Introduction to Quantum Computing

In this brief introduction to quantum computing, we’ll cover the following topics:

• What is quantum computing?
• Baby steps toward quantum computing
• Programming a quantum computer
• The future of quantum computing

What is quantum computing?

Quantum reality is very strange.

Imagine a spinning wheel that’s turning neither clockwise nor counterclockwise. Just by looking at the wheel, you set up a relationship between yourself and the wheel, and this makes the wheel turn in one direction or another.

Imagine two friends who travel to opposite ends of the Milky Way galaxy. Whatever one randomly decides to say upon landing on a distant planet, the other feels compelled to say too.

That’s the world of quantum mechanics. It’s what makes quantum computing so fascinating.

Here’s how we distinguish quantum computing from classical computing:

• Classical computing: A computational model in which the fundamental unit for calculation is a binary bit. Each bit’s value is either 0 or 1.

Every laptop, server, workstation, and smartphone is a kind of classical computer. Even the Frontier supercomputer with 600,000 cores in Oak Ridge, Tennessee is a classical...

Baby steps toward quantum computing

The idea for quantum computing came in 1981 with presentations by Paul Benioff and Richard Feynman at the First Conference on the Physics of Computation. Fast forward to 1998, when the world’s first quantum computer had only two qubits.

Tip

For more information about the First Conference, the two-qubit computer, and other topics in this Introduction, refer to this chapter's Further reading section. You wouldn’t buy a laptop whose chip could process only two bits. In the same way, you wouldn’t expect a two-qubit quantum computer to solve your puzzling mathematical problems.

By 2006, the world had 12-qubit quantum computers. And by 2017, we had 50-qubit computers. The number of qubits in most advanced quantum computers of the early 2020s is in the low-to-mid hundreds. Compare this with a typical laptop’s memory, which stores about 64 billion bits.

Of course, the answer you get when you ask for a count of...

Programming a quantum computer

Quantum computers don’t run independently. They receive input from classical computers and provide output to classical computers. In a sense, there’s no such thing as completely independent quantum computing. All quantum computing is part of a larger technology called hybrid computing.

When you work with quantum computers, you write code that runs on a classical computer. Based on your code, the classical computer feeds instructions to the quantum computer. There are many programming platforms designed specifically for quantum computing. They include Q# from Microsoft, Cirq from Google, OpenQASM from IBM, Ocean from D-Wave, and PennyLane, which is maintained by Xanadu.

In this book, we program using Qiskit – an open source software development kit. Qiskit (pronounced KISS-kit) is part of IBM’s family of quantum computing initiatives. Using Qiskit, you can run code for quantum computers on many different devices. Some...

The future of quantum computing

As far as we know, we’ll never trade in all our classical computers for quantum computing models. Quantum computers aren’t good for performing the mundane tasks that we assign to most computers today. You wouldn’t want to program a simple spreadsheet on a quantum computer, even if you could find a way to do it.

But to solve certain kinds of problems, a quantum computer with sufficiently many qubits will leave classical computers in the dust. Chapter 9 shows you how sufficiently powerful quantum computers will be able to factor 2,048-bit numbers. According to some estimates, a factoring problem that would take classical computers 300 trillion years to solve will require only 10 seconds of a quantum computer’s time. If we can achieve an advantage of this kind using a real quantum computer, we call it quantum supremacy.

In 2019, a team at Google claimed to have demonstrated quantum supremacy. Its 53-qubit quantum computer...

Further reading

1. Benioff, Paul A. (April 1, 1982). “Quantum mechanical Hamiltonian models of discrete processes that erase their own histories: Application to Turing machines”. International Journal of Theoretical Physics. 21 (3): 177–201. Bibcode:1982IJTP...21..177B. doi:10.1007/BF01857725. ISSN 1572-9575. S2CID 122151269.
2. Chuang, Isaac L.; Gershenfeld, Neil; Kubinec, Mark (April 13, 1998). “Experimental Implementation of Fast Quantum Searching”. Physical Review Letters. 80 (15): 3408–3411. Bibcode:1998PhRvL..80.3408C. doi:10.1103/PhysRevLett.80.3408. S2CID 13891055.
3. Feynman, R.P.(1982). Simulating physics with computers. Int J Theor Phys 21, 467–488 (1982). https://doi.org/10.1007/BF02650179
4. Quantum computers could crack today’s encrypted messages. That’s a problem: https://www.cnet.com/tech/computing/quantum-computers-could-crack-todays-encrypted-messages-thats-a-problem/
5. Arute, F., Arya, K., Babbush...

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