Experimental single-photon exchange along a space link of 7000 km

Extending the single-photon transmission distance is a basic requirement for the implementation of quantum communication on a global scale. In this work we report the single-photon exchange from a medium Earth orbit satellite (MEO) at more than $7000\text{km}$ of slant distance to the ground station at the Matera Laser Ranging Observatory. The single-photon transmitter was realized by exploiting the corner cube retroreflectors mounted on the LAGEOS-2 satellite. Long duration of data collection is possible with such altitude, up to 43 min in a single passage. The mean number of photons per pulse (${\mu }_{\mathrm{sat}}$) has been limited to 1 for 200 s, resulting in an average detection rate of $3.0\text{counts}/\text{s}$ and a signal-to-noise ratio of 1.5. The feasibility of single-photon exchange from MEO satellites paves the way to tests of quantum mechanics in moving frames and to global quantum Information.

Figure 1

Top: schematic of the two-way protocol. A pulsed laser beam is directed toward a satellite equipped with CCRs and redirected toward the ground station. The two satellite trajectories are drawn to reproduce the distances in scale of Ajisai and LAGEOS-2 from the Earth's surface. Bottom: measured radial velocity $v_R$ in natural units $\beta =v_R/c$. The corresponding Doppler effect on the pulse separation is schematized on the three boxes: when $\beta <0$ ($\beta >0$) the pulse separations at the receiver are smaller (larger) than the transmitted pulse separations. The Doppler effect affects in the same way the $10\text{Hz}$ SLR beam and the $100\text{MHz}$ beam used for single-photon generation.

Figure 2

Comparison of the measured time of detection with the expected time of photon arrival. The compatibility of the fit parameters for LEO and MEO satellites demonstrates the robustness of the analysis method independently from the satellite distance.

Figure 3

Top: satellite distance to the MLRO ground station, obtained with time-of-flight measurements of SLR pulses. Middle: single-photon detection rate averaged over 60 s compared with the global fit. The shaded area represents the 99.7% of confidence level of the fit. Bottom: detection rate of single photons averaged over $10\text{s}$ for different values of ${\mu }_{\mathrm{sat}}$: green circles, yellow circles, and red diamonds, respectively, correspond to ${\mu }_{\mathrm{sat}}\le 1,1<{\mu }_{\mathrm{sat}}\le 2$, and ${\mu }_{\mathrm{sat}}>2$.

Figure 4

Histograms of detections as a function of deviation $\mathrm{\Delta }$ from expected time $t_{\mathrm{ref}}$ for different values of ${\mu }_{\mathrm{sat}}$. When only intervals with ${\mu }_{\mathrm{sat}}<1$ are selected (top), a peak is observed at $\mathrm{\Delta }=0$ with statistical significance over background of $5.1\sigma$. If the analysis is extended to ${\mu }_{\mathrm{sat}}<2$ (bottom) the peak significance reaches $15.1\sigma$. Bin size is $0.4\text{ns}$. Robustness of the peaks has been verified by changing the bin size.