Explore the vast range of titles available.

A clock at a higher altitude ticks faster than one at a lower altitude, in accordance with Einstein’s theory of general relativity. The outstanding stability and accuracy of optical clocks, at 10−18 levels1–5, allows height differences6 of a centimetre to be measured. However, such state-of-the-art clocks have been demonstrated only in well-conditioned laboratories. Here, we demonstrate an 18-digit-precision frequency comparison in a broadcasting tower, Tokyo Skytree, by developing transportable optical lattice clocks. The tower provides the clocks with adverse conditions to test the robustness and a 450 m height difference to test the gravitational redshift at (1.4 ± 9.1) × 10−5. The result improves ground-based clock comparisons7–9 by an order of magnitude and is comparable with space experiments10,11. Our demonstration shows that optical clocks resolving centimetres are technically ready for field applications, such as monitoring spatiotemporal changes of geopotentials caused by active volcanoes or crustal deformation12 and for defining the geoid13,14, which will have an immense impact on future society.A pair of transportable optical lattice clocks with 10−18 uncertainty is developed. The relativistic redshift predicted by the theory of general relativity has been tested at the 10–5 level by the two optical clocks with a height difference of 450 m on the ground.

A pair of 87 Sr optical lattice clocks with a statistical agreement of 2 × 10 −18 within 6,000 s has been developed. To this end, the behaviour of the blackbody radiation—a major perturbation for optical lattice clocks—was directly investigated. The accuracy of atomic clocks relies on the superb reproducibility of atomic spectroscopy, which is accomplished by careful control and the elimination of environmental perturbations on atoms. To date, individual atomic clocks have achieved a 10 −18 level of total uncertainties 1 , 2 , but a two-clock comparison at the 10 −18 level has yet to be demonstrated. Here, we demonstrate optical lattice clocks with 87 Sr atoms interrogated in a cryogenic environment to address the blackbody radiation-induced frequency shift 3 , which remains the primary source of systematic uncertainty 2 , 4 , 5 , 6 and has initiated vigorous theoretical 7 , 8 and experimental 9 , 10 investigations. The systematic uncertainty for the cryogenic clock is evaluated to be 7.2 × 10 −18 , which is expedited by operating two such cryo-clocks synchronously 11 , 12 . After 11 measurements performed over a month, statistical agreement between the two cryo-clocks reached 2.0 × 10 −18 . Such clocks' reproducibility is a major step towards developing accurate clocks at the low 10 −18 level, and is directly applicable as a means for relativistic geodesy 13 .

The supreme accuracy of atomic clocks (Rosenband in Science 319:1808, 2008) relies on the universality of atomic transition frequencies. The stability of a clock, meanwhile, measures how quickly the clock's statistical uncertainties are reduced. The ultimate measure of stability is provided by the quantum projection noise (Itano in Phys. Rev. A 47:3554, 1993), which improves as 1/√ N by measuring N uncorrelated atoms. Quantum projection noise limited stabilities have been demonstrated in caesium clocks (Santarelli in Phys. Rev. Lett. 82:4619, 1999) and in single-ion optical clocks (Peik et al. in J. Phys. B 39:145, 2006, Chou et al. in Phys. Rev. Lett. 104:070802, 2010), where the quantum noise overwhelms the Dick effect (Santarelli in IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45:887, 1998) attributed to local oscillator noise. Here, we demonstrate a synchronous frequency comparison of two optical lattice clocks using 87 Sr and 88 Sr atoms (Akatsuka et al. in Nature Phys. 4:954, 2008), respectively, for which the Allan standard deviation reached 1 × 10 −17 in an averaging time of 1,600 s by cancelling out the Dick effect to approach the quantum projection noise limit. The scheme demonstrates the advantage of using a large number ( N  ≈ 1,000) of atoms in optical clocks and paves the way to investigating the inherent uncertainties of clocks and relativistic geodesy (Chou et al. in Science 329:1630, 2010) on a timescale of tens of minutes. Researchers cancel out the Dick effect through a synchronous frequency comparison between two optical lattice clocks based on 87 Sr and 88 Sr atoms. This scheme achieves an Allan standard deviation of around 10 −17 , which represents a significant advantage when using a large number (2,000) of atoms in an optical clock.

Time for a change Since the first practical caesium atomic clock was built in 1955 (by Louis Essen and John V. L. Parry, described in Nature 176 ; 280–285), work has gone on to improve the accuracy and stability of such devices. Trapped ions offer accuracy advantages, neutral atoms aid stability. A new device, the optical lattice clock, combines the best of both approaches and could point towards the next generation of atomic clocks. Using atoms trapped in an optical lattice as quantum references, this system has the potential to achieve a stability several orders of magnitude better than the caesium clock currently used to define the second as an SI unit. The precision measurement of time and frequency is a prerequisite not only for fundamental science but also for technologies that support broadband communication networks and navigation with global positioning systems (GPS). The SI second is currently realized by the microwave transition of Cs atoms with a fractional uncertainty of 10 -15 (ref. 1 ). Thanks to the optical frequency comb technique 2 , 3 , which established a coherent link between optical and radio frequencies, optical clocks 4 have attracted increasing interest as regards future atomic clocks with superior precision. To date, single trapped ions 4 , 5 , 6 and ultracold neutral atoms in free fall 7 , 8 have shown record high performance that is approaching that of the best Cs fountain clocks 1 . Here we report a different approach, in which atoms trapped in an optical lattice serve as quantum references. The ‘optical lattice clock’ 9 , 10 demonstrates a linewidth one order of magnitude narrower than that observed for neutral-atom optical clocks 7 , 8 , 11 , and its stability is better than that of single-ion clocks 4 , 5 . The transition frequency for the Sr lattice clock is 429,228,004,229,952(15) Hz, as determined by an optical frequency comb referenced to the SI second.

