Single Photon Quantum Cryptography

We report the full implementation of a quantum cryptography protocol using a stream of single photon pulses generated by a stable and efficient source operating at room temperature. The single photon pulses are emitted on demand by a single nitrogen-vacancy color center in a diamond nanocrystal. The quantum bit error rate is less that $4.6\%$ and the secure bit rate is $7700\mathrm{\text{bits}}/\mathrm{s}$. The overall performances of our system reaches a domain where single photons have a measurable advantage over an equivalent system based on attenuated light pulses.

Figure 1

Experimental setup.

Figure 2

Autocorrelation function of a single NV center on Alice’s side. The raw coincidences are given as a function of the delay between the arrival times of the photons at Alice’s correlation detection setup. The exciting laser has a repetition period of 187.5 ns, a pulse width of 0.8 ns, and an average power of 0.2 mW. The count rates are about $3.5\times {10}^4{\mathrm{s}}^{-1}$ on each avalanche photodiode, and the integration time is 166 s. The number above each peak represents its normalized area. The dots are experimental data. The line is an exponential fit for each peak and takes into account the background light.

Figure 3

These plots give theoretical evaluations obtained by using Eq. (1), together with the experimental parameters for our single photon source. (a) Calculated number of secure bit per time detection time gate $G$ as a function of the on-line losses for SPPs and WCPs, for a different average photon number per pulse $\mu$. The SPP traces correspond to our value of the zero time autocorrelation of $C=0.07$. (b) The maximum allowed on-line losses for secure communication are deduced from (a) and correspond to the attenuation for which $G={10}^{-6}$. This value is plotted as a function of the mean photon number per pulse $\mu$, for a WCP system, for a SPP system with our value of the zero time autocorrelation ($C=0.07$), and for an ideal SPP system with $C=0$. The vertical line corresponds to our source, i.e., $\mu =0.014$. (c) Number of secure bit per detection time gate $G$ as a function of the mean photon number per pulse $\mu$ for on-line losses of 12.5 dB. The SPP trace assumes that $C=0.07$, and the vertical line corresponds to $\mu =0.014$.