Ultrastable optical clock with two cold-atom ensembles
TL;DR: In this paper, a zero-dead-time optical clock based on interleaved interrogation of two cold-atom ensembles has been proposed to overcome the Dick effect, which results in an aliasing of frequency noise from the laser interrogating the atomic transition.
Abstract: Optical clocks with a record low zero-dead-time instability of 6 × 10–17 at 1 second are demonstrated in two cold-ytterbium systems. The two systems are interrogated by a shared optical local oscillator to nearly eliminate the Dick effect. Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with the ‘dead’ time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise from the laser interrogating the atomic transition1,2. Despite recent advances in optical clock stability that have been achieved by improving laser coherence, the Dick effect has continually limited the performance of optical clocks. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock that is based on the interleaved interrogation of two cold-atom ensembles3. This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability assessed to be for an averaging time τ in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce the standard quantum limit of clock stability, achieving a spectroscopy line quality factor of Q > 4 × 1015.
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TL;DR: Local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth are demonstrated and improved techniques allow the measurement of a frequency difference with an uncertainty of the order of 10–19 between two independent optical lattice clocks, suggesting that they may be able to improve state-of-the-art geodetic techniques.
Abstract: The passage of time is tracked by counting oscillations of a frequency reference, such as Earth’s revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10−17 level1–5. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer’s reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented values of three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4 × 10−18, measurement instability of 3.2 × 10−19 and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of [−7 ± (5)stat ± (8)sys] × 10−19, where ‘stat’ and ‘sys’ indicate statistical and systematic uncertainty, respectively. Although sensitivity to differences in gravity potential could degrade the performance of the clocks as terrestrial standards of time, this same sensitivity can be used as a very sensitive probe of geopotential5–9. Near the surface of Earth, clock comparisons at the 1 × 10−18 level provide a resolution of one centimetre along the direction of gravity, so the performance of these clocks should enable geodesy beyond the state-of-the-art level. These optical clocks could further be used to explore geophysical phenomena10, detect gravitational waves11, test general relativity12 and search for dark matter13–17. Improved techniques allow the measurement of a frequency difference with an uncertainty of the order of 10–19 between two independent atomic optical lattice clocks, suggesting that they may be able to improve state-of-the-art geodetic techniques.
492 citations
TL;DR: In this article, an optical atomic clock based on quantum-logic spectroscopy of the S 0↔ −3 P 0 transition in Al −+ was proposed, with a systematic uncertainty of 9.4×10 −19 and a frequency stability of 1.2×10−15 −15/sqrt[τ].
Abstract: We describe an optical atomic clock based on quantum-logic spectroscopy of the ^{1}S_{0}↔^{3}P_{0} transition in ^{27}Al^{+} with a systematic uncertainty of 9.4×10^{-19} and a frequency stability of 1.2×10^{-15}/sqrt[τ]. A ^{25}Mg^{+} ion is simultaneously trapped with the ^{27}Al^{+} ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous ^{27}Al^{+} clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous ^{27}Al^{+} clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.
464 citations
TL;DR: The first field measurement campaign with a transportable optical lattice clock was reported in this article, where the authors used it to determine the gravity potential difference between the middle of a mountain and a location 90 km away.
Abstract: Optical atomic clocks, due to their unprecedented stability and uncertainty, are already being used to test physical theories and herald a revision of the International System of Units. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations. Here, we report the first field measurement campaign with a transportable $^{87}$Sr optical lattice clock. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a $^{171}$Yb lattice clock also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.
350 citations
TL;DR: Two ultrastable lasers stabilized to single-crystal silicon Fabry-Pérot cavities at 124 K show unprecedented thermal noise-limited frequency instabilities of 4×10 and linewidths below 10 mHz.