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 Einstein's theory of relativity, the passage of time changes in a gravitational field 1 , 2 . On Earth, raising a clock by 1 cm increases its apparent tick rate by 1.1 parts in 10 18 , allowing chronometric levelling 3 through comparison of optical clocks 1 , 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 fibre 6 to synchronously operate 7 the distant clocks. Clocks operated under such phase coherence reject clock laser noise and facilitate proposals for linking clocks 8 , 9 and interferometers 10 . 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 gravimetry 11 . Our system demonstrates a building block for an internet of clocks, which may constitute ‘quantum benchmarks’, serving as height references with dynamic responses.

Quantum statistics fundamentally controls the way particles interact; bosons tend to bunch together, whereas fermions repulse each other. As a consequence, statistically different isotopes are found in different macroscopic quantum states at ultracold temperatures. This is related to the total atomic spin, which forces atoms to couple to ambient fields. In designing high-precision atomic clocks that operate at a fractional uncertainty of 10 −15 or less, quantum statistics and therefore the spins of the interrogated atoms have an essential role in determining the clocks’ ultimate performance. Here, we discuss the design of optical lattice clocks in view of the quantum statistics and lattice geometries. We propose two configurations that both make the interrogated atoms non-interacting: spin-polarized fermions in a one-dimensional (1D) and bosons in a 3D lattice. A 3D clock with bosonic 88 Sr is demonstrated for the first time, in addition to a 1D clock with fermionic 87 Sr. The sequential operation of the two clocks enables us to evaluate the clock stability with an uncertainty below 1×10 −15 and to determine the isotope shift with significant reduction of the uncertainty related to atomic collisions. Optical lattice clocks, in which trapped atoms serve as a frequency reference, are promising candidates for next-generation atomic clocks. Depending on whether bosons or fermions are loaded into the lattice, fundamentally different design principles apply, as has now been shown.

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 (refs 1-3), where full frequency descriptions are far beyond the reach of the SI second. Direct measurements of the frequency ratios of such super clocks, on the other hand, are not subject to this limitation. They can verify consistency and overall accuracy for an ensemble of super clocks, an essential step towards a redefinition of the second. Here we report a measurement that finds the frequency ratio of neutral ytterbium and strontium clocks to be R=1.207507039343337749(55), with a fractional uncertainty of 4.6x10-17 and a measurement instability as low as 4x10-16 ([tau]/s)-1/2 .

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 super(-18) (refs1-3), where full frequency descriptions are far beyond the reach of the SI second. Direct measurements of the frequency ratios of such super clocks, on the other hand, are not subject to this limitation. They can verify consistency and overall accuracy for an ensemble of super clocks, an essential step towards a redefinition of the second. Here we report a measurement that finds the frequency ratio of neutral ytterbium and strontium clocks to be =1.207507039343337749(55), with a fractional uncertainty of 4.610 super(-17) and a measurement instability as low as 410 super(-16) ( tau /s) super(-1/2).

The most accurate ratio of the clock transition frequencies between Yb and Sr is measured by using a pair of cryogenic optical lattice clocks. Through common mode rejection of the clock laser noise, a uncertainty of 4.6 × 10 −17 is achieved in 150 seconds. 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 (refs  1 – 3 ), where full frequency descriptions are far beyond the reach of the SI second. Direct measurements of the frequency ratios of such super clocks, on the other hand, are not subject to this limitation 4 , 5 , 6 , 7 , 8 . They can verify consistency and overall accuracy for an ensemble of super clocks, an essential step towards a redefinition of the second 9 . Here we report a measurement that finds the frequency ratio of neutral ytterbium and strontium clocks to be ℛ = 1.207507039343337749(55), with a fractional uncertainty of 4.6 × 10 −17 and a measurement instability as low as 4 × 10 −16 ( τ /s) −1/2 .

We consider designs of optical lattice clocks in view of the quantum statistics, relevant atomic spins, and atom-lattice interactions. The first two issues lead to two optimal constructions for the clock: a one-dimensional (1D) optical lattice loaded with spin-polarized fermions and a 3D optical lattice loaded with bosons. By taking atomic multipolar interactions with the lattice fields into account, an \"atomic motion insensitive\" wavelength is proposed to provide a precise definition of the \"magic wavelength.\" We then present a frequency comparison of these two optical lattice clocks: spin-polarized fermionic 87Sr and bosonic 88Sr prepared in 1D and 3D optical lattices, respectively. Synchronous interrogations of these two optical lattice clocks by the same probe laser allowed canceling out its frequency noise as a common mode noise to achieve a relative stability of 3×10−17 for an averaging time of τ 350 s. The scheme, therefore, provides us with a powerful means to investigate intrinsic uncertainty of the clocks regardless of the probe laser stability. We discuss prospects of the synchronous operation of the clocks on the measurement of the geoid height difference and on the search of constancy of fundamental constants.