Quantum cryptography: hacker attack pointless
Researchers at Ludwig Maximilian University in Munich and the University of Singapore have experimentally realized an advanced form of quantum cryptography for the first time. This will one day enable quantum networks in which absolutely secure communication should be possible between distant locations.
The Internet is teeming with highly sensitive information. Sophisticated encryption techniques usually ensure that such content cannot be intercepted and read. But in the future, powerful quantum computers in particular could crack the keys, sometimes in a matter of seconds.
Method from the 1990s
Quantum mechanical key exchange - known in technical jargon as "quantum key distribution (QKD)" - is tap-proof against attacks on the connection lines. QKD is thus immune even to quantum computers, but not to attacks or tampering with the devices themselves. The devices could issue a key that the manufacturer had already stored beforehand and possibly passed on to a hacker. The so-called "Device independent QKD", or DIQKD for short, now also checks the security of the devices. Theoretically, this method has been known since the 1990s, but now an international group of researchers led by LMU physicist Harald Weinfurter (https://xqp.physik.uni-muenchen.de/people/professor/weinfurter/index.html) and Charles Lim from the National University of Singapore (NUS) was realized experimentally for the first time.
Measuring quantum states of atoms
In the present experiment, the physicists used two entangled rubidium atoms located in two laboratories 400 meters apart on the LMU campus to exchange keys. The two sites are connected by a 700-meter fiber-optic cable that runs under the plaza in front of the university's main building. To exchange a key, the two parties measure the quantum states of their atoms. This is done randomly in two and four directions, respectively. If the directions match, the measurement results are identical due to entanglement and can be used to generate a secret key.
Tap-proof connections thanks to quantum cryptography
With the other measurement results, a so-called Bell's inequality can be evaluated. John Bell developed this inequality to test whether nature can be described with hidden variables. In the DIQKD, this test is now used to ensure that "there is no tampering with the instruments, i.e., hidden measurement results have not been stored in the instruments in advance," Weinfurter said. The NUS protocol now uses two measurement settings. "This makes it much more difficult to eavesdrop on information. So more noise can be tolerated and secret keys can be generated even with higher noise," says Charles Lim.
"With our method, we can now securely generate secret keys even with uncharacterized and potentially untrusted devices," Weinfurter explains. "Our work lays the foundation for future quantum networks in which absolutely secure communication is possible between distant locations," says Charles Lim.
