Recently, the subradiant states of one-dimensional two-level atom chains coupled to light modes were found to have decay rates obeying a universal scaling, and an unexpected fermionic character of the multiply-excited subradiant states was discovered. In this Letter, we theoretically obtain the singly-excited subradiant states, and by eliminating the superradiant modes, we demonstrate a relation between the multiply-excited subradiant states and the Tonks-Girardeau limit of the Lieb-Liniger model which explains the fermionic behavior. In addition, we identify a new family of states with correlations different from the fermionic ansatz.

We study quantum measurement with preselection and postselection, and derive the precise expressions of the measurement results without any restriction on the coupling strength between the system and the measuring device. For a qubit system, we derive the maximum pointer shifts by choosing appropriate initial and finial states. A significant amplification effect is obtained when the interaction between the system and the measuring device is very weak, and typical ideal quantum measurement results are obtained when the interaction is strong. The improvement of the signal-tonoise ratio (SNR) and the enhancement of the measurement sensitivity (MS) by weak measurements are studied. Without considering the probability decrease due to postselection, the SNR and the MS can be both significantly improved by weak measurements; however, neither SNR nor MS can be effectively improved when the probability decrease is considered.Comment: 9 pages, 4 figure

We show that chains of atoms coupled to a 1D waveguide support states with two excitations that have longer lifetimes than the most subradiant states with only a single excitation. These excitations form spatially correlated dimers where one excited atom effectively constitutes a defect (a site blocking further excitation) and establishes a localized mode for the other excitation. We investigate the properties of the dimer states, and we show that our results apply also to chains of atoms coupled to the free electromagnetic vacuum field in three dimensions.

No abstract

While the novel applications of weak values have recently attracted wide attention, weak measurement, the usual way to extract weak values, suffers from risky approximations and severe quantum noises. In this paper, we show the weak-value information can be obtained exactly in strong measurement with post-selections, via measuring the coupling-deformed pointer observables, i.e., the observables selected according to the coupling strength. With this approach, we keep all the advantages claimed by weak-measurement schemes and at the same time solve some widely criticized problems thereof, such as the questionable universality, systematical bias, and drastic inefficiency. PACS numbers: 03.65.Wj, 03.65.Ta, 42.50.Dv, 03.67.-a II. AHARONOV, ALBERT, AND VAIDMAN'S FORMALISM OF WEAK VALUEIn this section, we shall briefly introduce AAV's perturbation formalism of weak measurements, and the applications of weak value in quantum state tomography.