  We report on electron spin resonance spectroscopy measurements using a
superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out
using a frequency-tunable Josephson bifurcation amplifier, which leads to an
inferred measurement sensitivity of about 20 spins in a 1 s measurement. This
sensitivity represents an order of magnitude improvement when compared with
flux-qubit schemes using a dc-SQUID switching readout. Furthermore, noise
spectroscopy reveals that the sensitivity is limited by flicker ($1/f$) flux
noise.
