Quantum enhanced metrology in the optical lattice clock

Abstract

High-bandwidth, high-stability clocks offer unique chances to study fundamental physics beyond simple metrology. Today, the most stable clocks are optical lattice clocks and they can reach a fractional stability of 3.1 × 10^−18 at 1 s averaging time. Their applications are ranging from the redefinition of the SI time unit, the Second, to the possible detection of gravitational waves in space. The stability of optical lattice clocks is primarily limited by quantum projection noise and Dick noise. In this thesis, I present our effort in creating a platform for studying quantum enhanced metrology and its application towards optical lattice clocks. We demonstrated the laser cooling in an optical cavity with Raman-sideband cooling and achieved average vibration number of n = 0.2. We utilized cavity feedback squeezing to demonstrate unitary spin squeezing on magnetic sub-levels of 171Yb, achieving 6.5 dB quantum projection noise reduction over the standard quantum limit (SQL). With coherent optical state transfer by ultra stable laser pulses, we successfully transferred the spin squeezed state from magnetic sub-levels to the optical transition, and demonstrated quantum enhanced metrology in the optical lattice clock for the first time. We achieved 4.4 dB of metrological gain over the SQL, which corresponds to a reduction of the averaging time by a factor of 2.8 in clock operation. Furthermore, we studied quantum enhanced metrology with time-reversal-based protocols. We performed a signal amplification through time-reversed interaction protocol between the ground state magnetic sublevels of 171Yb, achieving the largest sensitivity improvement beyond SQL in any interferometers to date at 11.8dB. Besides that, we experimentally studied the connection between quantum information scrambling, by measuring out-of-time-order correlators (OTOCs), and quantum metrology, by measuring metrological gain. We found these values to agree with each other, and concluded that the time-reversal quantum metrology can saturate the attainable gain from the quantum information scrambling.