Sr Optical Clock Research Articles

Reducing statistical noise in frequency ratio measurement between Ca+ and Sr optical clocks with a frequency-synthesized local oscillator from a Sr optical clock

Optical frequency ratio measurement between optical atomic clocks is essential to precision measurement as well as the redefinition of the second. Currently, the statistical noise in frequency ratio measurement of most ion clocks is limited by the frequency instability of ion clocks. In this work, we reduce the statistical noise in the frequency ratio measurement between a transportable Ca+ optical clock and a Sr optical lattice clock down to 2.2×10−15/τ. The local oscillator of the Ca+ optical clock is frequency-synthesized from the Sr optical lattice clock, enabling a longer probe time for Ca+ clock transition. Compared to previous measurement using independent local oscillators, we achieve 10-fold reduction in comparison campaign duration.

A proposal for detecting weak electromagnetic waves around 2.6 μm wavelength with Sr optical clock

Infrared signal detection is widely used in many fields. Due to the detection principle, however, the accuracy and range of detection are limited. Thanks to the ultra stability of the 87Sr optical lattice clock, external infrared electromagnetic wave disturbances can be responded to. Utilizing the ac Stark shift of the clock transition, we propose a new method to detect infrared signals. According to our calculations, the theoretical detection accuracy in the vicinity of its resonance band of 2.6 μm can reach the order of 10−14 W, while the minimum detectable signal of common detectors is on the order of 10−10 W.

A complete analysis of the link uncertainty budget for pulsed laser time transfer on China space station

High-precision Time-Frequency Rack (HTFR) mission, a part of experiment module II of the China Space Station (CSS), was equipped with a Sr optical clock, H-maser, and laser-cooled microwave clock. To compare time and frequency across widely separated clocks, a microwave link, and a pulsed laser link were both established, which probably enables new possibilities for fundamental science, geodesy, and metrology. Based on the existing relativistic theory for time and frequency transfer, we derive a relativistic model of satellite-ground laser time transfer on CSS (CLT), which can be of direct use in CLT data processing. In addition to first and second-order Sagnac effects, it also includes relativistic rate shifts, atmospheric refraction, and turbulence, laser retroreflector array (LRA) range correction, and position correction between detector and reflector, satisfying the stability evaluation of CSS atomic clocks with fractional frequency stability of 3 × 10−17/10000 s. Additionally, we numerically investigate the link error magnitude and use Monte Carlo simulations to assess the impact of correcting for link errors on the stability of the CLT, taking into account the CSS orbit and attitude determinations as well as the CLT payload calibration parameters. It is shown that the CLT link can be considered to reach a TDEV of approximately 0.1 ps/300 s while meeting the CSS design objectives. The primary factors that influence the stability of the CLT are the position correction between detector and reflector, LRA range correction, and the coupling relation of these two error distributions.

Evaluation of Lattice Light Shift at Low 10^{-19} Uncertainty for a Shallow Lattice Sr Optical Clock.

A Wannier-Stark optical lattice clock has demonstrated unprecedented measurement precision for optical atomic clocks. We present a systematic evaluation of the lattice light shift, a necessary next step for establishing this system as an accurate atomic clock. With precise control of the atomic motional states in the lattice, we report accurate measurements of the multipolar and the hyperpolar contributions and the operational lattice light shift with a fractional frequency uncertainty of 3.5×10^{-19}.

Experimental Observation of the Suppression of the Dephasing in a Floquet Engineering Optical Lattice Clock

Accurately manipulating quantum states is a fundamental strategy for improving the performance of quantum metrology, computing, and simulation. However, the quantum state is susceptible to dephasing due to the temperature and density of the atomic ensembles. In this paper, we experimentally study the effect of Floquet engineering (FE) on the dephasing process in an 87Sr optical clock. By measuring the Rabi flopping process under different temperatures of the cold ensemble and numbers of atoms trapped in the lattice, our results show that the FE can suppress the dephasing due to high temperatures or a large number of atoms. Indeed, when the temperature and the number of atoms are 3.8 μK and 6300, respectively, the FE can obviously suppress the dephasing effect and improve the maximum excitation fraction of the Rabi spectrum by 15.4%.

A proof-of-concept model of compact and high-performance 87Sr optical lattice clock for space

A compact and robust design of an optical lattice clock with high stability and accuracy is needed for the implementation of optical clocks operating on the space station. Here, we present the proof-of-concept model of a 87Sr optical lattice clock for space constructed at the National Time Service Center. The design is different from that of a laboratorial optical clock in that it focuses on the aspects of compactness and robustness, such as the modular design, compact components, low power consumption, and all-fiber links. The physics package is placed on an aluminum alloy rack with dimensions of 40 × 55 × 28 cm3. As far as we know, this is the most compact physics package of the Sr optical clock reported. The time interleaved self-comparison frequency instability is determined to be 2.6 × 10−15/τ.

