Optical Clock Research Articles

We report on a 88 Sr + single-ion optical clock with an estimated fractional systematic uncertainty of 7.9× 10 − 19 . The low uncertainty is enabled by small rf losses, a thorough evaluation of the blackbody-radiation temperature, and our recent measurement of the differential polarizability. A detailed uncertainty evaluation is presented. We also report on two absolute frequency measurements: one against a remote cesium fountain clock, and one against International Atomic Time (TAI). The former lasted 12 d and resulted in a frequency value of 444 779 044 095 485.49(15) Hz. The latter spanned 10 months with monthly optical-clock uptimes between 68% and 99%, and yielded a frequency value of 444 779 044 095 485.373(44) Hz. With a fractional uncertainty of 9.8× 10 − 17 , it is, to our knowledge, the most accurate optical frequency measurement reported to date. Both frequency values are in agreement with other recent measurements, providing further evidence that the 2021 CIPM recommended frequency value is too high by 1.6 times its uncertainty.

The Pound-Drever-Hall (PDH) technique is widely used to stabilize the frequency of lasers. Here we report on a routinely underestimated source of error in PDH offset-locking: a shift in the lock point due to the unintended interaction between residual optical sidebands and higher-order spatial modes in misaligned Fabry-Pérot cavities. Significant frequency deviations—up to 50% of the cavity linewidth—can arise when the optical offset is obtained from a sinusoidally driven EOM. We measure this deviation experimentally, find agreement with a simple model, and show how a spectrally-pure frequency offset can reduce the deviation by an order of magnitude. Our findings draw attention to a systematic effect of importance to precision optical spectroscopy, optical clocks, and quantum information science.

Abstract Ytterbium (Yb) is used in cold-atom systems, including magneto-optical traps and optical lattice clocks. However, the long-term operation of such systems may be associated with substantial degradation of optical transmittance through vacuum chamber viewports due to Yb adsorption. Here, we show that coating the surface with tetracontane effectively suppresses such adsorption.

Relying on the nonlinear optical effect of difference-frequency generation, passively carrier-envelope stabilized comb systems are currently considered the simplest option for generating combs with zero carrier-envelope offset frequency ${f_{{\mathbf{ceo}}}}$, that is, without relying on electronic servo loops or additional acousto-optic shifters. However, recent measurements indicated statistically significant deviations from a perfect vanishing ${f_{{\mathbf{ceo}}}}$. Here, we show that temperature-drift-induced refractive index changes in subsequent distribution or booster amplifiers may give rise to an effective Doppler shift of the measured ${f_{{\mathbf{ceo}}}}$, thwarting precision frequency measurements up to the mHz level. Comparing this hypothesis with temperature-dependent refractive index data and gain spectra, we find excellent quantitative agreement with measurements. Mitigating these problems by allowing for sufficient warm-up times of the external amplifiers, the ${f_{{\mathbf{ceo}}}}$ mean value of a difference frequency comb is measured as -0.55±1.55 µHz, which is statistically equivalent to 0. The corresponding stability supersedes the current best optical clocks by an order of magnitude, with an overlapping Allan deviation of <1·10-17 at 1 s and 5.36×10-21 at 100,000 s. Previously often considered negligible, the temperature drift of external distribution or booster amplifiers plays a critical role in obtaining similar stabilities in frequency metrology for all kinds of ${f_{{\mathbf{ceo}}}}$-stabilized combs.

Quantum-amplified global-phase spectroscopy on an optical clock transition

The optical clock states of alkaline earth and alkaline earthlike atoms are the fundament of state-of-the-art optical atomic clocks. An important prerequisite for the operation of optical clocks is the magic trapping conditions where electronic and motional dynamics decouple. Here, we identify and experimentally demonstrate simultaneous magic trapping for two clock transitions in ^{88}Sr, realizing so-called triple-magic conditions at a specially chosen magic angle. Under these conditions, we operate an all-optical qutrit comprising the ground state ^{1}S_{0}, and the two metastable clock states ^{3}P_{0} and ^{3}P_{2}. We demonstrate fast optical control in an atom array using two- and three-photon couplings to realize high-fidelity manipulation between all qutrit states. At the magic angle, we probe the coherence achievable in magic-angle-tuned traps and find atom-atom coherence times between the metastable states as long as 715(30)ms. Our work opens several new directions, including qutrit-based quantum metrology on optical transitions and high-fidelity and high-coherence manipulation on the ^{88}Sr fine-structure qubit.

