Periodic Reporting for period 4 - XUV-COMB (High Resolution Extreme Ultraviolet Laser Spectroscopy)
Berichtszeitraum: 2021-12-01 bis 2022-05-31
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.
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.
The only currently available route to coherent extreme ultraviolet laser radiation is to employ a process called high order harmonic generation. This process takes place under suitable conditions when very short laser pulses are focused into a jet of noble gas. The emitted laser radiation may reach into a wavelength regime that is 100 times shorter than that of the driving laser. Even though high order harmonic generation has been shown to yield coherent light, no high resolution spectroscopy has been performed with it so far. One of the challenges for doing so is the very low efficiency of this process. In order obtain a useful power level in the extreme ultraviolet one has to explore new ways to improve the efficiency and at the same time use operate with a driving laser that has a large power level.
To this end, we have set up a laser system that generates a beam with 400 Watts of average power. This laser is pulsed with a pulse repetition rate of 40 MHz. We then designed a system that reduces the pulse duration down to 60 femtoseconds. This further increases the peak power. In a next step the pulses of the laser are introduced into an array of mirrors that guide them on a repetitive path. In this way each of the pulses can be used a multiple times so that the efficiency of the process again multiplies. In a last step the high order harmonic light has to be extracted from the array of mirrors. To do this we have developed mirrors with small slits that extracts the desired extreme ultraviolet radiation without perturbing the driving laser too much.
The extracted extreme ultraviolet radiation is then guided to a Paul trap that has been especially designed for this purpose. Since helium ions cannot be cooled with laser light like many other ions, we store them together with beryllium ions that can readily be cooled. Together these two ion species form crystals where all of them share the same very low temperature of the beryllium ions.
To this end, we have set up a laser system that generates a beam with 400 Watts of average power. This laser is pulsed with a pulse repetition rate of 40 MHz. We then designed a system that reduces the pulse duration down to 60 femtoseconds. This further increases the peak power. In a next step the pulses of the laser are introduced into an array of mirrors that guide them on a repetitive path. In this way each of the pulses can be used a multiple times so that the efficiency of the process again multiplies. In a last step the high order harmonic light has to be extracted from the array of mirrors. To do this we have developed mirrors with small slits that extracts the desired extreme ultraviolet radiation without perturbing the driving laser too much.
The extracted extreme ultraviolet radiation is then guided to a Paul trap that has been especially designed for this purpose. Since helium ions cannot be cooled with laser light like many other ions, we store them together with beryllium ions that can readily be cooled. Together these two ion species form crystals where all of them share the same very low temperature of the beryllium ions.
So far, we are still working on setting the systems up and testing components. However, we hope that we will eventually be successful with this apparatus and demonstrate high-resolution laser spectroscopy on hydrogen-like helium ions for the first time. If successful, we might even be able to employ the same apparatus for other target ions that have their transitions in the extreme ultraviolet region.