Efficient and complete spin state readout of two electrons in a double quantum dot

Spin-qubits based on gate defined semiconductor quantum dots are a promising platform for quantum computation and simulation. However the readout fidelity of the spin states is still too low to reach the quantum error correction threshold. The only readout method for spin qubits approaching the threshold consists in a spin to charge conversion using the Pauli spin blockade (PSB) principle. However reaching a high fidelity readout using this technique is a trade-off between the time needed to distinguish the two charge states and the lifetime of those states. We demonstrate the operation of a Singlet-Triplet (S-T) qubit by controlling the inter-dot tunnel coupling from a few Hz to tens of GHz. This level of control allowed us to perform a PSB projection followed by a freeze of the charge configuration preventing any T₁ relaxation. Using this new measurement protocol the PSB projection becomes only limited by the charge configuration readout and we reach a fidelity as high as 97.7 % in 500 µs. Moreover the spin measurement becomes repeatable and non-destructive enabling the possibility to perform a complete readout of the state discriminating between S, T₀, T₊ and T_-. Thanks to this complete readout we benchmark diverse operation of the qubit on a complete set of states.