Your First Circuit

There are two ways to build a circuit: the fluent CircuitBuilder API, or by parsing OpenQASM text.

With the builder

CircuitBuilder chains gate calls and runs the result. This builds a Bell pair, the two-qubit entangled state (|00⟩ + |11⟩) / √2:

#![allow(unused)]
fn main() {
use prism_q::CircuitBuilder;

let result = CircuitBuilder::new(2)
    .h(0)
    .cx(0, 1)
    .run(42)                       // seed = 42
    .expect("simulation failed");

let probs = result.probabilities.expect("no probabilities");
for i in 0..probs.len() {
    let p = probs.get(i);
    if p > 1e-10 {
        println!("|{i:02b}> = {p:.4}");
    }
}
// |00> = 0.5000
// |11> = 0.5000
}

Measurement is Z-basis by default. measure_in_basis(qubit, axis, bit) measures along X, Y, or Z, and measure_pauli_product(&terms, bit) records the parity of a Pauli string such as X0 Z1. Both lower onto the gates the backends already run, so every backend accepts them. A basis measurement leaves the qubit in the Z eigenstate of the recorded bit rather than rotating back, and a Pauli product accumulates its parity on one extra qubit appended past the declared register:

#![allow(unused)]
fn main() {
use prism_q::{CircuitBuilder, PauliAxis, PauliTerm};

let circuit = CircuitBuilder::new_with_classical(2, 2)
    .h(0)
    .cx(0, 1)
    .measure_in_basis(0, PauliAxis::X, 0)
    .measure_pauli_product(&[PauliTerm::z(0), PauliTerm::z(1)], 1)
    .build();
assert_eq!(circuit.num_qubits, 3);
}

A larger structurally similar circuit, the 5-qubit GHZ state, renders like this (diagram generated by PRISM-Q's own SVG renderer):

From OpenQASM

The same Bell pair, written in OpenQASM 3.0 and parsed:

#![allow(unused)]
fn main() {
use prism_q::circuit::openqasm;
use prism_q::simulate;

let qasm = r#"
    OPENQASM 3.0;
    include "stdgates.inc";
    qubit[2] q;
    h q[0];
    cx q[0], q[1];
"#;

let circuit = openqasm::parse(qasm).expect("failed to parse QASM");
let result = simulate(&circuit).seed(42).run().expect("simulation failed");
}

run_qasm(qasm, seed) is a shortcut that parses and simulates in one call. See the OpenQASM Support guide for the supported subset.

Next: sample measurement outcomes in Shots and Sampling.