Learn the Lingo – Quantum Information

Quantum information researchers use a lot of terms that would fit in perfectly with a Dungeons and Dragons campaign. From qubits to magic to oracles, let’s get up to speed on quantum information lingo with Perimeter researcher Alex May.  

Classical Computers, Quantum Computers, & Qubits

Classical computers process data stored as basic information chunks called binary digits, or bits. Bits can switch between one of two possible states: 0 and 1. They operate on an all-or-nothing system – bits are either 0 or 1, nothing in between.  

But quantum computers store data differently, as qubits. When you measure a qubit, you will find either a 0 or a 1, but before you look at them qubits exist as something stranger: a superposition of the 0 and 1 state. 

Superposition

When in superposition, a qubit is described by two numbers known as amplitudes. When you measure the qubit, these amplitudes determine the probabilities that you'll get 0 or 1. 

It’s tempting to view the qubit as being either in the 0 or 1 state before you measure it, and the probabilities reflecting the fact that you're not certain which state the qubit is in, says May. This isn't quite right though – amplitudes are something different than probabilities.  

“Being in a superposition does not mean that it’s already 0 or 1 and you just don’t know yet, but it also doesn’t mean it’s in both. It’s a different concept besides ‘or’ and ‘and’ that we don’t have a normal English word for.”

The ability to exist in superposition opens new ways qubits can behave and evolve compared to their classical counterparts. This ability could be the key feature – fingers crossed – that gives quantum computers an advantage.

Noise

Look up the definition of noise, and you’ll find it’s more than just sound. It can also refer to any disturbance that is unwanted.

There are plenty of things we might call noise in the ordinary, classical world, says May. “You’re talking over the phone, and it breaks up. That’s because there’s some noise in the system. In the classical world we have ways of dealing with this; for example, we could repeat ourselves on that phone call that's breaking up until our voice gets through."

In quantum information, noise refers to disturbances to a quantum system. Noise can come from a host of sources – everything from temperature to the machinery hosting the system.

May says quantum systems tend to be small and delicate. Because of their fragility, researchers have to protect them against noise. 

“One reason this is difficult is that if you try to look at your system and make copies to protect it against noise, you disturb your system. So you don’t get to make copies with quantum systems.” 

It’s as if in our choppy phone call, we were never allowed to repeat ourselves but still needed to make ourselves understood, says May. "You somehow have to battle noise without making copies.”

Magic 

Will quantum computers eventually completely replace classical computers? Not quite. Classical and quantum computers both have their strengths and weaknesses. 

May says researchers are still working on finding the line between what we can do classically and what needs quantum power. The key is in pinpointing what makes quantum different, and where its additional power lies.

One way to gauge the difference between classical and quantum computing is through a mathematical quantity called magic. In quantum information, magic isn’t the tool of witches and wizards; instead, it’s one of the conditions that contributes to how difficult it is to simulate a quantum computer problem on a classical computer.  

“In general, we think there’s no easy boundary between hard and easy to simulate, or one clear rule that delineates when quantum computing is better than classical computing,” says May. “But magic is one little piece of that puzzle that separates the two.” 

Oracle

In Greek mythology, an oracle provides supernatural prophecies for the future. In quantum information, an oracle provides a different insight: a way to probe the power of different computing models.

A function in mathematics takes an input and gives an output. A very simple example is something like f(x) = x² .You can input a number for x (let’s say 2) and receive an output. In this case, the output would be 4. 

“If you’re trying to compute something, there are two ways you can have access to a function,” says May. “One is, I give you the description of the function. That’s what’s happening when I say your function is to take the square, for example.”
The other option, May says, is to have oracle access to a function, where there is no description of the function. “Instead, you get a black box, where you can feed an input to the box and the output comes out,” he says. 

The very first problem where researchers proved a quantum computer was doing something a classical computer could not involved an oracle, says May. It’s called the Deutsch-Jozsa problem in honour of its creators, David Deutsch and Ricard Jozsa.

“In the Deutsch-Jozsa problem, we’re given oracle access to a function and asked to determine a certain property,” explains May. “Doing this in the classical world requires we ask the oracle to compute the function many times, but in the quantum world we can ask the oracle just once. The key is to give the oracle multiple inputs in superposition and carefully tease out properties of the function from the resulting superposition of answers.”

“This is interesting to researchers because it’s one example of quantum computing letting us do something that classical computing does not, but it’s in this funny oracle model that isn’t instantiated in the real world. It doesn’t necessarily tell us that quantum computing is useful for something in real life, but it’s suggestive.”

Researchers continue to explore which problems quantum computers can solve faster than classical computers, adds May. “Building on the insights first developed in the oracle setting, they’re finding methods to quickly solve a wide-ranging set of problems, including ones that occur out of the oracle setting and in the real world.”

Putting it into practice

Ready to dive deeper into the world of quantum information? Take what you’ve learned here and head on over to our Explorer profiles on Perimeter quantum information researchers Alex May, Beni Yoshida, and Sisi Zhou.