To put it simply, a quantum computer is a computer that utilizes quantum phenomena for calculations and computational purposes. But what does that even mean?

What are these “quantum phenomena”? What benefits does this new computational technique offer? What are its practical applications? And are quantum computers going to replace classical computers?

Well, here is a quick introduction to quantum computers discussing all these questions and much more.

- The Limit of Classical Computers – Why Quantum Computers are Necessary?
- How Quantum Computers Work?
- Quantum Computers and the Future

The classical computers of today can perform trillions of calculations per second. This empowers us to do amazing things like creating real-to-life graphics and sending rockets across the solar system.

However, their tremendous processing power is still limited when it comes to dealing with large amounts of information. Even with supercomputers, we struggle when analyzing and looking for patterns in big data, modelling molecular interactions, and the likes.

So let’s just build more powerful computers. That should solve the problem, right?

Well, here is the issue!

In the process of making computers that can perform more calculations/second, we need smaller transistors that are more power-efficient. This keeps the computer from getting too hot which is one of the main factors limiting overall performance.

Currently, we have transistors that are as small as 7 nm (nanometers). For reference, a human red blood cell is close to 10,000 nm across. Even a virus is around 100 nm.

So, as you can see, modern transistors are really small. And if we get smaller than 5 nm, the traditional laws of physics break down, and we start to experience quantum phenomena.

With transistors the size of a couple of atoms, properties like Heisenberg’s uncertainty principle take over – meaning the electrons can now potentially be outside the circuitry, or inside – we don’t know anymore. Furthermore, we also notice the effects of quantum tunnelling where the electrons will just “tunnel” through the transistors rendering them useless.

As such, because of quantum properties, the size and thereby the computation power of classical computers have already started to reach their limits.

And this is where quantum computers come into the picture.

Scientists have figured out ways to harness the quantum effects, and they have created computers that are exponentially more powerful than even our fastest supercomputers.

With quantum computers, we can now easily solve the above-mentioned problems involving big data, and modelling complex molecular structures in a practical amount of time, and that too without getting overheated.

Quantum computers take advantage of two primary quantum phenomena – Superposition and Entanglement.

In classical computers, bits are the smallest unit of information that has two possible states, either 0 or 1.

However, with quantum computers, we have qubits or quantum bits. These too can be set to 0 and 1. However, a qubit doesn’t have to be “either” of these two values. In fact, it can be both 0 and 1, or in any proportion of both states, at the same time. It is only when you measure the qubit that it has to decide to reduce itself to either 0 or 1.

This is called superposition, which opens up a richer set of states to explore.

For example, let’s say we have 4 classical bits, which can be in 2^4=16 different combinations. Out of these, you can only use 1 combination at a time. However, when dealing with 4 qubits, we still get 16 different combinations, but the qubits can store all of them in parallel at the same time.

This number grows at an exponential rate with every extra qubit. If you just have 20 qubits, then you can store a million values in parallel. As such, you get access to exponentially more vast and complex quantum information which allows for fast calculations and computation.

Next, we have quantum entanglement.

If we have two qubits entangled, then both of them will instantaneously react to a change in the other’s state, even if they are separated by a vast distance.

So when you measure the state of one entangled qubit, you will instantly know the state/property of the other one without even looking. Similarly, when you manipulate one entangled qubit, its twin will also get manipulated instantly without you having to lift a finger.

Classical computers take bits of data as a set of inputs, push them through a logic gate, and produce one definite output.

With quantum computers, you have a superposition of qubits as input, push them through a quantum gate to entangle them and produce another superposition as output, and then measure the outcome to collapse the superposition into an actual sequence of 0s and 1s.

However, when measuring the outcome, you will get just one result with a high probability that it’s the one you are looking for.

As such, you might want to run the quantum computation again to double-check the result. But even still, this setup allows you to get exponentially more calculations done in exponentially less time, making it more efficient than classical computers.

A quantum computer needs to be as isolated as possible. The slightest interference like a person sneezing or even a child walking by will cause the quantum state to wash away, a phenomenon called quantum decoherence.

Furthermore, to function properly, the quantum computer needs a super cold environment, as low as 0.01 degree Kelvin or -273.14 degree Celsius.

As such, quantum computers aren’t expected to replace classical computers anytime soon, let alone the PC on your desk. In fact, we are expecting a hybrid approach, where quantum computers are reserved for very niche, highly specialized computational work.

Here are a few situations where the computation power of quantum computers are absolutely invaluable:

- Searching through large databases, analyzing big data, and looking for patterns.
- Creating better models of financial services and analyzing risks.
- Modelling the molecular interaction of cancer cells and ultimately developing a cure.
- Simulating quantum particles to better understand quantum physics.

We are yet to understand the full extent of what is possible through quantum computing. Currently, it only serves as a specialized tool for solving certain problems. But it has the potential to start the next big revolution in human history.