1. Skolkovo Institute of Science and Technology, Nobel St. 3, Moscow 143026, Russia
2. Laboratory of Artificial Quantum Systems, Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
3. Physics Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom
4. National Physical Laboratory, Teddington TW11 0LW, United Kingdom