Quantum All-Star: Felix Leditzky

While researchers across the public and private sectors are figuring out how to build large-scale quantum computers, many others are already thinking a step beyond, envisioning how to put these machines to work once they’re available. IQUIST’s Felix Leditzky is among those who are peering into the future, as he works to understand the mathematical and computational aspects of how communication will work in quantum systems.

“The study of communication is a mathematical theory that’s actually very young,” says Leditzky, who is an assistant professor in the Department of Mathematics. “It was started by Claude Shannon just after World War II, in the late 1940s... to formalize the means of communicating information.”

At the time, the theory looked at classical, not quantum, communication. Our understanding of quantum principles had been growing for decades, but it wasn’t until the 1970s that people started thinking about how quantum theory could be used in communication.

Today, Leditzky says, we are asking the question, “When we try to communicate with someone, how can we incorporate quantum systems in this process, and what can we do with quantum systems that we couldn’t do otherwise, with purely classical systems?”

Researchers in this area look at new opportunities that arise from the correlation of physically distant particles through quantum entanglement. This correlation enables teleportation of quantum information between distant locations. “We can actually use this correlation to send quantum information from my home to your home, just using this bond... And it turns out that this is a very powerful idea that lets us do a lot of things in this quantum communication world.”

Looking at quantum communication channels

So what kinds of quantum communication problems are being tackled? A big one, which Leditzky has studied extensively, is communication through noisy quantum “channels.”

“I study quantum channels, which are mathematical models for when our communication link has some noise in it,” he says. “Think of a phone call where the signal is very bad, so you have a lot of white noise in the line, or something. And then you have the task of trying to convey information to the other person in such a way that the noise doesn’t affect it too much. A very simple example is... you tend to repeat what you say. This is the idea that if we make the information that we want to convey redundant, then we somehow protect it from this noise... The task for us is to come up with a way of protecting the information from getting destroyed too much... If you make it redundant, you can lose certain parts of it, but the information content is still somehow there.”

Some of his work has dealt with a curiosity dubbed the “platypus channel” by his former postdoctoral advisor. “This channel behaves very differently depending on the context in which you look at it,” Leditzky says; but it can be regarded as a combination of “two channels that are by themselves not very interesting, because we fully understand them, but... when you put them together in a certain way, you get this new channel which is very interesting.”

Why the name “platypus”? When Europeans first heard of the platypus—the bizarre duck-billed, egg-laying Australian mammal—they assumed it was a hoax. “They thought someone had stitched a beak onto some rodent or something,” says Leditzky. He doesn’t consider the platypus analogy very meaningful, but people find it memorable—and he’s fallen into the habit of saying that the two channels are “stitched” together!

What’s it like working in the field of quantum information science?

Leditzky says he enjoys many things about his job. “On the scientific side, [there’s] the variety of topics and methods that we can use in quantum information. You always learn a new theory when you look at a new problem. And this is great, because it keeps you engaged... And on the personal side, I have a lot of close colleagues, many of whom I also consider friends. And the atmosphere, the environment, in our field I think is a very good one. It’s very collaborative. You have a lot of fun going to conferences and engaging with people.”

He contributes to that fun atmosphere with an active Twitter account (https://twitter.com/felix_led) that has almost 2,000 followers, including many of his colleagues. “It’s a very good way to be in touch with people, in a way that’s a little more casual than via email,” he says. “You can have really good scientific discussions.” He adds that it’s also a great tool for engaging with students.

A potentially unsettling aspect of being a theoretician is the lack of experimental verification of your work. Can you be sure that what you’re doing is relevant to the real world? In that respect, Leditzky has been luckier than some: “The cool thing is that a couple of years ago, there was a team in China who actually conducted an experiment showing an effect that we predicted, which has to do with [a] communication problem. That was very gratifying.”

What should a young person know who’s curious about quantum?

“It’s a great time to join [this field],” says Leditzky. “The cool thing is that... quantum information science lives at the intersection of a lot of different fields. So I’m an applied mathematician; a lot of my colleagues are physicists. Then you have electrical engineering, you have computer science, you have materials science. So all of these different fields come together, and quantum information lives at the intersection of those. You can approach the field from very different angles depending on what you like... [and] a lot of good ideas come from people talking to each other, who approach it from different sides. So that’s great, because also it creates a lot of entry points into the field.”

To work on the kind of problems he studies, it’s important to have a strong interest in mathematics and to get a solid grounding in certain subjects, particularly linear algebra. With that being said, he notes that “a lot of [quantum communication] problems can actually be phrased in relatively simple mathematical terms. You do not need a lot of knowledge to start working on these problems, which is great, because it gets people early into the field.”

“You don’t need to be a genius!”

Leditzky is disappointed that many people find quantum intimidating.

“I’ve seen this a lot in conversations with people: whenever you say the word ‘quantum’ there are two broad reactions. Some people are very interested, and they start listening... But unfortunately also a lot of people internally switch off, and say, ‘ah, okay, quantum; this is something I will not understand.’ And you get a lot of... ‘Oh, you must be very smart!’... And I don’t think this is useful, this kind of ‘you need to be smart to do it.’ No! You need to be interested in it. And you need to have a certain drive to want to find out about these things. But you don’t need to be a genius. I’m not.”

He thinks the misplaced sense of awe is an unfortunate barrier. “In the end, it’s actually very simple ideas. They’re just weird! They’re not what we’re used to! And that’s why it took us so long to find out about them. But they’re actually very cool. And not too hard to understand.”

“I just invite everyone to try to read up about it, or get in touch with me via email or Twitter!”