As digital communication accelerates and cyber threats increase, researchers are working to develop more secure ways to transmit information. One of the most promising methods is quantum cryptography, which uses individual photons to generate encryption keys. A team of researchers from the Department of Physics at the University of Warsaw has developed and tested a new quantum key distribution (QKD) system within an existing city network. Their method uses advanced coding and is based on a well-known optical phenomenon called the Talbot effect. The findings were published in Quantum Optics, SERVICEand Physical Examination Used.
“Our research focuses on quantum key distribution (QKD) – a technology that uses a single photon to create a secure cryptographic key between two parties,” says Dr. Michał Karpiński, head of the Quantum Photonics Laboratory at the Department of Physics, University of Warsaw. “Typically, QKD uses so-called qubits – the simplest units of quantum information. Although this method has been tested well, it does not always meet the requirements of very demanding applications. That is why researchers are already working on a method of combining multiple variables. Instead of qubits, which provide one of the two results of a measurement that can take many complex results, we can use many complex data.
In the laboratory, scientists study time-bin superpositions of photons. In these states, the photon is not only found to arrive “early” or “late,” but exists as a combination of both possibilities. The exact time of detection is random, and the information is embedded in the phase relationship between these light waves.
“Until now, to successfully detect the superpositions of two pulses – before and after – was possible. We went one step further: we are interested in cases with more time, from two to four or more,” added Dr. Karpiński.
Applying the Talbot Effect to Quantum Communication
The team turned to the Talbot effect, an ancient phenomenon in optics first described in 1836 by Henry Fox Talbot.
“When light passes through a diffraction grating, its image repeats itself at times – as if it ‘restores’ in a certain area. It is interesting that the same effect occurs not only in space but also in time, as long as a regular train of light waves propagates in a dispersed area like an optical fiber,” explains Maciej Ogrodnik, PhD student of PhD.
By applying this effect to a sequence of light pulses, including a single photon, the researchers have developed a method by which signals can be efficiently organized over time in an optical fiber. The way these pulses combine and interfere depends on their phase, allowing different quantum states to be selected and measured.
“Thanks to the simulation of space-time in optics, we can apply the Talbot effect to short pulses of light, including a single photon – in this way we get a new ability to analyze and deal with quantum situations. In our case, the sequence of light pulses acts like a diffraction grating and can ‘reconstruct itself’ as time passes after scattering. their part, which allows us to see different types of superpositions.”
Design of a Simple Quantum Key Distribution System
Researchers have developed a QKD test system capable of working in four dimensions.
“Importantly, the entire configuration is built using commercially available components. The key trick is that the system needs only one photon detector to register the superpositions of multiple pulses – instead of a complex network of interferometers,” says Adam Widomski, PhD student in the Department of Physics, UW.
This design greatly reduces cost and technical complexity. It also removes the need for frequent and accurate calibration of the receiver, which is a major challenge in traditional systems.
“Typically, to detect the phase difference between the pulses, we use a setup of multiple interferometers – something like a tree, where the pulses are separated and delayed. Unfortunately, such methods do not work well, since some measurement results are not useful. The efficiency decreases with the number of pulses, and the receiver needs to measure well and stabilize,” explains Ogrodnik.
“The advantage of our method is its high performance, since all photon detection events are useful. The infection is almost a high degree of error rates. However, these do not prevent QKD, as we have shown in collaboration with researchers working in the theory of quantum cryptography. In addition, we do not need to rebuild the setup for different measurements of superpositions — we can detect 2D or 4D hardware without change 4D hardware. This is a huge advantage compared to earlier methods,” added Widomski.
Real World Testing and Security Updates
The system was tested both on laboratory fibers and across the existing fiber network of the University of Warsaw over several kilometers.
“Thanks to the new method of using the temporary effect of Talbot, we have successfully demonstrated QKD with two- and four-dimensional coding, using a transmitter and a receiver. Despite the errors related to the simple test method, our results confirm the success of high-level information of the system caused by high-level encoding,” says Widomski.
Quantum key distribution is considered important for its security which can be seen under certain assumptions. To ensure the robustness of their approach, the team collaborated with Italian and German experts specializing in QKD security testing.
“A closer examination shows that the common definition of many QKD protocols is incomplete, which attackers can use. Unfortunately, our method shares this vulnerability. We participated in the effort to solve this problem. Our colleagues found that a certain modification of the host allows to collect more data, thus eliminating the vulnerability. Evidence of the security of the latest application published in our App of Physical published in our Physical App in our test,” says Ogrodnik.
Advances in Quantum Photonics Research
In addition to demonstrating a new communication method, the project has strengthened the quantum photonics expertise developed at the University of Warsaw.
The work was carried out under the QuantERA international program on quantum technology, organized by the National Science Center (NCN, Poland). The researchers also used equipment at the National Laboratory for Photonics and Quantum Technologies (NLPQT) at the Department of Physics, University of Warsaw.
#200yearold #trick #revolutionized #quantum #encryption