What is a Quantum Computer?

TL;DR: A device that uses quantum mechanics to solve computational problems

If you have been following the latest technological advancements or bingeing on marvel movies, you have probably come across the word ‘quantum’ a lot. It has made it’s way into the list of adjectives like machine learning and block chain which can make anything sound like the next big thing. This ongoing hype drives curiosity but also clouds the boundary between fact and fiction. So coming to our topic, what is a quantum computer?

Let’s start with the question, what is a computer?

A computer is a device that takes some information as input and processes this input into an output. Here, the information is stored in a physical system, and the information processing is a physical process which is bound by the constraints of the computer architecture and fundamentally the laws of physics. Within this bound, we can write algorithms for information processing, and thereby solve a lot of real world problems.

The computers that most of us use have different ways of storing information and different architectures, which leads to different speed, size and efficiency. But the underlying physical system used for encoding and processing information is always a classical system constrained by the rules of classical mechanics. These are called classical computers or just computers.

Now, what is a quantum computer?

When the underlying physical system used for encoding and processing information is a quantum system, we have a quantum computer. These depend on the rules of quantum mechanics which are fundamentally different from classical mechanics. Hence, the basic paradigm of the information processing is different.

The important word here is ‘different’. It does not mean ‘better’ or ‘faster’. We don’t want to replace a classical computer with a quantum computer. A quantum computer allows us to solve a set of problems that are almost impossible to be solved on a classical computer, and only for these problems we will use a quantum computer. For many other tasks classical computers will continue to be a better choice.

IBM’s quantum computer

Bit vs Qubit

The fundamental unit of quantum information is called a quantum bit or qubit. A classical bit stores a value of either 0 or 1. The quantum bit stores a superposition value. It is neither 0 nor 1. It’s something else that we cannot intuitively imagine. But when measured it randomly takes a value of either 0 or 1, with an underlying probability distribution. Being a quantum state, any process that a qubit undergoes, is in accordance with the rules of quantum mechanics. This leads to qubits having some special quantum properties like superposition and entanglement, which can be taken advantage of, to solve certain type of computational problems.

A qubit can represent any value on the sphere, whereas a bit can represent only the poles

So, where can we use this ‘quantum’ computer?

In the early 1980s, Richard Feynman, a physics Nobel laureate talked about the kind of computer that will be used to simulate atoms.

“I’m not happy with all the analyses that go with just the classical theory, because nature isn’t classical, dammit, and if you want to make a simulation of nature, you’d better make it quantum mechanical,”

He was right. Atoms are quantum objects, and can be represented accurately only on a quantum bit. Representing quantum objects on a classical bit is kind of like representing a 3-D object with a 2-D image. A single image can’t represent the object. You can take multiple images from different angles but still can’t capture the full essence of it.

This looks like a niche physics issue that has no real world applications? You are wrong. Simulating atoms with all it’s quantum properties means simulating any chemical reaction. All the experiments conducted to create new materials, drugs and catalysts can be simulated on a quantum computer. This will revolutionize the pharmaceutical industry and other fields of applied chemistry and material science.

Simulation of atoms was one of the problems that gave birth to the idea of quantum computing. But computer scientists and mathematicians later figured out other use cases.

One important milestone was the development of Shor’s algorithm. The basis of modern cryptography, is the fact that it is effectively impossible to factorize a very large number. This is true for any algorithms developed for a classical computer. But Shor’s algorithms can potentially crack this, as soon as we have a powerful enough quantum computer. Cyber security, being an important concern for “national security”, let to the inflow of large amount of money and resources for research in this field.

Quantum computers can run better optimization algorithms which can revolutionize machine learning and other fields of applied math. In 1997 IBM’s computer Deep Blue defeated chess champion Garry Kasparov by examining 200 million possible moves each second. A quantum computer would be able to calculate 1 trillion moves per second.

We also have the the concept of quantum communication which will significantly improve data security and privacy. We have a lot more ideas and algorithms that could potentially ‘change the world’, if and when we have a quantum computer. But, this is a big ‘if’ and a long ‘when’.

Challenges ahead

We do have quantum computers today. I have even put an image of it in the previous section. It does look big and powerful, which it is not. Most of the hardware that you see, is what is called the dilution refrigerator. It is the cooling system that keeps the circuit (which is very small) at near zero kelvin temperature. This is done because qubits are very unstable and their superposition and entanglement can be destroyed with the most minimal disturbance. Even with this cooling we cannot keep the system stable for long enough.

A quantum chip

Due to the high error rate, we need quantum error correction algorithms, which add to the the complexity of the original problem to be solved. An algorithm that would have required 1000 qubits to solve, can require about a million qubits after accounting for quantum error correction. Today’s quantum computers have less than 100 qubits.

This is a pretty new field that still needs a lot of advancements in the field of physics, chemistry, mathematics, electronics, computers science and material science. We have a long way to go, and we don’t even know if we will ever built a scalable universal quantum computer. But, this has always been the case with any interdisciplinary cutting edge tech. As Jim Clarke of Intel puts it “The potential is too great, and the stakes are too high to quit at mile one of a marathon.”

Student @ BITS Pilani

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