Clock Applications Research Articles

Electromagnetically induced transparency in a Zeeman-sublevels Λ-system of cold 87Rb atoms in free space**Project supported by the National Basic Research Program of China (Grant No. 2011CB921504) and the National Natural Science Foundation of China (Grant No. 91536107).

We report the experimental investigation of electromagnetically induced transparency (EIT) in a Zeeman-sublevels Λ-type system of cold 87Rb atoms in free space. We use the Zeeman substates of the hyperfine energy states 52S1/2, F = 2 and 52P3/2, F′ = 2 of 87Rb D2 line to form a Λ-type EIT scheme. The EIT signal is obtained by scanning the probe light over 1 MHz in 4 ms with an 80 MHz arbitrary waveform generator. More than 97% transparency and 100 kHz EIT window are observed. This EIT scheme is suited for an application of pulsed coherent storage atom clock (Yan B, et al. 2009 Phys. Rev. A 79 063820).

Generalized mean-field approach to simulate the dynamics of large open spin ensembles with long range interactions

We simulate the collective dynamics in spin lattices with long range interactions and collective decay in one, two and three dimensions. Starting from a dynamical mean-field approach derived by local factorization of the density operator we improve the numerical approximation of the full master equation by including pair correlations at any distance. This truncations enable us to drastically increase the number of spins in our numerical simulations from about ten spins in case of the full quantum model to several ten-thousands in the mean-field approximation and a few hundreds if pair correlations are included. Extensive numerical tests help us identify interaction strengths and geometric configurations where these approximations perform well and allow us to state fairly simple error estimates. By simulating systems of increasing size we show that in one and two dimensions we can include as many spins as needed to capture the properties of infinite size systems with high accuracy, while in 3D the method does not converge to desired accuracy within the system sizes we can currently implement. Our approach is well suited to give error estimates of magic wavelength optical lattices for atomic clock applications and corresponding super radiant lasers.

Relativistically prolonged lifetime of the 2s2p 3P0 level of zero nuclear-spin beryllium-like ions

The E1M1 transition rate of the line in beryllium-like ions has been calculated within the framework of relativistic second-order perturbation theory. Both multiconfiguration and quantum-electrodynamical computations have been carried out independently to better understand and test for all major electron–electron correlation contributions in the representation of the initial, intermediate and final states. By comparing the results from these methods, which agree well for all ions along the beryllium isoelectronic sequence, the lifetime of the metastable 2s2p 3P0 level is found to be longer by about 2–3 orders of magnitude for all medium and heavy elements than was estimated previously. This makes the 3P0 level of beryllium-like ions to one of the longest living (low-lying) electronic excitations of a tightly bound system with potential applications for atomic clocks and in astro physics and plasma physics.

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications.

This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm.

High-power vertical-cavity surface-emitting lasers for atomic clock applications

High-power vertical-cavity surface-emitting lasers for atomic clock applications

Transitions between the4f-core-excitedstates inIr16+,Ir17+, andIr18+ions for clock applications

Iridium ions near $4f$-$5s$ level crossings are the leading candidates for a new type of atomic clocks with a high projected accuracy and a very high sensitivity to the temporal variation of the fine structure constant $\alpha$. To identify spectra of these ions in experiment accurate calculations of the spectra and electromagnetic transition probabilities should be performed. Properties of the $4f$-core-excited states in Ir$^{16+}$, Ir$^{17+}$, and Ir$^{18+}$ ions are evaluated using relativistic many-body perturbation theory and Hartree-Fock-Relativistic method (COWAN code). We evaluate excitation energies, wavelengths, oscillator strengths, and transition rates. Our large-scale calculations includes the following set of configurations: $4f^{14-k}5s^{m}5p^{n}$ with $(k+m+n)$ equal to 3, 2, and 1 for the Ir$^{16+}$, Ir$^{17+}$, and Ir$^{18+}$ ions, respectively. The $5s-5p$ transitions are illustrated by the synthetic spectra in the 180 - 200 \AA range. Large contributions of magnetic-dipole transitions to lifetimes of low-lying states in the region below 2.5 Ry are demonstrated.

