Unveiling the Nature of Genuine Many-Particle Phenomena

  “What is the interplay between the wave-like and particle-like effects in the mesoscopic or macroscopic regimes?”

Quantum mechanics, one of the pillars of modern physics, not only emerges prominently in the microscopic world but also persists within the phenomena at the macroscopic scale. Niels Bohr's complementarity principle conceptualizes the controversial dual nature of quantum objects, stating that they exhibit either wave-like or particle-like behavior but never both simultaneously due to their mutually exclusive descriptions. This is experimentally illustrated in the renowned double-slit experiment, indicating that particle character stems from the which-path information, while interference patterns denote wave behavior. Interference arises from the superposition principle in both classical (electromagnetic waves) and quantum (quantum wavefunctions) physics. At the quantum/classical frontier, any distinguishing information in a system's preparation, evolution, or measurement induces the collapse of the wavefunction and the interference vanishes.  Fundamentally, these phenomena appear to be old-fashioned yet experimentally proven concepts that originated early in the last century. However, any attempt to understand complex multi-particle interference phenomena through the well-known single- and two-particle principles have failed. This inevitably raises the question: What is the interplay between the wave-like and particle-like effects in the mesoscopic or macroscopic regimes?

“Since interference lies at the heart of cutting-edge quantum technologies, understanding a new realm of rich collective effects will help with the exploration of fundamental physics which we do not yet understand, as well as with technological developments in applications of quantum information science.”

The broad scientific community consensus holds that this century is evolving into the quantum era, with applications advancing from single-particle to many-particle effects. At the heart of this progression lies interference, a phenomenon driving cutting-edge quantum technologies. Recent research has made progress in the understanding and control of these complex quantum systems, including atomic, molecular, and optical platforms. Scaling the number of particles to demonstrate true quantum advantage will still require substantial effort. Yet, current systems can serve to study collective quantum features and their limitations.

   In this project, we will conduct foundational research into multi-particle wave-particle duality through interference experiments. In the light of quantum interference, indistinguishable particles routed through a scattering object can follow multiple paths to form the output state. However, any degree of distinguishability leads to incoherent evolution, degrading collective effects. Decoding and quantifying multi-particle phenomena is not straightforward, and it is not generally known which effects stem from genuine interference. Recent theoretical advances were able to define such quantitative measures, which we will comprehensively test for quantum state families of a small number of particles. This will be the first experimental observation of general multi-particle complementarity relations. Even then, measuring multi-particle systems presents open challenges in the field. The diagnostic tools established for single- or two-particle systems are insufficient to model collective effects, whereas full quantum state tomography becomes a practical impossibility. Therefore, we will identify new efficient quantifiers and optimal interferometric measurements to advance our understanding of collective multi-particle phenomena, laying the foundation for developing quantum information technologies and stimulating further research into new phenomena.

 

MSc Thesis Topic: 
Exploring Fundamental Multi-Particle Quantum Phenomena

Contact:

Josef Hlousek
Josef.Hlousek@uibk.ac.at

 

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