Coherent Optical Control of a Single Donor Electron Spin

In the quest to build a quantum internet, scientists are searching for a way to store information in light and matter. Imagine a network where data travels as photons, the particles of light, but must pause at specific nodes to be processed or remembered. For this to work, these nodes need a memory that can hold a quantum state long enough to be useful, yet interact with light fast enough to send and receive information. One promising candidate for this memory is the spin of a single electron. Spin is an intrinsic property of an electron, behaving like a tiny, invisible magnet that can point in different directions. If scientists can control this direction with light, they can create a bridge between stationary quantum memory and flying quantum information. However, achieving this control in a solid material is difficult. The environment is often too noisy, causing the delicate quantum state to collapse before it can be used. Researchers have long sought a material that offers bright, clear optical signals while protecting the electron's spin from the chaos of the surrounding atoms.

A team of researchers has now demonstrated a significant step forward by taming a single electron spin trapped inside a crystal of zinc selenide. They focused on a specific type of impurity, a chlorine atom embedded within the material, which acts as a trap for a single electron. This setup creates a system where the electron is bound to the chlorine atom, forming a stable structure that can be manipulated with lasers. The researchers successfully showed that they could use light to prepare the electron's spin, control its direction with extreme speed, and read out its final state. This achievement is notable because it combines the ability to see the electron clearly with the ability to control it coherently, meaning they can steer the spin without losing the delicate quantum information it carries.

The journey began with the creation of a tiny device. The scientists grew a thin layer of zinc selenide, a semiconductor material, sandwiched between layers of a similar material called zinc magnesium selenide. Within this thin layer, they introduced chlorine atoms at such a low density that, statistically, only one or two would appear in a tiny pillar they carved out of the material. This pillar, shaped like a post and topped with a lens, was designed to collect the faint light emitted by the single electron trapped at the chlorine site. When they shone a laser on the sample, they observed a specific color of light coming from a single chlorine atom, confirming that they were looking at one isolated system. By applying a magnetic field, they split the energy levels of the electron's spin, creating two distinct states that could be addressed separately with light.

To prove they could control the spin, the team first had to learn how to set it to a known starting position. They used a process called optical pumping, where they shone a laser tuned to a specific frequency that would excite the electron only if it was in one particular spin state. As the electron absorbed the light and then re-emitted it, it had a chance to fall back into either of the two spin states. By repeating this cycle, they effectively filtered the electrons, leaving only those in the desired state. Once the spin was prepared, they measured how long it stayed there. They found that the spin could remain in its state for about 18.5 microseconds before something caused it to change. Crucially, they determined that this change was not due to the spin flipping on its own, which would have been a fundamental limit, but rather because the electron occasionally escaped the chlorine trap entirely. This distinction is vital because it suggests that if they can stop the electron from escaping, perhaps by adjusting the electric fields in the material, the spin could last much longer, potentially for milliseconds.

With the spin initialized, the researchers moved to the most challenging part: steering it with light. Instead of using the slow, steady lasers used for preparation, they fired extremely short pulses of light, lasting only a few trillionths of a second. These pulses were tuned to a frequency slightly different from the electron's natural resonance, a technique that allows the light to influence the spin without actually exciting the electron to a higher energy level. By adjusting the power of these pulses, they could make the spin rotate. They demonstrated this by creating a pattern of oscillations, known as Rabi oscillations, where the spin flipped back and forth between its two states in a predictable rhythm. This proved they could control the spin with the same precision as a dial, turning it to any angle they desired.

To confirm that this control was truly quantum and coherent, they performed a more complex test called Ramsey interference. In this experiment, they applied two short laser pulses separated by a brief pause. During the pause, the spin was left to spin freely, like a gyroscope, under the influence of the magnetic field. When the second pulse arrived, it revealed how much the spin had turned during that pause. The results showed a clear wave-like pattern, proving that the spin maintained its quantum memory throughout the process. They measured how long this memory lasted, finding a coherence time of about 22 nanoseconds. While this seems incredibly fast, it is a significant duration for an electron in a solid material, and it matched theoretical predictions based on the interaction between the electron and the surrounding atomic nuclei.

Finally, the team explored the deeper structure of this system by looking at the interaction between the electron and the nucleus of the chlorine atom itself. The chlorine nucleus has its own spin, which acts like a tiny magnet that influences the electron. By increasing the power of their laser pulses, they were able to resolve the subtle effects of this interaction. Instead of seeing a single signal, they observed four distinct features, corresponding to the different possible orientations of the chlorine nucleus. This observation revealed that the nucleus could also be used as a memory, potentially extending the storage time even further. The ability to see and control these hyperfine interactions suggests that this single atom could serve as a multi-layered quantum memory, where the electron acts as a fast processor and the nucleus acts as a long-term storage drive.

This work establishes a new platform for quantum technology, showing that isolated donor atoms in zinc selenide can serve as robust, optically active qubits. The researchers have proven that they can initialize, manipulate, and read out a single electron spin with high fidelity, all while maintaining the coherence necessary for quantum networks. While challenges remain, such as preventing the electron from escaping the trap and extending the coherence time through material purification, the path forward is now clear. By combining efficient optical access with long-lived spin memory, these single donors offer a promising route toward scalable quantum information systems, turning a single atom in a crystal into a functional node for the future quantum internet.