Quantum Information Group, Hands-Free Operation
In Toshiba's key distribution system the bit information is encoded upon the phase of individual photons traveling through an interferometer. The interferometer provides a very sensitive means to measure the relative lengths of two fibre paths for the photons from source to detectors. Using a phase modulator, the sender can introduce a propagation delay in one path and thereby encode the photon with information.
Although such interferometers can be used to send quantum information over long distances, previously they have been very sensitive to fluctuations in the ambient temperature of just a fraction of 1 degree Celsius. Variations in the equipment temperature change the relative lengths of the two fibre paths within the interferometer, thereby obscuring the phase encoding of the photon. In the past this required that the path lengths in the interferometer are rebalanced every few minutes, which has meant that these systems require continual adjustment by an engineer.
Toshiba has developed a proprietary technology, which stabilizes the path lengths. This involves sending a bright reference pulse along with each weak qubit pulse. The reference is unmodulated and used to stabilize the relative lengths of the two arms of the interferometer.
As a result, the system can operate continuously without the need for any user adjustment, even in the most challenging operating conditions. In addition, the system is self-initialising, allowing a truly turn-key operation. These innovations mean that quantum key distribution can be used in real application environments.
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