48 -cm-long room-temperature cavities in vertical and horizontal orientations for Sr optical clock.

The development of an optical clock with ultimate accuracy and stability requires lasers with very narrow linewidths. We present two ultrastable laser systems based on 48-cm-long Fabry-Perot cavities made of ultralow expansion glass in horizontal and vertical configurations operating at 698 nm. Fractional frequency instability of the beat signal between the two lasers reaches 1.6×10-15 at the averaging time of 1 s. We experimentally characterized the contribution of the different noise sources (power fluctuations, residual amplitude modulation, the Doppler noise, and sensitivity to the shock impact) and found that in our case the laser frequency instability to a large extent is determined by an optoelectronic feedback loop. Although the vertical configuration was easier to manufacture and transport, it is much more sensitive to acoustics and horizontal accelerations compared to the horizontal one. Both laser systems were transported over a 60 km distance from the Lebedev Physical Institute to the All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology (VNIIFTRI), where they serve as local oscillators for spectroscopy of the clock transition in the recently developed strontium optical clock.

Test of Einstein Equivalence Principle by frequency comparisons of optical clocks

The Einstein Equivalence Principle (EEP) carries a pivotal role in understanding theory of gravity and spacetime. It guarantees the gravity to be understood as geometric phenomenon. Considering gravitational coupling of matter in the standard model extension, we propose a novel scheme using frequency measurements to limit the equivalence principle violations in normal matter. The proposal consists of the comparison of high-precision clocks, comoving with the freely falling frame. The experimental comparison of identical kind of clocks on Earth surface can be used to carry out the proposed test, which allows performing simultaneous tests of weak equivalence principle and gravitational redshift. From the existing experiments of Sr optical clocks, we present a simultaneous determination of Earth-dependent parameter βE and clock-dependent parameter ξSr at the level of 10−5, and in combination with the gravitational redshift experiments and lunar laser ranging, we also obtain a limit on standard model extension coefficients for Lorentz violation. This work provides another important fundamental physics application for the continuous-improvement accuracy of atomic or optical clocks.

Miniaturization of physics system in Sr optical clock

The compactness and robustness of the vacuum setup are the major limitations to develop transportable and space-borne optical clocks. The first step in the engineering challenge is to reduce volume and weight with respect to a stationary system. In this paper, we present the realization of a miniaturized vacuum system by building two anti-Helmholtz coils inside the vacuum magneto-optical-trap (MOT) chamber. The built-in coils are specially designed to minimize the distance between the coils, and in this way it is possible to reduce the current needed to realize a typical magnetic gradient of 40 Gs/cm required for blue MOT. When the MOT coil current is 2 A, an axial magnetic field gradient of 43 Gs/cm is obtained in the center of the MOT, which is enough for the first stage Doppler cooling. This novel design allows us to reduce size, weight and power consumption with respect to a traditional laser cooling apparatus, and simultaneously avoid complicating the water cooling equipment. Our vacuum system has a size of 60 cm×20 cm×15 cm, about 1/10 of the original system in the laboratory. In addition, the circularly polarized Zeeman slowing laser is sent to counter propagating atomic beam, and atoms at a few hundred meters per second are now routinely slowed down to velocities of tens of meters per second. As a result, about 16.4% of the atoms are actually trapped into the blue MOT. The final temperature of the blue MOT is approximately 10.6 mK, and the internal diameter is 1.5 mm by observing the expansion of the atoms from the MOT. The populations of cold atoms finally trapped in the MOT are 1.6×106 of 88Sr and 1.5×105 of 87Sr. The 1S0 → 1P1 transition used for the blue MOT is not perfectly closed due to the decay channel of the 5p1P1 → 4d1D2, and a part of atoms are stored in the 3P2 and 3P0 states. To prevent the atoms from losing, 707 and 679 nm repumping lasers are employed to recycle these atoms in the 3P1 state, and then the atoms decay to the ground state 1S0. Now the typical number of loaded atoms dramatically increases by 5 times compared with before. The slowing efficiency of Zeeman slower is also optimized for the operation with deceleration related to the parameter of magnet length, which uses more effectively available magnetic field distribution, in contrast to the usual constant deceleration mode. Our future work will focus on constructing a Zeeman slower combined with permanent magnets or an electric magnet for better tuning of the magnetic field.

Atom Interferometry with the Sr Optical Clock Transition.

We report on the realization of a matter-wave interferometer based on single-photon interaction on the ultranarrow optical clock transition of strontium atoms. We experimentally demonstrate its operation as a gravimeter and as a gravity gradiometer. No reduction of interferometric contrast was observed for a total interferometer time up to ∼10 ms, limited by geometric constraints of the apparatus. Single-photon interferometers represent a new class of high-precision sensors that could be used for the detection of gravitational waves in so far unexplored frequency ranges and to enlighten the boundary between quantum mechanics and general relativity.

