Tomoya Akatsuka

Geopotential measurements with synchronously linked optical lattice clocks

Real-time geopotential measurements with two synchronously linked optical lattice clocks are demonstrated. A height difference between the two clocks separated by 15 km is determined, with an uncertainty of 5 cm, by means of a gravitational redshift. According to Einsteins theory of relativity, the passage of time changes in a gravitational field1,2. On Earth, raising a clock by 1 cm increases its apparent tick rate by 1.1 parts in 1018, allowing chronometric levelling3 through comparison of optical clocks1,4,5. Here, we demonstrate such geopotential measurements by determining the height difference of master and slave clocks separated by 15 km with an uncertainty of 5 cm. A subharmonic of the master clock laser is delivered through a telecom fibre6 to synchronously operate7 the distant clocks. Clocks operated under such phase coherence reject clock laser noise and facilitate proposals for linking clocks8,9 and interferometers10. Taken over half a year, 11 measurements determine the fractional frequency difference between the two clocks to be 1,652.9(5.9) × 10−18, consistent with an independent measurement by levelling and gravimetry11. Our system demonstrates a building block for an internet of clocks, which may constitute ‘quantum benchmarks’, serving as height references with dynamic responses.

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

We report recent progress of optical lattice clocks with strontium, ytterbium and mercury atoms with an emphasis on their synchronous frequency comparison inside a laboratory and inter-laboratories connected by a phase-stabilized fiber link.

The measurement of time / La mesure du tempsFrequency ratios of Sr, Yb, and Hg based optical lattice clocks and their applicationsRapports de fréquence du Sr, Yb et Hg dans des horloges optiques à réseau et leurs applications

This article describes the recent progress of optical lattice clocks with neutral strontium (87Sr), ytterbium (171Yb) and mercury (199Hg) 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 an 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.Abstract 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 an 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.

Sub-hertz-linewidth diode laser stabilized to an ultralow-drift high-finesse optical cavity

An external cavity diode laser stabilized to a high-finesse rigid cavity made of ultralow-expansion (ULE) glass maintained at a zero-crossing temperature of −3.3 °C showed a linear frequency drift of 25 mHz/s, which was the lowest ever reported with ULE glass. The linewidth of the beat note between two independent laser systems stabilized to independent ULE glass cavities was narrower than 1 Hz, and the Allan deviation of the beat note around 1 s of averaging time was close to the Brownian thermomechanical noise limit.

30-km-long optical fiber link at 1397 nm for frequency comparison between distant strontium optical lattice clocks

We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at RIKEN and the University of Tokyo. We use a transfer laser at 1397 nm, which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1 × 10−17 for an averaging time of τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with a stability of 1 × 10−17(τ/s)−1/2 by synchronously operating two distant clocks.

A 30-km-long optical fiber link for frequency comparison between distant strontium optical lattice clocks

We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at Riken and the University of Tokyo. We use a transfer laser at 1397 nm which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1×10 17 for an averaging time τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with stability of 10 (τ/s) by synchronously operating the two distant clocks.

Prospects for frequency comparison of Sr and Hg optical lattice clocks toward 10 −18 uncertainties

We are developing optical lattice clocks with a scope of attaining 10-18 fractional uncertainty. Cryogenic silicon cavity targeting 2×10-17 stability at 1s, will allow full utilization of the potential stability of optical lattice clocks. In order to reduce the blackbody radiation shift, which is the most serious source of uncertainties, Sr clocks in cryogenic environment and Hg clocks are underdevelopment. We discuss prospects for clock comparison, no dead time operation of the clocks and fiber link of the clocks between Riken and the University of Tokyo.

Thermal calcium atom interferometer with a phase resolution of a few milliradians based on a narrow-linewidth diode laser

A symmetrical atom interferometer with a thermal calcium atom beam has been developed using a narrow linewidth diode laser stabilized to the resonance of a high-finesse cavity. The linewidth of the diode laser was estimated to be less than 1 Hz relative to the cavity resonance in noise measurement over the range of 100 Hz to 1 MHz, and the phase instability of the interference fringes obtained from the Allan deviation was improved to 2 mrad at an integration time of 300 s. Using this atom interferometer, the ac Stark phase shift between the {sup 1}S{sub 0} and {sup 3}P{sub 1} states of a Ca atom was measured as a function of a laser power near the resonance of the {sup 1}S{sub 0}-{sup 1} P{sub 1} transition at a wavelength of 423 nm. The decay rate of the {sup 1}P{sub 1} state was determined to be {gamma}=1.91(33)x10{sup 8} s{sup -1}.

Optical lattice clocks toward 10-17 uncertainty

The concept and recent progress of optical lattice clocks are reviewed. With the clock uncertainty of 10–17 in perspective, we discuss new challenges in optical lattice clocks and possible applications of such highly accurate and stable atomic clocks.