A team of researchers from the University of Vienna and the Austrian Academy of Sciences (ÖAW) has introduced a novel technique to detect entanglement even in large-scale quantum systems. Here is how the technique works: After a quantum system has been generated in the laboratory, the scientists choose specific quantum measurements that are then applied to the system. The results of these measurements lead to either confirming or denying the presence of entanglement. The scientists’ confidence in the existence of entanglement grows exponentially with the number of individual detection events, namely copies of the quantum state. “It is somehow similar to asking certain ‘yes/no’ questions to the quantum system and noting down the given answers. The more positive answers are given, the higher the probability that the system exhibits entanglement,” researcher Valeria Saggio said. Artistic impression of entanglement detection. The stream of green and red lights represents the answers required by the protocol, thus revealing the presence of entanglement between photons. Courtesy of Rolando Barry/University of Vienna. To benchmark their findings, the researchers experimentally verified the presence of entanglement in a photonic six-qubit cluster state generated using three single-photon sources operating at telecommunication wavelengths. The results showed that only a few experimental runs were needed to confirm the presence of entanglement with extremely high confidence. The team found that the presence of entanglement could be certified with at least 99.74% confidence by detecting 20 copies of the quantum state. Additionally, the team showed that genuine six-qubit entanglement could be verified with at least 99% confidence by using 112 copies of the state. Efficient methods for entanglement detection have been developed for systems containing only a few qubits; but verifying entanglement in large systems is challenging and time-consuming with existing methods, since many repeated experimental runs are required. The Vienna team’s protocol can be carried out with a relatively low number of copies and in the presence of experimental imperfections, making it a practical method to verify large-scale quantum devices. Moreover, in certain cases the number of questions needed to confirm entanglement is independent of the size of the system. According to the researchers, the new technique is orders of magnitude more efficient than conventional methods. While the physical implementation of a quantum computer still faces challenges, new advances such as efficient entanglement verification could move the field forward, thus contributing to the progress of quantum technologies. The research was published in Nature Physics (https://doi.org/10.1038/s41567-019-0550-4).