Cold Atom Clock Research Articles

Automatic compensation of magnetic field for a rubidium space cold atom clock**Project supported by the Ministry of Science and Technology of China (Grant No. 2013YQ09094304), the Youth Innovation Promotion Association, Chinese Academy of Sciences, and the National Natural Science Foundation of China (Grant Nos. 11034008 and 11274324).

When the cold atom clock operates in microgravity around the near-earth orbit, its performance will be affected by the fluctuation of magnetic field. A strategy is proposed to suppress the fluctuation of magnetic field by additional coils, whose current is changed accordingly to compensate the magnetic fluctuation by the linear and incremental compensation. The flight model of the cold atom clock is tested in a simulated orbital magnetic environment and the magnetic field fluctuation in the Ramsey cavity is reduced from 17 nT to 2 nT, which implied the uncertainty due to the second order Zeeman shift is reduced to be less than 2×10−16. In addition, utilizing the compensation, the magnetic field in the trapping zone can be suppressed from 7.5 μT to less than 0.3 μT to meet the magnetic field requirement of polarization gradients cooling of atoms.

Reaching 5.0×10-13τ-1/2 short term frequency stability of the integrating sphere cold atom clock

We present an improvement of short term frequency stability of the integrating sphere cold atom clock after increasing the intensities of clock signals and optimizing the feedback loop of the clock. A short term frequency stability of 5.0×10-13τ-1/2 has been achieved and the limiting factors have been analyzed.

Initial Tests of a Rubidium Space Cold Atom Clock**Supported by the National Key Scientific Instrument and Equipment Development Project of China under Grant No 2013YQ09094304, the Youth Innovation Promotion Association of Chinese Academy of Sciences, and the National Natural Science Foundation of China under Grant Nos 11034008 and 11274324.

We report the initial test results of a rubidium (87Rb) space cold atom clock (SCAC). The space-qualified 87Rb SCAC is composed of the physical package, the optical bench, the microwave synthesizer and the control electronics. After the system is integrated, about 108 87Rb cold atoms are captured by magneto-optical trap. The linewidth of the Ramsey fringe is about 10 Hz for the free evolution time of 50 ms on the ground, and the signal-to-noise ratio is measured to be larger than 300. We demonstrate a good medium-term fractional frequency stability of 1.5 × 10−14@1000 s in the closed-loop operation on the ground. The main effects of the noise on the stability are also presented, and the optimized operating parameter is analyzed for the operation of SCAC in the microgravity environment.

Highly reliable optical system for a rubidium space cold atom clock.

We describe a highly reliable optical system designed for a rubidium space cold atom clock (SCAC), presenting its design, key technologies, and optical components. All of the optical and electronic components are integrated onto an optimized two-sided 300 mm×290 mm×30 mm optical bench. The compact optical structure and special thermal design ensure that the optical system can pass all of the space environmental qualification tests including both thermal vacuum and mechanical tests. To verify its performance, the optical system is carefully checked before and after each test. The results indicate that this optical system is suitably robust for the space applications for which the rubidium SCAC was built.

Microwave interrogation cavity for the rubidium space cold atom clock**Project supported by the National Natural Science Foundation of China (Grant No. 11034008), the Fund from the Ministry of Science and Technology of China (Grant No. 2013YQ09094304), and the Youth Innovation Promotion Association, Chinese Academy of Sciences.

The performance of space cold atom clocks (SCACs) should be improved thanks to the microgravity environment in space. The microwave interrogation cavity is a key element in a SCAC. In this paper, we develop a microwave interrogation cavity especially for the rubidium SCAC. The interrogation cavity has two microwave interaction zones with a single feed-in source, which is located at the center of the cavity for symmetric coupling excitation and to ensure that the two interaction zones are in phase. The interrogation cavity has a measured resonance frequency of 6.835056471 GHz with a loaded quality factor of nearly 4200, which shows good agreement with simulation results. We measure the Rabi frequency of the clock transition of the rubidium atom in each microwave interaction zone, and subsequently demonstrate that the distributions of the magnetic field in the two interaction zones are the same and meet all requirements of the rubidium SCAC.

积分球冷原子钟探测光功率自动稳定实验研究

积分球冷原子钟探测光功率自动稳定实验研究

积分球冷原子钟相位调制Ramsey条纹研究

积分球冷原子钟相位调制Ramsey条纹研究

Scheme for a compact cold-atom clock based on diffuse laser cooling in a cylindrical cavity

We present a scheme for a compact rubidium cold-atom clock which performs diffuse light cooling, microwave interrogation, and detection of the clock signal in a cylindrical microwave cavity. The diffuse light is produced by laser light reflection at the inner surface of the microwave cavity. The pattern of the injected laser beams is specially designed to accumulate the majority of the cold atoms in the center of the microwave cavity. Microwave interrogation of the cold atoms in the cavity leads to Ramsey fringes, which have a linewidth of $24.5$ Hz with a contrast of $95.6%$ when the free evolution time is 20 ms. Recently, a frequency stability of $7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.28em}{0ex}}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved. The scheme of this physical package can largely reduce the cold-atom clock complexity and increase clock performance.

