Generation of Coherent Quantum Light from a Single Impurity-Bound Exciton

In the quest to build a future internet that is fundamentally secure and capable of solving problems beyond the reach of current computers, scientists are turning to the smallest possible units of light. These units, known as photons, can carry information in a state of delicate balance called coherence, where their wave-like properties are perfectly synchronized. To create a network based on this principle, researchers need a reliable source that can emit these photons one by one, on demand, while keeping their quantum state intact. A promising candidate for this role is a tiny defect within a solid crystal, specifically an atom trapped inside a semiconductor material. When light hits this trapped atom, it can form a bound pair with an electron, creating a particle called an exciton. If this pair is held tightly by a specific impurity atom, it can act as a stable, bright light source. However, for these sources to be useful in advanced quantum networks, they must be driven by a laser that matches their energy exactly, a technique known as resonant excitation. Previous attempts to use this method in certain materials often failed because the laser light would overwhelm the faint signal from the atom, or because the atom would lose its charge and stop glowing entirely.

A team of researchers has now successfully demonstrated a way to generate coherent quantum light from a single impurity-bound exciton in a crystal of zinc selenide. By carefully engineering a microscopic pillar of the material and using a precise laser tuning method, they managed to make a single chlorine atom embedded in the crystal glow with a steady, quantum stream of light. The experiment showed that when the laser is tuned to the exact energy needed to excite the atom, the resulting light maintains a perfect phase relationship with the laser itself, a crucial requirement for sending quantum information over long distances. The researchers found that this light source is remarkably efficient, with nearly all of the emitted energy appearing as a sharp, clean color rather than being lost to vibrations within the crystal. They also discovered that the atom behaves in a predictable way when hit by light, allowing them to turn the emission on and off with extreme speed by briefly applying a second, weaker laser. This work proves that these specific defects in zinc selenide can be controlled with the precision needed for future quantum technologies, offering a bright and stable path forward for building quantum networks.

The journey to this result began with a specific material: a thin layer of zinc selenide, a semiconductor crystal, grown on a gallium arsenide (GaAs) base. Inside this layer, the researchers placed a single chlorine atom, which acts as a trap for an exciton. To make this tiny trap visible and efficient at sending out light, they carved the crystal into a microscopic pillar, only a few hundred nanometers wide, and capped it with a tiny lens made of a specialized glass-like material. This structure acts like a funnel, gathering the light emitted by the single atom and directing it toward a detector. The team cooled the entire device to a frigid temperature of 3.2 Kelvin, just a few degrees above absolute zero, to calm the thermal vibrations that would otherwise scramble the delicate quantum signals.

The core challenge was to excite the atom without drowning out its signal. In earlier experiments, scientists used a broad, high-energy laser to pump the material, which worked but produced light that was jittery and lacked the precise timing needed for quantum computing. The new approach required a laser that could be tuned to the exact energy of the atom's transition. The researchers set up a system where a tunable laser, whose color could be adjusted with extreme precision, was fired at the nanopillar. To separate the faint glow of the atom from the blinding reflection of the laser itself, they used a clever trick involving polarization. By rotating the direction of the light waves, they could filter out the reflected laser light while letting the atom's emission pass through, achieving a level of clarity that had been difficult to reach before.

When they tuned the laser to the exact energy of the chlorine-bound exciton, the atom began to glow with a steady, bright light. The researchers measured this light and found that it was not just a random burst of photons, but a coherent stream that preserved the phase of the driving laser. This means the light waves were marching in step with the laser that created them, a property essential for creating entangled states and squeezing light for advanced communication. They also observed that the light was incredibly pure, with a specific ratio of clean, sharp emission to noisy, vibration-induced sidebands. By analyzing the spectrum of the light, they calculated a value known as the Debye-Waller factor, which turned out to be 0.94. This number indicates that 94 percent of the light is emitted in the desired sharp color, a figure that stands among the highest ever recorded for solid-state light sources.

The team also investigated how the atom behaved over time when hit by the laser. They discovered that the resonant light, while exciting the atom, also had a side effect: it could knock the atom out of its charged state, causing it to stop glowing. This process happened quickly, within a microsecond, but the researchers found a way to fix it. By briefly firing a second, weak laser that was not tuned to the atom's resonance, they could recharge the atom and restore its ability to emit light. This recovery happened in less than 10 nanoseconds, a timescale fast enough to allow for rapid switching of the light source. This ability to turn the quantum light on and off with such speed and reliability suggests that these impurities could be used as switches in future quantum circuits.

The researchers confirmed that the light they were seeing came from a single source by measuring how the photons arrived at their detectors. They found that the photons arrived one by one, never in pairs, which is the hallmark of a true single-photon emitter. They also mapped out how the light intensity changed as they adjusted the laser power, observing a nonlinear response where the light behaved differently at very low powers. This behavior hints at the potential for using these emitters to create optical gates that operate with just a few photons, a key step toward building logic gates for quantum computers.

Throughout the study, the researchers ruled out several alternative explanations for their observations. They demonstrated that the broadening of the light's color was not caused by the power of the laser itself, but rather by subtle fluctuations in the electrical environment around the atom. They also showed that the loss of the signal was not due to the atom simply running out of energy, but rather a specific process where the atom lost its charge to nearby traps. By systematically testing these possibilities, they built a clear picture of how the system works. The results suggest that zinc selenide with chlorine impurities is a robust platform for generating coherent quantum light, combining high efficiency with the ability to be controlled electrically and optically. This work provides a new set of tools for manipulating the quantum states of matter, paving the way for more complex experiments in quantum networking and information processing.