Schematic of deflection of a “charged” photon inside a parallel-plate capacitor where the electric field is directed downward in the z-direction.
Schematic diagram of the experimental setup. The strength of the electric field V/d across the parallel plates is modulated with the upper plategrounded, and the subsequent light “deflection” is measured from the lock-in output.
Calibration for the lock-in amplifier at a modulation frequency of 250 Hz. The larger the voltage applied to the PZT drive, the greater its displacement and hence the larger the output response from the lock-in. Care is taken not to drive the PZT with a negative voltage (an offset voltage is applied to ensure the modulation signal is always positive). Similar calibration data are also obtained at different modulation frequencies from 100 to 250 Hz.
Schematic of a Michelson's interferometer (top) and interference patterns (bottom). The intensity resulting from an interference between two light beams, one traveling directly into the photodiode and the other traveling a longer distance and being reflected from a mirror attached to the PZT. As the PZT moves the intensity varies from minimum (destructive) to maximum (constructive) with a visibility larger than 90%. Periodicity of , where nm enables a direct calibration of the PZT's applied voltage to the actual position displacement.
Circuit diagram of a relaxation oscillator (top) and period measurements (bottom). The period of the relaxation oscillator is proportional to the external capacitance: . A motorized actuator moves one of the plates thereby changing their separation distance. The absolute distance is not known a priori, and measured data are fit to a function , where pF in the limit of , d 0 is a point of contact to be determined from the fit, and dr is the relative position of the actuator. Finally, β contains the information regarding the effective size of the plate as well as the permittivity constant in between.
Schematic diagram to determine the speed of light (top) and phase change versus distance (bottom). The laser used in our experiment is modulated at 15 MHz and is split into two different paths. Delay times associated with different travel lengths are then measured in terms of phase changes between two photodetectors. 28 A linear fit to the data leads to m/s. Modulation frequencies ranging from 1 to 15 MHz are used, leading to similar results.
Status of upper limits on the photon charge. There have been several laboratory tests to constrain the photon charge, three employing electric fields 17,18,20 and the most recent laboratory test involving magnetic deflection. 16 Even the best laboratory limit 20,21 is about ten orders of magnitude weaker than the limit obtained from charge asymmetry in the CMB data, 14 which is the least stringent bound among those based on astrophysical observations.
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