We demonstrate a single-mode hybrid integrated laser architecture combining a waveguide Bragg grating fabricated on a 3 µm silicon-on-insulator (SOI) platform with a semiconductor optical amplifier (SOA) gain chip. Utilizing a Bragg grating with a controlled period of 224 nm, and duty cycle of 75%, the device achieves a narrow intrinsic linewidth of 4.89 kHz. With a compact 6-mm-long DBR grating, this design simultaneously yields an ultra-sharp spectral filtering with the full width at half maximum value of 30 pm and a low coupling constant of 0.68 cm−1. Leveraging the thick-SOI platform's advantages-including superior fiber-chip coupling efficiency and enhanced power handling capabilities - in conjunction with a high-power SOA design, the system delivers output power of 112 mW while maintaining kHz-level linewidth. This performance-optimized design establishes a manufacturable pathway for implementing high-power narrow-linewidth lasers in coherent optical networks, optical atomic clocks, and light detection and ranging (LiDAR) systems that require simultaneous high coherence and high-power output.

We demonstrate a straightforward frequency-locking technique that employs external modulation saturation absorption spectroscopy to achieve a high-performance 780nm laser system. By externally modulating the laser frequency through a fiber electro-optic modulator, this scheme can stabilize multiple transitions of the 87RbD2 line with excellent performance. Experimental results establish fractional frequency instability of 2.21 × 10-13 at 1s with a linewidth of 1.46kHz for the cycling transition 5S1/2(Fg = 2) → 5P3/2(Fe = 3). Notably, we have locked the system to the repumping transition 5S1/2(Fg = 1) → 5P3/2(Fe = 2), achieving a record-low instability of 1.98 × 10-12 at 1s averaging time. This simplified laser system, featuring exceptional short-term frequency stability and multi-frequency locking capability, serves as a critical subsystem for the compact cold-atom optical clock, enabling advanced precision metrology applications. Furthermore, its metrological performance enables immediate applications in other quantum sensors, such as Rydberg electrometry, atomic magnetometry, and matter-wave interferometry.

We present an International System of Units (SI)-traceable temperature calibration apparatus utilizing optical lattice clocks for precision metrology. The system employs a dual-blackbody radiation shield chamber with independent temperature control, enabling synchronous differential measurements of blackbody radiation (BBR)-induced frequency shifts in atomic ensembles. By correlating these shifts with chamber temperature, we propose absolute temperature determination traceable to the SI second through the optical clock frequency. Comprehensive uncertainty analysis demonstrates an absolute temperature uncertainty below 17 mK across the 200−350K range based on Sr87 optical lattice clock, representing an improvement of two orders of magnitude over current temperature measurements based on BBR-induced Rydberg state transitions. This advancement in primary thermometry offers significant improvements in precision, reproducibility, and versatility, with potential applications in metrology, fundamental physics, and industrial processes.

Soliton microcombs, generated in high-Q microresonators, have revolutionized integrated photonic technologies such as optical clocks, spectroscopy, and telecommunications. However, conventional pump schemes suffer from low pump-to-soliton conversion efficiency and high power threshold. Besides, parasitic nonlinear processes such as stimulated Raman scattering (SRS) may also affect the generation and stabilization of soliton states. Here, we realize a high-efficiency soliton microcomb with a low power threshold by exploiting the synergistic interplay between SRS and four-wave mixing (FWM) in a high-Q SiO2 microsphere. A dual-pump strategy-where a primary pump triggers the Raman gain and a secondary pump initiates soliton microcomb formation-enables 21.8% conversion efficiency from the pump laser to the soliton state. Experimental evidence reveals that the introduction of the secondary pump allows the tuning of the center frequency of the Raman soliton microcomb within the Raman gain region. This approach not only reduces the soliton formation threshold but also enables robust soliton stabilization via dynamic Raman gain compensation and thermal suppression. Our work provides a universal pathway for energy-efficient nonlinear photonics in Raman-active microresonators.

