Pioneering Quantum Optics at the Chip Scale

Christine Silberhorn is a physicist at the forefront of quantum optics and quantum information science. Based at the University of Paderborn in Germany, she is widely recognized for her pioneering work developing compact, chip-based systems that harness the quantum properties of light.

Silberhorn will be delivering a plenary talk at the World of Photonics Congress that will take place in Munich, Germany, from 22 to 27 June, in parallel with the Laser World of Photonics 2025.

Ahead of the Congress and as part of our coverage of the International Year of Quantum (IYQ), OPN talks to Silberhorn about integrated quantum devices, the biggest challenges facing the field today—and why initiatives like IYQ matter more than ever.

Can you tell us about your current research focus?

I’m based at the University of Paderborn and also work at the recently established Institute for Photonic Quantum Systems (PhoQS). Our mission is to develop photonic quantum technologies using a holistic approach—from theory and device design to fabrication and system integration—all under one roof. That’s something quite unique.

My current focus is on developing photonic integrated circuits, not using the typical χ(3) (chi-three) nonlinear materials, but instead using χ(2) (chi-two) materials. These materials offer additional functionalities, most importantly electro-optic modulation, which enables fast and efficient control of light. A well-known χ(2) material is lithium niobate, which is widely used for high-speed optical switches in telecommunications. However, when it comes to integrated photonics, lithium niobate processing is still relatively new—there haven't been large-scale foundries for long. What we’re doing in Paderborn is tailoring lithium niobate-based devices specifically for quantum applications, such as quantum circuits and quantum computing components.

Another major focus of my group is on system scaling. We're working on building photonic quantum simulators and processors. Right now, we’re particularly excited about constructing an academic-scale Gaussian boson sampling system. Back in 2017, we proposed a method for implementing this kind of sampling with integrated photonics, which is a promising pathway to demonstrating quantum advantage in photonic systems. It’s also a great example of how we can combine cutting-edge hardware development with highly theoretical concepts to build scalable quantum systems.

We're also investing heavily in using the frequency domain for information encoding. This is a particularly exciting area. Frequency encoding—such as what's used in WDM (wavelength-division multiplexing)—has revolutionized classical telecommunications. We're now exploring how similar principles can be applied in quantum photonics. For example, we’re developing devices that can operate on frequency superposition states or interfere across different frequency modes, with the goal of enabling more scalable quantum architectures.

How do you see the role of photonics in advancing quantum computing and quantum communication?

That’s a very good question. In Europe, we often talk about the four main pillars of quantum technologies: quantum communication, quantum simulation, quantum computation, and quantum metrology. In many areas, researchers tend to work within just one of those pillars.

What makes photonics unique is that the same photonic devices and circuits—sometimes even identical hardware—can be used across multiple pillars. That’s quite special. For example, in quantum communication, if you're trying to transmit quantum information over long distances, it’s hard to imagine doing that without light. Photonics is the natural platform for this, since light is already the primary information carrier.

Now, when you think about where the field is heading—toward multi-node quantum networks involving many users—you quickly get into the realm of distributed quantum computing. Again, photonics provides a natural solution, because if your system is already processing information with light, it's intuitive to use the same underlying hardware for both communication and computation.

If you're trying to transmit quantum information over long distances, it’s hard to imagine doing that without light. Photonics is the natural platform for this, since light is already the primary information carrier.

When it comes to quantum simulation and computation, we face different challenges. One advantage is that we can build on existing photonic integration technologies—PICs—and there are already semiconductor fabs that support this. However, these need to be customized for quantum applications, and that tailoring can be quite specific. The good news is that photonics does seem scalable, which is a big plus.

From my perspective, though, the algorithms we have today aren’t yet fully adapted to photonic platforms. In conventional computing, information is typically carried by matter—such as atoms or electrons—while processing is done using light. In photonics, we do the reverse: the information is carried by light, and the processing is done using matter—namely, the chip. This inversion presents unique challenges, particularly when it comes to modeling errors and ensuring fault tolerance. It has advantages and disadvantages, and we’re still exploring both.

That said, I’m confident that photonics has a strong role to play in quantum computing and simulation. Whether or not photonics will underpin the most universal quantum computer remains to be seen—it’s a difficult path. But I do believe we can build small-scale, and eventually medium-scale, quantum processors that serve very real and useful purposes. Even if they don’t become the core of a universal machine, these smaller processing units will make a meaningful contribution to the quantum ecosystem.

