22.12.2018difficulty level - Q

QuTech interview: Prof. Wolfgang Tittel

It is a cold Monday afternoon when we have our appointment with Wolfgang Tittel, professor in physics and specialized in photon entanglement, quantum teleportation, and quantum memory. As we expect from a busy professor, he is still in another meeting when we arrive, so we decide to wait two meters outside his door. When we go and check if he’s almost done with this other meeting, his office suddenly appears deserted. In a vivid demonstration of his expertise, he seems to have teleported away from his office…

Luckily, he reappears quickly and lets us into his spacious office. One wall is completely covered by a large whiteboard, covered in scribbled equations and diagrams. Clear signs of occupation by a physics professor. Professor Tittel himself welcomes us with a smile, clearly relishing the opportunity to talk about his work. What follows is an interview with professor Tittel, shortened and lightly edited for clarity.

What type of research do you do?

My research lies in the framework of the quantum internet. More precisely, it is about quantum key distribution (QKD) and the creation of quantum key distribution systems over very long links. This requires quantum repeaters. To create the quantum internet, we send photons down an optical fiber, but, just as in standard telecommunication, these photons get lost at some point. In standard telecommunication, you can use amplifiers to boost the signal level, but for quantum internet this doesn’t work because of the no-cloning theorem. Instead, we can use a so-called quantum repeater.

This research includes lots of very interesting technology to work with and to develop, ranging from sources of entangled photon pairs and quantum memories of light, which in my case are rare-earth-ion doped crystals cooled down to below 1 Kelvin. There’s a lot of interesting physics going all the way to fundamental questions, like how does a certain crystal doped with certain rare-earth ions react and what are their properties at low temperature? Can you modify the way they operate based on optimization of the magnetic field and its direction? This is really about the discovery of atomic dynamics at very, very low temperatures and that’s interesting in itself, but then of course you want to find or create something that allows you, or has the properties that allow you, to put technology around it – more precisely, technology for the quantum internet.

What are the devices that you surround yourself with daily?

There are many of those. First of all, people is a device [laughs]. And we use lots of lasers and various types of crystals. These are crystals that either create pairs of entangled photons out of a strong laser pulse, or, when cooled down to very low temperatures, allow us to store a photon; that is to absorb a photon and then to re-emit it sometime later with properties that aren’t changed. By `very low temperatures’ we mean below 1 Kelvin. We use a closed-cycle cooler for this, which is interesting, because we like to demonstrate things outside the lab. These cryostats are really more compact than dilution refrigerators and we bought them because of their transportability. We like to move the fridges around and show that experiments in the lab are not just interesting physics phenomena – maybe something that’s good for an evening conversation over a pint of beer – but really something that works. We did many such demonstrations when I was still in Calgary; we had cryostats in the town hall, for instance.

Are you also planning to do these kinds of demonstrations here in the Netherlands?

Yeah of course I will. I’ve done the first demonstrations of QKD and entanglement as a very young PhD student in 1996 and 1997 at the University of Geneva and since then continued along this direction, showing more and more difficult things that became mature enough to demonstrate outside the lab.

How did you get involved in such demonstration experiments?

It was part of my PhD thesis. When I joined the University of Geneva in 1996, the group did one experiment before that, in which they demonstrated the possibility to send photons that were polarized in particular directions under the Lake of Geneva. At that time it was sold as the first QKD experiment, but it wasn’t QKD. It was really just demonstrating that polarizations of photons are transmitted over fiber and are not changed. But at the time it was a new and really fascinating thing. Then, in Geneva, we continued building on that. The university of Geneva was one of the first who really pushed the demonstration of these quantum phenomena outside the lab. Of course now there are more people doing it. As a note now there are commercial systems that allow QKD outside the lab.

We continued these real-world demonstrations after I moved to Calgary. For instance, we demonstrated quantum teleportation across the city using the standard fibre network. One of the new things in this experiment was that the different pieces were located at widely separated places, the university of Calgary in the North-West, City Hall at the centre, and a datacentre in the South. This added a lot of complexity, but its exactly what will be required in a quantum internet.

Following up on your career history, did you switch directions along the career path at any point or was it a continuation?

