Fig. 2. Overview of the DIQKD system.

a, Alice’s equipment (Device 1 in Lab 1) is formed by a single-atom trap and a BSM set-up. Bob (Device 2 in Lab 2) uses a second single-atom trap together with a 90:10 (T:R) beam splitter (BS) and a single-photon detector (SPD). Each trap set-up contains a high numerical aperture (NA) objective to optically trap a single atom and collect atomic fluorescence into a single-mode (SM) fibre. The atoms are entangled in an event-ready scheme by synchronously exciting them, after which the spontaneously emitted photons are collected by high-NA objectives and guided to the BSM. Here, a coincidental photon detection on two detectors in the same output arm of the fibre BS heralds the entangled atom–atom state $|{\mathrm{\Psi }}^{+}⟩$, which is announced to both users by a ‘ready’ signal. After receiving the ready signal, two quantum random number generators (QRNGs) select the inputs to the devices, determining the polarization of a read-out pulse in a state-selective ionization scheme. The binary output of the devices is determined from a fluorescence measurement of atom presence after the ionization attempt, as ionized atoms are lost from the trap. The inputs and outputs of each round are stored locally using a trusted storage. In Lab 2 a spectral filter and shutter are implemented to avoid leakage of the inputs and outputs of the device. b, Map showing the main campus of the LMU in Munich, indicating the locations of the two laboratories. Map data in b are from Bayerische Vermessungsverwaltung .