arXiv subjects

Ichiro Ushijima

Publications and source records attributed to Ichiro Ushijima.

7 recordsLinked to original sources

arXiv2024

International comparison of optical frequencies with transportable optical lattice clocks

Optical clocks have improved their frequency stability and estimated accuracy by more than two orders of magnitude over the best caesium microwave clocks that realise the SI second. Accordingly, an optical redefinition of the second has been widely discussed, prompting a need for the consistency of optical clocks to be verified worldwide. While satellite frequency links are sufficient to compare microwave clocks, a suitable method for comparing high-performance optical clocks over intercontinental distances is missing. Furthermore, remote comparisons over frequency links face fractional uncertainties of a few $10^{-18}$ due to imprecise knowledge of each clock's relativistic redshift, which stems from uncertainty in the geopotential determined at each distant location. Here, we report a landmark campaign towards the era of optical clocks, where, for the first time, state-of-the-art transportable optical clocks from Japan and Europe are brought together to demonstrate international comparisons that require neither a high-performance frequency link nor information on the geopotential difference between remote sites. Conversely, the reproducibility of the clocks after being transported between countries was sufficient to determine geopotential height offsets at the level of 4 cm. Our campaign paves the way for redefining the SI second and has a significant impact on various applications, including tests of general relativity, geodetic sensing for geosciences, precise navigation, and future timing networks.

physics.atom-ph↗

arXiv2018

Operational Magic Intensity for Sr Optical Lattice Clocks

We experimentally investigate the lattice-induced light shift by the electric-quadrupole ($E2$) and magnetic-dipole ($M1$) polarizabilities and the hyperpolarizability in Sr optical lattice clocks. Precise control of the axial as well as the radial motion of atoms in a one-dimensional lattice allows observing the $E2$-$M1$ polarizability difference. Measured polarizabilities determine an operational lattice depth to be $72(2) E_R$, where the total light shift cancels to the $10^{-19}$ level, over a lattice-intensity variation of about 30%. This operational trap depth and its allowable intensity range conveniently coincide with experimentally feasible operating conditions for Sr optical lattice clocks.

physics.atom-ph↗

arXiv2016

Real-time geopotentiometry with synchronously linked optical lattice clocks

According to the Einstein's theory of relativity, the passage of time changes in a gravitational field. On earth, raising a clock by one centimetre increases its tick rate by 1.1 parts in 10$^{18}$, enabling optical clocks to perform precision geodesy. Here, we demonstrate geopotentiometry by determining the height difference of master and slave clocks separated by 15 km with uncertainty of 5 cm. The subharmonic of the master clock is delivered through a telecom fibre to phase-lock and synchronously interrogate the slave clock. This protocol rejects laser noise in the comparison of two clocks, which improves the stability of measuring the gravitational red shift. Such phase-coherently operated clocks facilitate proposals for linking clocks and interferometers. Over half a year, 11 measurements determine the fractional frequency difference between the two clocks to be $1,652.9(5.9)\times 10^{-18}$, or a height difference of 1,516(5) cm, consistent with an independent measurement by levelling and gravimetry. Our system is as a building block of an internet of clocks, consisting of a master and a number of slave clocks, which will provide "quantum benchmarks" that are height references with dynamic response.

physics.atom-ph↗

arXiv2016

Frequency ratio of Yb and Sr clocks with $5 \times 10^{-17}$ uncertainty at 150 s averaging time

