies to NoiseConfig to express different kinds of behavior on lost qubits

source/compiler/qsc_eval/src/backend/noise_tests.rs

@@ -10,7 +10,7 @@ use crate::{
 use expect_test::{Expect, expect};
 use num_bigint::BigUint;
 use num_complex::Complex;
-use qdk_simulators::noise_config::{NoiseConfig, NoiseTable, encode_pauli};
+use qdk_simulators::noise_config::{LossPolicy, NoiseConfig, NoiseTable, encode_pauli};
 use std::fmt::Write;
 
 #[test]
@@ -258,6 +258,7 @@ fn noise_config_with_single_qubit_fault(
         qubits: 1,
         pauli_strings: vec![encode_pauli(pauli)],
         probabilities: vec![1.0],
+        on_loss: LossPolicy::Skip,
     };
     set_gate(&mut config, table);
     config
@@ -274,6 +275,7 @@ fn noise_config_with_two_qubit_fault(
         qubits: 2,
         pauli_strings: vec![encode_pauli(pauli)],
         probabilities: vec![1.0],
+        on_loss: LossPolicy::Skip,
     };
     set_gate(&mut config, table);
     config
@@ -525,6 +527,7 @@ fn noise_config_mz_with_loss() {
     let mut config = NoiseConfig::NOISELESS;
     config.mz = NoiseTable {
         qubits: 1,
+        on_loss: LossPolicy::Skip,
         pauli_strings: vec![encode_pauli("L")],
         probabilities: vec![1.0],
     };
@@ -545,6 +548,7 @@ fn noise_config_gate_loss_causes_measurement_loss() {
     let mut config = NoiseConfig::NOISELESS;
     config.x = NoiseTable {
         qubits: 1,
+        on_loss: LossPolicy::Skip,
         pauli_strings: vec![encode_pauli("L")],
         probabilities: vec![1.0],
     };

source/compiler/stim_compiler/src/qir.rs

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-use qdk_simulators::noise_config::{NoiseConfig, NoiseTable, encode_pauli};
+use qdk_simulators::noise_config::{LossPolicy, NoiseConfig, NoiseTable, encode_pauli};
 
 use crate::parser::*;
 use miette::Diagnostic;
@@ -975,6 +975,7 @@ impl<'noise> Compiler<'noise> {
             qubits: columns.len() as u32,
             pauli_strings,
             probabilities,
+            on_loss: LossPolicy::Skip,
         };
         self.noise.intrinsics.insert(id, table);
 

source/qdk_package/qdk/_native.pyi

@@ -893,6 +893,31 @@ class QirInstruction: ...
 class IdleNoiseParams:
     s_probability: float
 
+class LossPolicy(Enum):
+    """
+    Specifies the behavior of a multi-qubit gate when at least one of its
+    qubit operands is lost.
+    """
+
+    # If any operand of a gate is lost, skip the gate entirely.
+    # This policy can apply to all multi-qubit gates.
+    SKIP = 0
+    # If any operand of a gate is lost, propagate the loss to the other operands.
+    # This policy can apply to all multi-qubit gates.
+    PROPAGATE = 1
+    # For multi-qubit rotations, degrade the unitary to its single-qubit version
+    # on the surviving operand (e.g. rxx -> rx). Falls back to SKIP for gates with
+    # no single-qubit reduction (cx, cy, cz, swap, and single-qubit gates).
+    # This policy only applies to the rxx, ryy, and rzz gates, in which case
+    # they degrade to rx, ry, and rz on the remaining qubit respectively.
+    DEGRADE = 2
+    # Skip the gate and instead apply an S adjoint to each surviving operand.
+    # This policy can apply to all multi-qubit gates.
+    RESIDUAL_S_DAGGER = 3
+    # This policy only applies to the swap gate, in which case the qubit states
+    # are exchanged, including their loss flags.
+    APPLY_ANYWAY = 4
+
 class NoiseTable:
     # Deprecated. Setting `loss` distributes the per-qubit loss probability
     # across the correlated loss fault strings ('L' for a single-qubit
@@ -901,6 +926,7 @@ class NoiseTable:
     # `loss` reconstructs that per-qubit probability. Prefer setting the loss
     # fault strings directly via `set_pauli_noise`.
     loss: float
+    on_loss: LossPolicy
 
     def __init__(self, num_qubits: int):
         """

source/qdk_package/qdk/simulation/__init__.py

@@ -15,6 +15,10 @@
   to individual gate intrinsics to model depolarizing, bit-flip, phase-flip,
   or correlated noise channels.
 
+- :class:`~qdk.simulation.LossPolicy` — selects how a gate behaves when one of
+  its qubit operands is lost. Assign it to a noise table's ``on_loss`` attribute
+  (e.g. ``noise.cx.on_loss = LossPolicy.SKIP``).
+
 - :func:`~qdk.simulation.run_qir` — simulates QIR as given in one of
   three backend simulators: clifford, gpu or cpu.
 
@@ -26,7 +30,7 @@
 """
 
 from .._device._atom import NeutralAtomDevice
-from ._simulation import NoiseConfig, run_qir
+from ._simulation import NoiseConfig, LossPolicy, run_qir
 from ._noisy_simulator import (
     NoisySimulatorError,
     DensityMatrixSimulator,
@@ -40,6 +44,7 @@
 __all__ = [
     "NeutralAtomDevice",
     "NoiseConfig",
+    "LossPolicy",
     "run_qir",
     "NoisySimulatorError",
     "Operation",

source/qdk_package/qdk/simulation/_simulation.py

@@ -15,6 +15,7 @@
     run_cpu_adaptive,
     run_cpu_full_state,
     NoiseConfig,
+    LossPolicy,
     GpuContext,
     try_create_gpu_adapter,
     Result,

source/qdk_package/src/interpreter.rs

@@ -24,7 +24,7 @@ use crate::{
     },
     noisy_simulator::register_noisy_simulator_submodule,
     qir_simulation::{
-        IdleNoiseParams, NoiseConfig, NoiseTable, QirInstruction, QirInstructionId,
+        IdleNoiseParams, LossPolicy, NoiseConfig, NoiseTable, QirInstruction, QirInstructionId,
         cpu_simulators::{
             run_clifford, run_clifford_adaptive, run_cpu_adaptive, run_cpu_full_state,
         },
@@ -105,6 +105,7 @@ fn verify_classes_are_sendable() {
     is_send::<NoiseConfig>();
     is_send::<NoiseTable>();
     is_send::<IdleNoiseParams>();
+    is_send::<LossPolicy>();
 }
 
 #[pymodule]
@@ -134,6 +135,7 @@ fn _native<'a>(py: Python<'a>, m: &Bound<'a, PyModule>) -> PyResult<()> {
     m.add_class::<NoiseConfig>()?;
     m.add_class::<NoiseTable>()?;
     m.add_class::<IdleNoiseParams>()?;
+    m.add_class::<LossPolicy>()?;
     m.add_function(wrap_pyfunction!(physical_estimates, m)?)?;
     m.add_function(wrap_pyfunction!(run_clifford, m)?)?;
     m.add_function(wrap_pyfunction!(try_create_gpu_adapter, m)?)?;