Special relativity was proposed by A. Einstein in 1905. Relativistic phenomena such as the time dilation and the rest mass energy has been confirmed in experiments. However, the relativistic Coulomb field around a highly energetic charged particles propagates in vacuum with nearly speed of light has never been demonstrated directly due to the lack of ultrafast field measurement with a temporal resolution of femtosecond. Here, we utilized a terahertz technique based on electro-optic sampling, and visualized relativistic Coulomb field around a highly energetic electron beam generated by a linac. The obtained spatiotemporal electric field profile shows the contracted electric field around an electron beam, as predicted by special relativity. Moreover, we studied the birth of the relativistic Coulomb field by passing the electron beam through a metallic foil. Our results provide a new experimental evidence of the theory of relativity.

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We have performed the gravitational redshift measurement by using a pair of transportable optical lattice clocks located on the ground floor and on the observatory floor of the Tokyo SKYTREE, separated by an altitude of 450 m. We have observed a corresponding gravitational redshift of 21.1771(18) Hz. By comparing the observed shift with the predicted gravitational redshift by general relativity, evaluated using the measurements of global navigation satellite system, spirit leveling, laser ranging and gravity, the deviation of the observed gravitational redshift is found to be α = (1.3 ± 9.1) × 10−5, which is two-orders of magnitude better than any other ground-based experiments and is comparable with the findings of space borne experiments.

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Ultra-precise optical atomic clocks show promise as the next-generation frequency standard, as well as quantum sensors for exploring new physics beyond the standard model. This article reviews our attempts to search for new physics through precise spectroscopy of neutral ytterbium atoms. Neutral ytterbium atoms have multiple clock transitions and many stable isotopes, making them suitable for the search for new physics. By measuring the isotope shift of the 1S0 − 3P0 transition at the Hz level, we can observe the nonlinearity of the generalized King’s plot. Additionally, a new clock transition from the ground state to the metastable 4f 135d6s2 (J = 2) state, which was theoretically predicted, is directly observed, and investigations are carried out on its isotope shift, g factor, hyperfine structure, magic wavelength, and trap lifetime. These achievements open up the possibility of future new physics searches.

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Experimental techniques to manipulate cold molecules have seen great development in recent years. The precision measurements of cold molecules are expected to give insights into fundamental physics. Here we use a rovibrationally pure sample of ultracold KRb molecules to improve the measurement on the stability of electron-to-proton mass ratio (μ = me/mp). The measurement is based upon a large sensitivity coefficient of the molecular spectroscopy, which utilizes a transition between a nearly degenerate pair of vibrational levels each associated with a different electronic potential. Observed limit on temporal variation of μ is (dμ/dt)/μ = (0.30 ± 1.0) × 10−14/year, which is better by a factor of five compared with the most stringent laboratory molecular limits to date.

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Precision spectroscopy has been developed for decades using different methods based on high-resolution laser spectroscopy. The ultimate results of precision spectroscopy are optical clocks with ultra-high coherence and accuracy. The redefinition of the second using optical clocks is being discussed and preliminarily scheduled in 2030. Optical clocks are also used to perform the research on the validation the constancy of fundamental constants, including the fine structure constant.

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Vacuum magnetic birefringence (VMB) is an anisotropy of the refractive index of the vacuum under a magnetic field. VMB is predicted to be induced by virtual electron-positron pairs or undiscovered light particles in the vacuum. Our OVAL experiment is trying to observe VMB with a high-finesse Fabry- Pérot cavity and a pulsed magnet. The setup and the current status of the OVAL experiment is reported.

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