Researchers at TU Berlin are developing quantum dot molecules, two accumulations of atoms in a semiconductor that emit single light particles and are able to store information. This will enable the creation of quantum repeaters, which form the basis for the quantum Internet of the future. The quantum Internet would be absolutely tap-proof and could avert the threat to our data currently posed by high-performance quantum computers. The Chair of Optoelectronics and Quantum Devices at the Institute of Solid-State Physics at TU Berlin will receive 1.8 million euros funding until 2024 as part of the Federal Ministry of Education and Research's Quantenrepeater (QR.X) joint research project.
In a just few years from now, powerful quantum computers capable of easily cracking our current encryption systems could create a worst-case scenario for data protection. This would not only affect sensitive data exchange, such as banking or messenger services. Many other secrets, still well protected today, would also become open books. In response to this threat, researchers worldwide are working flat-out on developing quantum cryptography, a science that exploits quantum mechanical properties to achieve inherent security. If they succeed, we could all benefit from a completely new technical infrastructure for the Internet - the quantum network. As this development is of such importance to us all, we would like to present its basic principles here in terms which a non-expert can also understand. However, we should not forget the words of Nobel Prize for Physics laureate Richard Feynman: "I think I can safely say that nobody understands quantum mechanics." The rules of quantum theory are extremely well described from a mathematical perspective, but often contradict our everyday experience. Anyone embarking on this adventure will gain an insight into how strange nature behaves at its very smallest level.
The encryption principle of quantum cryptography originally had nothing to do with physics and has been around for more than a hundred years. The information contained in a secret message encoded in zeros and ones (the "bits") is overwritten with a random key consisting of zeros and ones: Each "one" flips one bit (0 → 1 or 1 → 0), while each "zero" in the key leaves one bit of information unchanged. If the random key is chosen to be the same length as the message itself, the message is hidden by the principle of chance - it can never be deciphered without the key. However, the task now is to create large random keys and exchange them between the sender and receiver of a message so that they are tap-proof. The downside of this method is that each key can only be used once, as hackers would otherwise be able to guess it over time.
This is where quantum physics comes into play with two helpful principles. First, at the level of the smallest objects, such as atoms, electrons or light particles (photons), there are certain things that occur purely by chance, without direct cause. This makes it possible to generate the random key necessary for the procedure described above. Additionally, nature seems to allow strange "superposition states" for quantum objects, where two or more objects can be in the same superposition state regardless of their distance. "These superposition states always occur when nature should actually choose one of several alternatives, but is not actually forced to do so by a measurement," explains Stephan Reitzenstein, head of the Chair of Optoelectronics and Quantum Devices at TU Berlin. And it is this phenomenon which is ultimately responsible for tap-proof security in quantum cryptography.
In Reitzenstein's research group, such superposition states are generated with the help of so-called quantum dots. These are clusters of about one thousand atoms located within a semiconductor device. The researchers use a laser to raise two electrons in such a quantum dot to a higher energy level. After a time, their energy levels fall back - first to an intermediate level and then to their original energy level. During this process, the electrons each emit a small light wave - a photon - as a portion of energy. "The trick now is that nature has two intermediate levels and thus two different emission paths in our quantum dots," says Reitzenstein. These two paths differ in one important point. If the two electrons take one path, the light waves they emit both oscillate in a horizontal direction. If they take the other path, the photons oscillate in a vertical plane.
"Nobody can predict which path the electrons will take," explains Reitzenstein. Consequently, it is also undetermined which "polarization direction" - namely horizontal or vertical - the two emitted photons have. "Crucial in this is the fact that the polarization direction is not only unknown, it is still undetermined," adds Reitzenstein. "As incredible as this may sound it is only a later measurement that determines which path the two electrons have actually taken." As long as this does not happen, the two photons are in a common superposition state and are connected to each other as if by an invisible band, no matter how far away they are - they are "entangled," as physicists say. If you register horizontal polarization for one of the photons in a lab, you know immediately that the other photon also has horizontal polarization. This would also be the case if one of the photons were in Berlin and the other had been sent via a fiber optic able to Potsdam. A vertical polarization measurement in turn means a vertical direction of oscillation in the other proton as well.
By sending such entangled photons, the sender and receiver can now exchange a random but exactly identical numerical key. To do this, the transmitter and receiver simply define the "horizontal" measurement result as 0 and the "vertical" measurement result as 1. The procedure is tap-proof as anyone listening in on the line would be detected immediately. In order to identify the key, the person listening in would have to take a measurement. However, doing so would already determine the state of a photon before it reaches the receiver. A statistical evaluation of some of the measurement results, which the sender and receiver communicate with each other via the Internet, for example, would reveal the resulting errors. Such a security check must therefore always be carried out - the measurement results used for this purpose do not, of course, become part of the secret key.
"One major problem however is that over longer distances the photons are lost in the fiber optic cables," explains Reitzenstein. Over a distance of 100 kilometers, only one in a hundred photons would reach its target. "This is why we have to incorporate quantum repeaters in transmission lines for networks of over 100 kilometers." They don't amplify the light signals, as this would be tantamount to measuring them. Instead, their task is to generate entangled photon pairs and distribute them toward the two ends of the transmission link. Placing a quantum repeater in the middle would halve the distance that the photons have to travel.
Whole chains of quantum repeaters could be deployed over very long distances of several hundred kilometers. On the one hand, quantum dots inside these repeaters would generate entangled photons and send them off in both directions. Special measurements would also take place in the quantum repeaters with photons of neighboring stations measured together and thus also entangled in one operation. Performing these operations strategically makes it possible to build up a whole "entanglement chain" between the two ends of a transmission line. Anyone seeking to listen in, as it were, would be reliably detected by matching a few measurement results at the two ends of the line. As such it would not be necessary to specifically protect the repeaters against access by unauthorized persons. "They would be automatically protected by the principles of quantum physics - including against an attack using a quantum computer," Reitzenstein stresses.
However, as delays can occur when many repeaters are used, it is necessary that both the repeaters and the receiving stations for the photons at the ends of the link have caches for the superposition states of the photons. This is where Reitzenstein's research group's expertise in the area of quantum dot molecules comes into play. “Each quantum dot is a cluster of some thousand atoms. A quantum dot molecule simply consists of two neighboring quantum dots at a distance of about five to ten billionths of a meter from each other," Reitzenstein explains. Through their proximity, the quantum dots influence each other and form common energy levels, similar to when two atoms together result in a molecule with new electronic properties. This is precisely what is desired, because, as for the production of entangled photons, matching energy levels are also necessary for storing quantum information. When excited with a laser or electrically, these quantum dot molecules can now produce entangled photons as well as store the superposition states of light particles. The quantum dots are embedded in semi-conductor elements, which provide the necessary surface structures for the process.
The aim of the chair's work within Projekt QR.X is to equip the first quantum repeater networks using their technology. The researchers are now able to integrate their components directly into fiber optic cables. This will save both space and costs in any future quantum networks. These components are produced using a unique lithography system. Not only can this system inscribe the necessary structures into the semiconductor material using an electron beam and an etching process, it can also detect quantum dots using a light detector. This significantly increases efficiency in the production of the components. The process of forming quantum dots is self-organized, meaning that the quantum dots distribute themselves in a completely random manner in the semiconductor material. Until now, it was purely a matter of chance whether the structures introduced by the researchers located a quantum dot or even a quantum dot molecule. The new lithography system enables them to detect the quantum dots and introduce them into the semiconductor structures with precision. "This process means we can increase the efficiency of photons from approximately one percent to close to 100 percent. This is essential to achieve high data transfer rates in the quantum Internet of the future," Reizenstein concludes.