# Synopsis: Finding Quantum Keys in Noisy Fibers

“Quantum keys” can be sent long distances on high-traffic optical fibers.

A “quantum key” provides data security by alerting users of eavesdropping. Previous trials of transmitting quantum keys have employed a dedicated optical fiber, but this is expensive. A new filtering technique could, however, allow quantum keys to be sent alongside data on the same fiber. Experiments reported in Physical Review X demonstrate key transmission over $90$ kilometers, at a bit rate orders of magnitude higher than previous attempts.

Imagine Alice is sending an encrypted message to Bob. To read it, Bob will need a decryption key, which could be encoded into the quantum states of, for example, a stream of polarized photons. If a third party peeked at the key, the measurement would irrevocably disturb the quantum states, and Alice and Bob would be aware of a security breach. Current research efforts are aimed at incorporating keys with data on a single fiber, but the main obstacle has been retrieving the key signal out of the photon noise from the millionfold brighter data signal.

Ketaki Patel of Toshiba Research Europe Ltd in Cambridge, UK, and her colleagues have devised a temporal filtering scheme for separating keys from data. Using multiple wavelength channels, the researchers sent a key on a fiber with bidirectional data traffic. The dominant noise in the key channel came from inelastic (Raman) scattering out of the data channels. To filter this out, the team installed subnanosecond gated photodiodes that captured the key-carrying laser pulse, while ignoring much of the random (time-independent) scattering. The results support the prospects of quantum key distribution over metropolitan networks. – Michael Schirber

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## Subject Areas

Quantum Information

Optics

Soft Matter

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