The basics of Quantum Computers
By: Richard Maccaw
Tags: Science, Quantum Computing

Everyday computers are pretty damn smart. They helped us get to the moon, make driverless cars, augment reality and yet there are still some areas where they perform badly.

Let’s say you have ten friends over for dinner, and you want to create a seating plan based on who gets on best with who. Ten friends equals a seating plan with over 3.6 million (10 factorial) different combinations. Our classical computer will loyally chug through those combinations before returning a seating plan. Then another two friends arrive bringing the total guests to 12. That's now 479 million different possibilities for our computer to run.

The more possibilities, the longer it takes for our computer to process. You can quickly to see how certain jobs which have an exponential relationship could get out of hand for a classical computer.

Classical computers are weak at solving problems which involve exponential complexity

This shortcoming is simply a consequence of how we designed computers in the first place. Computers are built upon transistors which are like on or off switches (1 or 0). To make more powerful computers, you can make transistors smaller, so you can have more of them and hence more bits (1’s and 0's).

Historically we’ve been making smaller and smaller transistors. But there are some limits.

Transistors are already ridiculously small at 14nm. That's 50 times smaller than a red blood cell. There will come a point when we cannot make transistors any smaller. At these tiny sizes, we run into some weird quantum effects which mean our transistors don’t always work properly (quantum tunnelling).

Transistors, the building blocks of computers, are 50x smaller than a red blood cell.

These factors caused scientists to think about remodelling the fundamental building blocks of a computer… which is where quantum comes in.

Introducing Quantum

Quantum physics arose because there is an entirely different physical rulebook when dealing with things the size of atoms. Classical physics (Newtonian physics) goes out the window. Quantum computing aims to take advantage of two quantum theories. Superposition and quantum entanglement.

Schrodinger's thought experiment paves the way to understand them both (although this example is Newtons).

Schrodinger’s cat. Is it dead or alive? It’s both.

Imagine a cat and a barrel of gunpowder in a bunker. The gunpowder has a 50/50 chance of exploding and killing the cat. Before you look in the bunker to check the explosion, the cat has an equal chance of being dead or alive. So we can say the cat at that moment is both dead and alive or exists in a superposition of states. Now think about it from the cats perspective… its fate is tied or entangled with the explosion. If the cat sees the explosion it's dead if it doesn’t it's alive. The fate of the cat is tied to the explosion.

The same rules can be given to quantum bits which are the building blocks of quantum computers. Because a qubit can exist in a superposition of states, it gives quantum computers the potential to have enormous power.

A classical computer can make one operation per cycle.

A quantum computer can do multiple operations in parallel in a cycle.

So a quantum computer with 3 qubits is 8 times more powerful (2³) than a 3-bit classical computer. That might not seem that much at first, but a quantum computer with only 50 qubits will be more powerful than any classical computer ever created (an event known as quantum supremacy). 2⁵⁰ equals 1,000,000,000,000,000 parallel computations!

A quantum computer. The white cylinder is a fridge which is cooled to 300 nanoKelvin (minus 273 degrees).

So what does all that power mean?

First, it's important to know that there are some major hurdles in the development of quantum computers. For instance, qubits are very sensitive and its very easy for them to fall out of their superposition (1 and 0) state. Adding more qubits further increases the sensitivity of the system. But if we can solve these challenges, quantum computers will have a major impact.