Testing of laser beams to be used by Rede Rio Quântica to connect UFF to CBPFTatiana Azzi (NCS / CBPF)
A continuous green laser beam shines 6.8 kilometers (km) over Guanabara Bay, Rio de Janeiro, and into a window on the terrace of the Brazilian Center for Physics Research (CBPF) in the neighborhood of Urca. The light source is located in a room at the top of the Physics Institute of Fluminense Federal University (UFF), in the neighboring city of Niterói on the other side of the bay. The laser is the most visible element of an attempt to create the first experimental metropolitan communication network based on quantum mechanics in Brazil, dubbed Rio Quântica.
In addition to the aerial connection established by the photon beam between the two institutions, the network is also using almost 21 km of fiber optic cables that connect 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). The network, under development since 2021, is intended to use quantum cryptography to transmit data securely. It is currently in the testing and instrumentation phase.
In traditional cryptography, which is used in cell phones and computers around the world, information is encoded in the form of classic bits (a sequence of 0s and 1s) and transmitted on 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, and it can only represent one of two possible values: 0 or 1.
Quantum communication works with an equivalent of the classic bit called the qubit, which can assume two values simultaneously—it can represent both 0 and 1 at the same time. This characteristic, based on the phenomenon of quantum superposition, makes quantum cryptography almost inviolable, opening the door to endless possibilities. One particular type of superposition is entanglement, an additional feature used in the security of quantum communication networks. These networks are therefore seen as fundamental to ensuring the security of many tasks in the near future, from simple authentication in banking applications to the exchanging of sensitive messages for national security.
“Quantum cryptography is not used to encode a text, but rather to create and transmit the keys that are used to decode the messages securely,” explains physicist Antonio Zelaquett Khoury, head of Rio Quântica. Creating the quantum network is a complex task. So far, early tests show that at least some communication channels within the network are working satisfactorily.
Fiber optic cables were already in place connecting UFRJ, PUC-Rio, and CBPF thanks to previous investments by the Rio de Janeiro State Research Foundation (FAPERJ) and the National Education and Research Network (RNP), an office of the Brazilian Ministry of Science, Technology, and Innovation (MCTI). Unused fibers within this network were made available for use by Rio Quântica. Fiber pairs connect PUC-Rio to the CBPF and UFRJ (see map). The channel between PUC-Rio and the CBPF works well, but the one connecting the university to UFRJ has a weak signal and needs more work. The connection between the CBPF and IME, the last institution to join the project, is in the final phase of implementation. The distance between the two institutions is just 800 meters. In addition to the fiber optic connection, an additional aerial connection via laser is set to be installed between the CBPF and the IME.
This aerial link has so far proven to be most challenging aspect of setting up the Rio Quântica Network. The laser beam emitted from UFF needs to be captured by an optical receiver located in a room at the top of the CBPF built specifically to house the equipment. Any light loss can compromise the integrity of the transmitted information. At this moment in time, the green light particles are highly dispersed by the time they reach the CBPF terrace, a flaw that it is hoped will be corrected by the end of 2024. “Long distance links can really trip up the quantum approach. Even slight distortions or vibrations on the terraces misalign the light beam, not to mention the effect of environmental factors that attenuate the signal, such as heat, fog, or rain,” says Lieutenant Colonel Vítor Andrezo, a communications engineer at IME who specializes in free-space optical communication. Quantum systems are very fragile and any influence from the environment can interfere with how they function.
Once the infrastructure is set up and the communication channels are working, the next step is to implement a quantum cryptography protocol between the institutions, to put the fundamental objective of the project into practice: the remote generation of random cryptographic keys. “The goal is for two research stations named Alice and Bob to share a cryptographic key between them that encodes and decodes messages. Once used, the key must be discarded,” explains Guilherme Temporão, from the Electrical Engineering department at PUC-Rio, a member of the network.
