NIST QKD system at 1310 nm combines speed and distance
In the 1310-nm QKD system, the quantum key is encoded by 1310-nm photons using the B92 protocol, as shown in the figure below. The QKD system uses a custom printed circuit board with a field-programmable gate array (FPGA) to generate a random stream of quantum key data as well as to transmit and receive classical data, which will be encoded and decoded by the quantum key. The classical data is carried as an optical signal in the 1550-nm band.
To polarization-encode the quantum channel from the random quantum key, we first modulate a 1306-nm CW light into a 625-MHz pulse train which is evenly split into two polarization channels. Each pulse train is further modulated by one of two complementary 625-Mbit/s quantum channel data streams. The two quantum channels are combined by a 45-degree polarization-maintaining combiner and attenuated to a mean photon number of 0.1 per bit, then multiplexed with the classical channel and transmitted over a standard single-mode fiber.
At Bob, another WDM is used to demultiplex the quantum and the classical channels. The quantum channels are polarization-decoded and detected by the up-conversion single-photon detectors, generating the raw key. Bob's board informs Alice of the location of the raw key data via the classical channel. After reconciliation and error correction, Alice and Bob obtain a common version of the secure key, which is further used to encode and decode the classical signal.
In the 1310-nm QKD system we have achieved approximately 500 Kbit/s and 9.1 Kbit/s of secure-key rates at 10 km and 50 km, respectively. We also generated secure keys in real time for one-time-pad encryption at a continuous rate of 200 Kbit/s encrypted video transmission over 10 km (potentially the key rates can reach 500 Kbit/s).
The figure of merit of the 1310-nm QKD system can be described in three key characteristics. First, the dark count (noise) is small. Second, transmission of the quantum and the classical channel can be achieved using a single fiber.
Third, the chromatic dispersion is nearly zero at 1310 nm for the standard single-mode fiber. Hai Xu, Lijun Ma, Alan Mink, Barry Hershman and Xiao. Tang. "1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm", Optics Express, Vol. 15, Issue 12, pp. 7247-7260 (June 11, 2007).