Multi-Path Interference Tests of Quantum Mechanics

Quantum mechanics is one of the most successful theories in physics, governing practically all processes in the microscopic world and even affecting many phenomena on a mesoscopic scale. Its predictions have been in excellent agreement with all experimental observations made throughout what has been almost an entire century. Furthermore, quantum mechanics is the basis of many modern technologies like microelectronics. It is therefore of utmost importance to know the limits of the theory. One of the theorys key features is interference: if a particle can follow multiple paths towards a destination, all of these possibilities need to be taken into account to determine the chance of it ending up at the destination. However, this interference only considers all possible combina- tions of pairs of paths, but not three or more at a time. Our experiments using modern optical waveguide technology will test very precisely whether particles of light actually behave as quan- tum mechanics predicts, or whether there are deviations. At the same time we will explore theo- retically how alternative theories could look like and how their predictions could be tested.

Quantum mechanics is one of the most successful theories in physics, governing practically all processes in the microscopic world and even affecting many phenomena on a mesoscopic scale. One of the theory's key features is interference: if a particle can follow multiple paths towards a destination, all of these possibilities need to be taken into account to determine the chance of it ending up at the destination. However, this interference only considers all possible combinations of pairs of paths, but not three or more at a time. In our project we have been perfecting multi-path interference experiments in order to be able to investigate the interference of multiple photons and to be able to detect the tiniest deviations from the standard predictions of quantum theory. In particular we have been developing optical waveguide interferometers to minimize experimental imperfections. Two key results of our project are that we managed to quantify the interferences of entangled photon triplets and, in what has arguably been a breakthrough, we were able to establish a new theoretical law for the totally destructive interference of multiple particles. Furthermore, we were able to test this law experimentally in a configuration with up to four photons. Because optical quantum information processing relies mainly on the interference of many photons our law serves as an important and convenient tool to check such devices. In the main line of research within the project we investigated whether more than two paths at a time could have an influence on interference and whether we could detect minute effects of alternative formulations of quantum mechanics that use higher-dimensional numbers. The biggest challenge for such precision experiments is to identify experimental and technical shortcomings that could lead to false positives or biases in the results. To this end we have been developing three generations of integrated optical interferometers, each of which brought an improvement of at least one order of magnitude in accuracy over the previous one. Generation two showed that a hypothetical influence of higher-dimensional numbers in quantum mechanics is very small. The third generation has been implemented and will yield results in a follow-up project. Because quantum mechanics lays the ground for quantum information processing, which is becoming more and more practical, it is important both that we have powerful tools to verify (optical) quantum information processors and that we make sure that quantum mechanics works as we understand it now, without any deviations, however small they may be. In both areas our project achieved tremendous progress and can thus be considered very successful.