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Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission, and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such D-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit D-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2-km-long fiber and detected with high fidelities. In addition, three quantum-key-distribution protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as a vessel for the future quantum network

Raman scattering among conventional linearly polarized (LP) modes in single mode optical fibers is generally accepted as a promising way to achieve distributed amplification due to the fact that Raman amplification may provide gain at any wavelength, determined by the used pump wavelength, and excellent noise performance. Here, we show that Raman scattering among orbital angular momentum (OAM) modes in optical fibers have similar properties. We show theoretically that the Raman gain among OAM modes is independent on the topological charge of the OAM modes and that the gain efficiency when the pump and signal are parallel (orthogonally) polarized is similar to the Raman scattering among LP modes in parallel (orthogonal) states of polarization. In addition, we experimentally characterize Raman gain among OAM modes in a fiber supporting multiple OAM modes for both the pump and signal. Finally, we discuss the impact of polarization mode dispersion.

We implement the first fiber-based orbital angular momentum (OAM) state high-dimensional quantum key distribution protocol using four OAM modes, and demonstrate the highest secret key rate and longest transmission distance for OAM presented to date.

Quantum key distribution (QKD), a branch of Quantum Communications (QCs), provides ultimate security based on quantum mechanics laws [1,2]. Essential challenges of most QKD systems are the relatively short propagation distances and the low transmittable bit rates. A fundamental way to overcome these issues is represented by high-dimensional (HiD) quantum states, which allow increased information capacity and higher robustness against channel noise. This higher information efficiency has the benefit of increasing the robustness to channel noise, resulting in an increased error threshold [3–5]. The generation, transmission and detection of high-dimensional quantum states is very challenging and only a few experimental realizations have been achieved for Hi-D QC protocols [6,12]. Using the orbital angular momentum (OAM) of light is promising, as it provides a natural discrete Hi-D basis for quantum states [13]. However, OAM fiber transmission with more than two modes has only been used for classical communication so far [14]. We experimentally demonstrate the first transmission of Hi-D quantum states, encoded in four OAM modes and their superposition, over a 1.2 km long OAM fiber, by implementing a real-time decoy-state Hi-D QKD protocol, demonstrating the highest secret key rate and the longest transmission distance presented to date [15].

Simultaneous MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber is demonstrated. WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals. System performance is evaluated by measuring bit error rates, which are found to be below the soft FEC limit, and limited by inter-modal crosstalk. The crosstalk in the system is analyzed, and it is concluded that it can be significantly reduced with an improved multiplexer and de-multiplexer.

Simultaneous MIMO-free transmission of a record number (12) of orbital angular momentum modes over 1.2 km is demonstrated. WDM compatibility of the system is shown by using 60 WDM channels with 25 GHz spacing and 10 GBaud QPSK.