Quantum metrology with a scanning probe atom interferometer

Atom interferometers provide record precision in measurements of a broad range of physical quantities. Extending atom interferometry to micrometer spatial resolution would enable new applications in electromagnetic field sensing, surface science, and the search for fundamental short-range interactions. I present experiments where we use a small Bose-Einstein condensate on an atom chip as an interferometric scanning probe to map out a microwave field at distances down to $16$ micrometer from the chip surface with a few micrometers spatial resolution. By creating entanglement between the atoms, our interferometer overcomes the standard quantum limit of interferometry by 4 dB in variance and maintains enhanced performance for interrogation times up to 10 ms. This corresponds to a microwave magnetic field sensitivity of 77 pT/sqrt(Hz) in a probe volume of 20 cubic micrometer. High-resolution measurements of microwave near-fields, as demonstrated here, are important for the development of integrated microwave circuits for quantum information processing and applications in communication technology.
Quantum metrology with entangled atoms is particularly useful in measurements with high spatial resolution, since the atom number in the probe volume is limited by collisional losses. I analyze the effect of such density-dependent losses in high-resolution atom interferometry, and show that there is a strict upper limit on the useful number of atoms. Our experimental results indicate that even tighter limits on the particle number and interrogation time may arise from density-dependent dephasing, and provide a starting point for future studies towards the fundamental limits of coherence in Bose-Einstein condensates. Our experimental setup is ideally suited to experimentally address these questions, and provides a promising platform for further studies on quantum metrology and entanglement in many-particle atomic systems.