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United States of America – At the National Institute of Standards and Technology (NIST), researchers have successfully constructed grids which contain small clumps of atoms known as Quantum Dots. There, scientists have studied what happens when electrons are incorporated into these atomic islands. Observing how electrons react in such simple setups delivers insight as to how electrons would behave in complex real-world materials. Measuring such behaviors allows researchers to engineer powerful quantum computers and other dynamic technology.
About Quantum Dots
Quantum dots are tiny islands of confined electric charge. They act like interacting artificial atoms. Coupled quantum dots can deliver a robust quantum bit, or qubit, which is the fundamental unit of information for a quantum computer. The patterns of electric charge within the island are unable to currently be explained by quantum physics.
Quantum Verse Standard Comparisons
Standard computers rely on binary bits that have a fixed value of either “1” or “0.” These bits are used to store memory. On the other hand, a quantum computer stores and processes information through qubits, which take on a multitude of values. As a result, quantum computers perform larger and more complex operations as opposed to classical bits.
Artificial Complexity Of Quantum Optics
NIST’s researchers have published their work in Nature Communications. Their article explains how multiple 3-by-3 grids of precisely spaced quantum dots were formed. Each quantum dot was composed of one to three phosphorus atoms. The grids were attached with electrical leads and other material that permitted electrons to easily flow through. The grids served as playing fields where electrons behaved in ideal, textbook-like circumstances. Outside factors of the real-world environment were unable to penetrate through and cause detrimental effects.
The grids were injected with electrons and observed by researchers. Scientists also established varying conditions like increasing the spacing of the quantum dots, to study the altered behavior of electrons. When quantum dots were close to one another, electrons spread out and acted like waves, existing simultaneously in several areas. Consequently, when the dots had an increased distance between them, electrons would occasionally get trapped in a single quantum dot. This phenomenon occurs in materials with insulating properties.
Electrons orbit the center of an individual quantum dot in the same way they orbit atoms. Charged particles can occupy only specific energy levels. At each energy level, an electron can occupy a range of possible positions within the dot. A pair of coupled quantum dots can form a qubit by sharing an electron between them.
The NIST had researchers from the University of Maryland NanoCenter and the National Institute for Materials Science in Japan. Together the team used the tip of an incredibly sharp scanning tunneling microscope (STM) to fabricate the quantum dots.
When the voltage pulse moved through the graphene into an underlying layer of boron nitride, an electric field was generated. The electrons were stripped from atomic impurities within the layer and as a result, a mountain of electric charge was created. The pileup enclosed freely floating electrons into the graphene. There, they were confined to a tiny energy well.
The shape and distribution of the orbits which electrons could occupy was changed when researchers applied a magnetic field of 4 to 8 tesla. This would be approximately 400 to 800 times the strength of a small magnet. Instead of a single well, the electrons were altered. Electrons ended up residing within two sets of closely spaced rings that remained within the original well. The electrons were separated by a small empty shell. Subsequently, the two sets of rings for the electrons behaved as if they were weakly coupled quantum dots.
Improving and enhancing the grid would permit researchers to observe electron behavior in controllable conditions. Such studies would open opportunities for analog quantum simulators to unlock the secrets of exotic materials including high-temperature superconductors. Additionally, future research can reveal how to create specific materials by managing the geometry of the quantum dot arrangement. Scientists are especially interested in learning how to generate and control topological insulators.
The same NIST researchers published an additional piece in ACS Nano. Here, they discussed improvements to the fabrication method. Now, the institute is able to reliably formulate identical and equally spaced dots with one atom each. This establishes an ideal environment crucial for an accurate quantum simulator. The next step is to design a simulator with a larger grid of quantum dots. A 5×5 array of dots has the potential to create electron behavior that is impossible to simulate by even the most advanced computers.
The conduction of this study marks the first time that researchers have examined the inner workings of a coupled quantum dot system so deeply. Creating images of the distribution of electrons with atomic resolution has now been set as a precedent. The NIST team utilized the special relationship between the size of a quantum dot and the spacing of the energy levels occupied by the orbiting electrons. In doing so, researchers were able to produce high-resolution images and spectra of the system.
In a prior quantum dot study, the research team applied a smaller magnetic field using graphene. Ring structures centered on an independent quantum dot which is the origin of the concentric quantum dot rings. A STM tip was used to create dots which were half the diameter (100 nanometers) of dots that were previously studied. This study revealed the entire and completed structure of the coupled system.
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