A 200-year-old light trick just transformed quantum encryption

"Our research focuses on quantum key distribution (QKD) -- a technology that uses single photons to establish a secure cryptographic key between two parties," says Dr. Michał Karpiński, head of the Quantum Photonics Laboratory at the Faculty of Physics, University of Warsaw. "Traditionally, QKD employs so-called qubits -- the simplest units of quantum information. While this method is already well tested, it does not always meet the requirements of more demanding applications. That's why researchers are now working on multidimensional encoding. Instead of qubits, which yield one of two measurement outcomes, we use more complex quantum states that can take on multiple values."

At the lab, scientists study time-bin superpositions of photons. In these states, a photon is not simply detected as arriving "early" or "late," but exists as a combination of both possibilities. The exact detection time is random, and information is encoded in the phase relationship between these light pulses.

"Until now, efficient detection of superpositions of two pulses -- earlier and later -- was possible. We went a step further: we are interested in cases with more time bins, ranging from two to four or even more," adds Dr. Karpiński.

Using the Talbot Effect in Quantum Communication

The team turned to the Talbot effect, a classical optics phenomenon first described in 1836 by Henry Fox Talbot.

"When light passes through a diffraction grating, its image repeats itself at regular intervals -- as if it 'revives' at a certain distance. Interestingly, the same effect occurs not only in space but also in time, provided that a regular train of light pulses propagates in a dispersive medium such as an optical fiber," explains Maciej Ogrodnik, a PhD student at the Faculty of Physics, UW.

By applying this effect to sequences of light pulses, including single photons, the researchers created a system where signals can effectively reconstruct themselves over time as they travel through optical fiber. The way these pulses overlap and interfere depends on their phase, allowing different quantum states to be identified and measured.

"Thanks to the space-time analogy in optics, we can apply the Talbot effect to short light pulses, including single photons -- thereby gaining new capabilities for analyzing and processing quantum states. In our case, a sequence of light pulses acts like a diffraction grating and can 'self-reconstruct' in time under dispersion after traveling some distance in an optical fiber. Moreover, the way pulses interfere depends on their phase, which allows us to detect different types of superpositions."

Simpler Quantum Key Distribution System Design

The researchers built an experimental QKD system capable of operating in four dimensions.

"Importantly, the entire setup is built using commercially available components. The key trick is that the system requires only a single photon detector to register superpositions of many pulses -- instead of a complex network of interferometers," says Adam Widomski, a PhD student at the Faculty of Physics, UW.

This design significantly lowers both cost and technical complexity. It also removes the need for frequent and precise calibration of the receiver, which is a major challenge in traditional systems.

"Traditionally, to detect phase differences between pulses, we use a multi-interferometer setup -- something like a tree, where pulses are split and delayed. Unfortunately, such systems are inefficient, since some measurement outcomes are useless. The efficiency drops with the number of pulses, and the receiver requires precise calibration and stabilization," explains Ogrodnik.

"The advantage of our method is its high efficiency, as all photon detection events are useful. The drawback is relatively high measurement error rates. However, these do not prevent QKD, as we showed in collaboration with researchers working on the theory of quantum cryptography. Furthermore, we do not need to rebuild the setup for different dimensions of superpositions -- we can detect 2D and 4D superpositions without changing hardware or stabilizing the receiver. This is a huge advantage compared to earlier methods," adds Widomski.

Real-World Testing and Security Improvements

The system was tested both in laboratory fiber setups and across the University of Warsaw's existing fiber network over several kilometers.

"Thanks to the new method using the temporal Talbot effect, we successfully demonstrated QKD with two- and four-dimensional encoding, using the same transmitter and receiver. Despite errors inherent to the simple experimental approach, our results confirm the higher information efficiency of the system resulting from high-dimensional encoding," says Widomski.

Quantum key distribution is valued for its provable security under certain assumptions. To ensure the robustness of their approach, the team collaborated with experts in Italy and Germany who specialize in QKD security analysis.

"A closer analysis shows that the standard description of many QKD protocols is incomplete, which attackers could exploit. Unfortunately, our method shares this vulnerability. We took part in efforts to solve this issue. Our collaborators found that a certain modification of the receiver allows for collecting more data, thus eliminating the vulnerability. The security proof of the new protocol was published in Physical Review Applied, and in our latest paper we discuss its application to our experiment," says Ogrodnik.

Advancing Quantum Photonics Research

Beyond demonstrating a new communication method, the project strengthened expertise in advanced quantum photonics at the University of Warsaw.

The work was carried out under the QuantERA international program on quantum technologies, coordinated by the National Science Centre (NCN, Poland). Researchers also used facilities at the National Laboratory for Photonics and Quantum Technologies (NLPQT) at the Faculty of Physics, University of Warsaw.