Experimental quantum-key distribution with an untrusted source.

The photon statistics of a quantum-key-distribution (QKD) source are crucial for security analysis. We propose a practical method, with only a beam splitter and a photodetector, to monitor the photon statistics of a QKD source. By implementing in a plug and play QKD system, we show that the method is highly practical. The final secure key rate is 52 bit/s, compared to 78 bit/s when the source is treated as a trusted source.