Making Hacking Futile – Quantum Cryptography

There is a plenty of really private information online. Generally, advanced encryption methods ensure that such information cannot be intercepted and read. Future high-performance quantum computers, however, might be able to quickly crack these secrets. For this reason, it is fortunate that quantum mechanical techniques not only provide brand-new, significantly quicker algorithms but also highly effective cryptography.

According to the jargon, quantum key distribution (QKD) is secure from attacks on the communication channel but not from attacks or manipulations of the devices themselves. Because of this, the gadgets might output a key that the manufacturer had previously stored and might have given to a hacker. With device-independent QKD, it's an entirely other story (abbreviated DIQKD). The device has no impact on the encryption protocol. Although this technology has been theoretically understood since the 1990s, it has only just been empirically tested by a multinational research team led by physicists Harald Weinfurter of Ludwig Maximilian University of Munich and Charles Lim of the National University of Singapore (NUS).

Quantum mechanical keys can be exchanged using a variety of techniques. Either entangled quantum systems are used, or light signals are sent from the transmitter to the receiver. In the current experiment, two rubidium atoms that were quantum mechanically entangled were used in two labs located 400 meters apart on the LMU campus. A 700-meter fiber optic cable that connects the two buildings runs underneath Geschwister Scholl Square in front of the main structure.

The researchers first excite each atom with a laser pulse in order to establish an entanglement. Then, each atom releases a photon when it returns spontaneously to its ground state. Due to the conservation of angular momentum, the atom's spin is intertwined with the polarization of the photon it emits. The two light particles travel over the fiber optic cable to a reception station, where atomic quantum memory entanglement is discovered by combining measurements of the photons.                                                                                                           

Alice and Bob, as the two parties are commonly referred to by cryptographers, measure the quantum states of their respective atoms in order to exchange a key. This is carried out in each instance at random in two or four directions. The measurement outcomes are identical due to entanglement if the directions match, and this can be utilized to create a secret key. It is possible to assess a so-called Bell inequality using the other measurement data. These inequalities were initially created by physicist John Stewart Bell to see if nature could be explained using hidden variables.

According to Weinfurter, the test is used in DIQKD "particularly to check that there are no manipulations at the devices, that is, for example, that secret measurement results have not been kept in the devices beforehand."

The implemented protocol, which was created by researchers at NUS, differs from earlier methods in that it employs two measurement settings for key generation instead of just one. According to Charles Lim, "by introducing the additional setting for key generation, it becomes more difficult to intercept information, and therefore the protocol can tolerate more noise and generate secret keys even for lower-quality entangled states."

By contrast, security is only assured with conventional QKD approaches when the quantum devices utilized have been appropriately defined. Tim van Leent, one of the four lead authors of the article along with Wei Zhang and Kai Redeker, says that users of such protocols "had to rely on the specifications supplied by the QKD providers and trust that the device will not transition into another operational mode during the key distribution." According to van Leent, the vulnerability of earlier QKD devices to external hacking has been recognized for at least ten years.

According to Weinfurter, "With our technology, we may now produce secret keys with uncharacterized and potentially unreliable devices."

In fact, he first had his doubts that the experiment would be successful. He gratefully acknowledges that his team dispelled his concerns and considerably raised the experiment's quality. Another research team from the University of Oxford showed the device-independent key distribution alongside the collaboration initiative between LMU and NUS. The scientists accomplished this using a system made up of two entangled ions in the same laboratory.

According to Charles Lim, "These two projects set the groundwork for future quantum networks, in which completely secure communication is conceivable between far-off places."

The expansion of the system to include multiple entangled atom pairs is one of the remaining objectives. According to van Leent, "this would enable the generation of many more entanglement states, increasing the data rate and eventually the key security."

Additionally, the researchers want to widen the range. It was restricted in the current configuration by the loss of around half the photons in the fiber connecting the laboratories. In additional tests, the researchers were successful in reducing the photons' wavelength to a low-loss range appropriate for telecommunications. They were able to extend the quantum network connection's range to 33 kilometers in exchange for a small amount of additional noise in this method.

By LUDWIG-MAXIMILIANS-UNIVERSITÄT