What is (quantum) computing?
There has been a lot of interest in Quantum Computing in recent years, but what makes it different from regular computing? To understand that, let’s first try to ponder what “computing” is.
If one asks you what “computing” means to you, a natural answer would be anything that a computer can do for you. So what is a computer? Perhaps a device you carry with you that can run apps, browse the net, and communicate with people? This would include your laptop, smartphones, etc. It could also be something just as simple as a calculator - a device that does arithmetic for you. Or it could be even simpler, like a clock - a device that can measure time.
To be concrete, we can ask for a computer to be a device that can perform some “computation”. A computation is some task that given some input, produces some relevant outputs based on it. This disqualifies the clock as a computer, as it cannot behave differently based on some input (or can it?)
A really simple computer
To push our imaginative boundaries on what is the bare minimum to qualify as a computer, let us try a thought experiment.
Say you are in a multi-storey building where each floor is exactly $1$ meter high, so the $n$-th floor is at $n$ meters from the ground. You have two things: a stopwatch and a small stone. What happens if you drop the stone from the $n$-th floor and measure the time it takes to hit the ground? (for simplicity, assume $g = 10$ and negligible air resistance)
From elementary kinematics, we know that $h = \frac12 g t^2$, and therefore $t = \sqrt{\frac{n}{5}}$. Now if I was given an integer $x$, and wanted to find its square root, I could go to the $5x$-th floor, and measure the time for the stone to fall.
Therefore this box above is actually a computer! Just you, a stopwatch, a stone, and a multi-storey building.
Who is the real hero?
In the above setting, who actually performed the computation?
- Was it you? You just pressed start and stop, but could not control the actual behaviour of the objects.
- Was it the stopwatch? It only measured time.
- Was it the stone? It alone could not have done anything useful.
- Was it the building? It just provided a way for you to run the computation but did not “perform” the computation.
The actual hero here is hidden in the details, $h = \frac12gt^2$! Gravity is the actual driving force (pun intended) for the computation here. We were able to harness the laws of nature with gadgets we had (stopwatch, stone, building) to be able to perform a computation and extract useful information.
This is precisely what computing is as well! We harness the nature of systems that behave according to the laws of quantum mechanics. We “rephrase” the computation we are interested in into the equations that describe the dynamics of these systems, and let them “run” to perform the computation.
Can you think of other cool examples of a computer?
