18 April 2016

How often do you text on a smartphone? Research on a laptop? Use a GPS to find a place to eat? All of these devices contain some sort of computer, and computers wouldn’t exist without quantum mechanics.

Quantum mechanics is the branch of physics that describes the motion and interaction of subatomic particles. That might sound complicated, but as you can see from the examples above, you actually use things built on these principles every day!

While the components of your personal computer depend on principles of quantum mechanics, none of them actually use quantum mechanics to process information. A computer capable of doing this would be called a quantum computer. In theory, quantum computing would make it possible to process huge amounts of data thousands of times faster than the computers you use today.

Speed matters, because the world is producing more data than ever. These data are produced by things like surveillance systems, climate monitoring, weather forecasting, and even social media monitoring. And they are often too much for traditional computers to work with effectively. This is why government agencies, business groups, and other organizations are so interested in quantum computing.

**Did you know?** To calculate the number of possible of values for a series of bits, you compute the value of 2 raised to the power of the number of bits. For instance, two bits would have four possible values (2^{2} = 4). Ten bits would have 1024 possible values (2^{10} = 1024).

So how does a quantum computer work? Let’s begin by looking at the basic principles of quantum mechanics. One of these is **superposition**, which states that a subatomic particle is in all its possible states (or energy levels) at the same time. That is, until you actually observe it. You may have a hard time wrapping your head around this idea. It’s a little like you telling me that you are in both Vancouver and Toronto until I actually figure out where you are.

You may know that traditional computers store information in **bits**, and that each bit can be represented by a 0 *or* a 1. For instance, a series of two bits can have four possible values: 00, 01, 10, and 11. However, your computer can only perform computations on the one value that the two bits actually represent.

But thanks to superposition, quantum computers can use **quantum bits (qubits). **They store information as *both* a 0 *and* a 1… right up until an event forces the qubit to become *either* a 0 *or* a 1. That’s kind of like me discovering that you are, in fact, in Vancouver.

This means that quantum computers could perform calculations using *all four* of the possible values for a series of two bits! That may not sound like much. But computers regularly deal with much larger amounts of information. For example, 10 bits would have 1024 possible values. So you can imagine how quantum computers would be much more powerful than their traditional counterparts.

**Did you know? **The Heisenberg uncertainty principle is fundamental to quantum mechanics. It states that you cannot accurately measure both the position and the momentum of a subatomic particle at the same time.

It’s not easy to build a quantum computer. Traditional computers use tiny transistors to represent low and high voltages as 0 and 1. But a quantum computer needs access to its hardware’s subatomic characteristics so it can represent superposed qubits. Those are qubits that are *both* 0 *and* 1.

So to build a quantum computer, you need to choose materials that are capable of representing superposed qubits. One approach stores qubits using a lattice of superconducting circuits, cooled to a temperature just above absolute zero. Other labs have tried trapping ions using electromagnetic fields. Yet another suggested approach involves using silicon—the same material used in traditional computers.

Hardware is not the only challenge. Software also needs to be designed to take advantage of quantum mechanics. For example, algorithms (special problem-solving formulas) must use quantum superposition and account for uncertainty when performing computations.

Researchers have made steady progress in creating quantum computers that work on a small scale, or for specific tasks. For instance, quantum computers have been designed to quickly optimize water flow in complex networks of pressurized pipes. However, no one has yet built a *universal quantum computer. *That would be a quantum computer that can solve any computing task that a traditional computer can solve—only much more quickly.

How will we know when a universal quantum computer has been successfully built? One test would be whether the computer can quickly crack a public key encryption system. For example, RSA is a system widely used for securely transferring data—things like banking information, national defense secrets, and other top-secret information. It depends on math that is too complicated for classical computers to solve quickly.

In theory, Shor’s algorithm (a method for finding prime factors) could be used by a quantum computer to quickly crack a public key encryption system like RSA. Existing quantum computers have successfully used this algorithm to factor relatively small numbers. But no one really knows when there will be a functioning universal quantum computer that can use Shor’s algorithm to work on much larger numbers. In the meantime, government labs and universities around the world are continuing to work on the problem.

**Did you know? **Post-quantum cryptography involves finding algorithms that are secure against an attack by quantum computers.

Quantum computing could bring many positive developments. Instead of time-consuming experiments, new ideas in a variety of fields could be tested much more quickly using computer simulations. This could lead to things like safer transportation, more durable and energy-efficient machines, and more effective medical treatments.

On the other hand, if quantum computers capable of cracking public key encryption fell into the wrong hands, it could wreak havoc. For example, criminals could use quantum computers to steal military secrets or illegally transfer funds from banks.

Just as a qubit is simultaneously both a 1 and 0, quantum computing itself can simultaneously bring good and bad outcomes. Of course, this is true of any new technology. Hopefully, wise leadership from both industry and government will allow the good to prevail.

General information on quantum computing:

Crucial hurdle overcome in quantum computing (2015)

Wilson Da Silva, University of New South Wales

Quantum Computing 101 (2015)

Institute for Quantum Computing, University of Waterloo

Confused about the NSA’s quantum computing project? This MIT computer scientist can explain (2014)

T.B. Lee, The Washington Post

Issues related to quantum computing:

The Revolutionary Quantum Computer That May Not Be Quantum at All (2014)

Clive Thompson, Wired

Study doubts quantum computer speed (2014)

Paul Rincon, BBC News

Why nobody can tell whether the world’s biggest quantum computer is a quantum computer (2014)

Leo Mirani and Gideon Lichfield, Quartz

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