Classical bits vs qubits
A classical bit is 0 or 1. A qubit can exist in superposition of both simultaneously. Two qubits in superposition represent four states. n qubits represent 2-to-the-n states. This is the source of quantum's potential power.
Entanglement
Entanglement links two qubits so measuring one instantly determines the state of the other regardless of distance. Entanglement is a resource quantum algorithms exploit with no classical analogue.
Quantum gates and circuits
Quantum algorithms are expressed as circuits of gates manipulating qubit states. The Hadamard gate creates superposition. CNOT entangles two qubits. Unlike classical gates, quantum gates are reversible.
What quantum computers excel at
- Factoring large integers (Shor's algorithm) — threatens RSA encryption.
- Searching unsorted databases (Grover's algorithm) — quadratic speedup.
- Simulating quantum chemistry for drug discovery and materials science.
- Certain combinatorial optimisation problems.
The NISQ era
We are in the Noisy Intermediate-Scale Quantum (NISQ) era — machines with hundreds to thousands of qubits but high error rates. Fault-tolerant quantum computing requires millions of physical qubits. That milestone is years away.