Niedersachsen Quantum Link: Quantum Key Distribution (QKD) system installed between PTB and LUH

A quantum key distribution (QKD) system was installed between PTB in Braunschweig and LUH in Hannover as part of PTB's work to establish a quantum communication testbed between these two locations (the so-called "Niedersachsen Quantum Link"). The system was developed by Quantum Optics Jena first tested and characterized at PTB campus. Currently, it is operational between LUH (“Alice”) and PTB (“Bob”) and distributing secure quantum encrypted keys.

The QKD-system is based on the BBM92 protocol and relies on the laws of quantum mechanics and the non-cloning theorem, which prevents a quantum state from being copied or measured without disturbing it, to ensure a secure communication through an optical fiber.

The system communicates with polarization-entangled photon pairs generated by the nonlinear optical process of spontaneous parametric down-conversion. The source generates entangled photon pairs that consists of a signal at 1560 nm (telecom C-band) close to the absorption minimum of an optical fiber. At this wavelength, the loss reaches values as low as 0.2 dB/km, making it suitable for long distance optical fiber communication. The idler photons, at a wavelength of 807 nm, are efficiently detectable with a low-cost compact detector. The two wavelengths are spatially separated and filtered and finally coupled into optical fibers. The idler is transmitted to the local detector (Alice), located near the source in Hannover, where the four polarization states are detected with single photon avalanche diode (SPAD) detectors and the signal is transmitted through long optical fiber link to the remote detector (Bob) at PTB, where the polarization states are detected by a superconducting nanowire single-photon detector (SNSPD). The detection time of every photon is recorded by time taggers and the measurement data on both sides is shared via classical communication channel and processed by the system to synchronize and compensate for the polarization fluctuation during the optical fiber transmission, to reduce the mismatch probability of the signals sent and received between Alice and Bob and finally generate the secure quantum encryption key.

Contact: Ali Hreibi, Department Applied Radiometry, ali.hreibi@ptb.de

Picture of the QKD system monitoring interface showing the temporal fluctuation of the Quantum Bit Error rate and the generated secure Key rate after error detection and key sifting. The generated keys are continually accumulated in the Key Pool.