Probing fluctuations in photon emission statistics of spin injected quantum dots with time dependent electronic excitation

Light is usually thought of as a stream of tiny, independent particles. In the quantum world, however, these particles of light, called photons, can behave in ways that defy our everyday intuition. Scientists have long sought to create devices that emit exactly one photon at a time, a capability essential for the next generation of ultra-secure communication and powerful computing. Traditionally, creating these single-photon sources has required shining a bright laser onto a tiny crystal to trigger the emission. While effective, this method relies on bulky external equipment and optical triggers, making it difficult to integrate into the tiny, electrically driven chips that power modern electronics. Researchers have been working to replace the laser with a simple electrical current, hoping to build a light source that can be switched on and off with a wire, just like a standard light bulb.

The challenge lies in the fact that electrical currents are messy. Unlike a precise laser pulse, an electrical signal involves a chaotic rush of electrons that can be difficult to control. When these electrons hit a tiny semiconductor structure known as a quantum dot, they can create light, but the timing and the number of photons emitted can fluctuate wildly. A new study by researchers at the University at Buffalo and their international collaborators explores exactly how these electrical fluctuations affect the light. They focused on a specific type of quantum dot that can be injected with electrons that all share the same spin, a property that gives the electron a specific magnetic orientation. By controlling this spin, the team discovered they could not only dictate the color twist of the light but also uncover a surprising, hidden rhythm in how the photons are released.

The device at the heart of this research is a microscopic sandwich of materials. At its core lies a layer of quantum dots, which are nanoscale crystals of indium gallium arsenide, embedded within a semiconductor structure. Above this sits a special magnetic layer made of cobalt, iron, and boron, separated from the dots by a thin barrier of magnesium oxide. When the researchers apply a voltage pulse to this device, electrons are forced to tunnel through the barrier and into the quantum dots. Because of the magnetic layer, these electrons arrive with their spins aligned in a specific direction. As the electrons settle into the dots and recombine with holes, they emit photons. The alignment of the electron spins ensures that the emitted light is circularly polarized, meaning the light waves spiral as they travel, much like a corkscrew.

To see what was happening inside this tiny machine, the team did not just measure the total amount of light. Instead, they built a sophisticated detection system capable of counting exactly how many photons arrived at the same time. They used a setup with four sensitive detectors arranged to catch photons that might arrive in pairs or even triplets. By tagging the exact arrival time of every single photon with extreme precision, they could reconstruct the story of the light emission second by second. They ran the experiment with the magnetic layer fully magnetized, creating a stream of spin-aligned electrons, and then repeated it after scrambling the magnetism so the electrons arrived with random spins.

The results revealed a stark difference between the two conditions. When the electrons were spin-aligned, the emission of single photons followed the electrical pulse smoothly, rising and falling as expected. However, the emission of two photons at once told a different story. The researchers observed pronounced fluctuations in the timing of these two-photon events. These fluctuations appeared at specific moments within the electrical pulse, creating a jagged, irregular pattern that did not match the smooth curve of the single photons. Crucially, when the magnetic layer was demagnetized and the spin alignment was removed, these fluctuations vanished. The two-photon emission became smooth and predictable, mirroring the behavior of the single photons. This direct link proved that the chaotic fluctuations were not caused by the electrical noise itself, but were a direct consequence of the electrons' shared spin orientation.

To understand why this happens, the researchers turned to computer simulations. They modeled the quantum dots not as a single entity, but as a group of tiny emitters that could influence one another. Their model suggested that when the electrons are spin-aligned, the quantum dots begin to act in a coordinated way, a phenomenon known as cooperative emission. In this state, the dots are not just emitting light independently; they are somehow linked, causing them to release pairs of photons simultaneously in a burst that defies the random timing of standard electrical noise. The simulation, which included a mathematical description of this cooperative behavior, successfully reproduced the jagged fluctuations seen in the experiment. Without this cooperative link in the model, the simulation produced smooth curves, just like the demagnetized experiment.

The study confirms that spin injection does more than just control the polarization of light; it fundamentally alters the statistical nature of the emission. The researchers found that the presence of spin alignment introduces a new dynamic where the system can emit higher-order states of light, such as pairs of photons, with a distinct and fluctuating timing signature. This behavior was observed in an ensemble of quantum dots, where the average spin polarization was about twenty-one percent, and the density of dots was roughly eleven per square wavelength of light. The team used a pulse width of twenty nanoseconds, with rise and fall times that could be adjusted, to probe these dynamics. They collected hundreds of millions of single-photon events and tens of thousands of two-photon events to ensure their data was statistically robust.

This work suggests that the internal dynamics of electrically driven quantum light sources are more complex than previously thought. The fluctuations observed are not merely noise to be eliminated but are a signature of a deeper interaction between the emitters, driven by the spin of the injected carriers. While the exact microscopic mechanism that allows the dots to communicate and collapse together remains a subject for future investigation, the evidence points to a cooperative process that is enabled by spin. The findings open a new path for designing quantum light sources that can be controlled electrically, offering the potential to generate not just single photons, but also reliable streams of multi-photon states. By understanding and harnessing these spin-controlled fluctuations, scientists may one day build quantum devices that are smaller, faster, and more integrated into the electronic circuits of the future.