[add]: prelimenary noise layers

2 files changed, 562 insertions, 0 deletions

diff --git a/libpsi-core/src/core/mod.rs b/libpsi-core/src/core/mod.rs
index 343e319..c38cf38 100644
--- a/libpsi-core/src/core/mod.rs
+++ b/libpsi-core/src/core/mod.rs
@@ -3,6 +3,7 @@ pub mod classical_components;
 pub mod custom_gate;
 pub mod gates;
 pub mod kernel;
+pub mod noise;
 pub mod quantum_components;
 pub mod runtime;
 
@@ -11,5 +12,6 @@ pub use classical_components::*;
 pub use custom_gate::*;
 pub use gates::*;
 pub use kernel::*;
+pub use noise::*;
 pub use quantum_components::*;
 pub use runtime::*;
diff --git a/libpsi-core/src/core/noise.rs b/libpsi-core/src/core/noise.rs
new file mode 100644
index 0000000..6836c00
--- /dev/null
+++ b/libpsi-core/src/core/noise.rs
@@ -0,0 +1,560 @@
+use crate::{complex, Complex, Matrix};
+
+#[derive(Clone, Debug)]
+pub struct KrausOperator {
+    pub matrix: Matrix<Complex<f64>>,
+    pub name: String,
+}
+
+impl KrausOperator {
+    pub fn new(name: &str, matrix: Matrix<Complex<f64>>) -> Self {
+        Self {
+            matrix,
+            name: name.to_string(),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct NoiseChannel {
+    pub name: String,
+    pub operators: Vec<KrausOperator>,
+    pub num_qubits: usize,
+}
+
+impl NoiseChannel {
+    pub fn new(name: &str, operators: Vec<KrausOperator>, num_qubits: usize) -> Self {
+        Self {
+            name: name.to_string(),
+            operators,
+            num_qubits,
+        }
+    }
+
+    pub fn depolarising(p: f64) -> Self {
+        let sqrt_1_p = (1.0 - p).sqrt();
+        let sqrt_p3 = (p / 3.0).sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_1_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(sqrt_p3, 0.0),
+                complex!(sqrt_p3, 0.0),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        let k2 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(0.0, -sqrt_p3),
+                complex!(0.0, sqrt_p3),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        let k3 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_p3, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(-sqrt_p3, 0.0),
+            ],
+        );
+
+        Self::new(
+            "Depolarising",
+            vec![
+                KrausOperator::new("K0", k0),
+                KrausOperator::new("K1(X)", k1),
+                KrausOperator::new("K2(Y)", k2),
+                KrausOperator::new("K3(Z)", k3),
+            ],
+            1,
+        )
+    }
+
+    pub fn amplitude_damping(gamma: f64) -> Self {
+        let sqrt_gamma = gamma.sqrt();
+        let sqrt_1_gamma = (1.0 - gamma).sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(1.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_gamma, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(sqrt_gamma, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        Self::new(
+            "AmplitudeDamping",
+            vec![
+                KrausOperator::new("K0", k0),
+                KrausOperator::new("K1", k1),
+            ],
+            1,
+        )
+    }
+
+    pub fn phase_damping(gamma: f64) -> Self {
+        let sqrt_gamma = gamma.sqrt();
+        let sqrt_1_gamma = (1.0 - gamma).sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(1.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_gamma, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_gamma, 0.0),
+            ],
+        );
+
+        Self::new(
+            "PhaseDamping",
+            vec![
+                KrausOperator::new("K0", k0),
+                KrausOperator::new("K1", k1),
+            ],
+            1,
+        )
+    }
+
+    pub fn bit_flip(p: f64) -> Self {
+        let sqrt_1_p = (1.0 - p).sqrt();
+        let sqrt_p = p.sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_1_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(sqrt_p, 0.0),
+                complex!(sqrt_p, 0.0),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        Self::new(
+            "BitFlip",
+            vec![
+                KrausOperator::new("K0(I)", k0),
+                KrausOperator::new("K1(X)", k1),
+            ],
+            1,
+        )
+    }
+
+    pub fn phase_flip(p: f64) -> Self {
+        let sqrt_1_p = (1.0 - p).sqrt();
+        let sqrt_p = p.sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_1_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(-sqrt_p, 0.0),
+            ],
+        );
+
+        Self::new(
+            "PhaseFlip",
+            vec![
+                KrausOperator::new("K0(I)", k0),
