What are quantum computers today, and what are they not?
Few technologies attract as many exaggerations. Before going further, here are five common claims. For each one, decide whether it is true or false.
"A quantum computer tries every possible answer at once and picks the right one." False. Reading a quantum computer gives one result, at random. Quantum algorithms work by interference: they arrange things so that wrong answers cancel out and right ones become likely. You will see interference with your own eyes in module 5.
"Quantum computers will replace laptops and phones." False. They help with specific problems. Most of them need extreme conditions (temperatures close to absolute zero, ultra-high vacuum or lasers) and will be used remotely, like a specialised machine in a data centre.
"Quantum computers already exist, and anyone can use one online." True. Several companies give access through the cloud, some of them for free. You will run code on one in module 14.
"Today's quantum computers make many errors." True. Qubits are fragile: heat, vibrations and stray light disturb them. Today's machines have from a few dozen to around a thousand physical qubits, too noisy for the big algorithms. The race now is to build error-corrected logical qubits, which module 12 explains.
"Quantum computers are faster at everything." False. For most tasks an ordinary computer is faster, cheaper and simpler. The gain exists only for problems with a quantum algorithm, as the previous lesson showed.
So why learn all this now? Because the field is moving fast, companies are hiring people who understand it, and the ideas behind it (superposition, interference, entanglement) are among the most surprising in all of science.
The next module starts where quantum physics started: with light. Photons, the particles of light, are one of the ways to build qubits, and the easiest to see in a lab.
Quantum computers are real, fragile and specialised: they compute by interference, not by trying everything, and help only with certain problems.