We give a derivation for the indirect interaction between two magnetic dipoles induced by the quantized electromagnetic field. It turns out that the interaction between permanent dipoles directly returns to the classical form; the interaction between transition dipoles does not directly return to the classical result, yet returns in the short-distance limit. In a finite volume, the field modes are highly discrete, and both the permanent and transition dipole-dipole interactions are changed. For transition dipoles, the changing mechanism is similar with the Purcell effect, since only a few number of nearly resonant modes take effect in the interaction mediation; for permanent dipoles, the correction comes from the boundary effect: if the dipoles are placed close to the boundary, the influence is strong, otherwise, their interaction does not change too much from the free space case.Comment: 8 pages, 2 figure

We determine quantum precision limits for estimation of damping constants and temperature of lossy bosonic channels. A direct application would be the use of light for estimation of the absorption and the temperature of a transparent slab. Analytic lower bounds are obtained for the uncertainty in the estimation, through a purification procedure that replaces the master equation description by a unitary evolution involving the system and ad hoc environments. For zero temperature, Fock states are shown to lead to the minimal uncertainty in the estimation of damping, with boson-counting being the best measurement procedure. In both damping and temperature estimates, sequential prethermalization measurements, through a stream of single bosons, may lead to huge gain in precision.

A Near-Infrared (NIR) measurement method based on a digital orthogonal-vector lock-in amplifier (LIA) is presented in this paper. NIR sky background radiation is very weak; to detect the signals obscured by noise, our approach is to use a chopper to modulate the detected signal and demodulate it using an LIA . The effect of the 1/f noise of the detector, dark current and other noise sources can be reduced to improve Signal-to-Noise Ratio (SNR) and the detected signal will be obtained. The orthogonal vector LIA avoids phase shift and achieves high precision measurements using two orthogonal components. In order to simplify the system, data are digitized by an Analog-to-Digital Converter (ADC) and a digital algorithm, running in a Microcontroller Unit (MCU) with ARM cortex-M4, is adopted to implement the LIA . In our scheme, the signal of the detector is amplified and filtered. Then, Phase Sensitive Detection (PSD), Low-Pass Filtering (LPF) and amplitude phase calculation are performed using the digital LIA method. The digital method leads to a greatly simplified circuit design, and adjustment of the time constant of the LPF allows achieving different Equivalent Noise Bandwidths (ENBs) conveniently. The method has the advantage of high precision, flexible usage, simple implementation and low computational resource requirements. Using this method, weak infrared signal submerged in the noise can be picked up easily, which extremely improves the detection capability of the system.

No abstract