ﻻ يوجد ملخص باللغة العربية
Quantum state smoothing is a technique to construct an estimate of the quantum state at a particular time, conditioned on a measurement record from both before and after that time. The technique assumes that an observer, Alice, monitors part of the environment of a quantum system and that the remaining part of the environment, unobserved by Alice, is measured by a secondary observer, Bob, who may have a choice in how he monitors it. The effect of Bobs measurement choice on the effectiveness of Alices smoothing has been studied in a number of recent papers. Here we expand upon the Letter which introduced linear Gaussian quantum (LGQ) state smoothing [Phys. Rev. Lett., 122, 190402 (2019)]. In the current paper we provide a more detailed derivation of the LGQ smoothing equations and address an open question about Bobs optimal measurement strategy. Specifically, we develop a simple hypothesis that allows one to approximate the optimal measurement choice for Bob given Alices measurement choice. By optimal choice we mean the choice for Bob that will maximize the purity improvement of Alices smoothed state compared to her filtered state (an estimated state based only on Alices past measurement record). The hypothesis, that Bob should choose his measurement so that he observes the back-action on the system from Alices measurement, seems contrary to ones intuition about quantum state smoothing. Nevertheless we show that it works even beyond a linear Gaussian setting.
Quantum state smoothing is a technique for estimating the quantum state of a partially observed quantum system at time $tau$, conditioned on an entire observed measurement record (both before and after $tau$). However, this smoothing technique requir
Here, we are concerned with comparing estimation schemes for the quantum state under continuous measurement (quantum trajectories), namely quantum state filtering and, as introduced by us [Phys. Rev. Lett. 115, 180407 (2015)], quantum state smoothing
Quantum state smoothing is a technique to estimate an unknown true state of an open quantum system based on partial measurement information both prior and posterior to the time of interest. In this paper, we show that the smoothed quantum state is an
We develop a practical quantum tomography protocol and implement measurements of pure states of ququarts realized with polarization states of photon pairs (biphotons). The method is based on an optimal choice of the measuring schemes parameters that
Rather than point estimators, states of a quantum system that represent ones best guess for the given data, we consider optimal regions of estimators. As the natural counterpart of the popular maximum-likelihood point estimator, we introduce the maxi