Study of optical frequency transfer via fiber

Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.

Optical Сlocks and Thermal Fields Impact

<p>The sensitivity of the optical clocks to thermal radiation is shown in the research. The paper gives a brief overview of the optical clocks shift resonance reasons. The ways to investigate and decrease the thermal radiation impact on 87Sr optical clocks are found.</p>

A Hertz-Linewidth Ultrastable Diode Laser System for Clock Transition Detection of Strontium Atoms

The frequencies of two 698 nm external cavity diode lasers (ECDLs) are locked separately to two independently located ultrahigh finesse optical resonant cavities with the Pound—Drever—Hall technique. The linewidth of each ECDL is measured to be ∼4.6 Hz by their beating and the fractional frequency stability below 5 × 10−15 between 1 s to 10 s averaging time. Another 698 nm laser diode is injection locked to one of the cavity-stabilized ECDLs with a fixed frequency offset for power amplification while maintaining its linewidth and frequency characteristics. The frequency drift is ∼1 Hz/s measured by a femtosecond optical frequency comb based on erbium fiber. The output of the injection slave laser is delivered to the magneto-optical trap of a Sr optical clock through a 10-m-long single mode polarization maintaining fiber with an active fiber noise cancelation technique to detect the clock transition of Sr atoms.

Full phase stabilization of a Yb:fiber femtosecond frequency comb via high-bandwidth transducers

We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intracavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for f(rep) and f(ceo), producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f-2f interferometer and phase locked to an ultrastable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz-1 MHz), respectively. Alternatively, the comb was locked to two optical references at 698 nm and 1064 nm, obtaining 0.43 rad and 0.14 rad of integrated phase errors, respectively.

Resolved Atomic Interaction Sidebands in an Optical Clock Transition

We report the observation of resolved atomic interaction sidebands (ISB) in the (87)Sr optical clock transition when atoms at microkelvin temperatures are confined in a two-dimensional optical lattice. The ISB are a manifestation of the strong interactions that occur between atoms confined in a quasi-one-dimensional geometry and disappear when the confinement is relaxed along one dimension. The emergence of ISB is linked to the recently observed suppression of collisional frequency shifts. At the current temperatures, the ISB can be resolved but are broad. At lower temperatures, ISB are predicted to be substantially narrower and useful spectroscopic tools in strongly interacting alkaline-earth gases.

锶原子光晶格钟塞曼减速器的设计与实现

锶原子光晶格钟塞曼减速器的设计与实现

锶原子光晶格钟塞曼减速器的设计与实现

锶原子光晶格钟塞曼减速器的设计与实现

Optical clocks based on stimulated emission radiation

Atomic clocks operating at microwave region, are divided into two categories according to the mode of operation, namely the passive mode, in which the standard frequency of a clock signal is locked on to a very narrow atomic spectral line such as in cesium beam and rubidium gas cell clocks, and the active mode, where the atomic ensemble itself can produce a high stable and accurate standard frequency signal such as hydrogen maser. The hydrogen maser has been the most stable atomic clock for nearly half a century, and has been applied in a variety of scientific research fields. The optical clocks, operating at frequency domain by about five orders of magnitudes higher than that of the microwave clocks, have been expected to surpass the performance of microwave clocks both in stability and uncertainty. With the development and maturity of femtosecond optical frequency comb, very recently, the ion optical clock and neutral Sr optical clock have demonstrated better stability and uncertainty than any microwave clocks. However, all the optical clocks realized up to date are based on the “passive mode” of operation, this means that the laser frequency is locked to the very narrow atomic spectral line, but not the lasing light (the stimulated emission radiation) frequency itself which is very stable and can be used as a frequency standard——the active mode. Recently, Chen has proposed the concept of active optical clock (see ref. [6] in this issue), which is an optical clock based on the stimulated emission radiation. This is the first extension of the principle of hydrogen maser from microwave domain to optical domain. There are twofold advantages of this active optical clock. On one hand, the linewidth of the output laser from active optical clock determined by the modified SchawlowTownes formula, is much narrower than the linewidth due to transit time, which is the ultimate limit of absorption laser spectroscopy. On the other hand, utilizing bad cavity in active optical clock, the shift and noise of the centre frequency of active optical clock caused by cavity length change is reduced to a cavity pulling effect. Currently, the cavity length noise is the ultimate limit of existing lasers with coherence time longer than one second, and thus the laser linewidth is at mHz level. Active optical clocks provide several new possibilities of applications: (i) more stable optical clock than any current atomic clocks; (ii) sub-natural linewidth laser spectroscopy; (iii) long coherence time laser with linewidth at mHz level; (iv) Ramsey laser combining stimulated emission process and Ramsey separated oscillatory fields method. Therefore, the prospect of active optical clock is attractive for the research at the field of optical clock, laser spectroscopy and metrology.