Sensing past the quantum limit

Manijeh Razeghi, Gail J. Brown

Byoung Seung HAM

Interferometer-based precision measurements have been intensively studied for sensing and metrology over the past half century. In classical optics, the resolution and phase sensitivity of an optical signal are confined by diffraction limit and shot-noise limit (SNL), respectively. Highly entangled photon pairs, i.e., N00N states have been adapted to overcome SNL in quantum sensing over the last two decades. Recently, coherent light-excited quantum sensing has also been proposed and demonstrated for macroscopic quantum sensing to overcome the limited N scalability in N00N-based quantum sensing. Here, a Sagnac interferometer-based superresolution is proposed to solve environmental noises inevitable in an interferometer. Furthermore, a spatial light modulator takes over the role of phase-controlled quantum erasers to solve the linear optics-based complexity issue in the coherently-excited superresolution. Thus, the proposed Sagnac superresolution can beat the state-of-the-art ring laser gyroscope applied for inertial navigation and geodesy.

Lev Vaidman A

Although there is no consensus regarding the ``reality'' of the past of a quantum particle, in situations where there is only one trajectory with a nonvanishing quantum wave of the particle between its emission and detection points, it seems ``safe'' to associate the past of the particle with this trajectory. A method for analyzing the past of a quantum particle according to the weak trace it leaves is proposed. Such a trace can be observed via measurements performed on an ensemble of identically pre- and postselected particles. Examples in which this method contradicts the above common sense description of the past of the particle are presented. It is argued that it is possible to describe the past of a quantum particle, but the naive approach has to be replaced by both forward- and backward-evolving quantum states.

Mehrdad Tahmasbi, Matthieu R. Bloch

We formalize the problem of active covert sensing, in which a legitimate user wants to not only sense an unknown parameter but also remain undetectable from an adversary by actively controlling the actions that generate observations. We characterize the optimal achievable error exponent when the actions of the legitimate user do not depend on the past observations. When the actions are allowed to depend on the past observations, we provide an example showing the benefits of adaptivity.