Quantum cryptography: Hacking is futile



There is a wealth of really private information on the Internet. Such material is often protected from interception and reading thanks to sophisticated encryption mechanisms. High-performance quantum computers, however, may quickly decrypt these secrets in the future. Therefore, it is fortunate that quantum mechanical approaches not only allow for the development of new, much quicker algorithms but also extremely strong cryptography.

As the jargon goes, quantum key distribution (QKD) is safe against attacks on the communication channel but not from assaults or manipulations of the devices themselves. Therefore, the gadgets may produce a key that the manufacturer had previously kept and could have theoretically sent to a hacker. It is a different situation with device- independent QKD, or DIQKD. The cryptographic technique in this case is unaffected by the hardware being used. This strategy, which has been theoretically known since the 1990s, has now been practically implemented for the first time by an international research team led by Charles Lim from the National University of Singapore and LMU physicist Harald Weinfurter (NUS).

There are several methods for exchanging quantum mechanical keys. Either entangled quantum systems are employed, or light signals are conveyed from the transmitter to the receiver. Two rubidium atoms that were quantum mechanically entangled were employed in the current experiment. They were positioned in two laboratories on the LMU campus, 400 meters apart. A 700-meter fiber optic cable that connects the two places runs below Geschwister Scholl Square in front of the main structure.

The researchers initially use a laser pulse to stimulate each atom in order to produce an entanglement. The atoms then return to their ground state on their own, producing photons as they do so. The spin of the atom is entangled with the polarization of its released photon as a result of the conservation of angular momentum. A combined measurement of the photons at the reception station, when the two light particles arrive after traveling over the fiber optic cable, reveals an entanglement of the atomic quantum memory.

Cryptographers typically refer to the two parties as Alice and Bob. To exchange a key, they measure the quantum states of each atom. 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 may 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. Weinfurter states, "It turned out that it cannot. According to Weinfurter, the test is used in DIQKD "particularly to check that there are no manipulations at the devices—that is, to confirm, for example, that secret measurement data have not been recorded in the devices beforehand."

The implemented protocol, which was created by researchers at NUS, uses two measurement settings for key generation as opposed to one, as opposed to earlier methods' use of one: According to Charles Lim, adding the second parameter for key generation makes it harder to intercept data, allowing the protocol to withstand more noise and produce secret keys even for less-than-ideal entangled situations.

By contrast, security is only assured with traditional QKD approaches when the quantum devices utilized have been appropriately defined. Tim van Leent, one of the four primary 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 reservations about whether 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 experiments 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 cable 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.

Source:Ludwig-Maximilians-Universität München

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