quantrs2-core 0.1.3

Core types and traits for the QuantRS2 quantum computing framework
Documentation 
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//! Quantum Low-Density Parity-Check (LDPC) codes

use super::pauli::{Pauli, PauliString};
use super::stabilizer::StabilizerCode;
use crate::error::QuantRS2Result;

/// Quantum Low-Density Parity-Check (LDPC) codes
#[derive(Debug, Clone)]
pub struct QuantumLDPCCode {
    /// Number of physical qubits
    pub n: usize,
    /// Number of logical qubits
    pub k: usize,
    /// Maximum stabilizer weight
    pub max_weight: usize,
    /// X-type stabilizers
    pub x_stabilizers: Vec<PauliString>,
    /// Z-type stabilizers
    pub z_stabilizers: Vec<PauliString>,
}

impl QuantumLDPCCode {
    /// Create a bicycle code (CSS LDPC)
    pub fn bicycle_code(a: usize, b: usize) -> Self {
        let n = 2 * a * b;
        let k = 2;
        let max_weight = 6; // Typical for bicycle codes

        let mut x_stabilizers = Vec::new();
        let mut z_stabilizers = Vec::new();

        // Generate bicycle code stabilizers
        for i in 0..a {
            for j in 0..b {
                // X-type stabilizer
                let mut x_paulis = vec![Pauli::I; n];
                let base_idx = i * b + j;

                // Create a 6-cycle in the Cayley graph
                x_paulis[base_idx] = Pauli::X;
                x_paulis[(base_idx + 1) % (a * b)] = Pauli::X;
                x_paulis[(base_idx + b) % (a * b)] = Pauli::X;
                x_paulis[a * b + base_idx] = Pauli::X;
                x_paulis[a * b + (base_idx + 1) % (a * b)] = Pauli::X;
                x_paulis[a * b + (base_idx + b) % (a * b)] = Pauli::X;

                x_stabilizers.push(PauliString::new(x_paulis));

                // Z-type stabilizer (similar structure)
                let mut z_paulis = vec![Pauli::I; n];
                z_paulis[base_idx] = Pauli::Z;
                z_paulis[(base_idx + a) % (a * b)] = Pauli::Z;
                z_paulis[(base_idx + 1) % (a * b)] = Pauli::Z;
                z_paulis[a * b + base_idx] = Pauli::Z;
                z_paulis[a * b + (base_idx + a) % (a * b)] = Pauli::Z;
                z_paulis[a * b + (base_idx + 1) % (a * b)] = Pauli::Z;

                z_stabilizers.push(PauliString::new(z_paulis));
            }
        }

        Self {
            n,
            k,
            max_weight,
            x_stabilizers,
            z_stabilizers,
        }
    }

    /// Convert to stabilizer code representation
    pub fn to_stabilizer_code(&self) -> QuantRS2Result<StabilizerCode> {
        let mut stabilizers = self.x_stabilizers.clone();
        stabilizers.extend(self.z_stabilizers.clone());

        // Create logical operators (simplified)
        let logical_x = vec![
            PauliString::new(vec![Pauli::X; self.n]),
            PauliString::new(vec![Pauli::Y; self.n]),
        ];
        let logical_z = vec![
            PauliString::new(vec![Pauli::Z; self.n]),
            PauliString::new(vec![Pauli::Y; self.n]),
        ];

        StabilizerCode::new(
            self.n,
            self.k,
            4, // Typical distance for bicycle codes
            stabilizers,
            logical_x,
            logical_z,
        )
    }
}