  Topological spin textures such as magnetic skyrmions hold considerable
promise as robust, nanometre-scale, mobile bits for sustainable computing. A
longstanding roadblock to unleashing their potential is the absence of a device
enabling deterministic electrical readout of individual spin textures. Here we
present the wafer-scale realization of a nanoscale chiral magnetic tunnel
junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical
and multi-modal imaging techniques, we show that the MTJ nucleates skyrmions of
fixed polarity, whose large readout signal - 20-70% relative to uniform states
- corresponds directly to skyrmion size. Further, the MTJ exploits
complementary mechanisms to stabilize distinctly sized skyrmions at zero field,
thereby realizing three nonvolatile electrical states. Crucially, it can write
and delete skyrmions using current densities 1,000 times lower than
state-of-the-art. These results provide a platform to incorporate readout and
manipulation of skyrmionic bits across myriad device architectures, and a
springboard to harness chiral spin textures for multi-bit memory and
unconventional computing.
