This algorithm finds the prime factors (\)u\) and \(v\)) of an odd, composite integer \(n\), that is not a prime power.
(pseudo code)
Repeat
Randomly choose \\(a \in \{ 2, \dots, n - 1 \}\\)
Compute \\(d = gcd(a, n)\\)
If \\(d \geq 2\\) then
Return \\(u = d\\) and \\(v = n/d\\)
Else // We know \\(a \in \mathbb{Z}^*_N\\)
Let \\(r\\) be the order of \\(a\\) in \\(\mathbb{Z}^*_N\\) // Order finding algorithm
If \\(r\\) is even then
Compute \\(x = a^{r/2} - 1 (\mod n)\\)
Compute \\(d = \gcd(x, n)\\)
If \\(d \geq 2\\) then
Return \\(u = d\\) and \\(v = n/d\\)
Until a value is returned.
See https://cs.uwaterloo.ca/~watrous/LectureNotes.html
extern crate qcgpu;
extern crate rand;
use rand::{thread_rng, Rng};
use qcgpu::{gcd, State};
use qcgpu::gates::h;
fn main() {
let n = 15; // Number to factor
println!("Factoring {}.", n);
// Here we should check if the number is even, or if it is a power factor
let mut rng = thread_rng();
loop {
let mut a = rng.gen_range(2, n); // Randomly choose \\(a \in \{ 2, \dots, n - 1 \}\\)
let mut d = gcd(a, n);
if d >= 2 {
// Found the factors; No Quantum needed
println!(
"Factors are {} and {} (No quantum algorithm used)",
d,
n / d
);
break;
} else {
// We know \\(a \in \mathbb{Z}^*_N\\)
let r = find_order(a, n);
if r % 2 == 0 {
let x = (a.pow((r as f32 / 2 as f32) as u32) - 1) % n;
d = gcd(x, n);
if d >= 2 {
println!("Factors are {} and {}", d, n / d);
break;
}
} else {
println!("Period is odd");
}
}
}
}
Given a positive integer \(n \geq 2\) and an element \(a \in \mathbb{Z}_n^* \), Find the order of \(a\) in \(\mathbb{Z}_n^*\).
Order finding is the only quantum part in shors algorithm.
\(\mathbb{Z}_n\) is the set of integers from \({0,\dots, n - 1}\) or \(\mathbb{z} \mod n\). The set \(\mathbb{Z}_n^* = {a \in \mathbb{Z}_n : \gcd(a,n) = 1}\)
The set \( \mathbb{Z}_n^* \) forms a group wth the multiplication modulo \(n\) operation. Thus, for \( a \in \mathbb{Z}_n^* \) , \(\exists b \in \mathbb{Z}_n^*\) that uniquely satisfies
\[ ab \equiv 1 \mod n \]
The order of a given element \(a \in \mathbb{Z}_n^*\) is the smallest positive integer \(r\) such that
\[ a^r \equiv 1 \mod n \]
```rust fn find_order(a: i32, n: i32) -> i32 {
unimplemented!()
}