Research progress on active optical clocks

Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.

Optical atomic clocks

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.

Optical pumping and readout of bismuth hyperfine states in silicon for atomic clock applications.

The push for a semiconductor-based quantum information technology has renewed interest in the spin states and optical transitions of shallow donors in silicon, including the donor bound exciton transitions in the near-infrared and the Rydberg, or hydrogenic, transitions in the mid-infrared. The deepest group V donor in silicon, bismuth, has a large zero-field ground state hyperfine splitting, comparable to that of rubidium, upon which the now-ubiquitous rubidium atomic clock time standard is based. Here we show that the ground state hyperfine populations of bismuth can be read out using the mid-infrared Rydberg transitions, analogous to the optical readout of the rubidium ground state populations upon which rubidium clock technology is based. We further use these transitions to demonstrate strong population pumping by resonant excitation of the bound exciton transitions, suggesting several possible approaches to a solid-state atomic clock using bismuth in silicon, or eventually in enriched 28Si.

Design and Characterization of a Dual-Mode CMOS-MEMS Resonator for TCF Manipulation

A novel complimentary metal-oxide-semiconductor-microelectormechanical systems (CMOS-MEMS) composite ring resonator capable of a dual-mode operation has been proposed to enable temperature coefficient of frequency (TC $_{\!f})$ manipulation. To study the temperature dependence between dual modes, two resonant modes of a single resonator vibrating in the orthogonal axes (i.e., in-plane and out-of-plane) are chosen to enable a large difference of their TC $_{\!f}$ ’s while not to sacrifice its form factor. By adjusting the constituent ratio and position of the composed metals and dielectrics through the computer-aided-design layout, different TC $_{\!f}$ ’s have been successfully demonstrated in a single CMOS-MEMS resonator. By concurrently measuring the TC $_{\!f}$ ’s of the in-plane and out-of-plane modes with a divider-based scaling concept, estimated minimum first- and second-order temperature sensitivities (0.53 and 0.29 ppm/°C2, respectively) of their beat frequency can be obtained under proper scaling numbers for temperature-compensated clock applications. This paper also suggests that the first-order temperature coefficient of the beat frequency could be maximized under proper divider numbers. The process variations of the CMOS-MEMS resonators in terms of frequency, quality factor, and transmission magnitude are also intensively studied with an applicable amount of devices. The characterization result shows 1- $\sigma $ frequency variations of 2,574 and 5,414 ppm for in-plane and out-of-plane modes, respectively. [2014-0046]

Modelling the concentration dependence of doping in optical materials

As well as understanding the location of dopants in optical materials, it is also important to understand how much dopant can be added to a given material. A method for calculating the maximum concentration of dopants has been developed, and applied to dopants in mixed metal fluorides for optical and nuclear clock applications. Applications to rare earth doping in YLiF4, and Th doping in LiCaAlF6/LiSrAlF6 are described, and compared with available experimental data.

An on-chip frequency programmable test clock generation and application method for small delay defect detection

Small delay defects are posing a serious challenge to the quality and reliability of modern fabricated chips. A promising way for screening the timing-related defects in nanometer technology designs is faster-than-at-speed delay testing. However, the use of external automatic test equipment for faster-than-at-speed delay testing could be costly. In this paper, we present an on-chip frequency-programmable test clock generation method which facilitates faster-than-at-speed delay testing for both launch on capture and launch on shift test frameworks. With a reconfigurable launch-and-capture clock generator (LCCG) embedded on-chip, the required test clock, with a reconfigurable frequency and a high resolution, can be achieved by specifying the control information in the test patterns, which is then used to configure the LCCG. Similarly, the control information regarding test framework and clock signal selection can also be embedded in the test patterns. Experimental results are presented to validate the proposed method.

