Large magnetoresistance in a quantum dot between two reservoirs with spin accumulation

Devices with ferromagnetic contacts exhibit magnetoresistance that is limited by the finite-contact spin polarization. Here, a quantum dot connected to source and drain reservoirs with $s\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n$ $a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ is shown to exhibit a million-fold magnetoresistance. For parallel spin accumulations, the current vanishes due to Pauli spin blockade. Charge flows freely for antiparallel spin accumulations. The device, a spin qubit transistor, can operate as a spin transistor but also as a spin qubit by reconfiguring the source-drain bias and gate voltage.