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Quantum 101

These pages contain descriptions of commonly encountered terms in quantum technologies

Entanglement

A quantum phenomenon connecting two remote partners, and crucial for quantum technology

Qubit

In computing, a bit is the smallest unit of information, 0 or 1. In quantum computing, a quantum bit (or qubit) is the equivalent, and is physically encoded in quantum phenomena such as the spin of an atomic nucleus. However, thanks to quantum superposition, a qubit can be in a superposition of both 0 and 1 at the same time.

Superposition

In quantum physics, unlike classical physics, objects exist in all of their possible states simultaneously, but collapse into one of these states the instant they are measured. This is what lies behind the famous paradox of Schrödinger’s Cat. A cat is locked in a box alongside a radioactive source, a detector and a flask of poison. If the radioactive atom decays, the detector triggers a hammer to break the flask and the cat dies. If the atom does not decay, the cat remains alive. Since radioactive decay of a single atom is a probabilistic event, the atom has both decayed and not decayed until the moment it is measured. And therefore, until you check whether or not the cat has been poisoned, it remains both dead and alive at the same time. Quantum phenomena the scale of a cat have never been observed, but on quantum scales, this kind of behaviour is common.

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