As digital communication accelerates and cyber threats proceed to rise, researchers are working to develop safer ways to transmit information. Some of the promising approaches is quantum cryptography, which uses individual photons to generate encryption keys. A research team from the Faculty of Physics on the University of Warsaw has created and tested a brand new quantum key distribution (QKD) system inside existing city fiber networks. Their approach uses high-dimensional encoding and relies on a widely known optical phenomenon called the Talbot effect. The findings were published in Optica Quantum, Optica, and Physical Review Applied.
“Our research focuses on quantum key distribution (QKD) — a technology that uses single photons to ascertain a secure cryptographic key between two parties,” says Dr. Michał Karpiński, head of the Quantum Photonics Laboratory on the Faculty of Physics, University of Warsaw. “Traditionally, QKD employs so-called qubits — the only units of quantum information. While this method is already well tested, it doesn’t at all times meet the necessities of more demanding applications. That is why researchers are actually working on multidimensional encoding. As a substitute of qubits, which yield certainly one of two measurement outcomes, we use more complex quantum states that may tackle multiple values.”
On the lab, scientists study time-bin superpositions of photons. In these states, a photon isn’t simply detected as arriving “early” or “late,” but exists as a mixture of each possibilities. The precise detection time is random, and data is encoded within 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’re concerned with cases with more time bins, starting from two to 4 or much 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 identical effect occurs not only in space but additionally in time, provided that an everyday train of sunshine pulses propagates in a dispersive medium reminiscent of an optical fiber,” explains Maciej Ogrodnik, a PhD student on the Faculty of Physics, UW.
By applying this effect to sequences of sunshine 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 in which these pulses overlap and interfere is dependent upon their phase, allowing different quantum states to be identified and measured.
“Because of the space-time analogy in optics, we are able to apply the Talbot effect to short light pulses, including single photons — thereby gaining latest capabilities for analyzing and processing quantum states. In our case, a sequence of sunshine pulses acts like a diffraction grating and might ‘self-reconstruct’ in time under dispersion after traveling far in an optical fiber. Furthermore, the best way pulses interfere is dependent upon their phase, which allows us to detect various kinds of superpositions.”
Simpler Quantum Key Distribution System Design
The researchers built an experimental QKD system able to operating in 4 dimensions.
“Importantly, your complete setup is built using commercially available components. The important thing trick is that the system requires only a single photon detector to register superpositions of many pulses — as an alternative of a fancy network of interferometers,” says Adam Widomski, a PhD student on the Faculty of Physics, UW.
This design significantly lowers each cost and technical complexity. It also removes the necessity for frequent and precise calibration of the receiver, which is a serious 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 variety 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 downside is comparatively high measurement error rates. Nevertheless, these don’t prevent QKD, as we showed in collaboration with researchers working on the speculation of quantum cryptography. Moreover, we don’t must rebuild the setup for various dimensions of superpositions — we are able to detect 2D and 4D superpositions without changing hardware or stabilizing the receiver. It is a huge advantage in comparison with earlier methods,” adds Widomski.
Real-World Testing and Security Improvements
The system was tested each in laboratory fiber setups and across the University of Warsaw’s existing fiber network over several kilometers.
“Because of the brand new method using the temporal Talbot effect, we successfully demonstrated QKD with two- and four-dimensional encoding, using the identical transmitter and receiver. Despite errors inherent to the easy experimental approach, our results confirm the upper 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 make sure the robustness of their approach, the team collaborated with experts in Italy and Germany who concentrate on QKD security evaluation.
“A more in-depth evaluation shows that the usual description of many QKD protocols is incomplete, which attackers could exploit. Unfortunately, our method shares this vulnerability. We took part in efforts to unravel this issue. Our collaborators found that a certain modification of the receiver allows for collecting more data, thus eliminating the vulnerability. The safety proof of the brand 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 brand new communication method, the project strengthened expertise in advanced quantum photonics on 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 on the National Laboratory for Photonics and Quantum Technologies (NLPQT) on the Faculty of Physics, University of Warsaw.