+                KrausOperator::new("K1(Z)", k1),
+            ],
+            1,
+        )
+    }
+
+    pub fn bit_phase_flip(p: f64) -> Self {
+        let sqrt_1_p = (1.0 - p).sqrt();
+        let sqrt_p = p.sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_1_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(0.0, -sqrt_p),
+                complex!(0.0, sqrt_p),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        Self::new(
+            "BitPhaseFlip",
+            vec![
+                KrausOperator::new("K0(I)", k0),
+                KrausOperator::new("K1(Y)", k1),
+            ],
+            1,
+        )
+    }
+
+    pub fn generalised_amplitude_damping(p: f64, gamma: f64) -> Self {
+        let sqrt_p = p.sqrt();
+        let sqrt_1_p = (1.0 - p).sqrt();
+        let sqrt_gamma = gamma.sqrt();
+        let sqrt_1_gamma = (1.0 - gamma).sqrt();
+
+        let k0 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_p, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_p * sqrt_1_gamma, 0.0),
+            ],
+        );
+
+        let k1 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(sqrt_p * sqrt_gamma, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        let k2 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(sqrt_1_p * sqrt_1_gamma, 0.0),
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p, 0.0),
+            ],
+        );
+
+        let k3 = Matrix::new(
+            2,
+            2,
+            vec![
+                complex!(0.0, 0.0),
+                complex!(0.0, 0.0),
+                complex!(sqrt_1_p * sqrt_gamma, 0.0),
+                complex!(0.0, 0.0),
+            ],
+        );
+
+        Self::new(
+            "GeneralisedAmplitudeDamping",
+            vec![
+                KrausOperator::new("K0", k0),
+                KrausOperator::new("K1", k1),
+                KrausOperator::new("K2", k2),
+                KrausOperator::new("K3", k3),
+            ],
+            1,
+        )
+    }
+}
+
+#[derive(Clone)]
+pub struct DensityMatrix {
+    pub data: Vec<Complex<f64>>,
+    pub dim: usize,
+    pub num_qubits: usize,
+}
+
+impl DensityMatrix {
+    pub fn new(num_qubits: usize) -> Self {
+        let dim = 1 << num_qubits;
+        let mut data = vec![complex!(0.0, 0.0); dim * dim];
+        data[0] = complex!(1.0, 0.0);
+        Self {
+            data,
+            dim,
+            num_qubits,
+        }
+    }
+
+    pub fn from_state_vector(state: &[Complex<f64>]) -> Self {
+        let dim = state.len();
+        let num_qubits = (dim as f64).log2() as usize;
+        let mut data = vec![complex!(0.0, 0.0); dim * dim];
+
+        for i in 0..dim {
+            for j in 0..dim {
+                data[i * dim + j] = state[i] * state[j].get_conjugate();
+            }
+        }
+
+        Self {
+            data,
+            dim,
+            num_qubits,
+        }
+    }
+
+    pub fn get(&self, row: usize, col: usize) -> Complex<f64> {
+        self.data[row * self.dim + col]
+    }
+
+    pub fn set(&mut self, row: usize, col: usize, value: Complex<f64>) {
+        self.data[row * self.dim + col] = value;
+    }
+
+    pub fn trace(&self) -> Complex<f64> {
+        let mut sum = complex!(0.0, 0.0);
+        for i in 0..self.dim {
+            sum = sum + self.get(i, i);
+        }
+        sum
+    }
+
+    pub fn purity(&self) -> f64 {
+        let mut sum = complex!(0.0, 0.0);
+        for i in 0..self.dim {
+            for j in 0..self.dim {
+                let rho_ij = self.get(i, j);
+                let rho_ji = self.get(j, i);
+                sum = sum + rho_ij * rho_ji;
+            }
+        }
+        sum.real
+    }
+
+    pub fn is_pure(&self, tolerance: f64) -> bool {
+        (self.purity() - 1.0).abs() < tolerance
+    }
+
+    pub fn probabilities(&self) -> Vec<f64> {
+        (0..self.dim).map(|i| self.get(i, i).real).collect()
+    }
+
+    pub fn apply_unitary(&mut self, gate: &Matrix<Complex<f64>>, targets: &[usize]) {
+        let g = targets.len();
+        let gate_dim = 1 << g;
+
+        let target_bits: Vec<usize> = targets
+            .iter()
+            .map(|&t| self.num_qubits - 1 - t)
+            .collect();
+
+        let mut non_target_mask: usize = (1 << self.num_qubits) - 1;
+        for &pos in &target_bits {
+            non_target_mask &= !(1 << pos);
+        }
+
+        let mut new_data = vec![complex!(0.0, 0.0); self.dim * self.dim];
+
+        for i in 0..self.dim {
+            for j in 0..self.dim {
+                let mut sum = complex!(0.0, 0.0);
+
