By Andrea Hargraves 
Above Guanabara Bay, in Rio de Janeiro, a continuous green laser beam stretches 6.8 kilometers to a window on the balcony of the Brazilian Center for Physical Research (CBPF), in Rio’s Urca neighborhood. The light source is located in a room at the top of the building of the Institute of Physics at the Federal University of Fluminense (UFF), in neighboring Niterói, on the other side of the bay. The laser is the most visible part of the work to implement Brazil’s first experimental urban communications network based on the properties of quantum mechanics, Rio Quantica.
In addition to the aerial connection provided by the photon beam between these two institutions, the network connects the Federal University of Rio de Janeiro (UFRJ), the Pontifical Catholic University of Rio de Janeiro (PUC-Rio), the CBPF and, soon, the Military Institute of Engineering (IME). In operation since 2021, the network is in the testing and hardware phase and intends to use so-called quantum cryptography to securely transmit data.
In conventional cryptography, used in cell phones and computers around the world, information is encoded in the form of classical bits (a sequence of 0s and 1s) and transmitted over the same channel as the digital keys used to decode it. A bit is the smallest unit of information that can be stored and transmitted. It can represent only one of two possible values: 0 or 1.
Quantum communication works with a classical bit’s counterpart, the qubit, which can hold two values at the same time: being both 0 and 1. This property, based on a quantum phenomenon called superposition of states, expands the possibilities of quantum cryptography and makes it nearly unbreakable.
There is a particular type of superposition, entanglement, which is an additional feature used in the security of quantum communication networks. Therefore, these networks are seen as essential for ensuring the security of a range of tasks in the near future, such as simple authentication in a banking application or even the exchange of sensitive messages for national security purposes.
“Quantum cryptography is not used to encrypt text, but to create and transmit keys that must be used so that messages can be read securely,” comments physicist Antonio Zelakete Khoury, Rio Quantica’s coordinator. Assembling the network is a complex process. So far, the first tests ensure that at least some of the communication channels within the network are working satisfactorily.
The UFRJ, PUC-Rio and CBPF are already connected with fiber optic cables thanks to an investment made years ago by the State Research Support Foundation of Rio de Janeiro (Faperj) and the National Network for Education and Research (RNP) of the Ministry of Science, Technology and Innovation (MCTI). Since there are inactive fibers within this network, they have been transferred for use by Rio Quantica.
Two pairs of fibers leave from PUC-Rio, one for the CBPF and one for the UFRJ. The CBPF channel is working well, but the UFRJ channel has a weak signal and will need to be corrected. The connection between the QIFs and IME, the last institution to join the project, is in the final stage of implementation. The distance between them is only 800 meters. In addition to the fiber connection, an additional airborne laser connection is scheduled to be installed later this year between the CBPF and IME.
In the operation carried out by Rio Quantica, the part that has proven to be the most challenging is the communication over the air. The laser beam emitted by the UFF must be captured by an optical receiver located in a room built above the CBPF exclusively to house the equipment. Any missing amount of light could affect the integrity of the transmitted information. Currently, green light particles are still arriving overly scattered on the CBPF balcony, a flaw that should be corrected by the end of 2024.
“Long-distance links can significantly disrupt the quantum part. Even slight distortions or vibrations on the stands can distort the light beam, not to mention the effect of environmental factors that weaken the signal, such as heat, fog and rain,” comments Lieutenant Colonel Vitor Andrizo, IME communications engineer and specialist in free-space optics. Quantum systems are extremely fragile and any influence from the environment can interfere with their operation.
Once the infrastructure is in place, with the proper channels in place, the next step is to implement a quantum cryptographic protocol between institutions to put the project's primary goal into action: the remote generation of random cryptographic keys.
“The goal is for two research stations, called Alice and Bob, to share an encryption key between them that encrypts and decrypts messages. Once used, the key must be discarded,” explains Guilherme Temporão, from the Department of Electrical Engineering at PUC – Rio, a member of the network.