Progress towards primary frequency standard

The cesium fountain clock as primary frequency standard is widely used in the areas, such as time-keeping system, satellite navigation, fundamental physics research, etc. The principle of operation of cesium fountain clock is introduced. The noise source and frequency shift term are ananlyzed. The major noise source influencing frequency stability are cold atom loading time, microwave phase noise related to Dick effect, and detection laser frequency noise. The major frequency bias influencing frequency uncertainty is blackbody radiation frequency shift,cold atom collision frequency shift,distributed cavity phase frequency shift and microwave leakage frequency shift.The key technique to achieve highperformance cesium fountain clock is sumerized. The application of cesium fountain clock is presented. The status of space cesium clock and future primary frequency standard of optical clock are shown.

Laser Cooling of Lanthanides: from Optical Clocks to Quantum Simulators

We discuss current progress in laser cooling of lanthanides (Er, Yb, Dy, Tm etc.) focusing on applications. We describe some important peculiarities taking Thulium atom as an example: Two stage laser cooling, trapping in an optical lattice, anisotropic interactions and spectroscopy of narrow transitions. Specific level structure and presence of magic wavelengths make ultracold Thulium a favorable candidate for optical clock applications. On the other hand, abundance of Feshbach resonances allow to tune interactions in ultracold gases and thus reach quantum degeneracy. It opens intriguing perspectives for novel quantum simulators employing dipole-dipole interactions in an optical lattice.

Design and Implementation of iOS-based Mobile Application about Awakening by CNR

With the growing popularity of the iPhone, there is a constantly increasing demand of users for mobile applications of iPhone, in which the alarm clock application is contained. But existing alarm clock application is not favored by many users because of functional singleness. This paper presents an iOS-based mobile application on alarm clock with online radio support, and designs and implements its framework by virtue of Objective-C and SQLite in the Xcode 4.5.2 development environment, based on the popular three-tier MVC software design structure. After the application is installed and deployed on iPhone, it is available to users by providing some functions such as auto-playing radio, setting radio alarm clock, turning off the alarm by shaking iPhone, sleeping timer by radio, reserving radio program, binding and sharing microblog. This proposed iOS-based mobile application provides users with more choices and allows the user to enjoy a more colorful leisure time. The practical test from some people shows that the proposed application is very popular with users, which also illustrates its practicability and effectiveness.

Hunting for topological dark matter with atomic clocks

The cosmological applications of atomic clocks so far have been limited to searches of the uniform-in-time drift of fundamental constants. In this paper, we point out that a transient in time change of fundamental constants can be induced by dark matter objects that have large spatial extent, and are built from light non-Standard Model fields. The stability of this type of dark matter can be dictated by the topological reasons. We point out that correlated networks of atomic clocks, some of them already in existence, can be used as a powerful tool to search for the topological defect dark matter, thus providing another important fundamental physics application to the ever-improving accuracy of atomic clocks. During the encounter with a topological defect, as it sweeps through the network, initially synchronized clocks will become desynchronized. Time discrepancies between spatially-separated clocks are expected to exhibit a distinct signature, encoding defect's space structure and its interaction strength with the Standard Model fields.

Highly flexible short-pulse generation and high sensitivity sensing with stimulated Brillouin scattering in optical fibers

We demonstrate two novel types of applications using the same generator as a source for Brillouin multi-wavelength generation. The first is as a highly flexible picosecond pulsed laser source which is tunable in time duration, repetition rate and emission wavelength for optical clock applications in telecommunication. The second application is a high sensitivity distributed Brillouin sensors (DTS) to lower costs and widens the market sector. We demonstrate tunability of the pulsed laser source from ~15 ps down to ~3.5 ps over the whole telecommunications C-band by simply controlling the number of Stokes waves being generated forming a phase-locked Brillouin frequency comb. The repetition rate is the Brillouin frequency shift of ~10 GHz which can be tuned by changing the gain fiber within the cavity. An increase in the standard temperature sensitivity of DTS of ~1.3 MHz/°C by 6 fold is also demonstrated. This increase is of great importance in DTS, since the detection of any variation can be made faster, which can enhance the functionality of such sensors.

