Today quantum theory is used to describe essentially all observed physical phenomena, many of them find no or even incorrect explanations in classical physics. Many technological advances say in the semiconductor industries or time keeping, would not have been possible without the proper quantum mechanical description. With the so-called quantum technologies are expected to play an even stronger role in technologies that are anticipated such as quantum computers or quantum cryptography. Therefore, it is essential to better understand, verify and further explore the basic principles of this theory. Moreover, it is also interesting to explore the proper interpretation and even ask for the philosophical implications of this theory. Quantum mechanics allows for an unprecedented computational precision for some measurable quantities, while it predicts truly random outcomes for others. As of today, this is not fully understood and it is hoped to find new physical laws by discovering discrepancies between present theory and observations. These discrepancies have always been the trigger for new physical theories or completions of existing ones.
Due to the extreme precision of quantum mechanical prediction, discrepancies, if they exist, are expected to be extremely small. Otherwise, they would have already been discovered long ago. The discrete line spectrum of atomic hydrogen plays the key role in such a comparison between theory and experiment. As the simplest of all atoms, it allows for the most precise calculations. Classical physics fails completely for describing it. This discrepancy gave rise to the first quantum theory by Niels Bohr. In large parts it was key to the development of quantum mechanics. Subsequent refinements of the experimental capabilities lead to the quantum theories of Schrödinger and Dirac. Another famous discrepancy between theory and experiment discovered by Lamb has led to the most accurate theory in all of physics so far, the theory of quantum electrodynamics (QED).
New technologies like the frequency comb allowed to further increase measurement accuracy, which reaches almost 15 digits today. With advances in laser technology, it is now conceivable to perform measurements on an even better target than atomic hydrogen, singly ionized helium. The latter possess only electron, just like regular hydrogen. Therefore, one can exploit the same theoretical precision as for regular hydrogen. On top of that, higher order terms in this theory are very sensitive to the charge of the nucleus. Since the helium ion carries twice the charge of the hydrogen nucleus, it can be used to tests these higher order terms. On the experimental side helium ions are charged and can therefore be stored in electrometric traps such as the Paul trap. These traps have been used with other ions to perform some the most accurate measurements in all of physics. It is therefore expected that measurements on helium ions can be significantly more accurate than on atomic hydrogen.
This seemingly win-win situation between theory and experiment has so far been prevented by technological restrictions imposed by the available laser technologies. The required laser wavelength is way shorter than anything that has been possible with conventional laser technologies. To build a laser capable of producing the required extreme ultraviolet wavelength with the required power level and coherence, is the main challenge of this project. If successful, such a laser will almost certainly find other applications, for example in small structure photolithography as employed to make the most advanced integrated circuits. Another conceivable spin-off is to use such a laser as the driver in a thorium nuclear atomic clock. Several groups around the world are currently competing to set-up the first realization of such a clock.
