Classical computers store information in bits: 0 or 1. Quantum computers use qubits that can be prepared in superpositions of 0 and 1.

Two classical bits hold one of four values. Two qubits are described by amplitudes for four possible outcomes, and the state description grows exponentially - 300 qubits would involve more basis states than there are atoms in the observable universe. But measurement does not let you read all those outcomes at once. Quantum speedups come from algorithms that use interference to amplify useful answers and cancel unhelpful ones.

This lets them tackle some problems classical machines struggle with:

• Factoring large numbers, which threatens some encryption
• Simulating molecules for drug discovery
• Optimizing certain complex systems
• Modeling other quantum systems

The catch: qubits are extraordinarily fragile.