Physicists seek answers to a number of fundamental questions: Are natural constants really constant? Is nature governed by four and only four forces? What is dark matter? Researchers hope to find the answers by studying very meticulously simple quantum systems. In recent years, a new family of such quantum systems has received particular attention: molecular hydrogen ions, the simplest of all molecules.
A member of this family is the HD molecular ion+, composed of a proton and a deuteron linked by a single electron. It is therefore a very simple molecule which can on the one hand serve as a model system for more complex molecules and on the other hand serve as a convenient system for carrying out fundamental research on the properties and interactions of the electron , proton and deuteron.
Using this molecule, Helmholtz Prize winners from the Heinrich Heine University of Düsseldorf developed and tested an innovative technique for high-precision spectroscopy that can now be applied to other molecular ions. They have also succeeded in improving the accuracy of certain aspects of fundamental physics.
A characteristic shared by the vast majority of molecular ions is that cooling them directly with laser light is a complex and expensive undertaking. But overcoming this challenge is worth it, because precision molecular ion spectroscopy promises to provide complementary information to our knowledge of neutral atoms and molecules. In the early 2000s, researchers at Heinrich Heine University were the first to venture into this area of research.
Their approach is to capture two sets of different ion types – one consisting of molecular ions and the other atomic ions – in a single ion trap. Here, the atomic ions are cooled by laser (as is done in optical atomic clocks and quantum computers based on trapped ions) and they in turn cool the molecular ions by electrical interaction. This cooling considerably slows down the movement of molecular ions, which makes it possible to characterize their properties with great precision.
By applying this method in its award-winning work, the research group was able to make progress on four key fronts. First, they determined with unprecedented precision a combination of masses of protons, deuterons and electrons (a combination of three fundamental constants). Second, they demonstrated a low-energy quantum physics test involving baryon motion that is ten times more accurate than previous tests.
The group’s third achievement was to improve the force limits of a hypothetical fifth force between the proton and the deuteron. Finally, the group became the first to demonstrate a Doppler-free molecular electric quadrupole transition, achieving a line quality factor 40,000 times higher than previously achieved for molecular ions.
Fascinating holography – digital and more powerful than ever
We’ve all seen holograms: on banknotes, in passports, and in Star Wars! Holography is a powerful 3D photographic technique. Edoardo Vicentini and Nathalie Picqué of the Max Planck Institute for Quantum Optics (MPQ) in Garching have taken holography a step further by coupling it with frequency comb technology. This enables even better 3D imaging with previously unimaginable properties and opens up new avenues in optical diagnostics.
An optical frequency comb generator emits a regular train of short laser pulses. Its spectrum consists of a large number of sharp, evenly spaced spectral lines that together resemble a comb. Such frequency combs can be used to determine the frequency of light with great precision. For this invention, Theodor Hänsch, who heads the laser spectroscopy division at MPQ, received the Nobel Prize in Physics in 2005. Later, the MPQ group led by Nathalie Picqué developed what is called double comb spectroscopy.
This technique uses all the spectral lines of one frequency comb to interrogate a sample over a wide spectral range and combines this with a second frequency comb having slightly different spacing. The resulting interference pattern is then detected by the team using a fast photodetector.
With its new imaging technique called digital hyperspectral holography, the group extends the application of this interference method to holographic imaging. The basic principle of using two frequency combs remains unchanged, but for holographic applications a camera is used instead of a photodetector. The camera records a spatial interference pattern that changes over time as the two lasers emit their pulses at different intervals. A computer calculates the radio frequency spectrum of the interfering signal for each pixel. These spectra are then combined into a stack of digital holograms.
Double-comb interferometers produce stunning results in spectroscopy and ranging applications. The unique combination of wide spectral bandwidth, long temporal coherence and multi-heterodyne readout offers powerful new capabilities for holography. The technique developed by Vicentini and Picqué is likely to conquer new frontiers in non-scanning wavefront reconstruction and three-dimensional metrology. Beyond that, it may have potential applications in microscopy for biological samples.
Commenting on the laureates, Professor Joachim Ullrich, who is stepping down as President of the Physikalisch-Technische Bundesanstalt (PTB) and President of the Helmholtz Fund at the end of April 2022, said: “I am impressed by the extraordinary scientific innovation demonstrated by both groups.” His designated successor, Prof. Dr. Cornelia Denz, who will take over Ullrich’s two offices on May 1, 2022, added that “These two papers reveal how broad the spectrum covered by optics is – optics being one of the most important areas of physics in the 21st century.”
The Helmholtz Prize is awarded by the Helmholtz Fund for outstanding scientific and technological research in the field of precision measurement in physics, chemistry and medicine. The prize is awarded in two categories, fundamental research and applied metrology, with each prize being endowed with 20,000 euros. The award ceremony can again take place face-to-face this year and will take place on May 12, 2022 as part of the international Wilhelm and Else Heraeus seminar on “High-precision measurements and research into new physics” at the Physikzentrum in Bad Honnef. The event will be broadcast live.
The 2022 Laureates
Helmholtz Prize in the category “Precision Measurements in Basic Research”
Dr Soroosh Alighanbari, Dr Gouri Shankar Giri, Ivan Kortunov, Magnus Roman Schenkel, Prof. Dr Stephan Schiller For their article entitled “Most Accurate Determination of a Fundamental Mass Ratio and New Fundamental Physics Tests with the HD+ molecule using a new precision ion laser spectroscopy technique”