Quantum Computing Explained
By: Thomas Lawrence
Tags: Computer Science, Quantum Computing, Quantum Physics, Quantum, Computers

Quantum physics

Before understanding quantum computing we have to learn some basic quantum physics. It’s a bit complicated, but trust me you can do it!

Superposition

Superposition gives qubits (quantum bits- don’t worry we’ll talk about these later), their amazing exponential growth ability by allowing electrons to be in multiple states at once. If you’ve taken a physics class, you’ve probably heard that the electron is in a cloud around an atom somewhere. If not, then you’ve heard it here and if you don’t know what an electron is you can learn about that here.

The electrons are actually everywhere in the cloud, and in multiple states until they are measured. I know… confusing but stick with me. When being measured, electrons either have an up spin or a down spin; but, when electrons are not being measured, they can be in multiple states. They can be up and down at the same time. Think of it this way, you have a magic coin that when every you look away the coin is spinning but when are you look the coin is either head or tails. You can tell the coin is spinning when you’re not looking, because you can hear the coin spinning when you’re not looking at it. Electrons work the same way. When you are not measuring the electron, it can have an up spin a down spin, or any percentage of both. When you measure it, it has a down spin or an up spin. Also, we know that electrons can be in multiple states when we aren’t looking because we can observe the effects of this quantum mechanic. This was proven in 1927 when Clinton Davisson and Lester Germer did the double-slit experiment with electrons. The double-slit experiment is an experiment first performed in 1801 by Thomas Young to show the wave properties of light. The double-slit works like this: you have a screen with two slits and a surface behind it. Light is then shining through the screen and the waves of light pass through the slits, where a crest of the waves of light hits a trough, the light cancels out, and where a crest hits and amplifies, resulting in a pattern that looks like this:

If you do this with particles like sand, you get something that looks like this:

We already know this light is a wave and thus it makes a resulting pattern of a wave. Sand is a particle thus makes a particle pattern; but what about electrons? Electrons, as previously mentioned, can be in multiple states when we’re not measuring them, but when we measure them they collapse into a state. So when we do the double-slit experiment and we’re not measuring, the electron makes this pattern:

I know!! Confusing right? This is because electrons, although usually represented by a sphere or circle, are actually waves… but only when not being measured. What if we measured the electrons? What pattern would they make?

So if the electrons are being measured it collapses and it acts like particles.

Entanglement

Entanglement is weird! In fact, it is so weird that Einstein called it:

Spooky action at a distance — Albert Einstein

Entanglement is a principle that lets electrons communicate faster than the speed of light… theoretically. Entanglement happens when a parent electron with no spin decays into two electrons one with an up spin and the other with a down spin. (Note: it doesn’t have to be an electron it could be a photon, neutrino or almost any molecules). These electrons are now entangled, but what does that mean? When two electrons are entangled, they must be in opposite states at all times. For example, let’s imagine we had two of those magic coins from the start that are entangled. If I measure one of those coins and its heads, the other one will become tails instantly, even if they are on other sides of the world! That means quantum information can travel faster than the speed of light! As well, even when the coins are in a superposition, they must have an opposite spin. So does this mean classical physics is invalid now if quantum information can travel faster than the speed of light? While no, at least not yet, because we can prove the electron information is travelling faster than the speed of light. Our information only travels at the speed of light. For example, if lab 1 is talking to lab 2, and lab 1 tells lab 2 to “measure the electron now”, lab 2 will only get that information at the speed of light. The electron could have theoretically already transmitted the information by the time lab 2 measures the electrons. Even if we had two labs, and a computer in the middle of both labs that transmits a signal and at the same time causing the electrons to be measured, we can’t prove it was instant only faster than the speed of light. We can prove that the electron sends information faster than we can measure them. (which is at the speed of light)

Coherence & Decoherence

Coherence and decoherence are what make the electrons in superposition, entangled, or collapsed and untangle. If an electron is coherent, it is to say the wave of the election is consistent in-phase (the distance between a constant point) and waveform (shape of wave). If an electron is decoherent, the electron’s waveform and phase are not consistent.

Decoherence is caused due to noise in the environment. Noise can be anything in the environment that the electrons interact with, for example, other electrons, atoms, measurement systems, etc. The only known way to get rid of all noise in an environment for the electrons is to perfectly isolate the electron. If that can be done, the electron will be coherent forever. Unfortunately, we cannot do that. Decoherence is useful in quantum computing because during the decoherence time is when the quantum computer does its magic.