The protocol adopted by the Rio Quantica network uses a third agent, called Charlie, which can be trusted or not, and must be placed between Alice and Bob's stations, forming a kind of circuit.
The first attempt to implement the protocol will be made under experimental conditions between PUC-Rio, in the role of Charlie, and UFRJ and CBPF, who will play the roles of Alice and Bob, respectively. The group aims to change the roles of each institution participating in the network, in order to test new configurations.
“In the protocol, Charlie is responsible for sending out empty photons, without any information, which will be modified by Alice and Bob and sent back to Charlie and detected,” Temporao comments.
Rio Quântica received around R$3 million in 2022 from a partnership between Fapesp and MCTI for its initial implementation. Last year, it won another R$3 million from the National Council for Scientific and Technological Development (CNPq). The Financing Agency for Studies and Projects (Finep) also made R$23 million available to the Qatar National Fund, of which R$1 million was allocated to network operations and R$22 million to build a quantum technologies laboratory on the ground floor of the center. The new space is expected to be completed by 2025.
According to physicist Ivan Oliveira, project coordinator at the CBPF's Quantum Technologies Laboratory and a member of Rio Quântica, the new research area aims to manufacture materials useful for quantum computing.
“There are different candidates for devices to perform this type of computing, and the lab will build prototypes of quantum chips and other electronic components,” Oliveira says. “These devices need to operate at very low temperatures, close to absolute zero, about minus 273 degrees Celsius. We will have refrigerators to house the materials that will be built.”
Two other urban quantum networks are currently being built in Brazil, at an even more preliminary stage than the one in Rio de Janeiro, with funding from CNPq. The Recife Quantum Network is installing a fiber optic link between the Federal University of Pernambuco (UFPE) and the Rural University of Pernambuco (UFRPE). The distance between the institutions is about 5 km. However, delays in the release of resources have affected the progress of the project.
The second project involves the creation of a network of about 4 kilometers with three nodes in São Carlos, within São Paulo. The nodes of the network are the Federal University of São Carlos (UFSCar), the São Carlos Institute of Physics of the University of São Paulo (IFSC-USP) and the Wernher von Braun Center for Advanced Research, a local private institution working in the field of physics and electronics.
“We are currently analyzing the feasibility of connecting UFSCar to USP via optical cable and the latter to the von Braun Center headquarters,” says USFCar physicist Celso Villas-Boas, project coordinator. The quantum network in São Paulo is scheduled to become active within two years.
Networks abroad
Two decades ago, the idea of implementing metropolitan networks to investigate the potential of quantum communication began to become a reality. The first initiative in this regard emerged in 2003 in the United States and lasted for four years. It was a quantum encryption network using optical fibers to connect Boston University with the technology company BBN and Harvard University, both in Cambridge, a city next to Boston, Massachusetts.
Since then, other initiatives of this kind have emerged around the world.
In May of this year, three independent research groups based in China, the United States and Europe announced almost simultaneously that they had successfully transmitted entangled photons between different network points in urban areas connected by optical fiber.
This would be the first step towards creating a kind of quantum internet. In a state of entanglement, two or more particles (they can be photons, electrons or atoms) behave as if they were a single entangled entity, even if they are separated by any distance. The result of measurements made on one particle is linked to the value obtained for the other particle. This property can be used to transmit information.
The country currently investing the most in quantum communications is China, which has spent more than $15 billion in the emerging sector, more than all other competitors combined. The Asian giant has already implemented some quantum encryption networks. The largest currently connects four metropolitan areas via fiber optic cable — Beijing, Jinan, Hefei, and Shanghai — and has two quantum-enabled satellites in orbit, Micius and Jinan 1, communicating with ground stations. In total, the network covers more than 4,600 kilometers.
The Rio Quantica network and its counterparts in São Carlos and Recife represent the country's first steps towards creating qubit-based communications systems.