Researchers from the Institute of Physics of São Carlos at the University of São Paulo (IFSC-USP) have achieved, for the first time in Brazil, the trapping of cold ions using exclusively an electric field, through a Paul Trap. This achievement involved strontium atoms converted into electrically charged particles and held in an extremely reduced area, establishing a national experimental platform for the development and testing of qubits.
Summary of the work
This work is conducted by physicist Amilson Rogelso Fritsch, a Young Researcher supported by the São Paulo State Research Foundation (FAPESP) under the Quantum Technologies Initiative (QuTIa) program. The result was obtained after about eighteen months of project development. The subsequent phase will consist of progressing from mere particle confinement to controlling their quantum states, which is essential for executing information processing operations.
In conversation with Olhar Digital, Fritsch clarified that the country lacked any dedicated experiment on trapped-ion quantum computing. He mentioned that despite existing teaching initiatives or projects like Professor Cláudio Lenz Cesar's in Rio de Janeiro, focused on antimatter, Brazil needed its own infrastructure for quantum hardware. According to him, the most promising systems currently are those based on trapped ions and superconductors, attracting both research and investment, and recently, neutral atom systems have also gained prominence.
The success of this project also paves the way for applications in high-precision metrology. The physicist highlighted that in the near future, Brazil's capacity will not be limited to quantum computing but will also include accuracy measurements, especially in the definition standard of the second, since trapped-ion optical clocks hold the world record for precision and stability.
To keep the ions contained, the team employs a Paul Trap, a technique developed by German physicist Wolfgang Paul in the 1950s. This device operates with high-frequency oscillating electric fields, varying approximately 18 million times per second in the experiment. Due to the rapid change in field direction, the ions cannot keep up and escape, being held in the center by the resulting average force.
The confinement occurs in two distinct dimensions. Transversely, the oscillating electric field prevents the ions from moving away from the central axis. Longitudinally, a static electric field ensures they do not escape through the ends. The researcher pointed out that the biggest practical challenge does not lie in generating the oscillation, but rather in precisely positioning and stabilizing the atom at the center of the trap. He detailed that the main difficulty is placing the ion in the center, because if it deviates, it starts feeling the dynamic field and is forced to the sides.
To mitigate this problem, Fritsch's team uses static support fields and periodic adjustments to maintain system stability throughout the day. When multiple particles are captured, their positive charges cause mutual repulsion, organizing them into a small chain spaced by mere micrometers.
Before being inserted into the trap, scientists start with neutral strontium atoms. Laser beams remove one electron from each atom, transforming it into a positive ion. These particles are then confined in an ultra-high vacuum chamber, a crucial environment to prevent collisions with residual molecules from ejecting the ions or disturbing the quantum states that need to be monitored.
Although the chamber operates at room temperature, the ions are cooled using lasers. The goal is to reduce their movement to levels below one millikelvin, bringing the system closer to the fundamental quantum state. Less particle agitation results in greater precision for manipulating their information, as excessive thermal movements could interfere with operations on quantum states.
The collected images reveal bright spots corresponding to the trapped ions, with each spot being an individual particle. It is possible to control the number of ions, allowing work with a single particle or chains composed of several. This capability is relevant because lasers can be directed at specific ions, enabling modification of their states without affecting neighbors.
Fritsch emphasized that the high degree of isolation and laser focus allow for surgical precision individual manipulation. He stated he could focus a laser very narrowly and manipulate only one ion in the trap with a fidelity greater than 99.99%, without impacting the surrounding atom, adding that the system also supports simultaneous operation of the entire particle chain when necessary.
Direct connection to quantum computing
It is at this point that the experiment connects directly to quantum computing. While in traditional computers the smallest unit of data is the bit, which assumes only the values 0 or 1, in the trapped-ion system, two internal states of each particle fulfill this function, forming a qubit. The fundamental distinction is that, according to the laws of quantum mechanics, the qubit can exist in a superposition of the 0 and 1 states.
A simple analogy to illustrate this difference is comparing a bit to a coin that has already landed, showing heads or tails. The qubit, on the other hand, can be compared to the coin while it is spinning. Although this comparison has limitations, it helps in understanding that a quantum state can combine both values. However, at the moment of measurement, the result will always be 0 or 1.
Another essential property is quantum entanglement, which allows establishing correlations between two or more qubits, treating the system as a cohesive whole. Quantum algorithms can leverage these correlations, along with superposition and interference, to increase the probability of certain outcomes. This approach can bring benefits to specific problems that are difficult to solve efficiently with conventional computers.
This does not imply that quantum computers will merely be faster machines for all tasks. Their potential lies in specific types of challenges. Among the most researched examples is the simulation of molecules and atoms, which can drive research into new drugs and materials. The researcher reinforced that the quantum architecture is designed to deal with the fundamental nature of matter, stating that solving quantum problems will be much faster and more feasible if quantum technology is used, also citing future impacts on data security against fraud and complex financial transactions.
To reach this level, it is still essential to prove that the ions effectively function as controllable qubits. Lasers with carefully adjusted frequencies allow changing the internal states of the particles. Specific pulses can prepare superpositions and execute operations on the qubits. In a chain with multiple ions, the next objective is to generate entangled states and execute sequences of operations representing quantum calculations.
Strontium was selected due to its particularly advantageous characteristics for this type of experiment. Certain electronic transitions facilitate both the cooling and detection of the ions. Furthermore, an extremely narrow transition allows maintaining an excited state long enough for the information to be manipulated. Such attributes make the element an appropriate choice for experiments requiring precise control of single particles.
Advancement in Brazilian scientific capacity
The construction of the trap also represents an advance in Brazil's scientific training. Although the country already had a solid foundation in neutral atoms, the consolidation of an ion platform opens the way for the emergence of a new generation of specialists. Fritsch noted that having this technology within the University makes it possible to train students and create this research group, providing qualified labor for quantum computing, a central theme in current research and industry.
The existing infrastructure at IFSC-USP aided in this process. The Center for Optical and Photonic Research (CEPOF), coordinated by Professor Vanderlei Bagnato, concentrates expertise in optics, lasers, vacuum systems, and atomic cooling. This knowledge is applied directly to the new line of investigation. The trap was manufactured in Brazil and installed in a laboratory at the institute.
The project began in December 2024, dedicating the first months to equipment acquisition, system assembly, and trap construction. The first significant result was the production, capture, and observation of strontium ions. Although the experiment is still in a preliminary phase and without formal scientific publication on the finding, this achievement establishes the foundation for future tests.
According to Bagnato, this result positions Brazil at a favorable starting point to continue developing quantum computing. He stated in a release that with the success in trapping ions, Brazilian research has entered the map, being one of the necessary hardwares for the advancement of quantum computing.
It should be noted that the Paul Trap does not yet constitute a complete quantum computer; the next step is to use the trapped ions as qubits and perform the first quantum processing operations.
