Zhong-Kun Hu

Determination of fundamental frequency of a physical oscillator by the period fitting method

A period fitting method, which is a variant of the classical least-square fitting method, is proposed to determine the fundamental frequency of a physical oscillator. The root mean square deviation used as the criterion in this method is a single-parameter function of the fundamental frequency of the oscillator, so it makes the fitting process optimize the period fit at the expense of a lesser evaluation of the other parameters such as the amplitude and the phase. Theoretical analysis shows that this method is intrinsically independent of the disturbances of the high order harmonic frequencies oscillation, and the computer simulation experiments show that it is effective to overcome the disturbances of the finite quality factor and the monotonic drift of an oscillator system, as well as the white noise, and this method can determine the fundamental frequency or period of a physical oscillator with a relative precision of 10−7 orders.

Test of the Universality of Free Fall with Atoms in Different Spin Orientations

We report a test of the universality of free fall by comparing the gravity acceleration of the ^{87}Rb atoms in m_{F}=+1 versus those in m_{F}=-1, of which the corresponding spin orientations are opposite. A Mach-Zehnder-type atom interferometer is exploited to alternately measure the free fall acceleration of the atoms in these two magnetic sublevels, and the resultant Eötvös ratio is η_{S}=(0.2±1.2)×10^{-7}. This also gives an upper limit of 5.4×10^{-6}  m^{-2} for a possible gradient field of the spacetime torsion. The interferometer using atoms in m_{F}=±1 is highly sensitive to the magnetic field inhomogeneity. A double differential measurement method is developed to alleviate the inhomogeneity influence, of which the effectiveness is validated by a magnetic field modulating experiment.

Thermoelastic property of the torsion fiber in the gravitational experiments

The thermoelastic and the nonlinear properties of a torsion fiber were studied. A symmetric disk torsion pendulum was designed to measure the temperature coefficient of the torsion spring constant of a tungsten fiber at room temperature, and the result shows that the ambient temperature fluctuation with ±1 °C would introduce a considerable uncertainty about ∓165 ppm in the torsion spring constant of the fiber. It is suggested that the thermoelasticity of the torsion fiber should be measured in a precision torsion pendulum experiment.

Amplitude dependence of quality factor of the torsion pendulum

Abstract The amplitude dependence of the angular frequency ω and the quality factor Q of the torsion pendulum were measured. The result shows that Q is more sensitive to the amplitude than ω , and the amplitude dependence of Q should be considered in the torsion pendulum experiments.

Determination of the limit of photon mass and cosmic magnetic vector with rotating torsion balance

Abstract A rotating torsion balance method to detect the product of the photon mass squared and the ambient cosmic vector potential is proposed. This modulation method is efficacious whether the vector potential is fortuitously aligned with Earths rotation axis or not, and the experimental precision of the modulation method can be improved by at least an order of magnitude compared to the statistical method.

Note: A three-dimension active vibration isolator for precision atom gravimeters

An ultra-low frequency active vibration isolator, simultaneously suppressing three-dimensional vibration noise, is demonstrated experimentally. The equivalent natural period of the isolator is 100 s and 12 s for the vertical and horizontal direction, respectively. The vibration noise in the vertical direction is about 50 times reduced during 0.2 and 2 Hz, and 5 times reduced in the other two orthogonal directions in the same frequency range. This isolator is designed for atom gravimeters, especially suitable for the gravimeter whose sensitivity is limited by vibration couplings.

G measurements with time-of-swing method at HUST

We review the G measurements with time-of-swing method at HUST. Two independent experiments have been completed and an improved experiment is in progress. The first G value was determined as 6.6699(7)×10−11 m3 kg−1 s−2 with a relative standard uncertainty (ur) of 105 ppm by using a long period torsion pendulum and two cylindrical source masses. Later, this result was corrected to be 6.6723(9)×10−11 m3 kg−1 s−2 with ur=130 ppm after considering the density distribution of the cylinders and the air buoyancy, which was 360 ppm larger than the previous value. In 2009, a new experiment by using a simple block pendulum and spherical source masses with more homogeneous density was carried out. A series of improvements were performed, and the G value was determined to be 6.67349(18)×10−11 m3 kg−1 s−2 with ur=26 ppm. To reduce the anelasticity of the torsion fibre, fused silica fibres with Qs of approximately 5×104 are used to measure G in the ongoing experiment. These fibres are coated with thin layers of germanium and bismuth in turn to reduce the electrostatic effect. Some other improvements include the gravity compensation, reduction of the coating layer effect, etc. The prospective uncertainty of the next G value is 20 ppm or lower.

Limits on Lorentz violation in gravity from worldwide superconducting gravimeters

We investigated Lorentz violation through anisotropy of gravity using a worldwide array of 12 superconducting gravimeters. The Lorentz-violating signal is extracted from the difference between measured gravity and a tidal model. At the level of sensitivity we reach, ocean tides start to play an important role. However, most models available that include ocean tides are empirically based on measured gravity data, which may contain Lorentz-violating signal. In this work we used an ocean tides included tidal model derived from first principles to extract Lorentz-violating signal for the first time. We have bounded space-space components of gravitational Lorentz violation in the minimal standard model extension (SME) up to the order of

Low-phase noise and high-power laser for Bragg atom interferometer

10^{-10}

A general relativistic model for free-fall absolute gravimeters

, one order of magnitude improved relative to previous atom-interferometer tests.