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Quantum computers are devices capable of performing computations using quantum bits, or qubits.
The first thing we need to understand is what a qubit actually is. A “classical computer,” like the one you’re reading this on (either desktop, laptop, tablet, or phone), is also referred to as a “binary computer” because all of the functions it performs are based on either ones or zeros.
On a binary computer the processor uses transistors to perform calculations. Each transistor can be on or off, which indicates the one or zero used to compute the next step in a program or algorithm.
There’s more to it than that, but the important thing to know about binary computers is the ones and zeros they use as the basis for computations are called “bits.”
Quantum computers don’t use bits; they use qubits. Qubits, aside from sounding way cooler, have extra functions that bits don’t. Instead of only being represented as a one or zero, qubits can actually be both at the same time. Often qubits, when unobserved, are considered to be “spinning.” Instead of referring to these types of “spin qubits” using ones or zeros, they’re measured in states of “up,” “down,” and “both.”
Qubits can be more than one thing at a time because of a strange phenomenon called superposition. Quantum superposition in qubits can be explained by flipping a coin. We know that the coin will land in one of two states: heads or tails. This is how binary computers think. While the coin is still spinning in the air, assuming your eye isn’t quick enough to ‘observe’ the actual state it’s in, the coin is actually in both states at the same time. Essentially until the coin lands it has to be considered both heads and tails simultaneously.
A clever scientist by the name of Schrodinger explained this phenomenon using a cat which he demonstrated as being both alive and dead at the same time.
Qubits work on the same principle. An area of related study called “observation theory” dictates that when a quantum particle is being watched it can act like a wave. Basically the universe acts one way when we’re looking, another way when we aren’t. This means quantum computers, using their qubits, can simulate the subatomic particles of the universe in a way that’s actually natural: they speak the same language as an electron or proton, basically.