The ACES/PHARAO space mission

Proposed in 1997, the ACES/PHARAO experiment is a space mission in fundamental physics with two atomic clocks on the International Space Station, a network of ultra-stable clocks on the ground, and space-to-ground time transfer systems. The ACES flight instruments are near completion and launch in space is planned for the first half of 2017 for a mission duration of three years. A key element of the satellite payload is a cold-atom clock designed for microgravity environment, PHARAO, operating with laser-cooled cesium atoms. Here we first report on the design and tests of the PHARAO flight model, which is now completed and ready for launch. We then briefly present the status of development of the other instruments of the ACES payload, the Space Hydrogen Maser, the microwave time-transfer system (MWL), and the laser time transfer ELT. Proposée en 1997, l'expérience ACES/Pharao est une mission spatiale en physique fondamentale avec deux horloges atomiques sur la Station spatiale internationale, un réseau d'horloges ultra-stables à terre et de systèmes de transfert de temps de l'espace jusqu'au sol. Les instruments de vol ACES sont proches de l'aboutissement et leur lancement dans l'espace est planifié pour la première moitié de 2017 pour une durée de mission de trois ans. Un élément-clé de la charge utile du satellite est une horloge atomique à atomes froids conçue pour la microgravité, Pharao, qui fonctionne avec des atomes de césium refroidis par laser. Nous commençons par décrire la conception et les essais du modèle de vol Pharao, qui est maintenant opérationnel et prêt pour le lancement. Nous présentons ensuite brièvement l'état de développement des autres instruments de la plateforme ACES, le maser passif à hydrogène, le système de transfert de temps par liaisons micro-ondes (MWL) et le transfert de temps par lien laser (ELT).

Development of an ultra-high vacuum system for space cold atom clock

Space cold atom clock (SCAC) is developed for a series of experiments related to laser cooling of atoms and cold atom clock in space microgravity environment. An ultra-high vacuum (UHV) system is the key element for SCAC. This paper presents the design of the UHV system and also shows the status of an initial UHV system prototype with the design and a series of special vacuum treatments whose pressure distribution satisfies the special requirements for SCAC. A dual pumping system, including ion pump and getter, is introduced, and pressures of different positions have been calculated. Experiments have been carried out to acquire the performances of the initial prototype. It is proved that the initial prototype can not only self-sustain an equilibrium pressure for more than 2 months without power but also withstand the mechanical vibration test and space environment simulation test. Furthermore, an optimization design is used to get pressures which are much lower than the desired pressure (∼×10−8 Pa) in the interaction zone of the system has been demonstrated.

Non-resonant magneto-optical effects in cold atoms

We report a novel nonresonant magneto-optical effect in cold atoms and present the optimized parameters of the biased magnetic field, the incident probe light intensity, and the probe detuning to obtain the maximal signal of the magneto-optical rotation. This detection scheme may further improve the stability of the cold atom clock.

空间冷原子钟原理样机地面测试结果

空间冷原子钟原理样机地面测试结果

Evaluation of long term performance of continuously running atomic fountains

An ensemble of rubidium atomic fountain clocks has been put into operation at the US Naval Observatory (USNO). These fountains are used as continuous clocks in the manner of commercial caesium beams and hydrogen masers for the purpose of improved timing applications. Four fountains have been in operation for more than two years and are included in the ensemble used to generate the USNO master clock. Individual fountain performance is characterized by a white-frequency noise level below 2 × 10−13 and fractional-frequency stability routinely reaching the low 10−16 s. The highest performing pair of fountains exhibits stability consistent with each fountain integrating as white frequency noise, with Allan deviation surpassing 6 × 10−17 at 107 s, and with no relative drift between the fountains at the level of 7.5 × 10−19/day. As an ensemble, the fountains generate a timescale with white-frequency noise level of 1 × 10−13 and long-term frequency stability consistent with zero drift relative to the world's primary standards at 1 × 10−18/day. The rubidium fountains are reported to the BIPM as continuously running clocks, as opposed to secondary standards, the only cold-atom clocks so reported. Here we further characterize the performance of the individual fountains and the ensemble during the first two years in an operational environment, presenting the first look at long-term continuous behavior of fountain clocks.