Abstract: We report on two ultrastable lasers each stabilized to independent silicon Fabry-Perot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of 4×10^{-17} for integration times between 0.8 s and a few tens of seconds. We rigorously treat the notorious divergences encountered with the associated flicker frequency noise and derive methods to relate this noise to observable and practically relevant linewidths and coherence times. The individual laser linewidth obtained from the phase noise spectrum or the direct beat note between the two lasers can be as small as 5 mHz at 194 THz. From the measured phase evolution between the two laser fields we derive usable phase coherence times for different applications of 11 to 55 s.
340 citations
20 May 2019
TL;DR: In this paper, a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 1013.
Abstract: Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities for exploring fundamental physics and enabling new measurements. However, their size and the use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 1013. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.
294 citations
References
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TL;DR: In this article, the authors review the spectacular accuracy and stability gains that can be obtained when working with laser cooled ions or neutral atoms and discuss some important applications of these optical clocks, from geodesy to tests of fundamental theories to many body physics.
Abstract: Since 1967 the primary time standard is the cesium atomic clock, based on a hyperfine transition in the microwave domain The development of ultrastable laser sources now allows one to operate on electronic transitions in the optical domain, corresponding to a 5-order-of-magnitude increase in the clock frequency This article reviews the spectacular accuracy and stability gains that can be obtained when working with laser cooled ions or neutral atoms It also discusses some important applications of these optical clocks, from geodesy to tests of fundamental theories to many-body physics
1,393 citations
TL;DR: In this article, the Doppler effect results from the recoil momentum changing the translational energy of the radiating atom, and it is shown that the assumption that recoil momentum is given to the radii is incorrect if collisions are taking place.
Abstract: Quantum mechanically the Doppler effect results from the recoil momentum changing the translational energy of the radiating atom. The assumption that the recoil momentum is given to the radiating atom is shown to be incorrect if collisions are taking place. If the collisions do not cause broadening by affecting the internal state of the radiator, they result in a substantial narrowing of the Doppler broadened line.
1,243 citations
TL;DR: The development and operation of two optical lattice clocks are described, both using spin-polarized, ultracold atomic ytterbium, and an unprecedented atomic clock instability of 1.6 × 10–18 after only 7 hours of averaging is demonstrated.
Abstract: Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10 18 enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10 –18 after only 7 hours of averaging.
746 citations
TL;DR: The direct observation of quantum jumps between the 6-2 and 5-2 states is demonstrated, and the resulting ``telegraph signal'' provides a direct monitor of the atomic state.
Abstract: We demonstrate here the direct observation of quantum jumps between the ${6}^{2}$${\mathrm{S}}_{1/2}$ state and the ${5}^{2}$${\mathrm{D}}_{5/2}$ state of an individual laser-cooled ${\mathrm{Ba}}^{+}$ ions contained in a radio-frequency trap. The state detection and cooling are performed by two lasers which cause ${6}^{2}$${\mathrm{S}}_{1/2}$${\mathrm{\ensuremath{-}}6}^{2}$${\mathrm{P}}_{1/2}$${\mathrm{\ensuremath{-}}5}^{2}$ ${\mathrm{D}}_{3/2}$ transitions. Incoherent excitation to the ${5}^{2}$${\mathrm{D}}_{5/2}$ state (via the ${6}^{2}$${\mathrm{P}}_{3/2}$ level) causes the fluorescence from the ${6}^{2}$${\mathrm{P}}_{1/2}$ state to be suppressed for g30-sec lifetime of that state, after which the fluorescence reappears. The resulting ``telegraph signal'' provides a direct monitor of the atomic state.
719 citations
TL;DR: By preparing correlated states, here called squeezed spin states, the signal-to-noise ratio in spectroscopy can be increased by approximately N 1/2 in certain cases over that found in experiments using uncorrelated states.
Abstract: We investigate the quantum-mechanical noise in spectroscopic experiments on ensembles of N two-level (or spin-1/2) systems where transitions are detected by measuring changes in state population. By preparing correlated states, here called squeezed spin states, we can increase the signal-to-noise ratio in spectroscopy (by approximately ${\mathit{N}}^{1/2}$ in certain cases) over that found in experiments using uncorrelated states. Possible experimental demonstrations of this enhancement are discussed.
706 citations