Communications of the ACM
Quantum Cryptographers Set 400k Distance Record
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University of Science and Technology of China researchers say they have demonstrated a record-breaking form of device-independent quantum cryptography, sending a key over a distance of more than 100 kilometers at data rates measured in kilobits per second, and distances of more than 400 kilometers at lower data rates. "This is by far the longest distance reported for all kinds of quantum key distribution systems," the researchers say.
The new technique, called measurement-independent quantum cryptography, does not depend on the way the photons are detected, so there is no way a hacker can eavesdrop on the message by hacking into the photon detectors. Although it is still possible for the transmitters to be hacked, they can be tested in a safe laboratory before transmission, to ensure they are not compromised, which is why the researchers say the best practical compromise is to pursue measurement-independent quantum cryptography.
However, manufacturers of quantum transmitters or receivers could build in a quantum memory that stores information before it is transmitted and later reveal it to an eavesdropper during another transmission. Detecting this kind of hack would be impossible without destroying the device. The only way to counter a hack of this kind would be to make single-use quantum transmitters and receivers, but that option is so expensive and impractical it may never be possible, the researchers say.
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