How does a quantum computer differ from an ordinary computer?

Both kinds of machine take in information, process it, store it and output it. The difference lies in the basic unit. An ordinary computer uses bits, each of which is either 0 or 1. A quantum computer uses qubits, which obey quantum mechanics and can be in a superposition of 0 and 1 until they are measured. Qubits can also be entangled with one another. [1] [2]

Phones, servers and supercomputers all encode information as bits, for example representing the letter A as 01000001. Physically, a bit is something like a tiny magnet or electric switch placed in one of two states, and it usually stays put for a long time. A qubit can instead be any combination of 0 and 1, such as 40 percent 0 and 60 percent 1, as the [tudelft.nl](https://www.tudelft.nl/en/about-tu-delft/strategy/vision-teams/quantum-computing/what-is-quantum/comparing-computers) explainer puts it. [1] [3]

Mark Horowitz offers an analogy: a bit is like a coin lying on a table, heads or tails, while a qubit is like a ball that can point up, down or to the side. [4]

Two qubits can hold a superposition of four combinations of 0s and 1s, three qubits eight, four qubits sixteen, and so on, doubling with each added qubit. Combined with entanglement, this lets quantum computers do some operations that are hard for ordinary computers, and there are tasks known to run much faster on them. [1] [4]

A common misconception is that a quantum computer tries every solution at once. Stephen Jordan, a Google researcher, says that although computations can be done in superposition, this does not allow an efficient brute-force search over all solutions. Measurement at the end extracts only a small amount of information. [1]

Reading out a qubit always gives 0 or 1, with probabilities set by its superposition, like a spinning coin slapped flat on a table. Running the same procedure twice can therefore give different outcomes. Programmers must design algorithms that reduce the chance of wrong answers and amplify the chance of correct ones, so the final measurement is useful. Quantum logic gates also work differently: they manipulate the qubits directly rather than passing results to new bits. [3] [1]

Horowitz says conventional computers can hold millions or billions of bits and make errors extremely rarely. Quantum computers, by his account, have only around 100 qubits at present, and errors are not that infrequent, because controlling and rotating qubits precisely is much harder. This describes the state of the field when his interview was given, and the figures may have changed since. NIST also notes that some groups pursue a different approach, quantum annealing, aimed at certain physics and optimization problems. [4] [1]