The protocol adopted by the Rio Quântica Network uses a third agent, called Charlie, who may or may not be trustworthy and must be positioned between the Alice and Bob stations, forming a kind of circuit. A first attempt to implement the protocol under experimental conditions will involve PUC-Rio in the role of Charlie, and UFRJ and the CBPF as Alice and Bob respectively. The plan is to keep changing the role of the institutions involved in the network to test new configurations. “In the protocol, Charlie is responsible for sending blank photons containing no information, which will be modified by Alice and Bob and then sent back to Charlie and detected,” explains Temporão.
In 2022, Rio Quântica received approximately R$3 million in funding for the initial stage of implementation from a partnership between FAPESP and the MCTI. Last year, it received an additional R$3 million from the Brazilian National Council for Scientific and Technological Development (CNPq). The Brazilian Funding Authority for Studies and Projects (FINEP) also awarded R$23 million to the CBPF, of which R$1 million was allocated to the network’s operations and R$22 million to the construction of the Quantum Technologies Laboratory at the center. The new lab is scheduled to be completed by 2025.
According to physicist Ivan Oliveira, head of the CBPF’s Quantum Technologies Laboratory project and a member of Rio Quântica, the new research space is intended to manufacture materials for the quantum computing process. “There are different hardware candidates for this type of computing, and the lab will build prototypes of quantum chips and other electronic components,” explains Oliveira. “These are devices that need to operate at very low temperatures, close to absolute zero, which is around negative 273 degrees Celsius. We will have special freezers to house the materials that will be built.”
Two other metropolitan quantum networks are under development in Brazil with funding from CNPq, both at even more preliminary stages than the one in Rio de Janeiro. The Quântica Recife Network is installing a fiber optic connection between the Federal University of Pernambuco (UFPE) and the Rural University of Pernambuco (UFRPE), which are located about 5 km apart. A delay in the release of funds, however, has impacted the project’s progress.
The second project is a network of approximately 4 km with three nodes in São Carlos, São Paulo State: the Federal University of São Carlos (UFSCar), the São Carlos Physics Institute at the University of São Paulo (IFSC-USP), and the Wernher von Braun Center for Advanced Research, a private institution that focuses on physics and electronics. “We are currently analyzing the feasibility of connecting UFSCar to USP via optical cable, and USP to the Wernher von Braun center,” says UFSCar physicist Celso Villas-Bôas, coordinator of the project. The quantum network in the city of São Paulo plans to be active within two years.
Launch of the Micius satellite in 2016, one of two used by China for quantum communicationSTR/ AFP via Getty Images
Networks abroad
It was two decades ago that the idea of implementing a metropolitan area network to study the potential of quantum communication began to become a reality. The first initiative started in the USA in 2003, operating for four years. It was a quantum encryption network that used optical fibers to connect Boston University to the technology company BBN and Harvard University, both in the neighboring city of Cambridge, Massachusetts.
Since then, other similar projects have emerged around the world. In May of this year, at almost the same time, three independent research groups based in China, the US, and Europe, announced that they had successfully transmitted entangled photons between different network nodes connected by optical fiber in urban areas. This kind of local network represents the first step towards creating a quantum internet. In entanglement, two or more particles (which can be photons, electrons, or atoms) behave as if they were a single, entangled entity, even if separated by a large distance. The measured state of one particle is correlated to the value obtained for the others. This property can be used to transmit information.
The country investing the most in quantum communication is China, which has spent more than US$15 billion on this nascent sector, more than all of its competitors combined. The Asian giant has established a number of quantum cryptography networks. The largest currently connects four metropolitan areas via fiber optic cable — Beijing, Jinan, Hefei, and Shanghai — and has two satellites in orbit — Micius and Jinan 1 — that use quantum technology to communicate with ground stations. In total, the network covers more than 4,600 km. The Rio Quântica Network and its counterparts in São Carlos and Recife represent Brazil’s first steps towards setting up communication systems that use qubits.
The story above was published with the title “Qubits in Guanabara” in issue 342 of august/2024.
Project
Rio Quântica Network (nº 21/06823-5); Grant Mechanism Thematic Project; Agreement MCTI/MC; Principal Investigator Antonio Zelaquett Khoury (UFF); Investment R$2,369,806.41.
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