Readout Of Singlet-Triplet Spin Qubits At Large Magnetic Field Gradients

Singlet-triplet qubits are one particularly successful implementation of spin qubits that has recently demonstrated improved immunity to charge noise by applying intense magnetic field gradients. However, large magnetic field gradients allow for relaxation between qubit states during measurement, reducing readout visibility to almost zero. Here we present a new technique that is robust against these relaxation pathways and enables working above magnetic field gradients of 400 MHz, a regime that unaccessible using previously studied methods. This technique maps the qubit states onto readout states that are immune to relaxation by adding a new shelving step, immediately before measurement. Shelving transitions are enabled by spin dependent electron tunneling processes between the qubit and the lead. We demonstrate readout contrast up to the largest magnetic field gradients we can generate, almost 1 GHz, and expect this method should be continue to be robust against gradients several times as strong. We also experimentally probed this readout’s performance as a function of applied magnetic field, the magnitude and duration of the shelving pulse applied to the qubit. We developed a model that we use to simulate the readout and find strong agreement with the data. The model and concept of using a shelving step before measurement have the potential to improve readout in a wide variety of qubits.