Single-frequency fiber lasers (SFFLs) are vital for precision metrology, optical clocks, and ultra-dense wavelength division multiplexing (UD-WDM) due to their ultra-narrow linewidth and high coherence. However, traditional SFFLs based on static filters suffer from a fundamental trade-off between wavelength tunability and spectral purity, limiting their practical tuning ranges to below 40 nm, despite much broader gain bandwidths in rare-earth-doped fibers. Here, we report an all-optically reconfigurable Er3+-doped SFFL that achieves an unprecedented tuning range of 100.12 nm (1512.84-1612.96 nm), enabled by self-adaptive dynamic gratings (SADGs) formed through nonlinear intermodal coupling and spatial hole burning in unpumped fiber. Theoretical modeling and experiments reveal that SADGs provide automatic, mode-selective feedback that maintains continuous single-longitudinal-mode (SLM) without mechanical tuning. The laser features a 3 kHz linewidth, 16 pm tuning precision, 30 nm/s tuning speed, and <0.5% power fluctuation, demonstrating stable, narrow-linewidth emission across the S + C + L bands. This work substantially mitigates the long-standing tunability-coherence trade-off and establishes a generalizable framework for all-optically controlled high-coherence laser systems, paving the way for next-generation photonics in UD-WDM, quantum metrology, and reconfigurable optical networks.

We explore the limits of atomic coherence and measurement precision in a ^{87}Sr optical lattice clock. We perform a detailed characterization of key effects, including lattice Raman scattering and atomic collisions in a shallow lattice configuration, determining a 174(28)s ^{3}P_{0} clock state lifetime. Investigation of atomic coherence across a range of lattice depths and atomic densities reveals decoherence mechanisms related to photon scattering and atomic interaction. At a reduced density, we observe a coherence time of 118(9)s, approaching the fundamental limit set by spontaneous emission. Guided by this coherence understanding, we demonstrate a clock instability for an atomic ensemble of 1.5×10^{-18} at 1s in fractional frequency units. Our results are important for further advancing the state of the art of an optical lattice clock for fundamental physics applications.

We report the design and in-orbit demonstration of a compact optical system for a 87Sr optical lattice clock aboard the Chinese Space Station. This system adopts a compact and robust vertically stacked architecture with a total volume of 0.11m3 and a mass of 53.6kg. It passed thermal and vibration qualification tests and remained fully operational after launch. In orbit, it achieved automated multi-stage laser stabilization and a blue magneto-optical trap for 87Sr atoms. This marks a significant step toward operational optical clocks in space.

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The S10−P30 clock transition in strontium serves as the foundation for the world’s best atomic clocks and for gravitational wave detector concepts in clock atom interferometry. This transition is weakly allowed in the fermionic isotope Sr87 but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic Sr88 using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies of up to 50 kHz using W/cm2 laser intensities and Gauss-level magnetic field amplitudes. The absence of nuclear spin in bosonic isotopes offers decreased sensitivity to magnetic fields and optical lattice light shifts, enabling atomic clocks with reduced systematic errors. The collinear propagation of the laser fields permits the interrogation of spatially separated atomic ensembles with common laser pulses, a key requirement for dark matter searches and gravitational wave detection with next-generation quantum sensors.

Abstract We report on the trapping and imaging of individual ytterbium atoms in arrays of optical tweezers, loaded from a magneto-optical trap (MOT) formed by only five beams in an orthogonal configuration. In our five-beam MOT, operating on the narrow 1S0 → 3P1 intercombination transition, gravity balances the radiation pressure of a single upward-directed beam. This approach enables efficient trapping and cooling of the most common ytterbium isotopes (171Yb, 173Yb and 174Yb) to ≤ 20 μK at densities ~ 1011 atoms/cm3 within less than one second. This configuration allows for significantly reducing the complexity of the optical setup, potentially benefiting any ytterbium-atom based quantum science platform leveraging single-atom microscopy, from quantum processors to novel optical clocks. We then demonstrate the first single-atom-resolved imaging of the fermionic, large-spin isotope 173Yb (I=5/2), employing a two-color imaging scheme that does not rely on magic-wavelength trapping. We achieve a high single-atom imaging fidelity of 99.96(1)% and a large survival probability of 98.5(2)%, despite large differential light shifts affecting all nuclear spin sublevels of the excited 3P1 state involved in the cooling transition. The demonstrated capabilities will play a key role in future quantum simulations and computing applications with 173Yb arrays.