It was a continuation but there were big additions. When I started, the field was all about photons. QKD with photons, entangled photons, or QKD with entangled photons. Then I did a postdoc in Aarhus with Eugene Polzik. There, I started working with atomic clocks and learned about a photon echo – a variant on the Hahn-echo – you send a pulse of light into the material, you wait and you send another pulse of light. Then, a third pulse comes out which is actually a copy of the first one. Back then we had the idea that this could maybe be used as a quantum memory. Well, it turns out it can’t. But this idea triggered what we call now photon echo quantum memory protocols and that is something that we are still working on. It relies on atom-light interactions and that’s why we need these ion-doped crystals, cooled down to very low temperatures.

So, in hindsight you could say that it was a continuation: we started with photons went to pairs of photons, three photons for teleportation, four for entanglement swapping, then added the crystals for the memory and now it becomes something that goes more in the direction of quantum repeaters. In that sense everything was kind of a natural progression, but at some point I moved from working only with photons to also including things like light-matter interaction and that’s of course a completely different field.

It also sounds like the evolution of the research field was happening at the same time.

Exactly. I would say the application was a constant evolution. However, photons and dynamics in solid state materials are rather different, so the technology that was developed and the physical understanding needed for this changed. We had to dive into new fields and learn a lot of new things.

What’s the appeal of quantum internet for you?

One appeal for me is the motivation based on information security. I think nobody has to convince us that, if you can’t communicate securely anymore, society could just break down. No bank transactions anymore, no electronic stock exchange. If all these things break down from one day to the other, we go back to pen and paper. It would be a catastrophe. To me, from an application point of view, the quantum Internet is important because it’s the only proven possibility that overcomes the threat of a quantum computer.

So it’s really the application.

It’s the application but there’s much more. I enjoy building complicated things, where you have to build something that is intellectually challenging, where you have to keep track and think of lots of individual components and how they interact. Then maybe it’s more complicated, or maybe they’re two different views of the same thing, but I also like this challenge of pushing science, pushing technology and understanding the fundamental interactions happening in these crystals better and figuring out how can you modify a quantum repeater protocol to better take into account what can be achieved in nature and what cannot be achieved. All of this together is a very rich field to play around with. I like if a question has no simple answer and there are lots of variables that you somehow have to keep track of and optimize all at the same time.

It sounds like you still learn something every day.

You always learn more than you want I think [laughs]. Everything is more complicated than you think at the beginning. It’s a kind of richness — if you just set a research plan with an idea “let’s do this” and everything works directly as we imagined in the first place, it would almost be boring. Every day there’s something. Sometimes things are broken and easy to fix but sometimes it’s just nature that doesn’t work as you hoped it would. It’s difficult to force nature’s hand.

What is the biggest surprise that came up in your field in the past ten years?

It’s hard to say, because in this field, every day there is a little bit more, but rarely a huge step. Probably, if you’re not in the field and someone tells you what we can do now compared to a decade ago, it is amazing how complicated things have gotten – how many things you can entangle and from how many things you can send photons and they’re still entangled. But there is no single milestone, it never happens that on a certain day you turn on your laser/cryostat and boom, everything is different and clear. That would be cool but usually it is incremental steps and everything is difficult.

[pauses and starts smiling]

Although I have an example from when we started to have interactions between photons and the quantum memories. We tried many things, nothing really worked, but then we suddenly had a huge signal – usually you look at the screen, aren’t sure, do it again, maybe it’s gone, fit a line through it, followed by weeks of signal optimization, fiber splicing and removing loss and then you start to believe in what you see – it was just there, no doubt. One of my students just lost it and started running around the lab. But that is not what normally happens. So don’t worry, if your life seems difficult, that is just the way it is.

Quantum communication technology is commercially available, does it mean that quantum technology is already out of the lab and ready for commercial applications?

It is commercially available, so it is applicable. But only the simplest application of quantum internet technology. But still that is great, because you have to start walking before you run. There are many steps to be done before it will be widely applicable; create a market, deploy it, make it cost efficient.

You cannot develop a technology in the lab, in isolation, and when it is finished, you push a button and expect it to be adopted from one day to another. Also with quantum technology it is all about scaling up and creating the bridge between the people who create technology and the people who will use it. Therefore I’d like to emphasize that it is important to start and do this with technology that can be upgraded. The same happened with mobile phones. At the time you saw people walking around with huge bags with their phones – it was amazing. And right now everybody has that and everyone thinks that in quantum communication we need the same qubit rates as bit rates on the classical internet. And that of course makes no sense, since we had no time to develop the technology and optimize things.

