Quantum lock-in amplifier aims to extract an alternating signal within strong noise background by using quantum strategy. However, as the target signal usually has an unknown initial phase, we can't obtain the complete information of its amplitude, frequency and phase in a single lock-in measurement. Here, to overcome this challenge, we give a general protocol for achieving a quantum double lock-in amplifier and illustrate its realization. In analog to a classical double lock-in amplifier, our protocol is accomplished via two quantum mixers under orthogonal pulse sequences. The two orthogonal pulse sequences act the roles of two orthogonal reference signals in a classical double lock-in amplifier. Combining the output signals, the complete characteristics of the target signal can be obtained. As an example, we illustrate the realization of our quantum double lock-in amplifier via a five-level double-$Λ$ coherent population trapping system with $^{87}$Rb atoms, in which each $Λ$ structure acts as a quantum mixer and the two applied dynamical decoupling sequences take the roles of two orthogonal reference signals. Our numerical calculations show that the quantum double lock-in amplifier is robust against experimental imperfections, such as finite pulse length and stochastic noise. Our study opens an avenue for extracting complete characteristics of an alternating signal within strong noise background, which is beneficial for developing practical quantum sensing technologies.
Quantum double lock-in detection via sequential orthogonal quantum mixing
Shenzhen University（深圳大学）；Sun Yat-Sen University (Zhuhai Campus)（中山大学（珠海校区））；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area (Guangdong)（粤港澳大湾区量子科学中心（广东））
Over-8-dB squeezed light generation by a broadband waveguide optical parametric amplifier toward fault-tolerant ultra-fast quantum computers
NTT Device Technology Labs, NTT Corporation（NTT设备技术实验室，NTT公司）；Department of Applied Physics, School of Engineering, The University of Tokyo（东京大学工学部应用物理系）；Optical Quantum Computing Research Team, RIKEN Center for Quantum Computing（理研量子计算中心光学量子计算研究团队）
