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Nonlinear interferometry beyond classical limit facilitated by cyclic dynamics

Time-reversed evolution has substantial implications in physics, including prominent applications in refocusing of classical waves or spins and fundamental researches such as quantum information scrambling. In quantum metrology, nonlinear interferometry based on time reversal protocols supports entanglement-enhanced measurements without requiring low-noise detection. Despite the broad interest in time reversal, it remains challenging to reverse the quantum dynamics of an interacting many-body system as is typically realized by an (effective) sign-flip of the system's Hamiltonian. Here, we present an approach that is broadly applicable to cyclic systems for implementing nonlinear interferometry without invoking time reversal. Inspired by the observation that the time-reversed dynamics drives a system back to its starting point, we propose to accomplish the same by slaving the system to travel along a 'closed-loop' instead of explicitly tracing back its antecedent path. Utilizing the quasi-periodic spin mixing dynamics in a three-mode $^{87}$Rb atom spinor condensate, we implement such a 'closed-loop' nonlinear interferometer and achieve a metrological gain of $3.87_{-0.95}^{+0.91}$ decibels over the classical limit for a total of 26500 atoms. Our approach unlocks the high potential of nonlinear interferometry by allowing the dynamics to penetrate into deep nonlinear regime, which gives rise to highly entangled non-Gaussian state. The idea of bypassing time reversal may open up new opportunities in the experimental investigation of researches that are typically studied by using time reversal protocols.