Quantum computers must overcome the challenge of detecting and correcting quantum errors before they can fulfill their promise of sifting through millions of possible solutions much faster than classical computers.

Source: IBM Shows First Full Error Detection for Quantum Computers – IEEE Spectrum

Detecting quantum errors is anything but straightforward. Classical computers can detect and correct their bit-flip errors by simply copying the same bit many times and taking the correct value from the majority of error-free bits. By comparison, the fragility of quantum states in qubits means that trying to directly copy them can have the counterproductive effect of changing the quantum state.