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Boffins have worked out how to run quantum cryptography systems over a standard broadband fibre in a development that brings theoretically unbreakable encryption closer to mainstream use.

Traditionally it has been necessary to use dedicated fibre to send the single photons (particles of light) that are required for Quantum Key Distribution (QKD). This has restricted any applications of quantum cryptography technology to specialist and small-scale systems in banks and high-level government, essentially because of the extra inconvenience and cost required in allocating a dedicated fibre strand for quantum key distribution.

However, a breakthrough from Toshiba’s Cambridge Research Laboratory makes it possible to use existing telecoms networks to distribute secret keys, potentially slashing the price of using quantum cryptography in the process.

Researchers from Toshiba teamed up with boffins at Cambridge University Engineering Department to successfully create a rig that allowed them to extract the very weak signals used for quantum cryptography from ordinary telecom fibres, which transmit regular data traffic at a different wavelength.

The Cambridge team achieved their breakthrough using a detector that is sensitive only for a very brief window (100 millionths of a micro-second) at the expected arrival time of the single photon, which carries signals related to a quantum keys. The ultra-high shutter-speed snapshot detector responds largely to just the single photon signals and is insensitive to the scattered light caused by the other data signals. This allows the weak single photon signals to be recovered from the fibre.

Using the technique, the Cambridge team successfully ran quantum cryptography systems over ordinary telecom fibres while simultaneously transmitting data at 1Gbps in both directions. They demonstrated a secure key rate over 500kbps for 50km of fibre, about 50,000 times higher than the previous best value for this fibre length. The breakthrough was reported in the scientific journal, Physical Review X, on Tuesday.

Scattered light caused by the data signals would normally contaminate and overwhelm the single photon signals if sent along the same fibre. The disparity in the intensity of the signals is illustrated by the fact that one bit of data is carried by over one million photons in normal fibre optic networks, but one bit relates to just one polarised photon in quantum key distribution systems. Getting around the noise contamination problem without falling back on a dedicated fibre for quantum key exchange is therefore a massive breakthrough.

Dr Andrew Shields, assistant managing director at Toshiba Research Europe, said: “The requirement of separate fibres has greatly restricted the applications of quantum cryptography in the past, as unused fibres are not always available for sending the single photons, and even when they are, can be prohibitively expensive. Now we have shown that the single photon and data signals can be sent using different wavelengths on the same fibre.” ®

Boffin-note

Quantum Key Distribution (QKD) offers a high-security key exchange system that is theoretically uncrackable but still subject to potential implementation flaws. Secrets keys for one time key-pads are transmitted with one photon encoding one bit.

It is secure because any attempt by an eavesdropper to intercept and measure the photons alters their encoding, thanks to fundamental principals of quantum physics. This means that eavesdropping on quantum keys can be detected. Compromised key exchanges can be abandoned and the process repeated until a theoretically unbreakable key is exchanged.

The Toshiba QKD system is based on one-way optical propagation and the BB84 "Alice and Bob (PDF)" cryptography protocol with decoy pulses.

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