Phys. Rev. X 8, 021012 (2018)

It is a widely held view that a large number of qubits will be required for quantum computers to prove beneficial and offer supremacy over existing solutions. However, the generation and detection of complex multi-qubit entangled states remains an open challenge. Now, Nicolai Friis and co-workers from Austria and Germany have succeeded in generating and characterizing complex entangled states of 20 trapped-ion qubits. The 20 qubits were created from a one-dimensional string of 40Ca+ ions confined in a linear Paul trap. The string length was 108 μm. The qubit was encoded into two long-lived states of the outer valence electron in each ion. Laser beams were used to rotate the basis of individual qubits. The team succeeded in detecting genuine multipartite entanglement in groups of up to 5 qubits within the 20-qubit system. They found that every qubit simultaneously became genuine multipartite entangled with at least two of its neighbours and, in most cases, three and four of its neighbours. Each qubit could be individually controlled and qubit–qubit interactions could be tuned on and off as desired, opening the way for universal quantum simulation and quantum computation.