Microwave Interrogation Research Articles

Correcting heading errors in optically pumped magnetometers through microwave interrogation

Correcting heading errors in optically pumped magnetometers through microwave interrogation

Physics package based on intracavity laser cooling 87Rb atoms for space cold atom microwave clock

Abstract This article proposes a new physics package to enhance the frequency stability of the space cold atom clock with the advantages of a microgravity environment. Clock working processes, including atom cooling, atomic state preparation, microwave interrogation, and transition probability detection, are integrated into the cylindrical microwave cavity to achieve a high-performance and compact physics package for the space cold atom clock. We present the detailed design and ground-test results of the cold atom clock physics package in this article, which demonstrates a frequency stability of 1.2 × 10−12 τ −1/2 with a Ramsey linewidth of 12.5 Hz, and a better performance is predicted with a 1 Hz or a narrower Ramsey linewidth in microgravity environment. The miniaturized cold atom clock based on intracavity cooling has great potential for achieving space high-precision time-frequency reference in the future.

Detection of VOCs Using Doped ZnO Nanorods

Metal-oxides, such as ZnO, have been popular materials of choice for the detection of VOCs because they show a relatively quick response. In this study, ZnO nanorods were prepared, first, by depositing a seed layer via atomic layer deposition (ALD) on top of a silicon substrate, and then immersing that seed layer in a chemical bath solution to facilitate growth of the rods. To enhance the gas sensing properties of the rods, some of the nanorods were doped with metals such as Au and Co. The gas sensing materials were then probed with microwaves, using Broad band dielectric Spectroscopy (BDS), to observe their changing characteristics in the presence of a VOC analyte. BDS provides and advantage in that it avoids the need for physical electrical connections which lead to errors into the measurement. Additionally, we use Raman spectroscopy to not only confirm the presence of the dopants in the nanorods, but also to help explain the behavior of the materials under microwave interrogation.

Bimetal Temperature-Compensated Ramsey Cavity for Atomic Fountain Frequency Standards

A typical Ramsey cavity, commonly used in atomic fountain frequency standards (AFFSs) for microwave interrogation, is sensitive to ambient temperature variation. Then, it becomes one of the main limiting factors for the AFFSs operating in a narrow operating-temperature range and a temperature-well-controlled environment up to now. In this article, a new design of bimetal temperature-compensated Ramsey cavity (BTCRC) based on the thermal expansion self-compensating mechanism is presented and experimentally demonstrated. This cavity is a typical TE011-mode cylindrical microwave resonator with a titanium tube and two oxygen-free copper (OFC) caps. The resonance frequency (RF) of the cavity can be immune to temperature variations if the dimensions of the tube and caps are properly designed. To validate this self-compensating mechanism, a BTCRC resonating at the Rb clock frequency with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor of about 17000 is constructed. And its RF thermal coefficient is measured to be 0.73 kHz/°C, which is about 150 times superior to −115.9 kHz/°C of a typical OFC Rb cavity in the temperature range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$22.48~^{\circ }\text{C}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$37.48~^{\circ }\text{C}$ </tex-math></inline-formula> . During the operation of AFFSs, this BTCRC ensures that the microwave field amplitude inside the cavity, which dictates the transition probability and thus the contrast of the Ramsey fringes, will be stable in time. In addition, the potential temperature variation-induced frequency shifts due to cavity pulling are significantly minimized.

Microwave phase detection of coherent population trapping resonance in a Cs vapor cell

We report a phase-sensitive microwave detection setup for spectroscopy of coherent population trapping (CPT) resonance in a Cs vapor cell. In this scheme, the optically carried microwave interrogation signal that interacts with atoms is detected by a fast photodiode at the cell output and is then mixed with the local oscillator microwave signal itself. This detection scheme produces at the output of the microwave mixer a dispersive CPT signal that can be directly used for correction of the local oscillator frequency onto the atomic resonance. The evolution of the dispersive CPT resonance signal with the laser power is briefly reported. With this detection scheme, we demonstrate the operation of a CPT atomic clock with a short-term fractional frequency stability of 1 × 10 − 11 at 1 s, which is in good agreement with the slope of the frequency discriminator and the residual microwave phase noise of the setup.

Quantum estimation of a time-dependent perturbation

We analyze the estimation of a time-dependent perturbation acting on a continuously monitored quantum system. We describe the temporal fluctuations of the perturbation by a hidden Markov model, and we combine quantum measurement theory and classical filter theory into a time-evolving hybrid quantum and classical trajectory. The forward-backward analysis that permits smoothed estimates of classical hidden Markov models has a counterpart in the theory of retrodiction and past quantum states. As a specific example, we apply our hybrid trajectory and past-quantum-state theory to the sensing of a fluctuating magnetic field by microwave interrogation of a single quantum spin.

