Making Hacking Futile – Quantum Cryptography
An improved version of quantum key distribution.
The internet is full of highly sensitive data. In general, sophisticated encryption techniques ensure that this material cannot be intercepted and read. However, in the future, high-performance quantum computers could crack these keys in seconds. It is therefore fortunate that quantum mechanical approaches not only offer new, much faster algorithms, but also very efficient cryptography.
Quantum key distribution (QKD), as the jargon goes, is secure against attacks on the communication channel but not against attacks or manipulations of the devices themselves. As a result, devices may produce a key that the manufacturer had previously kept that could have been passed on to a hacker. It’s a different story with device-independent QKD (abbreviated as DIQKD). The cryptographic protocol is not affected by the device. This technology has been theoretically known since the 1990s, but it has only just been implemented experimentally by an international research team led by Harald Weinfurter, a physicist from the Ludwig Maximilian University of Munich, and Charles Lim from the University Singapore National Authority (NUS).
There are many methods for exchanging quantum mechanical keys. The transmitter sends light signals to the receiver, or entangled quantum systems are used. Scientists used two quantum-mechanically entangled rubidium atoms at two labs 400 meters apart on the LMU campus in the current experiment. The two facilities are connected by a 700 meter long fiber optic cable which runs under Geschwister Scholl Square in front of the main building.
To create a tangle, scientists first stimulate each atom with a laser pulse. Following this, the atoms spontaneously return to their ground state, each releasing a photon. The spin of the atom is entangled with the polarization of its emitted photon due to the conservation of angular momentum. The two light particles travel on the fiber optic cable to a receiving station, where a combined photon measurement reveals the entanglement of atomic quantum memory.
To exchange a key, Alice and Bob – as the two parties are commonly dubbed by cryptographers – measure the quantum states of their respective atoms. In each case, this is done randomly in two or four directions. If the directions match, the measurement results are identical due to entanglement and can be used to generate a secret key. Together with the other measurement results, a so-called Bell inequality can be evaluated. Physicist John Stewart Bell originally developed these inequalities to test whether nature can be described with hidden variables.
“It turned out that’s not possible,” Weinfurter says.
In DIQKD, the test is used “specifically to ensure that there are no manipulations on the devices – that is, for example, that hidden measurement results have not been saved in the devices beforehand,” says Weinfurter.
Unlike previous approaches, the implemented protocol, which was developed by NUS researchers, uses two measurement parameters for key generation instead of one: “By introducing the additional parameter for key generation, it becomes more difficult to intercept information, and therefore the protocol can tolerate more noise and generate secret keys even for entangled states of lower quality,” explains Charles Lim.
With classical QKD methods, on the other hand, security is only guaranteed when the quantum devices used have been sufficiently well characterized. “Thus, users of such protocols must rely on the specifications provided by QKD vendors and be sure that the device will not switch to another mode of operation when distributing the key,” explains Tim van Leent, l one of the four main authors of the paper alongside Wei Zhang and Kai Redeker. It’s been known for at least a decade that older QKD devices could easily be hacked from the outside, van Leent continues.
“With our method, we can now generate secret keys with uncharacterized and potentially unreliable devices,” says Weinfurter.
In fact, he initially had doubts about the success of the experiment. But his team proved his apprehensions unfounded and dramatically improved the quality of the experience, as he readily admits. Alongside the cooperative project between LMU and NUS, another research group from Oxford University demonstrated device-independent key distribution. To do this, the researchers used a system comprising two entangled ions in the same laboratory.
“These two projects lay the foundations for future quantum networks, in which absolutely secure communication is possible between remote sites,” says Charles Lim.
One of the next goals is to extend the system to incorporate multiple pairs of entangled atoms. “This would generate many more entanglement states, which would increase data throughput and ultimately key security,” van Leent says.
In addition, researchers would like to increase the range. In the current configuration, it was limited by the loss of about half of the photons in the fiber between the laboratories. In other experiments, the researchers were able to transform the photon wavelength into a low-loss region suitable for telecommunications. This way, for just a little extra noise, they managed to increase the quantum network connection range to 33 kilometers.
Reference: “A device-independent quantum key distribution system for remote users” by Wei Zhang, Tim van Leent, Kai Redeker, Robert Garthoff, René Schwonnek, Florian Fertig, Sebastian Eppelt, Wenjamin Rosenfeld, Valerio Scarani, Charles C.-W. Lim and Harald Weinfurter, July 27, 2022, Nature.