Multidimensional Entanglement Generation with Multicore Optical Fibers

Trends in photonic quantum information follow closely the technical progress in classical optics and telecommunications. In this regard, advances in multiplexing optical communications channels have also been pursued for the generation of multidimensional quantum states (qudits), since their use is advantageous for several quantum information tasks. One current path leading in this direction is through the use of space-division multiplexing multicore optical fibers, which provides a platform for efficiently controlling path-encoded qudit states. Here, we report on a parametric down-conversion source of entangled qudits that is fully based on (and therefore compatible with) state-of-the-art multicore-fiber technology. The source design uses modern multicore-fiber beam splitters to prepare the pump-laser beam as well as measure the generated entangled state, achieving high spectral brightness while providing a stable architecture. In addition, it can be readily used with any core geometry, which is crucial since widespread standards for multicore fibers in telecommunications have yet to be established. Our source represents a step toward the compatibility of quantum communications with the next-generation optical networks.

Figure 1

(a) A photograph of a multicore fiber with four cores (a 4CF) taken with a camera and a fiber microscope (left) and a diagram of the path-encoding strategy for defining the logical states in dimension $d=4$ (right). (b) The demultiplexer device, coupling single-mode fibers to a multicore fiber (here, 4CF). (c) A $4\times 4$ beam splitter constructed within a 4CF.

Figure 2

(a) The schematics of the experimental setup. Entangled qudit states are generated by resorting to coherent illumination of multiple regions of a nonlinear crystal and MCF technology. The generated entangled qudits are sent to one of the detection systems. (b) The detection system for $Z$ measurements, in which each core of the source output MCF is coupled to a SMF via a demultiplexer DM. Single-photon detectors register coincidences between the photon pairs. (c) The detection system for $X_j$ basis measurements. In this case, the output of the entanglement source is first coupled to a $4\times 4$ MCF-BS and its output fiber is the coupled to SMFs using a DM.

Figure 3

Normalized single counts while scanning the vertical ($y$-axis) transverse direction of the PPLN crystal, recorded by a pointlike single-photon detector. The shaded regions correspond to the dimensions of the crystal and the image size of the 4CF.

Figure 4

The coincidence rate in each independently illuminated core as a function of the crystal temperature. The optimal temperature to satisfy the phase-matching conditions for all cores is ${112}^{\circ }\mathrm{C}$.

Figure 5

The spectrum of frequency components of the coincidence counts between different detectors using the detection system shown in Fig. 2, taken at different times. Fluctuations due to phase changes occur on a time scale of several minutes.

Figure 6

The joint probability distributions obtained while measuring both photons in the (a) $Z$ basis, (b) $X_0$ basis, (c) $X_1$ basis, (d) $X_2$ basis, and (e) $X_3$ basis. The error bars are obtained considering Gaussian error propagation and Poissonian photocount statistics.

Figure 7

The xperimental values for the EPR-steering criterion $S$ for steering from $A$ to $B$, denoted $(B|A)$ and from $B$ to $A$, denoted $(A|B)$. When $S_{JK}^{(PQ)}<0$, we can confirm that the state is entangled in the one-sided device independent scenario.