  Efficient detection of magnetic fields is central to many areas of research
and has important practical applications ranging from materials science to
geomagnetism. High sensitivity detectors are commonly built using direct
current-superconducting quantum interference devices (DC-SQUIDs) or atomic
systems. Here we use a single artificial atom to implement an ultrahigh
sensitivity magnetometer with a size in the micron range. The artificial atom
is a superconducting two-level system at low temperatures, operated in a way
similar to atomic magnetometry. The high sensitivity results from quantum
coherence combined with strong coupling to magnetic field. By employing
projective measurements, we obtain a sensitivity of $2.7\,
\t{pT}/\sqrt{\t{Hz}}$ at 10 MHz. We discuss feasible improvements that will
increase the sensitivity by over one order of magnitude. The intrinsic
sensitivity of this method to AC fields in the 100 kHz - 10 MHz range compares
favourably with DC-SQUIDs and atomic magnetometers of equivalent spatial
resolution. This result illustrates the potential of artificial quantum systems
for sensitive detection and related applications.