Quantum communication is secure, but probably also more expensive, where do you see this trade off between cost and security?

I’m not sure if I agree with the statement that quantum communication is more expensive than classical communication. It is just that a lot of infrastructure for classical communication does already exist. But a lot of money has already been invested in building this infrastructure (e.g. putting fibres in the ground). Therefore, classical software upgrades are cheaper than new quantum key distribution infrastructure. But the question is how long we can trust these classical systems. Two years ago, the NSA openly declared that we have to move to quantum resilient cryptography algorithms.

[pauses a bit and then smiles and continues]

Of course, the technology is expensive now. But look at your car, if you look at technology inside a car, it is amazing for how little money you can buy that. The reason for that is, of course, the market; as a company you need to produce reasonably priced cars to be able to keep up with the competitors. If the same happened in the quantum world, a QKD system wouldn’t be so expensive anymore. What is the most expensive part of a QKD system? Currently the single photon detectors. Why? Because there are only a handful of companies that create them.

You’ve mentioned that we can build on the existing infrastructure, used for classical communication, can we actually use the existing fiber networks?

That’s what we do, yes. Since well, back in 1994 in Geneva. There’s nothing new. Although we use so-called dark fibres; fibers to which no classical equipment is connected so that no additional photons, that can mask our photons, are sent down the fibre. We are looking to demonstrate the possibility for our QKD system to have classical and quantum communication co-existing in the same fibre by using different wavelengths. But the problem is Raman scattering, which creates photons at different wavelengths, including the wavelength of the quantum communication photons, such that you cannot distinguish between classical and quantum communication anymore. We’re looking into that, because using dark fibre would make QKD very expensive. And if it works not only for our system, but also for e.g. NV-centers it would make quantum communication not so costly anymore and move it into the practical world.

What’s the yet to be demonstrated result that you have longed to see the most in your field?

I think the next big result for us is what we call an elementary quantum repeater link. Without going to much into detail about quantum repeaters, it basically means to split a connection into shorter links. The challenge is to develop those shorter links in such a way that they work efficiently. Once that is done, you can scale up. First, we have to demonstrate inefficient operation of such links, thereafter we make it more and more efficient. Once you have those very sophisticated repeaters, the concatenation of the links is not too difficult anymore. And that is something I’d like to see; this shorter link is a major step that I hope to achieve soon.

Do these elementary links need a high fidelity?

That depends on what you want to do with it. The fidelity for a quantum repeater with the purpose of QKD does not be too high, since you can run classical error correction. Therefore, QKD would be a good first application of quantum repeaters. When you want to connect quantum processors, either based on NV-centers, superconducting qubits or quantum dots, you’ll need higher fidelities and possibly entanglement-purification.

What is your first physics memory?

One of the first physics memories I have is when I was quite small, about 5 or 6, and I visited my father’s lab at an open house of the university – he was a professor in classical optics. [Smiling] You see, kids do what their parents do. He had a lab with huge tables with optics and lots of lasers. While talking about the lasers, at one point my father asked one of the visitors to light a cigarette and blow the smoke into the setup – I would go crazy if someone did that now. Then of course you see all laser beams reflected on the small particles and that was just fantastic and I’ll never forget. I’m not sure if this was my first memory, I visited his lab many times, but it is definitely something that really marked me.

What do you like to do when you’re not doing physics?

I like to spend time with my family. I have two little kids, 3 and 5 year old. Also, I really like climbing, skiing and mountain biking, so a lot of things you cannot do in the Netherlands. You can scratch your head and wonder whether I looked at the map before I moved here, but I was aware of that. I really like being in Europe again and there are lots of climbing gyms, with usually a bar nextdoor, which makes for an interesting setting. And I can always are a plane, or a car and go somewhere near the mountains.


About Wolfgang Tittel:

Wolfgang Tittel (far left) with his team.
Image credit: Justin Quaintance from the Gauntlet, Calgary

Wolfgang Tittel got his PhD from the university of Geneva, did postdoctoral work in Aarhus and in 2006 started his own lab in quantum communication with photons and rare earth ions in Calgary. About nine months ago he moved to the Netherlands, where he is a professor now at the Delft University of Technology.


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