How do you see the role of photonics in advancing quantum computing and quantum communication?

When it comes to photonics in quantum technologies, the biggest issue is loss—loss, loss, and more loss. If you encode information in photons and then lose those photons, the information is lost as well. Of course, we can try to build redundancy through clever encoding schemes, but that goes somewhat against the spirit of quantum information. So, one of the major technical challenges is that all of our devices need to be extremely low-loss.

In many standard photonic applications, a 5–10% loss might be acceptable. But in quantum systems, we’re aiming for losses of less than 1%, or ideally even below one part per thousand. That’s incredibly demanding and requires highly specialized fabrication and design.

Scaling is another significant challenge. While we do have integrated photonic circuits and can scale to a certain degree, building large, complex systems with many channels or modes remains difficult. That’s why we’re exploring different encoding strategies—like using frequency, time-bin, and spatial modes—to increase capacity and functionality without making systems physically unwieldy. Expanding and controlling larger systems in a scalable and reliable way is a major hurdle.

Then there’s the challenge of quantum state preparation. We need to generate not just single photons, but also bring them together in controlled ways to create entangled states. Producing multiphoton entangled states is extremely challenging because photons don’t naturally interact with each other. We have techniques for enabling interactions through measurement-based methods, but these approaches require substantial resources and can be inefficient.

So, overall, we face challenges across multiple dimensions: reducing loss, achieving scalability, preparing complex quantum states, and finding efficient methods to create interactions. And none of this can happen in isolation—we need advances on both the technological and theoretical sides. We need better hardware, but we also need new algorithms that are well suited to photonic platforms. Only by combining those efforts can we push the field forward.

This year we're celebrating the International Year of Quantum (IYQ). How do you think an initiative like this shapes the public perception of quantum science?

It should absolutely be seen as a good thing. We need to create a positive and inspiring atmosphere around quantum science. And honestly, many people are genuinely excited about quantum, which is wonderful to see. Quantum physics and quantum optics are fascinating areas. They’re counterintuitive, which makes them intriguing, but also means we have to do a good job of explaining them to the public. We need to communicate that this field isn’t just about abstract or theoretical ideas—it’s about real, measurable science.

It’s amazing that we can now control individual particles, even individual photons. That level of precision and manipulation is extraordinary. At the same time, we should be clear that quantum science isn’t just mysterious or undefined. People often hear that “anything can happen” in quantum mechanics, but that’s not the case. Quantum physics is a rigorous, well-defined scientific framework. Yes, it poses deep philosophical questions, but it’s not arbitrary—it follows strict principles.

So I believe it’s very important to convey all of this, especially to the younger generation—children, students, etc. We have a chance to offer them a new and inspiring perspective on physics and science in general. The IYQ brings attention, sparks curiosity, and opens the door for more people to understand and get excited about what quantum science is really about.

Do you think the IYQ will have a lasting impact on the quantum field?

That’s always a difficult question. I believe it does make a difference—but we probably won’t see the effects right away. For example, if you inspire a 12-year-old this year, and that child chooses to study physics five years later, that impact is real—but it’s not something you can immediately measure. The goal of IYQ is to raise awareness and spark curiosity about quantum science, which still isn’t widely understood. If someone hears about it and thinks, “That’s cool, I’ve heard of that,” then we’ve planted a seed. And those seeds matter, even if the results take time to emerge.

So when you ask whether it will have a lasting effect—yes, I think it will. But like many long-term impacts, it’s difficult to quantify. These kinds of outreach efforts are extremely important.

And especially when we talk about quantum technologies and quantum engineering, we need highly educated people. Right now, we’re already facing a shortage of skilled professionals in this area. So in that sense, yes—I believe IYQ can contribute meaningfully to the future of the field. But we may not be able to point to this exact year and say, “That’s when everything changed.” The effect is likely to be subtle, cumulative, and long-term.

What do you think about the relationship between academia, industry, and policymakers in the quantum field? Are we successfully bridging the gap between those groups—and is IYQ helping in that effort?

Quantum is a particularly fascinating and unique field. We often speak of the “second quantum revolution,” which implies a kind of transformation—but unlike traditional technology development, this revolution doesn't evolve in a steady, linear way. It’s not something that progresses predictably, step by step.