+                for k in 0..gate_dim {
+                    for l in 0..gate_dim {
+                        let mut src_i = i & non_target_mask;
+                        let mut src_j = j & non_target_mask;
+
+                        for (idx, &pos) in target_bits.iter().enumerate() {
+                            if (k >> (g - 1 - idx)) & 1 == 1 {
+                                src_i |= 1 << pos;
+                            }
+                            if (l >> (g - 1 - idx)) & 1 == 1 {
+                                src_j |= 1 << pos;
+                            }
+                        }
+
+                        let mut tgt_i = 0usize;
+                        let mut tgt_j = 0usize;
+                        for (idx, &pos) in target_bits.iter().enumerate() {
+                            if (i >> pos) & 1 == 1 {
+                                tgt_i |= 1 << (g - 1 - idx);
+                            }
+                            if (j >> pos) & 1 == 1 {
+                                tgt_j |= 1 << (g - 1 - idx);
+                            }
+                        }
+
+                        let u_ik = gate.data[tgt_i * gate_dim + k];
+                        let u_jl_dag = gate.data[tgt_j * gate_dim + l].get_conjugate();
+                        let rho_kl = self.get(src_i, src_j);
+
+                        sum = sum + u_ik * rho_kl * u_jl_dag;
+                    }
+                }
+
+                new_data[i * self.dim + j] = sum;
+            }
+        }
+
+        self.data = new_data;
+    }
+
+    pub fn apply_noise_channel(&mut self, channel: &NoiseChannel, target: usize) {
+        if channel.num_qubits != 1 {
+            panic!("Only single-qubit noise channels are currently supported");
+        }
+
+        let target_bit = self.num_qubits - 1 - target;
+        let mut new_data = vec![complex!(0.0, 0.0); self.dim * self.dim];
+
+        for kraus in &channel.operators {
+            let k = &kraus.matrix;
+
+            for i in 0..self.dim {
+                for j in 0..self.dim {
+                    let i_target = (i >> target_bit) & 1;
+                    let j_target = (j >> target_bit) & 1;
+
+                    for ki in 0..2 {
+                        for kj in 0..2 {
+                            let src_i = (i & !(1 << target_bit)) | (ki << target_bit);
+                            let src_j = (j & !(1 << target_bit)) | (kj << target_bit);
+
+                            let k_elem = k.data[i_target * 2 + ki];
+                            let k_dag_elem = k.data[j_target * 2 + kj].get_conjugate();
+                            let rho_elem = self.get(src_i, src_j);
+
+                            new_data[i * self.dim + j] =
+                                new_data[i * self.dim + j] + k_elem * rho_elem * k_dag_elem;
+                        }
+                    }
+                }
+            }
+        }
+
+        self.data = new_data;
+    }
+
+    pub fn measure_probability(&self, qubit: usize, outcome: usize) -> f64 {
+        let target_bit = self.num_qubits - 1 - qubit;
+        let mut prob = 0.0;
+
+        for i in 0..self.dim {
+            if (i >> target_bit) & 1 == outcome {
+                prob += self.get(i, i).real;
+            }
+        }
+
+        prob
+    }
+
+    pub fn fidelity_with_pure_state(&self, state: &[Complex<f64>]) -> f64 {
+        let mut sum = complex!(0.0, 0.0);
+
+        for i in 0..self.dim {
+            for j in 0..self.dim {
+                sum = sum + state[i].get_conjugate() * self.get(i, j) * state[j];
+            }
+        }
+
+        sum.real
+    }
+}
+
+impl std::fmt::Display for DensityMatrix {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "DensityMatrix ({} qubits, {}×{}):", self.num_qubits, self.dim, self.dim)?;
+        writeln!(f, "  Trace: {:.6}", self.trace().real)?;
+        writeln!(f, "  Purity: {:.6}", self.purity())?;
+        writeln!(f, "  Pure: {}", self.is_pure(1e-10))?;
+        writeln!(f, "  Probabilities: {:?}", self.probabilities())?;
+        Ok(())
+    }
+}
+
+impl std::fmt::Debug for DensityMatrix {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "DensityMatrix {}×{}:", self.dim, self.dim)?;
+        for i in 0..self.dim {
+            write!(f, "  [")?;
+            for j in 0..self.dim {
+                let val = self.get(i, j);
+                if j > 0 {
+                    write!(f, ", ")?;
+                }
+                write!(f, "{:.4}+{:.4}i", val.real, val.imaginary)?;
+            }
+            writeln!(f, "]")?;
+        }
+        Ok(())
+    }
+}
+