Advances in Single Photon Production Using Tungsten Diselenide
Solving the mystery of quantum light in thin layers
Excerpts and salient points ~
+ When a current is applied to a thin layer of tungsten diselenide, it begins to glow in a highly unusual fashion. In addition to ordinary light, which other semiconductor materials can emit, tungsten diselenide also produces a very special type of bright quantum light, which is created only at specific points of the material. It consists of a series of photons that are always emitted one by one—never in pairs or in bunches. This anti-bunching effect is perfect for experiments in the field of quantum information and quantum cryptography, where single photons are required. However, for years, this emission has remained a mystery.
Tungsten diselenide is a two-dimensional material that forms extremely thin layers. Such layers are only three atomic layers thick, with tungsten atoms in the middle, coupled to selenium atoms below and above. “If energy is supplied to the layer, for example by applying an electrical voltage or by irradiating it with light of a suitable wavelength, it begins to shine,” explains Lukas Linhart from the Institute of Theoretical Physics at the TU Vienna. “This in itself is not unusual, many materials do that. However, when the light emitted by tungsten diselenide was analyzed in detail, in addition to ordinary light a special type of light with very unusual properties was detected.”
+ Researchers at TU Vienna have now explained this: A subtle interaction of single atomic defects in the material and mechanical strain are responsible for this quantum light effect. Computer simulations show how the electrons are driven to specific places in the material, where they are captured by a defect, lose energy and emit a photon. The solution to the quantum light puzzle has now been published in Physical Review Letters.
+ This special nature quantum light consists of photons of specific wavelengths—and they are always emitted individually. It never happens that two photons of the same wavelength are detected at the same time. “This tells us that these photons cannot be produced randomly in the material, but that there must be certain points in the tungsten diselenide sample that produce many of these photons, one after the other,” explains Professor Florian Libisch, whose research focuses on two-dimensional materials.
+ Explaining this effect requires detailed understanding of the behavior of the electrons in the material on a quantum physical level. Electrons in tungsten diselenide can occupy different energy states. If an electron changes from a state of high energy to a state of lower energy, a photon is emitted. However, this jump to a lower energy is not always allowed: The electron has to adhere to certain laws—the conservation of momentum and angular momentum.
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