A Luminescent Organic Triradical as an Optically Addressable Qudit

In the quest to build machines that can solve problems far beyond the reach of today's computers, scientists are turning to the smallest possible units of information. For decades, the focus has been on the qubit, a tiny switch that can exist in two states at once, much like a coin spinning on a table that is both heads and tails until it lands. However, nature offers a richer palette than just two options. Just as a musical scale has more than two notes, quantum systems can be designed to hold multiple states simultaneously. These multi-state units, known as qudits, could unlock new ways to process information and sense the world with extreme precision. The challenge lies in finding materials that can hold these complex states stably while also allowing scientists to read and write information using light, a method that is fast and precise.

A team of researchers has now created a new molecular building block that brings this concept closer to reality. They designed a single, stable organic molecule that acts as a light-emitting qudit. This molecule is constructed from three identical radical groups—chemical structures with an unpaired electron—arranged around a central ring. By connecting these three parts in a specific way, the scientists forced the unpaired electrons to align their spins in a cooperative manner, creating a ground state with four distinct energy levels instead of the usual two. This structure is not just a theoretical curiosity; it glows with light when excited, and the color and timing of that glow reveal the internal state of the electrons. The researchers demonstrated that they could use a laser to prepare the molecule in a specific state, control it with microwaves, and read out the result through its light emission, with optical initialization and readout possible at room temperature.

The molecule, which the researchers call a triradical, is built by attaching three trityl groups to a central phenyl ring. Trityl groups are known for being stable and having unique magnetic properties, but combining three of them into a single glowing unit required careful design. The team used a chemical process to link the three parts, ensuring they were spaced just right to interact without destroying each other's stability. When they examined the molecule, they found it possessed two distinct sets of energy states: a doublet state with two possible configurations and a quartet state with four. Crucially, the ground state quartet sits slightly lower in energy than the doublet by about 1 meV, resulting in a near-equal population of doublets and quartets at room temperature. This near-equal mix is vital because it allows the system to access a larger range of information states than a simple two-state system could ever provide.

When the researchers shone a blue laser on the molecule, it absorbed the energy and began to glow. They observed two distinct colors of light emerging from the sample: a glow at 633 nanometers and a deeper red at 690 nanometers. By measuring how long each color lasted, they discovered that the 633 nanometer light came from the quartet state and faded away quickly, while the 690 nanometer light came from the doublet state and lingered longer, dominating the delayed photoluminescence. This separation allowed them to distinguish between the two types of states simply by looking at the color and timing of the light. The team also found that the molecule could be manipulated by magnetic fields. When they applied a weak magnetic field of just 5 millitesla, the brightness of the light changed by up to 3 percent. This sensitivity suggests the molecule could act as a highly effective sensor for detecting tiny magnetic fluctuations in its environment.

To prove that the molecule could function as a quantum device, the scientists performed a series of tests to see if they could control the electron spins. They used a laser flash to prepare the molecule in a specific spin configuration, a process known as optical initialization. Immediately after the flash, they used microwave pulses to probe the molecule's state. They found that the spins could be coherently controlled, meaning the researchers could steer the quantum state with precision. The molecule held onto this information for a surprisingly long time, with the spin-lattice relaxation time reaching over 150 microseconds, setting an upper limit on how long the state could persist. While the coherent control experiments were performed at low temperatures to extend these times, the results indicate the material is a viable candidate for storing information. Furthermore, they demonstrated that they could address the different parts of the quartet state separately, a capability that is essential for using the molecule as a multi-state processor rather than a simple on-off switch.

The researchers also explored how the molecule behaves under different conditions, such as varying temperatures and magnetic fields. They found that the energy gap between the doublet and quartet states was incredibly small, only about 1 millielectronvolt. This tiny difference means that thermal energy at room temperature is enough to keep the molecule moving between these states, which is unusual for quantum systems that often require freezing temperatures to remain stable. The team confirmed that the molecule's behavior matched their theoretical models, ruling out the possibility that impurities or incomplete chemical reactions were causing the observed effects. They also showed that the molecule could be used for optically detected magnetic resonance, a technique where microwaves are used to change the magnetic state of the molecule, which is then detected by a change in the light it emits. This method worked effectively at room temperature, a significant advantage over many other quantum materials that must be kept near absolute zero.

This work represents a significant step forward in the development of organic quantum materials. By creating a molecule that combines stability, light emission, and multi-state control, the researchers have provided a new tool for quantum sensing and computing. The ability to read and write information using light, combined with the molecule's robustness at room temperature, opens up possibilities for integrating quantum technology into everyday devices. While the molecule is currently a laboratory prototype, its design principles offer a clear path for future improvements. The researchers suggest that by tweaking the symmetry of the molecule, they could further separate the energy levels, making it even easier to control individual states. This molecule is not just a new chemical compound; it is a demonstration that complex quantum behavior can be engineered into simple, organic structures, bringing the dream of molecular-scale quantum devices one step closer to reality.