Further progress is documented by two interesting projects, around which the Czechs are also involved. The first of them concerns quantum entanglement of photons at CTU. The second is related to the so-called quantum teleportation, which was implemented by the operator Deutsche Telekom (T-Mobile), or rather its research section T-Labs, led by a Czech. Both technological demonstrations may in the future lead to the construction of a quantum internet, when quantum computers will be able to communicate with each other.
Quantum entanglement at CTU
A number of difficult-to-explain things are happening at the Faculty of Electrical Engineering at CTU. For example, mysterious dwarfs have been appearing there for decades. When descending into the basement, it starts to get dark. On the first underground floor is the Nanolab laboratory focused on chips and semiconductors, which was modernized a few years ago, including a room where dust does not penetrate. And in the dungeon one floor below, a quantum laboratory was recently put into operation.
This lab is part of the broader CZQCI project, the aforementioned national backbone quantum network built by the CyberSecurity Hub organization. This network connects several cities and in the future (if the state releases money) will be connected to the pan-European EuroQCI network.
The network is used for the so-called quantum key distribution (QKD). We explained the principle here and described the CZQCI network itself here. In short, two devices (Alice and Bob) exchange a symmetric key for encrypted data running on the traditional internet via a quantum channel over an optical network. The state of a photon is used. Such systems cannot be eavesdropped on, or rather, attempts to do so will be detected.
The QKD backbone network is intended, among other things, to serve as a space for research, development and preparation for the future. That is why the aforementioned laboratory was created at the FEE CTU, where, in addition to students and academics, other interested parties are invited. They can test the practical use of a quantum key and communication.
The laboratory has Alice and Bob-type devices from two manufacturers, namely HEQA Security and Luxquanta. Today, we can say that these are already “common” devices and a developing business sector, where several commercial suppliers compete. For example, the national backbone network runs on Toshiba elements and CETIN has chosen the European ID Quantique (IonQ).
However, the lab also includes three significantly more exotic “boxes”, which were supplied by the Austrian company Zerothird (formerly Quantum Industries). These devices (our gallery) are a practical example of how quickly the field of quantum communication is developing. It is possible to place technologies using quantum entanglement of photons without any problems in ordinary server cabinets (racks). Zerothird’s work is thus, among other things, the materialization of the 2022 Nobel Prize in Physics, which was awarded to Alain Aspect, John Clauser and Anton Zeilinger.
Zerothird technology is used for quantum key distribution, but using entangled photon pairs. While conventional QKD systems often use the BB84 protocol, the Austrian product is based on the more advanced BBM92 protocol. The system consists of a source of entangled photons, a receiver (Alice and Bob), and a classical communication channel for synchronization and final key calculation.
The source contains a crystal into which a powerful laser is shone, which causes a photon to split into two entangled photons with lower energy. These photons are sent to Alice and Bob. Until they are measured, their state is random.
Alice and Bob measure the polarization of the incoming photons, choosing a measurement basis for each individual photon at random. If they choose the same basis, the results will be correlated. If they choose different bases, the results will be random and uncorrelated. Alice and Bob do not tell each other what they measured, but only in which basis they measured. Results where the bases did not agree are discarded. Where the same base is hit, an identical sequence of bits remains, the so-called Sifted Key. Zerothird then takes a randomly selected part of the Sifted Key and publicly compares it, thereby determining the error rate (QBER – Quantum Bit Error Rate). Error correction is solved through algorithms.
Quantum teleportation in a normal environment
The German operator Deutsche Telekom (DT), which owns the Czech T-Mobile, presented its tests of so-called quantum teleportation at the Mobile World Congress in Barcelona this year. These are in charge of the T-Labs section, largely rooted in Prague and led by a Czech. The operator has mastered quantum teleportation of information (transfer of the quantum state of particles) in real conditions of the city's data infrastructure.
DT used devices from the American company Qunnect, which it deployed in its data network in Berlin, which is approximately 30 kilometers long. The photons ran as part of normal operation, so normal internet data also flowed through the optical cables. You can see the device in our gallery from Barcelona.
This teleportation is also based on quantum entanglement. The device from Qunnect creates a pair of entangled photons. One of the photons remains at the start and the other is sent through optics to the final station. At the start, a third photon (the information carrier) is then taken and a joint measurement is made with the first photon. This contact destroys the original information at the start. Thanks to the entangled bond, the state of the photon at the end changes immediately. So that the receiver knows
, how exactly this state is to be read and modified, the sender sends him the usual information about the measurement result via the classic Internet. The receiver modifies the particle according to these instructions and obtains an exact copy of the original information.
The process uses the so-called Bell measurement, which takes a photon carrying secret information and lets it collide with the first photon from the entangled pair on a special mirror. During this controlled collision, the two photons are mixed and their original separate identity (and the transmitted information) is definitively destroyed, but the detectors record one of four possible outcomes of their mutual relationship. Due to the instantaneous interplay of quantum entanglement, this destroyed information is “spilled” at the same moment into the second photon at the end of the optical cable kilometers away. Although it immediately rearranges itself into the desired state, the receiver can only read and decipher it correctly when the sender sends it the two-bit measurement result.