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FOR BEGINNERS

 

A brief introduction to the NAMEQUAM project

For more than a decade now scientists have been trying to harness the power of quantum physics in order to create a new kind of information technology known as “quantum information” or “quantum computation”. The promises of this technology are great, but the obstacles are many. In particular, physicists have yet to find the ideal “raw material” for quantum computation, i.e., a physical system that behaves quantum mechanically and can still be reliably and efficiently controlled and manipulated. The NAMEQUAM project aims at designing novel “quantum materials” that fulfil these requirements by engineering and controlling nanoscopic and mesoscopic objects such as atoms and molecules.

Creating “quantum matter”

Sixty years ago the invention of the transistor signalled the breakthrough that eventually led to modern electronics and computers. The possibility to scale down the components of an electronic circuit meant that millions of them could eventually fit into a few square millimetres. What had been tried and tested on a large scale was, essentially, just shrunk to a microscopic size, but the principle of operation remained the same.

The search for a raw material for quantum information faces a different and much tougher challenge. As is well known, everyday objects such as tables and chairs do not behave quantum mechanically – that is, they do not exist in a superposition of several states at the same time, and they do not “tunnel” through closed doors. Atoms and other nanoscale particles, on the other hand, do just that, which is why they are ideal candidates for the building blocks of a quantum computer. These quantum bits or “qubits” can represent the values 0 and 1 of a calculation simultaneously and thus enable a massively parallel processing of data. Unfortunately, single atoms are very sensitive objects and difficult to handle individually. Even if individual handling is achieved, one still has to make those atoms “talk” to each other in a controlled way in order to make them perform computations. In short, in order to build a quantum computer it is necessary to create a quantum material in which one has complete control over its individual atoms or molecules. Since such a material does not exist in nature, one has to build it from scratch. This is what NAMEQUAM tries to do.


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Crucial interactions

One particularly tricky issue is that of interactions between the quantum particles. While in naturally occurring materials such as solids individual atoms “feel” one another strongly, for quantum information purposes they have to be isolated from each other and only interact in a controlled way. At the same time, when needed, this interaction has to be sufficiently strong so that the calculation effected by it is fast enough. These two requirements are generally very difficult to satisfy simultaneously. NAMEQUAM tries to reconcile them by taking neutral atoms or molecules and trapping them inside a light-induced artificial crystal structure called an “optical lattice”. In this way, the quantum particles are located at well-known and controllable positions and are almost completely isolated from the surrounding world.

The particles can then be made to talk to one another by “switching on” interactions between them in a controlled way. One of several approaches that will be explored by NAMEQUAM is to excite the atoms to so-called Rydberg states in which they feel each other strongly. This interaction can then be used, for example, to create “entangled states”. These quantum states are the starting point for quantum computation and exhibit weird and wonderful properties – measuring the state of one particle immediately influences all the other particles even if they are far apart.


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General vs special purpose

The quantum matter created as described above can then be used in two ways. One of them is to build a “general purpose” quantum computer, i.e., a device which can solve an arbitrary computational problem if it is provided with the appropriate “software”, just like an ordinary (or, more correctly speaking, “classical”) computer. Less universal, but equally useful, is the “special purpose” quantum computer. In such a computer the problem to be solved is hard-wired into its architecture. Since the architecture of a quantum computer is itself strictly quantum mechanical, this means that it can, if properly designed, help to solve particular mathematical problems that arise when dealing with quantum systems composed of a large number of particles.

An example for such a system is the high-Tc superconductor, i.e. a material that conducts electrical currents without any resistance at (relatively) high temperatures (and is, therefore, of great interest for technical applications). While so far it has not been possible to solve the mathematical equations necessary for understanding how these superconductors work, a special purpose quantum computer (or “quantum simulator”) should be able to do the necessary calculations by directly mimicking the quantum mechanical properties of the superconductor through its quantum mechanical components.