Fig. 1: Nonlocal quantum gates based on fiber network in the Hefei city.

a Node A is located on the east campus of the University of Science and Technology of China, and node B is located on the foot of the mountain DaShuShan which is 7.0 km away from node A. Pulsed nondegenerate entangled photon pairs are generated through a spontaneous parametric down-conversion (SPDC) process in a periodically-poled lithium niobate (PPLN) waveguide pumped by a 421-nm laser gated with an acousto-optic modulator (AOM). The 580-nm and 1537-nm photons are separated by a dichroic mirror (DM) and spectrally filtered using cascaded etalons. The 580-nm photon is stored in the quantum memory which is implemented using ¹⁵³Eu³⁺: Y₂SiO₅ crystal cooled by a vibration-isolated cryostat and prepared with modulated laser at 580 nm, while the 1537-nm photon is sent to node B over fiber channel (QC). The control qubit and target qubit (A₁ and B₄) are encoded in the polarization degree of freedom (DOF) while the local two-qubit gates (gate C₂₁ and C₄₃) are implemented between the polarization DOF (A₁ and B₄) and the path DOF (A₂ and B₃). The results of polarization analysis (PA) in node B are sent back to node A through another fiber channel (CC) and determine the operations on A₁ which is realized using an electro-optic modulator (EOM). FC: fiber collimator, BS: beam splitter, PBS: polarizing beam splitter, HWP: half-wave plate, QWP: quarter-wave plate, TSS: time synchronization system, SNSPD: superconducting nanowire single-photon detector, and TDC: time-to-digital converter. The satellite map is from Google Earth (map data from Airbus and Maxar Technologies). b The quantum circuit of nonlocal two-qubit gates between node A and node B. The circuit includes the entanglement between A₂ and B₃, quantum memories, local two-qubit operations C₂₁ and C₄₃, measurements of A₂ in the X basis and B₃ in the Z basis, and classical communication and feedforward operations. c The coincidence counting histograms for the quantum storage of entanglement. The red line and the black line represent the transmission of the 580-nm photons, as measured with a 24-MHz transparency window and a 24-MHz AFC memory, respectively. The blue line is the coincidence counts after the 80.315-μs AFC storage. The data to the left of the black dotted line is multiplied by 10⁻². The retrieved echo peak duration is 72 ns, which is marked as a pair of orange dashed lines.