quaco.js/src/Qubit.js

A lightweight modular Quantum Computing Simulator in JavaScript. Supports qubits, quantum gates, entanglement, circuits, algorithms, and visualization.

// src/Qubit.js

import { Complex } from './utils/Complex.js';
import { MathUtils } from './utils/MathUtils.js';

/**
 * Represents a single quantum bit (qubit).
 * State is a 2-dimensional vector of complex numbers.
 */
export class Qubit {
    constructor() {
        // Start in |0> state
        this.state = [
            new Complex(1, 0), // α amplitude (for |0⟩)
            new Complex(0, 0)  // β amplitude (for |1⟩)
        ];
    }

    /**
     * Applies a single qubit gate represented by a 2x2 matrix.
     * @param {Complex[][]} gateMatrix 2x2 matrix of Complex numbers
     */
    applyGate(gateMatrix) {
        const newState = [
            gateMatrix[0][0].mul(this.state[0]).add(gateMatrix[0][1].mul(this.state[1])),
            gateMatrix[1][0].mul(this.state[0]).add(gateMatrix[1][1].mul(this.state[1]))
        ];
        this.state = newState;
    }

    /**
     * Measures the qubit, collapsing it to |0⟩ or |1⟩ based on probability.
     * @returns {number} 0 or 1
     */
    measure() {
        const prob0 = this.state[0].abs2();
        const rand = Math.random();
        if (rand < prob0) {
            // Collapse to |0>
            this.state = [
                new Complex(1, 0),
                new Complex(0, 0)
            ];
            return 0;
        } else {
            // Collapse to |1>
            this.state = [
                new Complex(0, 0),
                new Complex(1, 0)
            ];
            return 1;
        }
    }

    /**
     * Prints the current state of the qubit in Dirac notation.
     */
    printState() {
        const [alpha, beta] = this.state;
        console.log(`|ψ⟩ = (${alpha.toString()})|0⟩ + (${beta.toString()})|1⟩`);
    }
}