A current of spin-polarized electrons senses and controls the magnetic state of nanostructured materials1. Obtaining similar electrical access to quantum spin systems, such as single-molecule magnets, is still in its infancy. Recent progress has been achieved by probing the spin system near thermal equilibrium. However, it is the elusive non-equilibrium properties of the excited states that govern the time evolution of such structures and will ultimately establish the feasibility of applications in data storage and quantum information processing. Here we use spin-polarized scanning tunnelling microscopy to pump electron spins of atoms on surfaces into highly excited states and sense the resulting spatial orientation of the spin. This electrical control culminates in complete inversion of the spin-state population and gives experimental access to the spin relaxation times of each excited state. The direction of current flow determines the orientation of the atom’s spin, indicating that electrical switching and sensing of future magnetic bits is feasible in the quantum regime.
A universal quantum simulator is a controlled quantum device that reproduces the dynamics of any other many-particle quantum system with short-range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open-system evolution. Here we propose that laser-excited Rydberg atoms in large-spacing optical or magnetic lattices provide an efficient implementation of a universal quantum simulator for spin models involving n-body interactions, including such of higher order. This would allow the simulation of Hamiltonians of exotic spin models involving n-particle constraints, such as the Kitaev toric code, colour code and lattice gauge theories with spin-liquid phases. In addition, our approach provides the ingredients for dissipative preparation of entangled states based on engineering n-particle reservoir couplings. The basic building blocks of our architecture are efficient and high-fidelity n-qubit entangling gates using auxiliary Rydberg atoms, including a possible dissipative time step through optical pumping. This enables mimicking the time evolution of the system by a sequence of fast, parallel and high-fidelity n-particle coherent and dissipative Rydberg gates.