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//! Sampling a circuit with a Pauli frame, independently of any error model.
//!
//! The frame records how this shot differs from a noiseless run. For a Clifford
//! circuit under Pauli noise that is all a detector needs: it reads a parity the
//! noiseless circuit fixes, so it fires exactly when the frame flips an odd
//! number of the records it reads. Noise is sampled with each channel's true
//! (disjoint) semantics, not the error model's independent approximation, and
//! nothing here looks at the model, so a model that is wrong about the circuit
//! shows up as disagreement between the two rather than agreeing with itself.
//!
//! Resets and measurements randomise the component of the frame that the state
//! is insensitive to, as Stim's frame simulator does. It costs a draw and changes
//! nothing for a deterministic detector; for a non-deterministic one it makes the
//! failure visible as a coin flip instead of hiding it.
use crate::circuit::{Basis, Circuit, Instr};
use crate::surface_code::Xorshift;
pub struct Shot {
pub detectors: Vec<bool>,
pub observables: u64,
}
pub struct FrameSampler {
instrs: Vec<Instr>,
num_qubits: usize,
num_measurements: usize,
detectors: Vec<Vec<usize>>,
observables: Vec<Vec<usize>>,
}
fn apply(x: &mut [bool], z: &mut [bool], q: usize, pauli: u8) {
if pauli & 1 != 0 {
x[q] ^= true;
}
if pauli & 2 != 0 {
z[q] ^= true;
}
}
impl FrameSampler {
pub fn new(circuit: &Circuit) -> Result<FrameSampler, String> {
let res = circuit.resolve()?;
Ok(FrameSampler {
instrs: res.instrs,
num_qubits: res.num_qubits,
num_measurements: res.num_measurements,
detectors: res.detectors,
observables: res.observables,
})
}
pub fn num_detectors(&self) -> usize {
self.detectors.len()
}
pub fn num_observables(&self) -> usize {
self.observables.len()
}
pub fn sample(&self, rng: &mut Xorshift) -> Shot {
let nq = self.num_qubits;
let mut x = vec![false; nq];
let mut z: Vec<bool> = (0..nq).map(|_| rng.next_u64() & 1 == 1).collect();
let mut rec = Vec::with_capacity(self.num_measurements);
// Whether the current chain of correlated errors has fired.
let mut chain = false;
let mut pauli_obs = 0u64;
for ins in &self.instrs {
match ins {
Instr::Reset { basis, qubits } => {
for &q in qubits {
let q = q as usize;
match basis {
Basis::Z => {
x[q] = false;
z[q] = rng.next_u64() & 1 == 1;
}
Basis::X => {
z[q] = false;
x[q] = rng.next_u64() & 1 == 1;
}
}
}
}
Instr::H(qubits) => {
for &q in qubits {
let q = q as usize;
std::mem::swap(&mut x[q], &mut z[q]);
}
}
Instr::Cx(pairs) => {
for &(c, t) in pairs {
let (c, t) = (c as usize, t as usize);
if x[c] {
x[t] ^= true;
}
if z[t] {
z[c] ^= true;
}
}
}
Instr::Cz(pairs) => {
for &(a, b) in pairs {
let (a, b) = (a as usize, b as usize);
if x[a] {
z[b] ^= true;
}
if x[b] {
z[a] ^= true;
}
}
}
Instr::Measure { basis, reset, flip, qubits } => {
for &q in qubits {
let q = q as usize;
let mut r = match basis {
Basis::Z => x[q],
Basis::X => z[q],
};
if *flip > 0.0 && rng.next_f64() < *flip {
r = !r;
}
rec.push(r);
match basis {
Basis::Z => z[q] = rng.next_u64() & 1 == 1,
Basis::X => x[q] = rng.next_u64() & 1 == 1,
}
if *reset {
match basis {
Basis::Z => {
x[q] = false;
z[q] = rng.next_u64() & 1 == 1;
}
Basis::X => {
z[q] = false;
x[q] = rng.next_u64() & 1 == 1;
}
}
}
}
}
Instr::PauliError { pauli, p, qubits } => {
for &q in qubits {
if rng.next_f64() < *p {
apply(&mut x, &mut z, q as usize, *pauli);
}
}
}
Instr::Depolarize1 { p, qubits } => {
for &q in qubits {
if rng.next_f64() < *p {
apply(&mut x, &mut z, q as usize, 1 + (rng.next_u64() % 3) as u8);
}
}
}
Instr::PauliChannel1 { px, py, pz, qubits } => {
for &q in qubits {
let u = rng.next_f64();
let pauli = if u < *px {
1
} else if u < px + py {
3
} else if u < px + py + pz {
2
} else {
0
};
apply(&mut x, &mut z, q as usize, pauli);
}
}
Instr::Depolarize2 { p, pairs } => {
for &(a, b) in pairs {
if rng.next_f64() < *p {
let r = 1 + rng.next_u64() % 15;
apply(&mut x, &mut z, a as usize, (r & 3) as u8);
apply(&mut x, &mut z, b as usize, (r >> 2) as u8);
}
}
}
Instr::S(qubits) => {
for &q in qubits {
let q = q as usize;
z[q] ^= x[q];
}
}
Instr::Correlated { p, paulis, chained } => {
let fires = rng.next_f64() < *p && !(*chained && chain);
chain = if *chained { chain || fires } else { fires };
if fires {
for &(q, pauli) in paulis {
apply(&mut x, &mut z, q as usize, pauli);
}
}
}
Instr::PauliChannel2 { probs, pairs } => {
for &(a, b) in pairs {
let u = rng.next_f64();
let mut acc = 0.0;
if let Some(k) = probs.iter().position(|p| {
acc += p;
u < acc
}) {
const CODE: [u8; 4] = [0, 1, 3, 2];
apply(&mut x, &mut z, a as usize, CODE[(k + 1) / 4]);
apply(&mut x, &mut z, b as usize, CODE[(k + 1) % 4]);
}
}
}
// Relative to the noiseless run, a record's flip flips its Pauli; a sweep bit
// moves only the reference.
