Quantum technology is on the cusp of revolutionizing our daily lives, impacting everything from communication and sensing to security and computing. At the forefront of this transformation are Joseph Lukens, Senior Director of Quantum Networking at Arizona State University, and Hsuan-Hao (Peach) Lu, a Research Scientist at Oak Ridge National Lab.
Lukens and Lu’s collaborative work in frequency-bin quantum information — recently highlighted in the flagship journal Optica — has gained increasing recognition over the last seven years and provides a novel approach for harnessing photon properties for quantum information processing.
In this Q&A, Lukens and Lu break down this technology in layman’s terms and discuss its potential to revolutionize everyday technology. Their progression from theoretical concepts to potential real-world applications exemplifies the kind of innovation and collaboration championed by the Quantum Collaborative.
How do you explain the topic of your paper, frequency bin quantum information processing to your friends and family? And why is this important in the quantum field?
Lukens: Photons, or particles of light, can travel over long distances, like in optical fiber – that’s how we communicate over the internet today. Because photons are very flexible particles, they offer many ways to transmit quantum information as well. One common example is polarization, the direction the photons oscillate — horizontal or vertical — which is encountered in our day-to-day lives with devices like polarized sunglasses or an LCD screen. You can use polarization for a quantum bit, where horizontal is zero and vertical is one. However, polarization isn’t natively suited to optical fiber and can get scrambled. In contrast, a feature that works really well in optical fiber is frequency or wavelength. In optical communications today, wavelength division multiplexing is common – sending light of different colors down the fiber. How these ideas have impacted quantum information is our focus in this review.
We put together an exciting team of expert coauthors, including Marco Liscidini from the University of Pavia, Italy, Alex Gaeta from Columbia University, and Andy Weiner from Purdue, who was both of our PhD advisors. This review was a chance to look back at what we’ve accomplished and speculate on what’s next. We tried to balance explaining the field, reviewing our progress, and proposing future directions.
Lu: I’ll echo Joe’s points and add two more. When Joe discussed encoding information in photons using optical frequency or wavelength, which are interchangeable, we can describe it even simpler – as colors. Imagine a photon in two colors at once – that’s a frequency qubit. If it can be in multiple colors, that’s a qudit—a high-dimensional version of qubit.
Compared to polarization, frequency encoding was niche for many years, but it’s gaining attention. Classical networks rely on wavelength multiplexing – multiple light streams in a fiber, each a different color. A future quantum network will likely integrate this approach, and thus how frequency encoding will be utilized in future quantum networks is an intriguing question. So, our review in Optica offers an introduction to this area and our vision, and discusses the challenges of this technology.
What might be the real-world applications? Which industries could it impact? What new pathways could it open?
Lukens: In our review, we pointed to a couple of areas where this could have a major impact. One is quantum interconnects, a way to connect quantum computers or systems in different locations quantum mechanically. The frequency degree of freedom is easy to transmit through optical fiber, so if quantum systems emit photons of different wavelengths, it should be possible to entangle them with quantum frequency processing (QFP) technology which Peach and I have been involved in. In the review, we put together concepts of how this could be done with current technology and a bit more work.
Another transformative application is quantum computing on a chip. Analogous to miniaturization in electronics, many of the experiments we do today with big tabletop optical systems can be shrunk to tiny chips that are both cheaper and offer better performance. With frequency encoding in particular, it’s easy to generate entangled quantum states on a chip using a microresonator.
I’m excited about building integrated photonic circuits that are compact and could be used for quantum computing as well as quantum networking. This is a second arena where I think frequency encoding has a unique contribution to the development of quantum information science.
Speaking of the review, you mentioned various challenges encountered. Can you walk me through some of the significant ones?
Lu: Sure. We like frequency because it’s stable in fiber transmission, but that stability also makes it hard to control. For example, polarization changes when you touch the fiber, but frequency doesn’t. In quantum information processing, you need to change the quantum state in a controlled way. That’s the challenge Joe tackled in his 2017 theory paper — a set of tools that can change frequency states reliably. Even now, with the QFP technology and other methods discussed in this review paper, it’s still relatively hard to manipulate frequency bins. As Joe mentioned, moving to chip-based systems could be a future solution for scaling to more complex systems.
Let’s take a look toward the future of this research. Could you share your thoughts on the short-term and long-term impacts?
Lukens: In the short-term, the field is expanding, with more researchers taking frequency bin encoding seriously, which is both exciting and challenging due to increased competition. This growth in the field is significant, showing that the work is being accepted more broadly.
At ASU, at the new Quantum Networking Lab, we’re working on a scalable quantum network testbed. So far, our frequency encoding experiments have been confined to a single lab. We have our optics table, we have all of our equipment, we do an experiment. What we haven’t done is a frequency encoding experiment between different locations. We aim to extend these experiments between different locations, which involves logistical and technological challenges like fiber connections and precise synchronization. We’re optimistic about entering a new phase where deployed quantum experiments utilize frequency bin capabilities for practical quantum networking.
Hsuan-Hao, your thoughts on the long-term – 50 years out into the future?
Lu: Joe’s giving me the tough questions! Long term, 50 years is hard to predict. No one knows exactly what the future quantum network will look like. Classical networks rely on wavelength multiplexing, so frequency bin quantum information should play a significant role in future quantum networks, either as a main carrier of information (for encoding) or in a supporting role (for multiplexing). The technology and knowledge we’re developing now will likely help in various ways in the future.
