Quantum Key Extended
Technique keeps information private over greater distances

Source: “Experimental Quantum Key Distribution with Decoy States”
Yi Zhao et al.
Physical Review Letters 96: 070502

Results: Researchers at the University of Toronto have dramatically increased the distance that a quantum key – photons that represent bits of data for encoding and decoding secret communications – can travel by modifying a commercial quantum encryption system.

Why it matters: Quantum encryption is, in theory, a perfectly secure method for communications. Perfect security would require sending an encryption key on the backs of polarized photons, which would be transmitted – either via fiber optics or through the air – one at a time. The peculiar laws of quantum mechanics dictate that, if an eavesdropper picked off just one of these photons, the entire transmission would be altered, prompting the sender to transmit another key along a more secure path.

Unfortunately, no single-photon emitter actually exists. So scientists use the next best thing: strong filters that can winnow a laser pulse down to approximately one photon. Long-distance transmissions, however, require laser pulses of particularly high intensity, which increases the proba-bility that two photons per pulse will slip past the filter. If two identically polarized photons are sent at once, an eavesdropper can pick off one of them without disturbing the other.

Hoi-Kwong Lo and his group used the duplicate photons to their advantage. They purposely generated extra photons that contained no information about the key by passing a laser beam through a weak filter instead of a strong one. An eavesdropper wouldn’t know which photons held the key and which were decoys.

Methods: The researchers modified a commercial quantum encryption system by adding a modulator that adjusts the intensity of both decoy and key-carrying laser pulses. The filter allows the person who sends the key to randomly blend both types of pulses into the transmission.

Next Steps: On their first pass, the researchers were able to transmit a blended light signal over 60 kilometers of telecommunication fibers. With some slight modifications, the scientists say, their quantum decoy system will be able to securely send key-carrying photons 120 kilometers, a range comparable to that of commercial systems with less-rigorous security.