Development of an Integrating Sphere Cold Atom Clock

The Ramsey fringes with a linewidth of 45 Hz and a signal-to-noise ratio of 120 are demonstrated for a integrating sphere atom clock. The cycle time of the atom clock is reduced to 80 ms with the help of a pulsed cooling method. This result indicates that the short-term stability of the clock is in the order of 10−12.

Ramsey interaction with transverse decay

The Ramsey fringe contrast of a pulsed optically pumped cold atom clock is strongly affected by the transverse decay of the atomic sample. This letter calculates the Ramsey fringe with focus on transverse decay, and analyzes the Ramsey fringe contrast with different transverse decay rates. By fitting the experimental data, we obtain the transverse decay rate in a cold atom sample at an approximate value of 30.5 s-1, which is much smaller than that in a cell.

Quantum information at the interface of light with atomic ensembles and micromechanical oscillators

This article reviews recent research towards a universal light-matter interface. Such an interface is an important prerequisite for long distance quantum communication, entanglement assisted sensing and measurement, as well as for scalable photonic quantum computation. We review the developments in light-matter interfaces based on room temperature atomic vapors interacting with propagating pulses via the Faraday effect. This interaction has long been used as a tool for quantum nondemolition detections of atomic spins via light. It was discovered recently that this type of light-matter interaction can actually be tuned to realize more general dynamics, enabling better performance of the light-matter interface as well as rendering tasks possible, which were before thought to be impractical. This includes the realization of improved entanglement assisted and backaction evading magnetometry approaching the Quantum Cramer-Rao limit, quantum memory for squeezed states of light and the dissipative generation of entanglement. A separate, but related, experiment on entanglement assisted cold atom clock showing the Heisenberg scaling of precision is described. We also review a possible interface between collective atomic spins with nano- or micromechanical oscillators, providing a link between atomic and solid state physics approaches towards quantum information processing.

Kinetic behavior of cold 87Rb atoms in integrating sphere

Kinetic behavior of cold 87Rb atoms in integrating sphere is presented under adiabatic release. Decay time of cold atom cloud is deduced by calculation of residual cold atom number evolution under adiabatic release and agrees well with our experimental observation. Number evolutions of residual cold atoms under different kinds of acceleration have been studied in detail. Sequence laser pulse cooling is key technique for the successive operation of cold atom clock. The process of laser pulse cooling is numerically simulated and fitted with experiments, optimized distribution of cooling and dead time to get enough cold atoms is put forward to satisfy the demand of cold atom clock.

Cooling of rubidium atoms in pulsed diffuse laser light

This paper reports an experiment on laser cooling of 87Rb atoms in pulsed diffuse light, which is the key step towards a compact cold atom clock. It deduces an empirical formula to simulate the pulse cooling process based on the loading of cold atoms in cooling time and the loss in the dead time, which is in agreement with the experimental data. The formula gives a reference to select the parameters for the cold atom clock.

HORACE: A compact cold atom clock for Galileo

HORACE (HOrloge à Refroidissement d’Atomes en Cellule = clock based on atoms cooled from vapour cell) is a compact cold caesium atom clock developed in SYRTE at Paris Observatory. This clock can operate both on ground and in microgravity environment. Design of HORACE is based on isotropic light cooling, allowing performing the whole clock sequence (cooling, atomic preparation, Ramsey interrogation and detection) at the same place. Compared to more conventional cold atom clocks such as atomic fountains, the use of isotropic light cooling simplifies both the optical part of the setup and the detection sequence, and leads to a drastic size reduction of the physics package. Very good short-term performances have been demonstrated at SYRTE since relative frequency instability of 2.2 × 10 −13 τ −1/2 has been obtained. Optimization of the long term stability is still under progress and current results show relative frequency instability around 3 × 10 −15 in 10 4 s of integration. With these performances, HORACE appears as a good candidate both for Galileo’s ground segment clock and for onboard Galileo clock.

High-stability compact atomic clock based on isotropic laser cooling

We present a compact cold-atom clock configuration where isotropic laser cooling, microwave interrogation, and clock signal detection are successively performed inside a spherical microwave cavity. For ground operation, a typical Ramsey fringe width of 20 Hz has been demonstrated, limited by the atom cloud's free fall in the cavity. The isotropic cooling light's disordered properties provide a large and stable number of cold atoms, leading to a high signal-to-noise ratio limited by atomic shot noise. A relative frequency stability of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved, averaged down to $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ after $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ s of integration. Development of such a high-performance compact clock is of major relevance for on-board applications, such as satellite-positioning systems. As a cesium clock, it opens the door to a new generation of compact primary standards and timekeeping devices.