Spin blockade and exchange in Coulomb-confined silicon double quantum dots

Electron spins confined to phosphorus donors in silicon are promising candidates as qubits(1) because of their long coherence times, exceeding seconds in isotopically purified bulk silicon(2). With the recent demonstrations of initialization, readout(3) and coherent manipulation(4) of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling(1,5,6) in two tunnel-coupled phosphorus donors. Spin-to-charge conversion(3,7) via Pauli spin blockade(8,9), an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (similar to 45 meV), requiring large electric fields (>1 MV m(-1)) to transfer both electron spins onto the same donor(10). Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems.