#### Overview

Every so often a new technology surfaces that enables the bounds of computer performance to be pushed further forwards. From the introduction of valve technology through to the continuing development of VLSI designs, the pace of technological advancement has remained relentless. Lately, the key to improving computer performance has been the reduction of size in the transistors used in modern processors. This continual reduction however, cannot continue for much longer. If the transistors become much smaller, the strange effects of quantum mechanics will begin to hinder their performance. It would therefore seem that these effects present a fundamental limit to our computer technology, or do they?

In 1982, the Nobel prize-winning physicist Richard Feynman thought up the idea of a ‘quantum computer’, a computer that uses the effects of quantum mechanics to its advantage. For some time, the notion of a quantum computer was primarily of theoretical interest only, but recent developments have bought the idea to everybody’s attention. One such development was the invention of an algorithm to factor large numbers on a quantum computer, by Peter Shor (Bell Laboratories). By using this algorithm, a quantum computer would be able to crack codes much more quickly than any ordinary (or classical) computer could. In fact, a quantum computer capable of performing Shor’s algorithm would be able to break current cryptography techniques in a matter of seconds. With the motivation provided by this algorithm, the topic of quantum computing has gathered momentum and researchers around the world are racing to be the first to create a practical quantum computer.

It’s fascinating to think about the power in our pocket—today’s smartphones have the computing power of a military computer from 50 years ago that was the size of an entire room. However, even with the phenomenal strides we made in technology and classical computers since the onset of the computer revolution, there remain problems that classical computers just can’t solve. Many believe quantum computers are the answer.

#### What is Quantum Computing?

Quantum computers are incredibly powerful machines that take a new approach to processing information. Built on the principles of quantum mechanics, they exploit complex and fascinating laws of nature that are always there, but usually remain hidden from view. By harnessing such natural behaviour, quantum computing can run new types of algorithms to process information more holistically. They may one-day lead to revolutionary breakthroughs in materials and drug discovery, the optimization of complex manmade systems, and artificial intelligence. We expect them to open doors that we once thought would remain locked indefinitely. Acquaint yourself with the strange and exciting world of quantum computing.

#### How Quantum Computing Works and Why It’s Important

Computers have radically changed society. Shortly after the end of World War II, scientists were using computers to solve all sorts of problems. Progress was unbelievably fast. By the 1970s, the home computer was born.

Yet for all that progress, some problems are still really hard. No matter how good computers get, challenges like factoring large numbers or optimizing courier routes remain difficult.

But bits are not the only way to compute. Quantum mechanics — the rules that govern the world of atoms and molecules — can also be used to compute. And those computations are performed in a remarkably different manner.

The hope is that some day these “quantum computers” will be able to solve hard problems. The pressure is on the computer industry to find ways to make computing more efficient, since we reached the limits of energy efficiency using classical methods. By 2040, according to a report by the Semiconductor Industry Association, we will no longer have the capability to power all of the machines around the world. That’s precisely why the computer industry is racing to make quantum computers work on a commercial scale. No small feat, but one that will pay extraordinary dividends.

#### What Makes a Quantum Computer Different?

To illustrate the difference between quantum and traditional computing, Daniel Lidar, a professor of physical theoretical chemistry at the University of Southern California, uses the following analogy

Imagine searching for a black ball in a box full of white balls, and you can’t see inside the box. To find the black ball, you blindly grab a ball, examine the color, and throw it away if it isn’t black. You might grab the black ball on the first try, or you might pick it last.

The most likely outcome: You destroy the box in frustration.

Now let’s switch to a quantum algorithm. Your quantum hands reach into the box, but they don’t grab a ball. Instead, these hands hold the probabilities of having picked each ball — including the black ball. If the box has 10 balls, your quantum hands hold 10 equal probabilities.

Next, you run a quantum algorithm that *increases* the probability that the ball is black. Afterward, you check your hand: Disappointingly, the ball is white. You reach back into the box. But this time the probabilities are not equal: The probability of you finding the black ball is higher now than for the other balls.

It is as if the previous attempt threw away an extra white ball along with the one you found. This occurs for every attempt, so the chance of finding the black ball rapidly increases. The key to the way these probabilities change is in how quantum states—or “qubits,” in the case of computing—are manipulated.

#### Let’s break down that box-of-balls story to see how it all works.

The quantum hand reaches into the box and grabs probabilities. In traditional computing, information is stored as bits that have definite values. A bit is either a one or a zero. Checking the value of a bit doesn’t modify it in any way.

But a qubit doesn’t directly represent the value of the bit; it holds the probability of the qubit being a one or a zero. This is called a “quantum superposition state.”

When we check the value of the qubit, however, we don’t get the probability. The measurement reveals a one or a zero—the choice randomly determined from the probabilities of the superposition. Measuring sets the value of the qubit. If we measure the qubit and get a one, checking again will also result in a one.

When we reach into the box, we are actually taking a set of qubits — enough to represent all the balls. The qubits are put into a superposition state that holds the probabilities of finding each ball. Since the search is completely random, each ball is represented with equal probability.

Now** **we run an algorithm that increases the probability of finding the black ball.

You might ask: How can you increase a probability without sneaking a peak? The answer lies in how a qubit holds probabilities. A probability is represented by a number between zero and one. But qubits hold probability *amplitudes*, which can be positive or negative.

As Lidar puts it: “[T]his is where there is a real difference. There is no notion of negative probability [in classical physics], that’s meaningless…But in the quantum case, we can have [a] negative [probability] amplitude cancelling the positive [probability] amplitudes. It is through the manipulations of these interferences that we can start to understand how quantum computing can get an advantage.”

Two key points are hidden in that quote. When a negative amplitude meets a positive amplitude, the net result is something closer to zero, so the probability of that particular outcome goes down; if two positive amplitudes meet, the chance of that outcome increases. That is, we can manipulate the probability of a particular outcome without measuring the qubit. (Remember, making a measurement will destroy the superposition state.)

More important, qubits can be made to do this to themselves. When we talk of a positive amplitude meeting a negative amplitude, these amplitudes may be from the *same qubit*. And if that doesn’t cause your mind to bend and creak a little, nothing will.

As a result, a quantum computer can quickly reduce the probability of obtaining an incorrect answer and increase the odds of getting the correct answer. This is exactly the sort of trick a quantum computer uses to increase the probability of finding the right ball.

#### How to Build a Quantum Computer?

There are several basic approaches to building a quantum computer. The most common approach is much like we build computers now, called the circuit model of quantum computing.

Each program is broken into a series of specific logic operations, most of which modify probability amplitudes of one qubit, depending on the probability amplitudes of a second qubit. A circuit-based quantum computer takes in a starting set of qubits and performs each operation in the program sequentially. After running the program, the qubit states are read to obtain an answer.

#### How our world will change with quantum computing?

It’s difficult to predict how quantum computing will change our world simply because there will be applications in all industries. We’re venturing into an entirely new realm of physics and there will be solutions and uses we have never even thought of yet. But when you consider how much classical computers revolutionized our world with a relatively simple use of bits and two options of 0 or 1, you can imagine the extraordinary possibilities when you have the processing power of qubits that can perform millions of calculations at the same moment.

What we do know is that it will be game-changing for every industry and will have a huge impact in the way we do business, invent new medicine and materials, safeguard our data, explore space, and predict weather events and climate change. It’s no coincidence that some of the world’s most influential companies such as IBM and Google and the world’s governments are investing in quantum computing technology. They are expecting quantum computing to change our world because it will allow us to solve problems and experience efficiencies that aren’t possible today.