Quantum-Assisted Optical Spectroscopy

The QUAOS project is dedicated to the wave-particle duality, a phenomenon that is often referred to as the most characteristic manifestation of the laws of quantum mechanics. It deals with the fact that light as well as matter show properties of waves in some experiments while they appear corpuscular in others. Even larger and more complex molecules, such as the well-known C60 fullerenes, vitamins and peptides show wave properties under certain experimental conditions, in particular diffraction and interference. This phenomenon can also been shown for much larger molecules while the world record as measured in Vienna is at about 10,000 times the mass of a hydrogen atom. While modern experimental quantum mechanics is still searching for answers, such as how the transition from classical to quantum mechanics may be understood, the wave nature of microscopic matter can still be used to extract valuable molecular information. It turns out that wave properties of molecules e.g. their phases are very sensitive to external perturbations. They shift when the molecules interact with light for instance. This effect can now be used to investigate their optical behaviour. Interesting test candidates for this novel spectroscopy method are biologically relevant molecules like vitamins, peptides and small proteins. Their optical behaviour can be considered as kind of a fingerprint of their internal structure and therefore its determination is of great importance for a deeper understanding of fundamental concepts in a variety of relevant biomolecular processes.

The wave-particle duality is a ubiquitous phenomenon in quantum physics and it still triggers puzzlement when we see how it is realized in complex matter, even though it has been known for nearly a century: how can an object be delocalized in a wave-like manner? If quantum physics is a universal theory: how complex can an object be to still observe this counterintuitive effects? Does it still apply to larger lumps of matter, or even to the building blocks of life? The QUAOS project lead to new quantum experiments with natural biomolecules at the University of Vienna, supported by quantum chemical modelling at Stanford University and thereby helped to find answers to these questions. QUAOS showed the quantum wave nature of the complex antibiotic polypeptide gramicidin for the first time using a very sensitive technique known as time-domain Talbot-Lau interferometry. Quantum fringe patterns have been recorded and show that the molecular coherence was delocalized over more than 20 times the size of the molecules, which can only be explained by quantum mechanics. This conclusion is corroborated by additional high-level quantum chemical calculations, in collaboration with Todd J. Martinez from Stanford University, predicting electronic structure properties that enter quantum phase-space simulations to model the interference process. QUAOS further supported the development of sophisticated tools to launch, detect, and interfere complex molecules. It finally allowed to test quantum physics with long amino acid chains, which had remained prohibitive up to now. The challenges were mainly related to the generation of sufficiently intense beams of biopolymers, in order to isolate them in high vacuum from perturbing environments, and to establish coherent tools to probe their quantum nature. A key to this success was the use of ultrafast and intense laser light to desorb the peptides before they could decompose and matter-wave interferometry exploiting diffraction elements based on quantum measurement. These new techniques will pave the way to study even more complex biological nanomaterials from proteins to DNA and even enzymes. The research was driven by the fundamental interests in exploring the limits of quantum physics and in establishing novel quantum-enhanced technologies as minimally invasive analytical tools for individual biomolecules isolated in the gas phase. QUAOS enabled detailed studies of the quantum properties of biomolecules, opens the door for a new kind of optical spectroscopy of biologically relevant molecules in the near future, and further establishes the new research field of quantum-assisted molecule metrology.