The relentless progress in the field of photonic quantum technologies has revealed how powerful the paradigm of encoding information in single particles of light is. Several institutions around the world are investing in facilities to host quantum networks, motivated by advantages in secure communication and distributed computing. Whether this vision will ultimately lead to a quantum internet, it depends on the development of basic hardware elements with several crucial features - device scalability, on-demand operation and entanglement distribution above all.
While no operating quantum network is currently based in Italy, Sapienza hosts a unique set of different know-hows in quantum communication. By merging quantum optics, nanoscale optoelectronics, and nonlinear photonics we envision the opportunity of building an original approach to quantum networking.
We propose the development of a free-space optical communication system connecting the Marconi and Fermi buildings of the Physics Department at Sapienza. The infrastructure transfers quantum signals with minimal losses in an urban environment and is easily interfaced with existing equipment for the generation and detection of photons. Its purpose is to support entanglement distribution between the two end nodes with high channel fidelity. While this enables functionalities beyond first-generation quantum networks, we also devise a strategy aimed at scaling down the basic building blocks of the network, combining semiconductor nanostructures for signal generation and integrated photonic circuits for processing. The challenge of facing realistic conditions involving atmospheric turbulence is backed up by strong theoretical expertise in the modeling of optical wave propagation in non-linear media. The successful demonstration of a quantum link between the Marconi and Fermi buildings will open the way towards the construction of a functional quantum network at Sapienza, the first in an Italian University.
The Physics Department of the Sapienza University of Rome is currently not equipped with a free-space optical communication system. However, the preliminary tests made earlier this year (see Fig. 1) show that the construction of an optical link for 100% secure quantum communication is feasible with key devices that allows the issues related to atmospheric turbulences to be overcome. The equipment is composed of a sending and a receiving station that are intended to be installed on the top of the Marconi and Fermi buildings, in close proximity to the lab spaces of the experimental parties. It is important to notice that the proponents have already contacted the responsible of the "Prevention and Protection Office" at Sapienza, who have authorized the experiments. The stations are actively isolated and are equipped with adaptive optics to maximize the quality of the optical link, thereby reducing losses and noise related to atmospheric turbulences.The novelty of the equipment and the specific need for the experiments is discussed further below.
Due to the inherently lower optical power of a stream of photons encoding qubits of information with respect to classical transmission systems, strict requirements are placed on the tolerance to signal losses and environmental noise. In fact, refractive index variations caused by turbulent layers in the atmosphere causes random fluctuations of the beam intensity and its location at the receiver fluctuates on the millisecond time scale. The combination of image motion and intensity variation results in a low signal to noise ratio of the optical signal propagating in free space, a limitation that clearly appeared in the preliminary tests and it turns out to be unacceptable for real-life applications of quantum communication. Among the available solutions, we specifically consider those based on adaptive optics, in which wafefront distortion and beam motion are corrected via deformable mirrors, wavefront sensor and feedback systems. This is particularly relevant during daylight operation (so far impossible in the preliminary experiments) and also when orbital angular momentum encoding [1-3] is to be used for quantum key distribution [4]. The stages of signal generation and preparation, as well as of elaboration and detection, will be connected from independent stands by means of short links based on standard single-mode optical fibers. In addition to being a versatile and functional approach, the use of a single-mode fiber in reception is a practical way to decouple the sources of noise in the free-space channel from the rest of the apparatus, potentially without affecting the degree of entanglement of the transmitted photons. This advantage comes at the cost of the aforementioned stability requirements. In our proposal, they are fulfilled by active vibration isolation.
Sender and receiver have distinct implementations in this infrastructure and are initially planned to be placed at the Marconi and Fermi building respectively. However, they are designed in a modular near-symmetrical fashion, so that they can be exchanged with relatively little effort and can be conveniently upgraded to provide full-time two-way operation. In addition to that, it is important to note that due to the unique quantum property of entanglement (the asymmetry between sender and receiver simply restricts the position of the entangled photon source at the sender) qubits can be safely exchanged in both directions via the teleportation protocol thanks to the shared entanglement resource.
Additionally, we want to point out that even if the specific characteristics of the infrastructure are tailored for the foreseen application, it is easily interfaced and applies a simple principle of operation, namely transmission of near-infrared radiation in an atmospheric medium. Moreover, this infrastructure would be naturally assembled upon the existing one, and the experimental test of entanglement distribution is preparatory to investigate the largely unexplored implementation of multi-node communication protocols.
To summarize, the successful development of a functional quantum communication channel over the proposed free-space optical communication system will pave the way towards the realization of a quantum network at Sapienza: similar equipment could be indeed installed at different departments/buildings of Sapienza, and the dream of the first secure quantum communication network in an Italian University could become a reality in the near-future.
[1] G. Vallone, ..., F. Sciarrino, and P. Villoresi, Phys. Rev. Lett. 113, 060503 (2014).
[2] F. Steinlechner, ..., and R. Ursin, Nat. Commun. 8, 15971 (2017).
[3] W. Zhang, ..., and L. Chen, Phys. Rev. Appl. 10, 044014 (2018).
[4] A. Carrasco-Casado, N. Denisenko, and V. Fernandez, Opt. Eng. 53, 084112 (2014).