title: A self-sustaining atomic magnetometer with τ(-1) averaging property.
abstract: Quantum measurement using coherent superposition of intrinsic atomic states has the advantage of being absolute measurement and can form metrological standards. One example is the absolute measurement of magnetic field by monitoring the Larmor precession of atomic spins whilst another being the Ramsey type atomic clock. Yet, in almost all coherent quantum measurement, the precision is limited by the coherence time beyond which, the uncertainty decreases only as τ(-1/2). Here we show that by non-destructively measuring the phase of the Larmor precession and regenerating the coherence via optical pumping, the self-sustaining Larmor precession signal can persist indefinitely. Consequently, the precision of the magnetometer increases with time following a much faster τ(-1) rule. A mean sensitivity of 240  from 1 Hz to 10 Hz is realized, being close to the shot noise level. This method of coherence regeneration may also find important applications in improving the performance of atomic clocks.
authors.affiliation: Joint Institute for Measurement Science (JMI), Tsinghua University, Beijing 100084, China.|Department of Physics, Tsinghua University, Beijing 100084, China.
authors.forename: C
authors.initials: C
authors.lastname: Xu
authors.affiliation: Joint Institute for Measurement Science (JMI), Tsinghua University, Beijing 100084, China.|State Key Laboratory of Precision Measurement Technology and Instrument, Department of Precision Instruments, Tsinghua University, Beijing 100084, China.
authors.forename: S G
authors.initials: SG
authors.lastname: Wang
authors.affiliation: Joint Institute for Measurement Science (JMI), Tsinghua University, Beijing 100084, China.|State Key Laboratory of Precision Measurement Technology and Instrument, Department of Precision Instruments, Tsinghua University, Beijing 100084, China.
authors.forename: Y Y
authors.initials: YY
authors.lastname: Feng
authors.affiliation: Joint Institute for Measurement Science (JMI), Tsinghua University, Beijing 100084, China.|School of physics, Beihang University, Beijing 100191, China.
authors.forename: L
authors.initials: L
authors.lastname: Zhao
authors.affiliation: Joint Institute for Measurement Science (JMI), Tsinghua University, Beijing 100084, China.|Department of Physics, Tsinghua University, Beijing 100084, China.|State Key Laboratory of Precision Measurement Technology and Instrument, Department of Precision Instruments, Tsinghua University, Beijing 100084, China.
authors.forename: L J
authors.initials: LJ
authors.lastname: Wang