Higher dimensional communications in optical fiber enables new possibilities including increased transmission capacity and hyperentangled state transfer. Mode coupling among channels during transmission however causes interference among channels and limits detection. In classical optical communications, MIMO (modes in modes out) is a means to deal with this issue, however it is not possible to utilize this technology in quantum communications due to power limitations. Principal mode transmission is a another means to deal with mode coupling and signal interference among channels. Conceptually, this can be used in quantum communications with some limitations. In this report, we numerically simulate this process using the time delay method and show how it can be implemented using 2 and 4 higher dimensional quantum states, such as W or GHZ states. These numerical simulations are very illustrative of how the implementation proceeds.
Higher dimensional communications in optical fiber enables new possibilities including increased transmission capacity and hyperentangled state transfer. Mode coupling among channels during transmission however causes interference among channels and limits detection. In classical optical communications, MIMO (modes in modes out) is a means to deal with this issue, however it is not possible to utilize this technology in quantum communications due to power limitations. Principal mode transmission is a another means to deal with mode coupling and signal interference among channels. Conceptually, this can be used in quantum communications with some limitations. In this report, we numerically simulate this process using the time delay method and show how it can be implemented using 2 and 4 higher dimensional quantum states, such as W or GHZ states. These numerical simulations are very illustrative of how the implementation proceeds.
Higher dimensional communications are of interest for multiple reasons including increasing classical transmission capacity and more recently the quantum state transfer through these fibers using the many modes within the fibers. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error threshold and enabling hyperentanglement transfer among other possibilities. High dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer medium is limited by mode coupling among the various spatial and polarization modes. Here we consider how this mode coupling impacts the transfer process. We consider the fiber's modal properties including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components as well as fiber properties that mitigate the deleterious effects of mode coupling. We propose methods to circumvent this mode coupling using optical fibers, optical components and devices that are commercially available today or those that are undergoing intense global research such as Flat Optics.
We simulate the propagation of a W states through an optical fiber in the presence of mode coupling. We illustrate the propagating quantum state graphically on a group of higher order Poincaré spheres. The spheres show the propagation of the light in time and in distance as the transmission proceeds. Thus, the amplitudes and the relative phases of the modal propagation can be visualized throughout the transmission, which is novel and very useful to understand the propagation. At the fiber output we show how to recover the input quantum state using the simulated quantum state information displayed on the multiple spheres. The geometry of these states is an SU(N) quantum geometry. Applications include higher dimensional quantum communications, quantum cryptography, and quantum networks, and longer-term quantum optical computing.
In this study, we simulate the propagation of a single photon propagating within a modal group, traversing a multimode optical fiber in the presence of mode coupling. We illustrate the propagation graphically on a group of higher-order Poincaré spheres. The spheres display the propagation of the light within the modal group, including polarization in time and in distance as the transmission proceeds. Thus, the amplitudes and the relative phases within the group can be visualized throughout the transmission, which is novel and very useful for understanding the propagation. At the fiber output, we show how to recover the input of a classical state using the simulated propagation information displayed on such multiple spheres. Once a classical modal path is established from end to end, one can transmit quantum states, for example, as a spatial–time binned QKD (quantum key distribution) code. A quantum state follows this classical path. Hence, it can include binary or qudit information. Accessing a photon output state is important for many quantum network applications, including quantum key distribution, routing, and entanglement swapping. In addition, one can use other modal groups within the same fiber to multiplex other quantum channels as well as multiplex within a modal group using principal modes, a more complicated communication method. This situation is also discussed while considering the use of the spheres. Their applications include higher-dimensional quantum communications, quantum cryptography, and quantum networks. It is important to point out that in today’s commercial quantum communications systems, quantum states are single-photon states, but they are not entangled. This includes both discrete and continuous variable quantum key distribution systems. Thus, in this report, we also address single-photon systems that are not entangled, unless stated otherwise.
Quantum communication is based on the generation of quantum states and exploitation of quantum resources for communication protocols. Currently, photons are considered as the optimal carriers of information, because they enable long-distance transition with resilience to decoherence and they are relatively easy to create and detect. Entanglement is a fundamental resource for quantum communication and information processing, and it is of particular importance for quantum repeaters. Hyperentanglement, a state where parties are entangled with two or more degrees of freedom (DoFs) simultaneously, provides an important additional resource because it increases data rates and enhances error resilience. However, in photonics, the channel capacity, i.e., the ultimate throughput, is fundamentally limited when dealing with linear elements. We propose a technique for achieving higher transmission rates for quantum communication by using hyperentangled states, based on multiplexing multiple DoFs on a single photon, transmitting the photon, and eventually demultiplexing the DoFs to different photons at the destination, using Bell state measurements. Following our scheme, one can generate two entangled qubit pairs by sending only a single photon. The proposed transmission scheme lays the groundwork for novel quantum communication protocols with higher transmission rates and refined control over scalable quantum technologies.
Multiplexing is a strategy to augment the transmission capacity of a communication system. It consists of combining multiple signals over the same data channel and it has been very successful in classical communications. However, the use of enhanced channels has only reached limited practicality in quantum communications (QC) as it requires the manipulation of quantum systems of higher dimensions. Considerable effort is being made towards QC using high-dimensional quantum systems encoded into the transverse momentum of single photons, but so far no approach has been proven to be fully compatible with the existing telecommunication fibers. Here we overcome such a challenge and demonstrate a secure high-dimensional decoy-state quantum key distribution session over a 300-m-long multicore optical fiber. The high-dimensional quantum states are defined in terms of the transverse core modes available for the photon transmission over the fiber, and theoretical analyses show that positive secret key rates can be achieved through metropolitan distances.
Abstract In recent years, there has been a rising interest in high‐dimensional quantum states and their impact on quantum communication. Indeed, the availability of an enlarged Hilbert space offers multiple advantages, from larger information capacity and increased noise resilience, to novel fundamental research possibilities in quantum physics. Multiple photonic degrees of freedom have been explored to generate high‐dimensional quantum states, both with bulk optics and integrated photonics. Furthermore, these quantum states have been propagated through various channels, for example, free‐space links, single‐mode, multicore, and multimode fibers, and also aquatic channels, experimentally demonstrating the theoretical advantages over 2D systems. Here, the state‐of‐the‐art on the generation, propagation, and detection of high‐dimensional quantum states is reviewed. Quantum communication with states living in d ‐dimensional Hilbert spaces, qudits, yields great benefits. However, qudits generation, transmission, and detection is not a simple task to accomplish. This review presents the state‐of‐the‐art on the generation, propagation, and measurement of high‐dimensional quantum states, highlighting their advantages, issues, and future perspectives.