How Do Quantum Computers Work?
"Things on a very small scale behave like nothing you have any direct experience about... or like anything that you have ever seen." (Richard Feynman, Six Easy Pieces, p116.)
In order to understand anything Quantum, it’s best to approach it with an open mind. So, while you may know what a computer is and maybe even understand that quantum refers to interactions on a very small scale, in order to grasp the idea of quantum computers, I suggest we start at the beginning.
How do normal computers work?
Computers can’t handle big tasks but they can do lots of small tasks very quickly, so they break things down in to very small parts.
Normal computers use binary which is the smallest amount of data available to computers and it is represented by 0s and 1s, we call them bits.
The 0s and 1s just relate to the computer sending an electric current through a wire or not. 1 is current, 0 is no current. So basically there are only two options and you can only choose one at a time.
A quick example: Pressing ‘A’ on your keyboard.
Your keyboard has 7 wires, the letter A has a 7-digit combination of 0s and 1s:
To tell the computer, type the letter A, the following happens:
Wire 1 – 1 – Send current
Wire 2 – 0 –No current
Wire 3 – 0 –No current
Wire 4 – 0 –No current
Wire 5 – 0 –No current
Wire 6 – 0 –No current
Wire 7 – 1 – Send current
Once the processor of the computer receives that information, it processes it using binary logic, for example, using a series of AND, OR, NOT, XOR. These logic operations are implemented using transistors, or in other words, controllable electronic switches.
So how do quantum computers work?
There are two fundamental concepts of Quantum mechanics that allow quantum computers to work so well.
The equivalent of a bit in quantum computing is called a qubit (short for quantum bit). Rather than it being a small piece of data like it is for a regular computer, scientists use quantum objects like electrons, molecules, photons or even small electronic circuits displaying quantum effects.
According to a quantum mechanical effect called superposition, a particle behaves as if can be in multiple states at once. This means that a qubit behaves as if it was in 0 and 1 at the same time. This is a consequence of the wave-particle duality, which says that although they are particles, quantum objects have a behaviour akin to waves.
The other effect that we utilize in quantum computing is called quantum entanglement. Quantum entanglement is a special type of superposition between two or more qubits. Entangled particles are also correlated in a strange and powerful way! Today we will focus on superposition, but stay tuned, as the next post will be about what entanglement is and why it is important for quantum computers!
Let’s look at superposition closer:
Imagine you have two doors in front of you and someone tells you that your friend is behind one of those doors.
Superposition: Until you open one of the doors and check, in quantum mechanics your friend behaves as if they were behind both doors at once. You may think, well they’re either behind one or not. Nope, in the quantum world of small particles, the math suggests to us that they are behind both.
Disclaimer: This does not happen at the human level, you cannot be both at work and not at work – sorry.
However, superposition is one of the two rules in this game. The other rule is measurement.
Measurement: When you open the door, you force one of the options to become true. Physicists say the other possibility has collapsed. For example, if you open one door and see that your friend is not there, then you know they must be behind the other door – they are no longer in a superposition behind both doors. The key thing about quantum measurement is that it is completely random: there is no way to predict behind which door your friend will be. Essentially, quantum mechanics is very much probabilistic. But worry not, quantum scientists have found clever ways to harness this randomness to perform useful tasks.
So how does superposition help quantum computers?
Quantum computers are completely different than regular computers. Superposition allows quantum computers to make “new moves” when they process information. Superposition is in fact a property of waves and in the same way that water waves interfere to create a new wave, quantum computers also exploit interference. When we ask a quantum computer to do a calculation, there are certain paths that the computer needs to explore and, thanks to superposition, they can be investigated simultaneously. If we do things right, we can process the information such that the paths leading to the wrong answers destructively interfere and the computer never measures those answers. The paths leading to the right answer constructively interfere and so the quantum computer essentially only calculates the right answer!
Using superposition, interference and entanglement to process information doesn’t mean that a quantum computer is better at everything. But in some very important instances, like searching databases, factoring numbers and performing simulations of molecular and chemical processes, which would take a regular computer millions of years to do, a quantum computer could perform those tasks in just a couple of seconds.