We theoretically propose a tunable implementation of symmetry-protected topological phases of matter in a synthetic superlattice, taking advantage of the long coherence time and exquisite spectral resolutions offered by gravity-tilted optical lattice clocks. We describe a protocol similar to Rabi spectroscopy that can be used to probe the distinct topological properties of our system. We then demonstrate how the sensitivity of clocks and interferometers can be protected from unwanted experimental imperfections offered by the underlying topological robustness. The proposed implementation opens a path to exploiting the unique opportunities offered by symmetry-protected topological phases in state-of-the-art quantum sensors.

Controlling the Stark perturbation from ambient thermal radiation is key to advancing the performance of many atomic frequency standards, including state-of-the-art optical lattice clocks (OLCs). We demonstrate a cryogenic OLC that utilizes a dynamically actuated radiation shield to control the perturbation at 1.7×10^{-20} fractional frequency, a factor of ∼40 beyond the best OLC to date. Our shield furnishes the atoms with a near-ideal cryogenic blackbody radiation (BBR) environment by rejecting external thermal radiation at the part-per-million level during clock spectroscopy, overcoming a key limitation with previous cryogenic BBR control solutions in OLCs. While the lowest BBR shift uncertainty is realized with cryogenic operation, we further exploit the radiation control that the shield offers over a wide range of temperatures to directly measure and verify the leading BBR Stark dynamic correction coefficient for ytterbium. This independent measurement reduces the literature-combined uncertainty of this coefficient by 30%, thus benefiting state-of-the-art Yb OLCs operated at room temperature. We verify the static BBR coefficient for Yb at the low 10^{-18} level.

Chip-integrated optical frequency combs (OFCs) based on Kerr nonlinear resonators are of great significance given their scalability and wide range of applications. Broadband on-chip OFCs reaching visible wavelengths are especially valuable as they address atomic clock transitions that play an important role in position, navigation, and timing infrastructure. Silicon nitride (SiN) deposited via low-pressure chemical vapor deposition (LPCVD) is the usual platform for chip-integrated OFCs, due to its low absorption and repeatable dispersion, and such fabrication is now standard at wafer sizes up to 200 mm. However, the LPCVD high temperature and film stress pose challenges in scaling to larger wafers and integrating with electronic and photonic devices. Here, we report the linear performance and broadband frequency comb generation from microring resonators fabricated on 300 mm wafers at AIM Photonics, using a lower temperature, lower stress plasma-enhanced chemical vapor deposition process suitable for thick (≈700 nm) SiN films and compatible with electronic and photonic integration. The platform exhibits consistent insertion loss, high intrinsic quality factor, and thickness variation of ±2% across the whole 300 mm wafer. We demonstrate broadband soliton microcomb generation with a lithographically tunable dispersion profile extending to wavelengths of common alkali atom transitions. These results are a step towards more highly integrated and mass-manufacturable devices, enabling advanced applications including optical clocks, LiDAR, and beyond.

Photonic integrated circuits (PICs) are gaining significant attention in the visible and short near-infrared spectral regions for diverse applications such as spectroscopy, optical atomic clocks, quantum optics, and optical communications. Among various material platforms, silicon nitride (SiN) is particularly noteworthy due to its wide bandgap, low propagation losses, broad transparency, and well-established fabrication processes. Furthermore, thin-film lithium niobate (LN) has emerged as a promising platform for high-speed, power-efficient modulation in these spectral regimes, complementing the capabilities of SiN. Consequently, the integration of SiN and LN is well-suited for optical communication transmitters. However, on-chip receivers for such systems remain unexplored. We present the first demonstration of an optical coherent communication system at a short near-infrared window, utilizing a SiN-based integrated 90° optical hybrid. Our demonstration operates over a 13.9-THz bandwidth with 10-Gbaud binary phase-shift-keyed signals, which is comparable to or even more than the bandwidth of the conventional fiber-optic systems. Furthermore, we demonstrate a 2.2-m free-space optical transmission using the SiN chip as an application of visible and short near-infrared coherent communication. This demonstration highlights the potential of fully integrated high-performance coherent transceivers for new wavelength windows.