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‘Talking’ quantum dots could be used as qubits


Illustration of two quantum dots talking to each other
It’s good to talk: illustration showing two quantum dots "communicating" with each other by exchanging light. (Courtesy: Helmholtz Centre for Materials and Energy)

New atomic-scale computer simulations of how quantum dots “talk” to each other could lead to a wide range of practical applications ranging from quantum computing to green energy.

The research was done by Pascal Krause and Annika Bande at the Helmholtz Centre for Materials and Energy in Germany and Jean Christophe Tremblay at CNRS and the University of Lorraine in France, who modelled the absorption, exchange, and storage of energy within pairs of quantum dots. With further improvements to the model, the use of quantum dots could be expanded to include a diverse array of real-world applications.

Quantum dots are tiny pieces of semiconductor crystal contain thousands of atoms. The dots are quantum systems that behave much like atoms, having electron energy levels that can absorb and emit light at discrete wavelengths. For example, when illuminated with ultraviolet light a quantum dot can be excited to a higher energy state. When it drops back down to its ground state, it can emit a visible photon – allowing quantum dots to produce glow with vivid colours.

More complex behaviours can occur when two or more quantum dots are close enough together to interact with each other. For example, interactions can stabilize excitons, which are quasiparticles that comprise an electron and a hole and are created when electrons are excited. Long-lasting excitons can have applications ranging from photocatalysis to quantum computing

Sheer complexity

So far, computer simulations of quantum dot interactions have been limited by their sheer complexity. Since the processes involve thousands of atoms, each hosting multiple electrons, the characteristics of exciton formation and recombination cannot be fully captured by even the most advanced supercomputers. Now, Krause, Bande and Tremblay have approximated the process through simulations of scaled-down quantum dots, each containing just hundreds of atoms.

In their study, the trio successfully modelled the behaviour of the quantum dots at the femtosecond scale. Their simulations revealed how the quantum dot pairs absorb, exchange, and store light energy. They also found how excitons can be stabilized by applying a sequence of ultraviolet and infrared pulses to quantum dots. While an initial ultraviolet pulse can generate an exciton in one quantum dot, a subsequent infrared pulse can shift the exciton to a nearby quantum dot – where the energy it contains can be stored.

The team simulated interactions between three pairs of germanium/silicon quantum dots, which had different shapes and sizes. They now plan to create more realistic simulations that will allow them to model how environmental factors such as temperature can affect interactions. Through further improvements, their results could lead to a wide range of applications for quantum dots including quantum bits (qubits) that can reliably store and read out quantum information and photocatalysts that absorb sunlight, facilitating reactions that produce hydrogen gas as a carbon-free fuel source.

The research is described in the Journal of Physical Chemistry A.


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