Quantum communication based on next-generation telecommunication optical fibers

Guilherme B. Xavier

The general aim of this project is to support the development and construction of an experimental platform able to assist new experiments in quantum communication based on recently developed spatial division multiplexing (SDM) optical fibers. More specifically these fibers are designed to support several different spatial mode profiles of a light beam, thus allowing information multiplexing which is a highly interesting physical property for quantum communication experiments.

Background

The Internet, as we know it today, would not have been possible without the advent of the optical fiber. Optical fiber systems are used to carry the bulk of information being transported through high-capacity networks and long-haul links. The latest development in optical fiber technology is to take advantage of the spatial properties of light as a further degree of freedom for information encoding. In contrast to standard modern optical fibers, which only support one transverse propagation mode, newly developed fibers are designed to allow a few spatial modes, with each one carrying a separate data channel. This is called space-division multiplexing (SDM) [1]. SDM-based fibers are divided into two main subgroups: multi-core fibers (MCFs) and few-mode fibers (FMFs). In the first type, several single-mode cores are embedded in the same cladding where each core operates independently. In the second case, more transverse propagation modes are supported in a single core through a larger cross-sectional area, and as such, one channel is encoded in each mode. It is also possible to combine both types, yielding a fiber that has several cores, with each one supporting more than one spatial mode [2].

Quantum information is another major unfolding technological revolution [3]. It has been shown that some information tasks performed on physical systems that directly follow the laws of quantum mechanics can have major advantages compared to their classical counterparts. For instance, it is well known that quantum computation can provide exponential speed gains when solving some problems when compared to classical computers [3]. It is also possible to improve or complement data security using quantum key distribution (QKD) [4], opening up new possibilities in cryptography.

The main goal of this project is to develop techniques and methods to enable the widespread use of quantum communication technologies through optical fibers recently developed for spatial-division multiplexing (SDM) [1]. These fibers are expected to be the foundation of all optical communication systems in the near future, further motivating their use for quantum communication systems. Meanwhile, quantum technologies (communication and computation) are undergoing tremendous growth, and its success is dependent on integration with current technology, which will drive implementation costs down. We aim to experimentally demonstrate that high-dimensional photonic quantum systems (qudits) can be successfully generated and propagated through long-distances in these new optical fibers aided by the development of novel control optoelectronic subsystems. These qudits are created on the transverse spatial profile of light beams, while taking advantage of the geometry of few-mode and multi-core fibers (FMFs and MCFs) [5] respectively.

Short project description

A qudit is the quantum analogy to a classical dit, that is a d-level system capable of storing a symbol with d different values. Although the state of polarization of a single photon is a popular resource for quantum information processing it cannot be scaled to a dimension space higher than 2. There are many processing tasks that can only be done in higher dimensions, such as quantum contextuality [6], loophole-free Bell tests with lower efficiency requirements [7] and increasing the rate of secret key generation in QKD [8]. The majority of experiments have been performed in 2 dimensions, so the field of experimental high-dimensional quantum information is still relatively unexplored.

The basic block of the proposal is the successful generation, manipulation and transmission of high-dimensional spatially encoded photonic quantum states over FMFs. The core diameter and the operating wavelength determine the number of supported modes. Below a certain value, only the fundamental linearly polarized mode LP₀₁ is supported, which has the Gaussian profile. By increasing the diameter, more modes can propagate: the next ones are the degenerate higher-order modes LP_11a and LP_11b. Each of these three profiles are orthogonal, and therefore a natural candidate to build a 3-dimensional photonic quantum system (a qutrit). It is worth noting that the first experimental long-distance demonstration of a QKD session based on multi-core fibers has been co-led by the PI (Principal Investigator) of this proposal and executed in Chile. It has recently been published [9]. The knowledge and expertise acquired in this experiment will be used for the successful execution of this project.

References

[1] D. J. Richardson, J. M. Fini and L. E. Nelson, Nature Photon. 7, 354 (2013).

[2] K. Saitoh and S. Matsuo, Nanophotonics 2 441 (2013).

[3] M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information, (Cambridge University Press, Cambridge, 2000).

[4] N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Rev. Mod. Phys. 74, 145-195 (2002).

[5] G. Li, et al, Adv. Opt. Phot., 6, 413 (2014).

[6] G. Cañas et al, Phys. Rev. Lett. 113, 090404 (2014).

[7] T. Vertesi, S. Pironio and N. Brunner, Phys. Rev. Lett. 104, 060401 (2010).

[8] S. Etcheverry et al, Sci. Rep, 3, 2316 (2013).

[9] G. Cañas et al, Phys. Rev. A, 022317 (2017).