Two weeks ago we embarked on a little adventure to find out more about quantum computing. Last week I was busy being bombarded by hail atop a mountain for four days. Today, we finally continue on our quest. The first one, not the camping.
So what exactly makes quantum computing so special? In my previous post we discussed the fact that modern day computers work by using a series of 0s and 1s to represent an instruction, which is then processed by an algorithm. Essentially, all an algorithm does is manipulate this string of bits (0s and 1s) and then outputs another string which encodes the result.
Quantum computation makes use of the fact that, on a very small scale, things can simultaneously take on states that we would normally deem mutually exclusive. This quantum effect is known as superposition, and it allows tiny particles such as electrons or photons to be in several places at once. However, we never directly see this phenomena in the ordinary macroscopic world because as soon as the system is observed or measured, it collapses into a single state based on probability. Some of you may be familiar with the Schrödinger’s Cat analogy in which there is a cat in a box which, when opened, gives a 50% chance that the cat is either dead or alive. Until the box is opened, there is no way of determining the state of the cat, so it exists in a superposition of its two possible states, a mixture of both life and death simultaneously. When a measurement is taken by opening the box, the outcome of the cat is determined and it collapses into one of the two states.
So this idea of superposition widens the binary constraints – instead of representing bits, particles are used in quantum computation to represent quantum bits, or as the cool kids call it, qubits. These qubits can represent 0, or 1, or any superposition of those two qubit states; a pair of qubits can be in any superposition of 4 states, and 3 qubits in any of 8. In general, a quantum computer with n qubits can be in a superposition of up to 2n different states simultaneously, which massively improves upon the single state that a normal computer can use. This inherent parallelism allows quantum computers to perform a million computations at once, whereas a normal desktop PC can only perform one, giving quantum computers the potential to be millions of times more powerful than today’s most powerful supercomputers.
Quantum computers also use another aspect of quantum mechanics to their advantage, known as entanglement. As we mentioned earlier, a particle’s quantum state can be collapsed from its superposition of many states by observing it in any way. This poses a problem – if we try to look at subatomic particles in our quantum computer, this will bump them and change their value. Qubits in superposition, holding many simultaneous values, will assume the value of either 0 or 1, when we look at them. This effectively turns our fabulous quantum computer into a boring, mundane digital computer. To overcome this, entanglement provides a potential answer which makes it able to make measurements of the qubits indirectly so that the system’s integrity is preserved. Quantum entanglement is where two or more particles are generated or interact in such ways that they share a quantum state. For example, if we have two entangled particles that are spinning in opposite directions such that their total spin is known to be zero, and we measure one of the particles spinning anticlockwise, we now know, without further measurement, that the other will be spinning clockwise. Using this phenomena, scientists are able to know the value of qubits without actually observing them. Neat, huh?
Stay tuned, there’s more to come eventually. After a short interlude. But it will come, I promise.