That’s part of the challenge. We're expecting groundbreaking, even disruptive advances. But for industry—and I think sometimes also for policymakers—it can be difficult to understand how to plan for that kind of progress. Of course, we have roadmaps, but it's not like you can just invest a little, see some growth, invest a bit more, and continue in that fashion. Quantum technologies don’t always follow that incremental path, because we’re building entirely new capabilities.

That’s why I believe this year, the International Year of Quantum, is so important. It provides an opportunity to raise awareness and reach policymakers. There’s growing recognition that quantum has enormous long-term potential, and it’s encouraging to see that politicians are paying attention. Ultimately, we’re moving toward a future where everything becomes smaller, and quantum effects will become increasingly relevant in everyday technologies—so it’s essential to understand and support that transformation now, even if the short-term gains aren’t immediately visible.

Industry faces similar challenges. The landscape also differs by region. In the U.S., there’s a strong startup culture, and a willingness to take financial risks. That’s incredibly helpful in quantum, because developing these technologies requires significant investment and a high tolerance for uncertainty. In Europe we tend to be more cautious about investing in early-stage technologies. There’s less appetite for risk, which makes it harder to support long-term, high-impact innovation. And of course, we’ve all seen the pattern: initial hype, followed by a "valley of death," and then, hopefully, a resurgence. Right now, we’re either still on the peak of that hype or just beginning to descend into the difficult middle phase.

The key is to communicate clearly—especially with industry and policymakers—that while the potential of quantum is real, it may take time to fully realize. We need to manage expectations and build sustained support, even when results don’t come as quickly as people hope.

You mentioned that we might be entering the so-called "valley of death" phase for quantum technologies. Looking ahead, is there anything exciting on the horizon? 

In the case of quantum communication, we’re already seeing commercial systems in action, which is quite impressive. While they still face challenges, especially with scaling, they’re functional for small-scale use cases—which is great progress.

But I think the next big thing will be the emergence of real applications for quantum computers. There’s been a lot of effort in that area—many demonstrations and ongoing research to identify useful applications. The real excitement will come when we reach the point of having a full-fledged quantum computer that can perform tasks we can’t accomplish otherwise. And we’re starting to see early signs of that—applications are slowly emerging. That’s very promising.

What you’re asking touches on the intersection of engineering, industry, and science. From a scientific perspective, breakthroughs in complexity theory and quantum algorithms are fascinating—but they can be difficult to explain to people outside the field. On the applied side, excitement comes when quantum systems allow us to do something genuinely new.

As for the “next big milestone,” that really depends on how specifically you define it. It’s hard to pinpoint a single breakthrough, but we’re definitely seeing encouraging progress across different fronts.

Is there a specific project you're currently working on that you're particularly excited about?

Yes, definitely! As I mentioned earlier, we’re in the process of building our first larger photonic quantum system, and we’re currently benchmarking it. It’s an exciting phase because the system is still unpublished, so we’re exploring lots of new ideas and possibilities. We’re starting to generate data that’s already becoming challenging to process, which is a good sign—it means we’re entering a regime where the machine is doing something complex and interesting.

This gives us a unique opportunity to experiment and learn what such a system actually looks like in practice. We’re investigating how to design photonic quantum computers that may not be universal but can scale up significantly and perform useful tasks. That’s a very exciting direction for us right now.

In parallel, we’re also entering the era of miniaturization. We now have our first circuits with multiple functionalities integrated on a single chip. The next big step is to move beyond having individual components—like just a photon source or just a switching element—to creating complete systems on-chip. This includes sources, circuits, switches, and potentially even feed-forward mechanisms. It’s a really exciting time in the lab.

And for young people who are just entering—or thinking about entering—the field of quantum technology, what advice would you give them?

I would say: be curious and stay open-minded. Quantum technology is a rapidly evolving field, and there's no fixed path. It combines fundamental physics with engineering, computer science, and even materials science—so having an interdisciplinary mindset is a real advantage.

Also, don’t be intimidated by how complex it all sounds at first. Everyone feels that way when they start. What’s important is to dive in, ask questions, and be willing to learn across different areas. Right now, we're in a phase where there's room for new ideas and fresh perspectives, and young researchers can really make a difference.

Finally, I’d encourage students to get hands-on experience—whether in the lab, in simulations, or by joining collaborative research projects. Understanding how these systems behave in practice is just as valuable as knowing the theory behind them. This is a field where creativity, collaboration, and curiosity can take you far.