The texts below are from some tutorial, and it says (in bold format below) a qubit can be both 0 and 1 at the same time. It sounds very strange to me, can you please explain why?
Quantum cryptography utilises the physics of photons (light energy according to the formula E = hf) and their physical quantum properties to produce a virtually unbreakable encryption system. This helps protect the security of data being transmitted over fibre optic cables. Photons oscillate in various directions and produce a sequence of random bits (0s and 1s) across the optical network. Sending encryption keys across a network uses quantum cryptography – a quantum key distribution (QKD) protocol (one of the most common is BB84). QKD uses quantum mechanics to facilitate the secure transmission of encryption keys. Quantum mechanics use a qubit (quantum bit) as the basic unit of quantum data. Unlike normal binary (which uses discrete 0s and 1s), the state of a qubit can be 0 or 1, but it can also be both 0 and 1 simultaneously. Figure 17.5 shows a representation of a photon and how a photon can be affected by one of four types of polarising filter.
Plus, if some of you ever learned A-level computer science, can you please give some recommendation for A-level computer science books, and exercises?