Quantum foundation experiments with active feed-forward

Abstract

This work consists of two different experiments from the field of quantum foundations and quantum information. Both are based on an active feed- forward protocol [1], with which the path of individual photons in the experimental setup can be controlled. Since these two experiments were performed independently, both are briefly summarized in separate paragraphs.Maxwell’s demon is a thought experiment from 1867 in which a demon can lower the total entropy of two initially equalised thermal baths, without performing any work on the system [2]. The discussions that arose initially due to the apparent violation of the second law of thermodynamics have continued to this day. One of the reasons for this is that the thought experiment provides a direct link between quantum information processing and thermodynamics. In this work, we transferred the thought experiment into a photonic domain based on fiber optic components. Therefore, we use thermal states of light in place of thermal baths. This gives us access to new tools, like ultra fast optical switches and the ability to create more interesting states. Moreover, we have verified experimentally, that certain correlations between the thermal baths can tremendously enhance the ’power’ of Maxwell’s demon, while others eliminate his influence completely.The field of indefinite causal order in quantum mechanics has seen more and more interest in the past years, both theoretically [3] and experimen- tally [4]. In such processes, different parties act in a superposition of different orders. Since the first experimental realization of a process with an indefinite causal order, the quantum SWITCH, several protocols taking ad- vantage of this new resource have emerged. In previous experiments, the causal non-separability of two parties has been verified by measuring a so- called ’causal witness’ [5]. Nevertheless, the corresponding process matrix has only been evaluated theoretically. Here, we experimentally reconstruct the process matrix of a new passively-stable fiber-based architecture for the quantum SWITCH based on time-bin encoded qubits, which can readily be scaled to more parties. We perform a full characterization of this new type of quantum SWITCH by implementing causal process tomography for the first time. We then compare the tomography results to those obtained by directly measuring several different causal witnesses. Finally, we present the first measurement of the fidelity of our experimental quantum SWITCH to the ideal quantum SWITCH.