Quantum vs. Classical Computing

A classical computer processes bits through logic gates (AND, OR, NOT) that are deterministic but mostly not reversible: an AND gate maps both (0,0) and (0,1) to the same output, 0, so there's no way to work backward from that output alone to know which inputs produced it, and classical computing throws that information away on purpose because it doesn't need it. A quantum computer processes qubits through quantum gates that must be reversible (a consequence of the underlying physics, not a design choice), and it uses interference, which allows the "wrong" answers in a superposition to cancel out and the "right" answers to reinforce, as its actual computational trick. That reversibility requirement is a real structural difference, not a faster version of the same model.

Today's quantum computers (often called NISQ, Noisy Intermediate-Scale Quantum, devices) have real limitations: qubits lose their quantum state after a short time (decoherence), and every operation introduces some error. Large-scale, fully error-corrected quantum computers, the kind that could reliably outperform classical computers on hard, useful problems, don't exist yet publicly at meaningful scale. Most credible current results are either small proof-of-concept demonstrations or narrow "quantum advantage" experiments on problems specifically chosen to favor quantum hardware, not general business computing.

The realistic picture most researchers hold: quantum and classical computers will work together, with a classical computer handling the parts it's good at and calling out to quantum hardware for the specific sub-problems (chemistry simulation, certain optimizations) where it may eventually help. "Quantum will replace classical computing" is not the claim serious researchers are making.