A new approach to optical phase diffusion

Stephen G. Benka

Hongwei Xiong, Shujuan Liu, Guoxiang Huang +1 more

We investigate the phase diffusion of a Bose–Einstein condensate (BEC) confined in the combined potential of a magnetic trap and a one-dimensional optical lattice. We show that the phase diffusion of the condensate in the whole optical lattice is evident and can be measured from the interference pattern of expanding subcondensates after the optical lattice is switched off. The maximum density of the interference pattern decreases significantly due to the mixing of the phase diffusion that appears in different subcondensates. This suggests a way to detect experimentally the notable phase diffusion of BECs.

Ben Wang, Minghao Mi, Huangqiuchen Wang +2 more

Accurate phase estimation plays a pivotal role in quantum metrology, yet its precision is significantly affected by noise, particularly phase-diffusive noise caused by phase drift. To address this challenge, the joint estimation of phase and phase diffusion has emerged as an effective approach, transforming the problem into a multi-parameter estimation task. However, the incompatibility between optimal measurements for different parameters prevents single-copy measurements from reaching the fundamental precision limits defined by the quantum Cramer-Rao bound. Meanwhile, collective measurements performed on multiple identical copies can mitigate this incompatibility and thus enhance the precision of joint parameter estimation. This work experimentally demonstrates joint phase and phase-diffusion estimation using deterministic Bell measurements on a two-qubit system. A linear optical network is employed to implement both parameter encoding and deterministic Bell measurements, achieving improved estimation precision compared to any separable measurement strategy. This work proposes a new framework for phase estimation under phase-diffusive noise and underscores the substantial advantages of collective measurements in multi-parameter quantum metrology.

B. Havelka, J. Blabla

P. D. Drummond, M. D. Reid

We calculate the squeezing or quadrature spectrum of a nondegenerate parametric amplifier in an interferometer. Squeezing is found to occur below and above threshold. The new feature in the above threshold behavior is the existence of spontaneous phase diffusion in the transmitted signal, even with a stable pump. A novel technique is developed to calculate the quadrature fluctuations in the presence of phase diffusion. We find that squeezing occurs but is restricted to relatively high frequencies due to the phase diffusion. Another application of our theory is in the case of a subharmonic oscillator with a phase-diffusing pump. Squeezing is maintained here also, except at low frequencies. The presence of phase diffusion in quadrature phase sensitive detection can be expected to be universal. In all cases it tends to reduce or destroy the low-frequency squeezing and thereby increases low-frequency noise in a phase sensitive measurement. Our new stochastic technique, therefore, has a wide range of uses.

Lambertus Hesselink