Hunting for topological dark matter with atomic clocks

The cosmological applications of atomic clocks 1‐3 so far have been limited to searches for the uniform-in-time drift of fundamental constants 4 . We point out that a transient-in-time change of fundamental constants can be induced by darkmatter objects that have large spatial extent, such as stable topologicaldefects 5 builtfromlightnon-StandardModelfields. Networks of correlated atomic clocks, some of them already in existence 6 , such as the Global Positioning System, can be used as a powerful tool to search for topological defect dark matter,thusprovidinganotherimportantfundamentalphysics application for the ever-improving accuracy of atomic clocks. During the encounter with an extended dark-matter object, as itsweepsthroughthenetwork,initiallysynchronizedclockswill becomedesynchronized.Timediscrepanciesbetweenspatially separated clocks are expected to exhibit a distinct signature, encoding the defect’s space structure and its interaction strength with atoms. Despite solid evidence for the existence of dark matter ( 25% of the global energy budget in the Universe and DM’0.3GeVcm 3 in the neighbourhood of the Solar system 7 ), its relationship to particles and fields of the Standard Model (SM) remains a mystery. Although searches for particle dark matter (DM) are being actively pursued 8 , there is also significant interest in alternatives, among which is DM composed from very light fields. Depending on the initial field configuration at early cosmological times, such light fields could lead to dark matter via coherent oscillations around the minimum of their potential, and/or form non-trivial stable field configurations in physical three-dimensional space if their potential allows such a possibility. This latter option, which we will generically refer to as topological defects (TDs), is the main interest of our paper. The light masses of fields forming the TDs could lead to a large, indeed macroscopic, size for a defect. Their encounters with the Earth, combined with the DMSM coupling, can lead to novel signatures of dark matter expressed generically in terms of ‘transient eects’. These eects, coherent on the scale of individual detectors, are temporary shifts in the frequencies and phases of measuring devices, rather than large energy depositions as is the case for microscopic DM. In this paper we suggest the possibility of a new search technique for the topological defect dark matter (TDM), based on a network of atomic clocks. Atomic clocks are arguably the most accurate scientific instruments ever built, reaching a 10 18 fractional inaccuracy 1,2 .

Divalent Rydberg atoms in optical lattices: Intensity landscape and magic trapping

We develop a theoretical understanding of trapping divalent Rydberg atoms in optical lattices. Because the size of the Rydberg electron cloud can be comparable to the scale of spatial variations of laser intensity, we pay special attention to averaging optical fields over the atomic wavefunctions. Optical potential is proportional to the ac Stark polarizability. We find that in the independent particle approximation for the valence electrons, this polarizability breaks into two contributions: the singly ionized core polarizability and the contribution from the Rydberg electron. Unlike the usually employed free electron polarizability, the Rydberg contribution depends both on laser intensity profile and the rotational symmetry of the total electronic wavefunction. We focus on the $J=0$ Rydberg states of Sr and evaluate the dynamic polarizabilities of the 5s$n$s($^1S_0$) and 5s$n$p($^3P_0$) Rydberg states. We specifically choose Sr atom for its optical lattice clock applications. We find that there are several magic wavelengths in the infrared region of the spectrum at which the differential Stark shift between the clock states (5s$^2$($^1S_0$) and 5s5p($^3P_0$)) and the $J=0$ Rydberg states, 5s$n$s($^1S_0$) and 5s$n$p($^3P_0$), vanishes. We tabulate these wavelengths as a function of the principal quantum number $n$ of the Rydberg electron. We find that because the contribution to the total polarizability from the Rydberg electron vanishes at short wavelengths, magic wavelengths below $\sim$1000 nm are ``universal" as they do not depend on the principal quantum number $n$.

Phase Synchronization of Two Anharmonic Nanomechanical Oscillators

We investigate the synchronization of oscillators based on anharmonic nanoelectromechanical resonators. Our experimental implementation allows unprecedented observation and control of parameters governing the dynamics of synchronization. We find close quantitative agreement between experimental data and theory describing reactively coupled Duffing resonators with fully saturated feedback gain. In the synchronized state we demonstrate a significant reduction in the phase noise of the oscillators, which is key for sensor and clock applications. Our work establishes that oscillator networks constructed from nanomechanical resonators form an ideal laboratory to study synchronization--given their high-quality factors, small footprint, and ease of cointegration with modern electronic signal processing technologies.