quantrs2_device/photonic/

error_correction.rs

//! Photonic Quantum Error Correction
//!
//! This module implements quantum error correction specifically designed for photonic systems,
//! including loss-tolerant codes and measurement-based error correction.

use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use thiserror::Error;

use super::continuous_variable::{CVError, CVResult, GaussianState};
use super::{PhotonicMode, PhotonicSystemType};
use crate::DeviceResult;

/// Errors for photonic quantum error correction
#[derive(Error, Debug)]
pub enum PhotonicQECError {
    #[error("Loss rate too high: {0}")]
    LossRateTooHigh(f64),
    #[error("Insufficient redundancy: {0}")]
    InsufficientRedundancy(String),
    #[error("Syndrome extraction failed: {0}")]
    SyndromeExtractionFailed(String),
    #[error("Correction application failed: {0}")]
    CorrectionFailed(String),
}

/// Photonic error correction codes
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum PhotonicErrorCorrectionCode {
    /// Loss-tolerant encoding using redundant photons
    LossTolerant { redundancy: usize },
    /// Measurement-based error correction
    MeasurementBased { cluster_size: usize },
    /// Continuous variable error correction
    ContinuousVariable { code_type: CVQECType },
    /// Hybrid photonic-matter error correction
    Hybrid {
        photonic_modes: usize,
        matter_qubits: usize,
    },
}

/// CV quantum error correction types
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum CVQECType {
    /// Squeezed state encoding
    SqueezedState,
    /// Displaced squeezed state encoding
    DisplacedSqueezed,
    /// Multi-mode encoding
    MultiMode,
}

/// Photonic error correction engine
pub struct PhotonicQECEngine {
    /// Active error correction codes
    pub active_codes: Vec<PhotonicErrorCorrectionCode>,
    /// Error statistics
    pub error_stats: HashMap<String, f64>,
}

impl PhotonicQECEngine {
    pub fn new() -> Self {
        Self {
            active_codes: Vec::new(),
            error_stats: HashMap::new(),
        }
    }

    /// Apply error correction to a photonic state
    pub fn apply_error_correction(
        &mut self,
        state: &mut GaussianState,
        code: &PhotonicErrorCorrectionCode,
    ) -> Result<(), PhotonicQECError> {
        match code {
            PhotonicErrorCorrectionCode::LossTolerant { redundancy } => {
                self.apply_loss_tolerance(state, *redundancy)
            }
            PhotonicErrorCorrectionCode::ContinuousVariable { code_type } => {
                self.apply_cv_error_correction(state, code_type)
            }
            _ => Ok(()), // Placeholder for other codes
        }
    }

    /// Apply loss-tolerant error correction
    fn apply_loss_tolerance(
        &mut self,
        state: &mut GaussianState,
        redundancy: usize,
    ) -> Result<(), PhotonicQECError> {
        if redundancy < 2 {
            return Err(PhotonicQECError::InsufficientRedundancy(
                "Loss tolerance requires redundancy >= 2".to_string(),
            ));
        }

        // Simplified loss correction - in practice this would involve
        // complex syndrome extraction and correction operations
        for mode in 0..state.num_modes.min(redundancy) {
            if let Ok(loss_rate) = state.average_photon_number(mode) {
                if loss_rate < 0.1 {
                    // Apply correction by adjusting covariance
                    state.covariance[2 * mode][2 * mode] *= 1.1;
                    state.covariance[2 * mode + 1][2 * mode + 1] *= 1.1;
                }
            }
        }

        Ok(())
    }

    /// Apply CV error correction
    fn apply_cv_error_correction(
        &mut self,
        state: &mut GaussianState,
        code_type: &CVQECType,
    ) -> Result<(), PhotonicQECError> {
        match code_type {
            CVQECType::SqueezedState => {
                // Apply squeezing-based error correction
                for mode in 0..state.num_modes {
                    if state.squeeze(0.1, 0.0, mode).is_err() {
                        return Err(PhotonicQECError::CorrectionFailed(format!(
                            "Failed to apply squeezing correction to mode {mode}"
                        )));
                    }
                }
            }
            CVQECType::DisplacedSqueezed => {
                // Apply displacement and squeezing correction
                for mode in 0..state.num_modes {
                    let _ =
                        state.displace(super::continuous_variable::Complex::new(0.1, 0.0), mode);
                    let _ = state.squeeze(0.05, 0.0, mode);
                }
            }
            CVQECType::MultiMode => {
                // Multi-mode error correction (placeholder)
                // In practice, this would involve entangling operations between modes
            }
        }

        Ok(())
    }

    /// Detect and extract error syndromes
    pub fn extract_syndrome(
        &self,
        state: &GaussianState,
        code: &PhotonicErrorCorrectionCode,
    ) -> Result<Vec<bool>, PhotonicQECError> {
        match code {
            PhotonicErrorCorrectionCode::LossTolerant { redundancy } => {
                let mut syndrome = Vec::new();

                // Simple loss detection based on photon number variance
                for mode in 0..state.num_modes.min(*redundancy) {
                    if let Ok(avg_photons) = state.average_photon_number(mode) {
                        syndrome.push(avg_photons < 0.5); // Loss detected
                    } else {
                        syndrome.push(false);
                    }
                }

                Ok(syndrome)
            }
            _ => Ok(vec![false; 4]), // Placeholder syndrome
        }
    }

    /// Get error correction statistics
    pub fn get_statistics(&self) -> PhotonicQECStatistics {
        PhotonicQECStatistics {
            active_codes: self.active_codes.len(),
            total_corrections_applied: self.error_stats.get("corrections").copied().unwrap_or(0.0)
                as usize,
            loss_correction_rate: self
                .error_stats
                .get("loss_corrections")
                .copied()
                .unwrap_or(0.0),
            phase_correction_rate: self
                .error_stats
                .get("phase_corrections")
                .copied()
                .unwrap_or(0.0),
            overall_fidelity: self.error_stats.get("fidelity").copied().unwrap_or(0.95),
        }
    }
}

impl Default for PhotonicQECEngine {
    fn default() -> Self {
        Self::new()
    }
}

/// Statistics for photonic quantum error correction
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct PhotonicQECStatistics {
    pub active_codes: usize,
    pub total_corrections_applied: usize,
    pub loss_correction_rate: f64,
    pub phase_correction_rate: f64,
    pub overall_fidelity: f64,
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::photonic::continuous_variable::GaussianState;

    #[test]
    fn test_qec_engine_creation() {
        let engine = PhotonicQECEngine::new();
        assert_eq!(engine.active_codes.len(), 0);
    }

    #[test]
    fn test_loss_tolerant_correction() {
        let mut engine = PhotonicQECEngine::new();
        let mut state = GaussianState::vacuum(2);
        let code = PhotonicErrorCorrectionCode::LossTolerant { redundancy: 3 };

        let result = engine.apply_error_correction(&mut state, &code);
        assert!(result.is_ok());
    }

    #[test]
    fn test_syndrome_extraction() {
        let engine = PhotonicQECEngine::new();
        let state = GaussianState::vacuum(2);
        let code = PhotonicErrorCorrectionCode::LossTolerant { redundancy: 2 };

        let syndrome = engine
            .extract_syndrome(&state, &code)
            .expect("Syndrome extraction should succeed");
        assert_eq!(syndrome.len(), 2);
    }
}