Instr::Feedback { pauli, control, qubit } => {
if let crate::circuit::Control::Rec(k) = control {
if rec[rec.len() - *k as usize] {
apply(&mut x, &mut z, *qubit as usize, *pauli);
}
}
}
Instr::Heralded { probs, qubits, .. } => {
for &q in qubits {
let u = rng.next_f64();
let mut acc = 0.0;
let case = probs.iter().position(|p| {
acc += p;
u < acc
});
rec.push(case.is_some());
if let Some(k) = case {
apply(&mut x, &mut z, q as usize, [0, 1, 3, 2][k]);
}
}
}
Instr::Pad { flip, values } => {
for _ in values {
rec.push(*flip > 0.0 && rng.next_f64() < *flip);
}
}
// Signs, annotations and ticks leave a frame alone; loops and gates are
// flattened away.
// A Pauli target reads the frame: X anticommutes with its Z part, Z with its X.
Instr::Observable { index, paulis, .. } => {
for &(q, p, _) in paulis {
let q = q as usize;
if (p & 1 != 0 && z[q]) != (p & 2 != 0 && x[q]) {
pauli_obs ^= 1u64 << index;
}
}
}
Instr::Pauli { .. }
| Instr::SweepX(_)
| Instr::Detector { .. }
| Instr::QubitCoords { .. }
| Instr::ShiftCoords(_)
| Instr::Tick
| Instr::Repeat { .. }
| Instr::Gate { .. } => {}
}
}
let parity = |recs: &[usize]| recs.iter().fold(false, |acc, &m| acc ^ rec[m]);
let detectors = self.detectors.iter().map(|r| parity(r)).collect();
let mut observables = pauli_obs;
for (k, r) in self.observables.iter().enumerate() {
if parity(r) {
observables |= 1u64 << k;
}
}
Shot { detectors, observables }
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::dem::Dem;
use crate::fixtures::REP3;
/// Marginals follow from the model exactly: a detector fires when an odd
/// number of its independent mechanisms do, (1 − Π(1 − 2p))/2. The sampler
/// never reads the model, so agreement is a check on both.
#[test]
fn detector_marginals_match_the_error_model() {
let circuit = Circuit::parse(&REP3.replace("0.01", "0.05")).unwrap();
let dem = Dem::from_circuit(&circuit).unwrap();
let sampler = FrameSampler::new(&circuit).unwrap();
let n = 200_000usize;
let mut counts = vec![0usize; sampler.num_detectors()];
let mut obs = 0usize;
let mut rng = Xorshift::new(11);
for _ in 0..n {
let shot = sampler.sample(&mut rng);
for (i, &b) in shot.detectors.iter().enumerate() {
counts[i] += b as usize;
}
obs += (shot.observables & 1) as usize;
}
let predict = |pick: &dyn Fn(&crate::dem::Mechanism) -> bool| -> f64 {
let prod: f64 = dem.mechanisms.iter().filter(|m| pick(m)).map(|m| 1.0 - 2.0 * m.p).product();
(1.0 - prod) / 2.0
};
for (d, &c) in counts.iter().enumerate() {
let q = predict(&|m| m.detectors.contains(&(d as u32)));
let sigma = (q * (1.0 - q) / n as f64).sqrt();
let rate = c as f64 / n as f64;
assert!((rate - q).abs() < 5.0 * sigma, "D{d}: sampled {rate}, model {q}");
}
let q = predict(&|m| m.observables & 1 == 1);
let rate = obs as f64 / n as f64;
assert!((rate - q).abs() < 5.0 * (q * (1.0 - q) / n as f64).sqrt(), "L0: sampled {rate}, model {q}");
}
#[test]
fn noiseless_circuits_never_fire() {
let circuit = Circuit::parse(&REP3.replace("0.01", "0")).unwrap();
let sampler = FrameSampler::new(&circuit).unwrap();
let mut rng = Xorshift::new(3);
for _ in 0..2000 {
let shot = sampler.sample(&mut rng);
assert!(shot.detectors.iter().all(|&b| !b));
assert_eq!(shot.observables, 0);
}
}
}