Some phenomena of quantum mechanics might have a huge impact on human technology in the future, particularly in the form of quantum computers. A quantum computer is a computer using quantum superposition and quantum entanglement to improve its computing power. How?
In order to understand quantum computers, we first need to take a look at classical computers. The basic unit of information of classical computers is a bit. A bit can take one of two values: 1/0 (yes/no, on/off). Two bits may take one of four values (11/10/01/00), three bits one of eight values, four bits one of sixteen values, and so forth.
Quantum computers use a slightly different unit of information – a qubit (quantum bit). Qubits are similar to bits, but with one significant difference – due to quantum superposition, a qubit may take more values simultaneously! A qubit can thus be in a superposition of values 1 and 0. We could, for instance, create a qubit using an electron’s spin. Spin ½ could be assigned the value 1, spin −½ the value 0 (or vice versa). As long as the electron is not observed, its qubit has both possible values.
However, if we add another qubit, the whole situation becomes even more interesting. Due to quantum entanglement, both qubits enter a superposition of four states. Qubits now take all four possible values (11, 10, 01 and 00) simultaneously. If we add another qubit, the whole quantum system of these three qubits can take eight values at the same time, and so forth. Each time we add a qubit, the number of possible superposed states doubles. The main difference between a classical and a quantum computer is thus in the number of states it takes. While any set of bits can only take one possible value at a time, the same set of qubits can take all of these values simultaneously.
But what is the consequence of this difference? Speed. Quantum computer is capable of solving certain tasks even a million times faster than a classical computer of comparable size – for instance, a quantum computer composed of just twenty qubits can take 1048576 states simultaneously!
This may sound terrific, but there is a downside. Despite its tremendous speed, quantum computers will probably never entirely replace classical computers. The reason is simple – any time a quantum system is observed, the wave function of this system collapses. This means that anytime we tried to use a quantum computer, there would inevitably be an interaction between us and the computer. This interaction would cause the superposition within the quantum computer to collapse, and its qubits would suddenly become mere classical bits.
Unfortunately, quantum computers are only suitable for specific, usually complex computations. During the computation, they must be isolated from their surroundings to prevent the superposition of their qubits from collapsing. A quantum computer basically divides each problem into many simpler calculations, which it then solves in parallel. Once the computation is finished, the computer is observed, which causes its superposition to collapse, and it provides us with just one result.
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