In addition to entanglement based QKD, AIT is also pursuing the development of CV-QKD. The advantages of such an implementation are: a much higher key rate (up to MHz), photonic integration (PIN diodes suffice as detectors) and the coherent modulation scheme, which is very similar to the classical digital modulation schemes in use nowadays. In CV–QKD weak coherent pulses are used and the information is encoded in the quadratures of the EM field.
The two weaknesses of CV-QKD, namely the need for a local oscillator (LO) signal and efficient post-processing, have been overcome by AIT. A true local LO is used to provide the highest power available at the receiver without contaminating the quantum channel. Phase correction is performed using a weak pilot signal. AIT also developed fast post-processing algorithms for CV-QKD based on Graphics Processing Unit (GPU) implementations.
The system is currently modified to fit the 19-inch form factor. All electro-optical components, as well as hardware to control the system and post-process the key, will be installed in standard racks and can therefore be deployed in the field.
The AIT CV-QKD system consists of the following parts:
- FPGA signal generator for QPSK modulation of weak coherent pulses with 100 Mbaud rate
- Polarization-multiplexed pilot tone generation at 800 MHz, used for phase correction
- Electronically controlled attenuator for optimal signal powers
- Local LO laser for maximum signal leverage
- Polarization controller
- 90°pol-multiplexed hybrid with heterodyne detection of quantum signal and pilot tone
- Analogue signal processing to stabilize polarization, frequency offset and clock recovery
- Digital signal processing of pilot tone for phase correction.
- 1 Mbit/s of secure key rate at 40km in fibre spools
- Low excess noise measured at 0.01 SNU
- Demonstrated full post-processing of 200 Mbaud throughput