Quantum Sensors, like atom interferometers (AI), can be employed forhigh-precision measurements of inertial forces, including their applicationas gravimeters, gradiometers, accelerometers, and gyroscopes.Their measurement principle relies on ultracold atoms that are preparedin quantum-mechanical superposition states in external degreesof freedom. These states can be prepared by a momentum transfer ofa Raman laser. Then the superposition state senses the effect of an inertialforce, which induce a corresponding relative phase. The phase isread out by a final coupling which converts the interferometric phaseinto a atom number difference between the two states. The differenceprovides an estimate of the interferometric phase and the correspondingquantity of interest. The quantum mechanical noise of the atomicensemble cause a fundamental uncertainty of this estimation, which Ianalyze for generic AIs. For small atomic densities, the quantum phasenoise of the ensemble limits the interferometric sensitivity. For largedensities, quantum number fluctuations generate density fluctuations,which generates phase noise. I show that these two competing effectsresult in an optimal atom number with a maximal interferometer resolution.Squeezed atomic samples allow for a reduction of the quantumnoise of one quantity at the expense of an increased noise along of aconjugate quantity. Phase and number are such quantities which obeyto a variant of Heisenberg’s uncertainty principle. Neither phase nornumber squeezing can improve the maximal interferometer resolution.As one main result of this thesis, I show how an optimal squeezingin between number and phase squeezing, allows for a fundamentalimprovement. I evaluate possible experimental paths to implementthe proposed protocol.Concepts for a squeezing-enhanced operation of external-degreeAIs have not yet been demonstrated. I propose and implement anatomic gravimeter, which is designed to accept spin-squeezed atomicstates as input states. The interferometer is designed such that theinterferometer couplings are performed in spin space, while the phaseaccumulation is performed in momentum states. For this interferometer,the squeezed input can be directly obtained from spin dynamicsin spinor Bose-Einstein condensates. The main noise contributions inthe experiment are analyzed, which results in a factor of 84 abovethe relevant quantum limit, preventing a squeezing enhancement sofar. I outline a suppression of the main noise source, uncontrolledAC Stark shift on the squeezed mode and propose future importantapplications, including test of spontaneous collapse theories and animprovement of large-scale, high-precision gradiometers.