Quantum networking with time-bin encoded qu-d-its using single photons emitted on demand from an atom-cavity system

Summary form only given. One of today´s challenges to realize computing based on quantum mechanics is to reliably and scalably encode information in quantum systems. This is particularly crucial when dealing with promising approaches such as linear optics quantum computing (LOQC) in photonic circuits [1,2]. Although this method is in principle scalable, in practice it is limited by the probabilistic nature of spontaneous parametric down-conversion (SPDC) sources used to seed the circuits.Here, we demonstrate a novel tool to deterministically deliver quantum bits of information for LOQC circuits with the help of single photons emitted from a strongly coupled atom-cavity system (Fig. [1a]). Due to the ≈ 500ns coherence time of these photons [3] and our ability to arbitrarily shape their amplitude and phase profile we time-bin encode the information within one photon. Therefore, the preparation of a single photon sub-divided in d time bins allows for the delivery of arbitrary qu-d-its. RCPWe verify the fidelity of the quantum state preparation with time-resolved quantum-homodyne measurements [4], performed by sending single signal and reference photons into an elementary photonic circuit. Photon correlations monitored in a time-resolved manner then allow the state to be partially reconstructed. To obtain more insight into the time-bin encoding of information within one photon, it is helpful to define a `virtual´ photonic circuit (Fig. [1b]) where temporal modes are displayed as spatial ones. Due to the photons being much longer than the detector time resolution, the absolute time of each photo-detection is used to associate the event with one of these virtual detectors. In this way, the respective number of cross-correlation signals for qubits and qutrits, respectively, can be found (Fig. [1c]). We obtain photon production efficiencies up to 85% and state-preparation fidelities of F=96% [4]. Due to the timebin encoding taking place upon the photon´s generat- on process, our technique is inherently non-probabilistic, versatile, reconfigurable and not subject to systematic photon losses. Time-bins as an additional degree of freedom in LOQC experiments and an their deterministic photon-generation scheme can be regarded as a big step towards large-scale quantum computing in photonic networks.