Crystals and optical devices for quantum information processing

The idea of quantum information was born in the early 1980s from the fact that it was difficult to simulate the behavior of a quantum system using classical computers. This led to the design of theoretical information processing algorithms based on the laws of quantum mechanics. Their practical implementation required both the identification of a physical system sufficiently well mastered to be governed by quantum laws, and the adaptation of the algorithms to the reality of the experimental field.

This quest for the ideal system has naturally involved several fields of physics, from atomic physics to solid state physics. Chemistry is not to be outdone, making an essential contribution to the development of new materials. The development of optically active crystals in this context is emblematic of an interdisciplinary approach.

Experimental materialization of quantum objects is always a fundamental challenge, since it enables us to answer questions that the founding fathers of quantum mechanics could only address through reflection.

In the specific case of optical crystals, the problem is stimulating new developments at a time when their use as laser crystals seemed to have brought them to their apogee. New constraints have led to advances not only in their manufacture, but also in understanding and controlling the dynamics of optically active atoms.

Regardless of the physical system involved, applications are being developed in a wide variety of fields: directly, since a number of SMEs offer products based on quantum information processing for cryptography, random number generation and, very recently, quantum computing; and indirectly, since the miniaturization of circuits means that the behavior of electrons is becoming quantum. To ensure that the blurred boundary between the classical and quantum worlds does not turn out to be a technological lock, it is important to gain a better understanding of its laws right now.

The aim of this article is to introduce the concept of quantum information and the tools for manipulating it, and to show the benefits of rare-earth-doped crystals in this context.

We take a look at some of the state-of-the-art achievements in this up-and-coming field.