Transition frequencies of atoms and ions are among the most accurately accessible quantities in nature, playing important roles in pushing the frontiers of science by testing fundamental laws of physics, in addition to a wide range of applications such as satellite navigation systems. Atomic clocks based on optical transitions approach uncertainties of $10^{-18}$, where full frequency descriptions are far beyond the reach of the SI second. Frequency ratios of such super clocks, on the other hand, are not subject to this limitation. They can therefore verify consistency and overall accuracy for an ensemble of super clocks, an essential step towards a redefinition of the second. However, with the measurement stabilities so far reported for such frequency ratios, a confirmation to $1 \times 10^{-18}$ uncertainty would require an averaging time $τ$ of multiple months. Here we report a measurement of the frequency ratio of neutral ytterbium and strontium clocks with a much improved stability of $4 \times 10^{-16} (τ/s)^{-1/2}$. Enabled by the high stability of optical lattice clocks interrogating hundreds of atoms, this marks a 90-fold reduction in the required averaging time over a previous record-setting experiment that determined the ratio of Al+ and Hg+ single-ion clocks to an uncertainty of $5.2 \times 10^{-17}$. For the Yb/Sr ratio, we find R = 1.207 507 039 343 337 749(55), with a fractional uncertainty of $4.6 \times 10^{-17}$.

physics.atom-ph↗

arXiv2015

Frequency ratios of Sr, Yb and Hg based optical lattice clocks and their applications

This article describes the recent progress of optical lattice clocks with neutral strontium ($^{87}$Sr), ytterbium ($^{171}$Yb) and mercury ($^{199}$Hg) atoms. In particular, we present frequency comparison between the clocks locally via an optical frequency comb and between two Sr clocks at remote sites using a phase-stabilized fibre link. We first review cryogenic Sr optical lattice clocks that reduce the room-temperature blackbody radiation shift by two orders of magnitude and serve as a reference in the following clock comparisons. Similar physical properties of Sr and Yb atoms, such as transition wavelengths and vapour pressure, have allowed our development of a compatible clock for both species. A cryogenic Yb clock is evaluated by referencing a Sr clock. We also report on a Hg clock, which shows one order of magnitude less sensitivity to blackbody radiation, while its large nuclear charge makes the clock sensitive to the variation of fine-structure constant. Connecting all three types of clocks by an optical frequency comb, the ratios of the clock frequencies are determined with uncertainties smaller than possible through absolute frequency measurements. Finally, we describe a synchronous frequency comparison between two Sr-based remote clocks over a distance of 15 km between RIKEN and the University of Tokyo, as a step towards relativistic geodesy.

physics.atom-ph↗

arXiv2015

Frequency Ratio of ${}^{199}$Hg and ${}^{87}$Sr Optical Lattice Clocks beyond the SI Limit

We report on a frequency ratio measurement of a ${}^{199}$Hg-based optical lattice clock referencing a ${}^{87}$Sr-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of $7.2 \times 10^{-17}$ for the Hg clock. The frequency ratio is measured to be $ν_{\rm Hg}/ν_{\rm Sr}=2.629\, 314\, 209\, 898\, 909\, 60(22)$ with a fractional uncertainty of $8.4 \times 10^{-17}$, which is smaller than the uncertainty of the realization of the SI second, i.e., the SI limit.

physics.atom-ph↗

arXiv2014

Cryogenic optical lattice clocks with a relative frequency difference of $1\times 10^{-18}$

Time and frequency are the most accurately measurable quantities, providing foundations for science and modern technologies. The accuracy relies on the SI (Systéme International) second that refers to Cs microwave clocks with fractional uncertainties at $10^{-16}$. Recent revolutionary progress of optical clocks aims to achieve $1\times 10^{-18}$ uncertainty, which however has been hindered by long averaging-times or by systematic uncertainties. Here, we demonstrate optical lattice clocks with $^{87}$Sr atoms interrogated in a cryogenic environment to address the blackbody radiation-induced frequency-shift, which remains the primary source of clocks' uncertainties and has initiated vigorous theoretical and experimental investigations. The quantum-limited stability for $N \sim 1,000$ atoms allows investigation of the uncertainties at $2\times 10^{-18}$ in two hours of clock operation. After 11 measurements performed over a month, the two cryo-clocks agree to within $(-1.1\pm 1.6)\times 10^{-18}$. Besides its contribution to fundamental science and the redefinition of the SI second, such a speedy comparison of accurate clocks provides a means for relativistic geodesy.

physics.atom-ph↗