Large-Scale Quantum Network over 66 Orbital Angular Momentum Optical Modes

Multipartite entanglement (ME) is the fundamental ingredient for building quantum networks. The scale of ME determines its quantum information carrying and processing capability. Most of the current efforts for boosting the scale of ME focus on increasing the number of entangled nodes. However, the number of channels for broadcasting ME is also an important index for characterizing its scale. In this Letter, we experimentally exploit orbital angular momentum multiplexing and the spatial pump shaping technique to simultaneously and deterministically generate 11 channels of individually accessible and mutually orthogonal continuous variable (CV) spatially separated hexapartite entangled states over 66 optical modes in a single quantum system. These results suggest that our method can greatly expand the scale of ME and provide a new perspective and platform to construct a CV quantum network.

Figure 1

The principle for engineering large-scale ME. The initial state is a bipartite entanglement (BE). By the spatial pump shaping technique, the number of the entangled parties increases to 6, forming a hexapartite entanglement. Then with orbital angular momentum multiplexing, the number of the multiplexed channels increases to 11, leading to the generation of large-scale ME over 66 OAM optical modes.

Figure 2

Schematic of generating and detecting the large-scale ME. (a) The seven concurrent FWM processes happen in a ${}^{85}\mathrm{Rb}$ cell and the generated OAM multiplexed hexapartite states (beams 1 to 6) from it are detected by six homodyne detections (HDs). ${\mathrm{Pump}}_1$ and ${\mathrm{pump}}_2$ are intense pump beams. 1, 2, 3 denote conjugate beams while 4, 5, 6 denote probe beams. The injected probe beam coincides with beam 5. ${\mathrm{LO}}_i$ ($i=1$, 2, 3, 4, 5, 6) is the local oscillator for the corresponding ${\mathrm{HD}}_i$. Two home-made single pole six-throw switches (Switch) allow us to select any two of the photocurrents from the six HDs and to measure the correlation between their quadrature components by spectrum analyzer (SA). Polarization beam splitter (PBS), half wave plate (HWP), quarter wave plate (QWP), high reflectivity mirror (Mirror), D shaped mirror (D-Mirror), acousto-optic modulator (AOM), spatial light modulator (SLM), beam splitter (BS), piezo-electric transducer (PZT) which is used to scan the phase between signal light and LO. (b) Interaction structure of the six LG beams. (c) Double-$\mathrm{\Lambda }$ energy level configuration of the FWM process. (d) The camera-captured intensity pattern of six OAM modes when the topological charge of the injected probe beam 5 is 5.

Figure 3

Witnessing the large-scale ME. (a)–(b) All 31 bipartitions possess an eigenvalue below 1 for different cases of the topological charge $\ell$ ($\ell =-5$ to 5), indicating complete inseparability for the six OAM modes in each case. (c)–(d), The corresponding computer-generated holograms loaded on the SLM and the intensity patterns of the six LG beams.

Figure 4

Witnessing the HE for three types of coherent OAM superposition modes. The PPT test of HE for (a), ${\mathrm{LG}}_{\ell }+{\mathrm{LG}}_{-\ell }$ modes. (b), $\sqrt{\kappa }{\mathrm{LG}}_{-1}+\sqrt{1-\kappa }{\mathrm{LG}}_{-3}$ modes. (c), ${\mathrm{LG}}_{-1}+e^{2i\theta }{\mathrm{LG}}_1$ modes. (d)-(f), The corresponding computer-generated holograms loaded on SLM and the intensity patterns of the six LG beams.