Toward an “unbreakable” message exchange

Single particles of light produced and implemented into a quantum key distribution (QKD) link could pave the way for unbreakable communication networks.

The single photons were produced by a collaboration of German scientists using two devices made of semiconductor nanostructures that emitted a photon each time they were excited by an electrical pulse. Composed of different semiconductor materials, the two devices emitted photons of different colors.

QKD is not new; one of its first uses was to encode national election ballot results in Switzerland in 2007. The process enables two parties, “Alice” and “Bob,” to share a secret key that can protect data they want to send to each other. The secret key is made up of streams of photons that spin in different directions according to the senders’ preferences.

The laws of physics state that it is not possible to measure the state, or spin, of a particle such as a photon without altering it, so if “Eve” attempted to intercept the key that was sent between “Alice” and “Bob,” Eve’s activity would become instantly noticeable.

“The random nature of emission events from strongly attenuated lasers sometimes results in the emission of two photons very close to each other,” said project coordinator Dr. Sven Höfling of the University of Würzburg. “Such multiple photon events can be utilized by an eavesdropper to extract information.”

Today, the technique is used commercially and relies on lasers to create the source of photons. Researchers, however, soon hope to further increase the efficiency of QKD by returning to the original concept of using single photons for generating a secure key.

“The nature of light emitted by lasers is very different from light emitted by single-photon sources,” Höfling said. “Whereas the emission events in lasers occur completely random in time, an ideal single-photon source emits exactly one photon upon a trigger event, which in our case is an electrical pulse.”

In the scientists’ experiment, the single photons were produced with high efficiency, then made into a key and successfully transmitted from the sender to the receiver across 40 cm of free space in the laboratory.

For the experiment to become more practical and commercially viable, it must be scaled up so that quantum keys can be sent over larger distances, the researchers say. To achieve this, quantum repeater stations must be incorporated into the network to amplify the message.

Quantum keys have been sent over 500 m of free space atop roofs in Munich, Höfling said.

Several projects have received funding to further develop the technology.

The work appeared in the Institute of Physics and German Physical Society’s New Journal of Physics (doi: 10.1088/1367-2630/14/8/083001).