As digital communications accelerate and cyber threats continue to grow, researchers are working to develop more secure ways to transmit information. One of the most promising approaches is quantum cryptography, which uses individual photons to generate encryption keys. A research team from the University of Warsaw’s Faculty of Physics has created and tested a new quantum key distribution (QKD) system within an existing urban fiber network. Their approach uses high-dimensional encoding and is based on a well-known optical phenomenon called the Talbot effect. The survey results are quantum optics, opticaland Physical review applied.
“Our research focuses on quantum key distribution (QKD), a technique that uses single photons to establish secure cryptographic keys between two parties,” said Dr. Michał Karpinski, head of the Quantum Photonics Laboratory at the Faculty of Physics at the University of Warsaw. “Traditionally, QKD uses the simplest unit of quantum information, the so-called qubit. Although this method is already well-tested, it does not always meet the requirements of more demanding applications. That is why researchers are now working on multidimensional coding. Instead of a qubit, which produces one of two measurement results, they use more complex quantum states that can take on multiple values.”
In the lab, scientists study the superposition of time bins of photons. In these states, photons are not detected simply as arriving “early” or “late”, but exist as a combination of both possibilities. The exact detection time is random and the information is encoded in the phase relationship between these light pulses.
“Until now, it has been possible to efficiently detect the superposition (before and after) of two pulses. We are interested in going a step further and having more time bins, ranging from 2 to 4 or even more,” added Dr. Karpiński.
Using the Talbot Effect in Quantum Communication
The research team focused on the Talbot effect, a classic optical phenomenon first described by Henry Fox Talbot in 1836.
“When light passes through a diffraction grating, its image repeats itself at regular intervals, as if it were ‘resurrected’ at a certain distance. Interestingly, when a regular train of light pulses propagates through a dispersive medium such as an optical fiber, the same effect occurs not only in space but also in time,” explains Maciej Ogrodnik, a doctoral student in the Department of Physics at Washington State.
By applying this effect to sequences of light pulses containing single photons, the researchers created a system that can effectively reconstruct the signal itself over time as it passes through an optical fiber. How 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 containing single photons, which gives us new abilities to analyze and manipulate quantum states. In our case, a series of light pulses acts like a diffraction grating and, after traveling some distance in an optical fiber, can ‘self-reconstruct’ in time under dispersion.” Furthermore, the way the pulses interfere depends on their phase, so different types of superpositions can be detected. ”
Designing a simpler quantum key distribution system
The researchers built an experimental QKD system that can operate in four dimensions.
“Importantly, the entire setup is built using off-the-shelf components. The key trick is that the system requires only a single photon detector to record the superposition of many pulses, rather than a complex network of interferometers,” said Adam Widamski, a doctoral student in the Washington State Department of Physics.
This design significantly reduces both cost and technical complexity. It also eliminates the need for frequent and accurate calibration of the receiver, a major challenge with traditional systems.
“Traditionally, to detect the phase difference between pulses, we used a multiple interferometer setup, like a tree in which the pulses are split and delayed. Unfortunately, such systems are inefficient, because some measurements are useless. Efficiency decreases with the number of pulses, so the receiver requires precise calibration and stabilization,” explains Ogrodnik.
“The advantage of our method is that it is efficient, as every photon detection event is useful. The disadvantage is that the measurement error rate is relatively high. However, as we have shown in collaboration with researchers working on the theory of quantum cryptography, QKD is not hindered. Moreover, different There is no need to rebuild the dimensional superposition setup; we can detect 2D and 4D superpositions without changing the hardware or stabilizing the receiver, which is a huge advantage compared to previous methods,” adds Widomski.
Real-world testing and security improvements
The system was tested both in a laboratory fiber setup and across the University of Warsaw’s existing fiber network spanning several kilometers.
“Thanks to a new method using the temporal Talbot effect, we have successfully demonstrated QKD with two- and four-dimensional encoding using the same transmitter and receiver. Despite the errors inherent in simple experimental approaches, our results confirm the higher information efficiency of the system with high-dimensional encoding,” Widomski said.
Quantum key distribution has been evaluated as provable security under certain assumptions. To ensure the robustness of the approach, the team collaborated with Italian and German experts specializing in QKD security analysis.
“Detailed analysis revealed that the standard descriptions of many QKD protocols are incomplete and could be exploited by attackers. Unfortunately, this vulnerability also exists in our method. We participated in an effort to resolve this issue. Our collaborators discovered that making certain changes to the receiver allowed it to collect more data, thereby eliminating the vulnerability. Proof of the new protocol’s security is based on a Physical Review Applied , and our latest paper discusses its application to our experiments,” Ogrodnik said.
Advances in quantum photonics research
The project not only demonstrated a new communication method, but also strengthened the University of Warsaw’s expertise in advanced quantum photonics.
This study was carried out under the international program QuantERA on Quantum Technologies, coordinated by the National Science Center (NCN, Poland). The researchers also used the facilities of the National Laboratory for Photon and Quantum Technologies (NLPQT) at the Faculty of Physics at the University of Warsaw.
