Laser-Induced Vibrations Generate Coherent Signals in Silicon Chips

A group of researchers from the University of Tsukuba analyzed an innovative process of using ultrashort laser pulses to generate coherent lattice waves within silicon crystals.

The researchers used theoretical calculations in combination with experimental results obtained at the University of Pittsburgh to demonstrate that it is possible to maintain coherent vibrational signals within the samples. This study could result in quantum computers based on current silicon devices with the potential to quickly carry out tasks that cannot be achieved even by the existing fastest supercomputers.

In everyday life, computers play a central role, such as from home PCs to business servers, and their power continues to widen at an incredible rate. However, for classical computers, there are two huge impending challenges. Firstly, there is a basic restriction on the number of transistors that can be packed into a single processor. In the long run, there will be a need for a completely new strategy if their processing capacity has to be increased continuously.

Secondly, even the most robust computers struggle with some of the crucial problems, for example, cryptographic algorithms that maintain the safety of credit card numbers on the Internet, or the optimization of routes for delivering packages.

Quantum computers could be the solution to both the challenges since they make the most of the rules of physics that govern very small length scales, as is the case with electrons and atoms. In the quantum realm, electrons behave more like waves than billiard balls, where positions are “smeared-out” and not definite.

Moreover, different components could become entangled in a way that it is not possible to completely describe the characteristics of each one without reference to the other. For a quantum computer to be effective, it has to maintain the coherence of the entangled states sufficiently long to carry out calculations.

In this study, researchers from the University of Tsukuba and Hrvoje Petek, RK Mellon Chair of Physics and Astronomy at the University of Pittsburgh, excited electrons within a silicon crystal by using very short laser pulses.

The use of existing silicon for quantum computing will make the transition to quantum computers much easier.

Dr Yohei Watanabe, Study First Author, University of Tsukuba

The energetic electrons gave rise to coherent vibrations in the silicon structure in a way that the motions of the silicon atoms and the electrons became entangled. Then, a second laser pulse was used to investigate the state of the system after a variable delay time.

Using their theoretical model, the researchers could explain oscillations seen in the charge produced as a function of delay time. “This experiment reveals the underlying quantum mechanical effects governing the coherent vibrations,” stated Prof. Muneaki Hase, senior author who conducted the experiments. “In this way, the project represents a first step towards affordable consumer quantum computers.”