A friend asked me over a few beers recently if I could explain how a regular old computer like we have at home is different than the quantum computers that they’d heard about in the news. But the catch was that I had to explain it as briefly as possible in layman’s terms, which, of course, was easier said than done. I managed to get through it without experiencing that dreaded glazed-over look that we as Computer Scientists occasionally encounter when we talk tech, so I figured it may be useful for others if I share my summary here as well.
The main difference between classical computers and quantum computer is that classical computers perform calculations using bits, which contain a value of either 0 or 1, while quantum computers perform calculations using qubits, which can be both multiple values of 0 or 1, or both, or a combination simultaneously, in potentially an infinite number of times. Because of this, the power of classical computers increases linearly as the number of bits (transistors) it contains increases, while the power of classical computers increases exponentially as the number of qubits increase.
Classical computers are very reliable and operate with low error rates at room temperature. Quantum computers, however, have much higher error rates and need to be kept at extremely cold temperatures, close to absolute zero, to be exact, in order to achieve the superconductivity that qubits require to function. This is all but impossible outside of the most highly-controlled laboratory conditions.
If ever established and made to be reliably stable, quantum computers would be useful tools for tasks such as developing new drugs or medical treatments, developing new materials, machine learning and artificial intelligence, running large-scale simulations, and solving currently often insurmountable problems such as cryptography or predicting and forecasting trends on Wall Street. Any one of these would be a monumental and potentially world-changing accomplishment if successfully achieved. Hopefully we’ll be able to see this happen in our lifetimes.