Optical-Relayed Entanglement Distribution Using Drones as Mobile Nodes

Entanglement distribution has been accomplished using a flying drone, and this mobile platform can be generalized for multiple mobile nodes with optical relay among them. Here we develop the first optical relay to reshape the wave front of photons for their low diffraction loss in free-space transmission. Using two drones, where one distributes the entangled photons and the other serves as relay node, we achieve entanglement distribution with Clauser-Horne-Shimony-Holt $S$ parameter of $2.59\pm 0.11$ at 1 km distance. Key components for entangled source, tracking, and relay are developed with high performance and are lightweight, constructing a scalable airborne system for multinode connectio and toward mobile quantum networks.

Figure 1

Quantum network and physical realization with mobile drone-based nodes. (a) A practical quantum network consists of multinode structure and should provide access for users at any time. (b) If link distance is over the Rayleigh length limit, the beam aperture will be divergent and become larger than the receiver telescope, which inevitably brings loss to the system. Using relayed transceivers between nodes is an economical solution to keep each link distance within Rayleigh length limit to minimum the diffraction loss. (c) Diagram of the optical relay. The photon is transmitted from node $A$ to the relay node and then retransmitted to another node $B$. After coupling into a SMF fiber and going through a PCS system for polarization compensation, the photons are collimated for retransmission to node $B$. Such diagram can be cascaded with unlimited optical relay nodes, in principle. (d) Diffraction loss versus beam aperture at link distances of 200 and 400 m, respectively, as used in our experiment. (e) Illustration scheme of a mobile quantum network using drones from local to wide-area scales. For local-area network, plug-and-play drone nodes can establish connections among mobile and fixed nodes promptly. Whereas for wide-area network, high-altitude UAVs can build multinode mobile links, which is an important supplement to the satellite and fiber-based networks.

Figure 2

Structure diagram and performance of the AEPS and the APT system. (a) Structure diagram of the AEPS. The 405 nm pump laser is a self-injection laser diode locked by a volume holography Bragg (VHG) grating. The entangled photon pair is generated by the PPKTP crystal inside a Sagnac loop. LD, laser diode; DM, dichroic mirror; ISO, isolator; LPF, long-pass filter; $M$, mirror; QWP, quarter-wave plate; $C$, collimator. (b) Two-photon correlation curves measured in lab. The interference visibilities with one photon projected to $|H⟩$, $|V⟩$, $|D⟩$, and $|A⟩$ states are 98.6%, 99.1%, 98.4%, and 98.0%, respectively. The error bars represent one standard deviation in the counts. (c) Schematic of the whole APT system. Coarse tracking is realized by a coaxial zoom camera loaded on each APT system to image the 940 nm LD of the opposite telescope. The fine tracking device is integrated inside the telescope as in the enlarged picture. CMOS, complementary metal oxide semiconductor; FSM, fast steering mirror; OAPM, off-axis parabolic mirror. (d) Tracking errors of transceivers in the 200 m drone-to-drone link and 400 m drone-ground links, respectively.

Figure 3

The setup of the experiment. Illustration of the experimental setup for the optical-relayed entanglement distribution experiment. Drone 1 distributes the entangled photon pairs to Alice and Drone 2 (Bob) stations through mobile links. State projection measurement is performed at each ground station and a classical communication link is used for coincidence measurements. BF, band-pass filter; SPAD, single-photon avalanche detector; TC, transceiver; TDC, time-to-digital converter.

Figure 4

Scheme and measurement results of the relayed entanglement distribution. (a) Illustration of the experimental setup for the relayed entanglement distribution experiment. Drone 1 generates the entanglement with AEPS and drone 2 collects the photons from drone 1 and retransmits them to Bob for optical relay. The distance between Alice and Bob stations exceeds 1 km. Inset: relay link in the experiment. (b) Measured correlation function results for the CHSH inequality calculation. The error bars represent 1 standard deviation in the counts.