4 ordered couplings, partial control and decoherence. Acknowledgments . This work was supported by the NSF, ITAMP, DARPA and the Packard Foundation. 2 0 4 6 8 0.2 0.4 0.6 0.8 1.0 2 0 4 6 8 0.2 0.4 0.6 0.8 1.0 0 2 4 6 0.2 0.4 0.6 0.8 1.0 0 2 4 6 0.2 0.4 0.6 0.8 1.0 n$_{c}$=8  n$_{c}$=12  n$_{c}$=25  n$_{c}$=50  time time time time FIG. 3: Simulations. Normalized signal decay for the pro- posed (red, solid lines), spin-echo sequence (blue, dashed lines) and no control (black, dotted lines). We assumed per- fect delta function pulses. A leading order cluster expansion was used [23]. 20 dark spins were randomly placed in a cube of side-length 3 √$_{20 with the sensor spin at the center.}$ We set gµ$_{B}$ ≡ 1 [ m ] 3 / $^{2}$[ s ] − 1 / 2 to use dimensionless quantities. WAHUHA sequences [17] with n$_{c}$ =8, 12, 25 and 50 cycles per echo interval were simulated. Each curve is an average over 10 Monte Carlo simulations. For simplicity we set P = 0. quence is applied (for the same environment the decay is now described by the dephasing time T ∗ $_{2}$). We sim- ulated a spin bath composed of spin 1/2 paramagnetic impurities, undergoing a WAHUHA sequence (which is designed to refocus the dipole-dipole coupling of the en- vironment spins, but does not cancel out the coupling to the external field [17, 26]). A different limitation on the sensing time τ is set by the fact that the orientation of the dark spin will not be static as assumed. Dipole-dipole couplings among dark spins during each τ / 4 period of free evolution rotate each spin away from the initial direction. This rotation means that the spins cease to build up a phase difference between the two arms of the interferometer for time scales comparable to the correlation time of the dark spin bath τ d c . The optimum sensing time is thus T ∼ min { T s $_{2}$, τ d c } . Since in most systems τ d c ≥ T s 2 [17], the optimum sensing time of this pulse sequence is comparable to that of spin-echo based magnetometry, thus the sensitivity enhancement is roughly the same as the signal strength enhancement. In conclusion, we proposed a scheme to enhance pre- cision measurement by exploiting the possibility to co- herently control the ancillary qubits. In solid state implementations we are able to exploit dark spins in the bath while preserving roughly the same coherence times as in spin-echo based magnetometry. Thus sig- nal enhancement leads directly to sensitivity enhance- ment. For trapped ion implementations we can use im- perfect phase gates and still achieve Heisenberg-limited sensitivity. Our method has the potential to be applied more generally, using different systems and more sophis- ticated pulse sequences [26]. 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However the improve- ment in coherence times for spin echo scales poorly as ∼ 4 √$_{1}$ − P 2 and homonuclear decoupling sequences such as WAHUHA can be used to reduce the flip-flop effects.