Loaded Microwave Cavity for Compact Vapor-Cell Clocks.

Vapor-cell devices based on microwave interrogation provide a stable frequency reference with a compact and robust setup. Further miniaturization must focus on optimizing the physics package, containing the microwave cavity and atomic reservoir. In this article, we present a compact cavity-cell assembly based on a dielectric-loaded cylindrical resonator. The loaded cavity resonating at 6.83 GHz has an external volume of only 35 cm3 and accommodates a vapor cell with 0.9-cm3 inner volume. The proposed design aims at strongly reducing the core of the atomic clock, maintaining, at the same time, high-performing short-term stability ( σy(τ) ≤ 5×10-13 τ-1/2 standard Allan deviation). The proposed structure is characterized in terms of microwave field uniformity and atom-field coupling with the aid of finite-element calculations. The thermal sensitivity is also analyzed and experimentally characterized. We present preliminary spectroscopy results by integrating the compact cavity within a rubidium clock setup based on the pulsed optically pumping technique. The obtained clock signals are compatible with the targeted performances. The loaded-cavity approach is, thus, a viable design option for miniaturized microwave clocks.

Compact Cold-Atom Clock for Onboard Timebase: Tests in Reduced Gravity

We present a compact atomic clock using cold rubidium atoms based on an isotropic light cooling, a Ramsey microwave interrogation and an absorption detection. Its technology readiness level is suitable to industrial transfer. We use a fibre optical bench, based on a frequency-doubled telecom laser. The isotropic light cooling technique allows us to cool down the atoms in 100 ms and works with a cycle time around 200 ms. We carried out measurements in simulated microgravity and obtained the narrowest fringes ever recorded in microgravity.

Rubidium-beam microwave clock pumped by distributed feedback diode lasers**Project supported by the National Natural Science Foundation of China (Grant No. 11174015).

A rubidium-beam microwave clock, optically pumped by a distributed feedback diode laser, is experimentally investigated. The clock is composed of a physical package, optical systems, and electric servo loops. The physical package realizes the microwave interrogation of a rubidium-atomic beam. The optical systems, equipped with two 780-nm distributed feedback laser diodes, yield light for pumping and detecting. The servo loops control the frequency of a local oscillator with respect to the microwave spectrum. With the experimental systems, the microwave spectrum, which has an amplitude of 4 nA and a line width of 700 Hz, is obtained. Preliminary tests show that the clock short-term frequency stability is 7 × 10−11 at 1 s, and 3 × 10−12 at 1000 s. These experimental results demonstrate the feasibility of the scheme for a manufactured clock.

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.

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.

Comment on ``Ramsey spectroscopy, matter-wave interferometry, and the microwave-lensing frequency shift''

Determination of the second in the International System of Units (SI) and realization of international atomic time (TAI) are among the most precise measurements of any kind and have some of the greatest impacts on technology and research, largely because the careful physical measurements are complemented by well-established and tested theories. Reference [1] describes the theory of a frequency shift from which the author calculates “corrections” to various primary frequency standards: the laser-cooled atomic microwave fountain clocks that are used to determine TAI and the SI second. According to Ref. [1], the magnitude of this frequency shift (δf/f )i s on the order of the current frequency uncertainty of the best primary frequency standards df/f ∼ 1 × 10 −16 . Corrections for this shift (microwave lensing) are currently applied to some (but not all) of the primary frequency standards that help determine TAI and the SI second [2–4]. The frequency shift described in Ref. [1], thus introduces a significant bias in TAI compared to TAI realized without the proposed shift applied. Our group has performed independent theoretical investigations into the theory of a microwave frequency shift from lensing [5], and our theory shows significantly different results than those claimed in Ref. [1]. The effect described in Ref. [1] is the result of “clipping” of the atomic wave function by limiting apertures in a primary frequency standard (PFS). It is postulated that differential clipping of the two dressed states of the atomic wave function occurs as a result of a very small transverse force on the atoms caused by a gradient in the microwave field during microwave interrogation. This small transverse force generates a displacement between the wave functions of the two dressed states on the order of a few nanometers, which is much less

Design and test of the microwave cavity in an optically-pumped Rubidium beam frequency standard**Project supported by the National Natural Science Foundation of China (Grant No. 11174015).

We are developing a compact rubidium atomic beam frequency standard with optical pumping and detection. The cavity for microwave interrogation is an important part of the clock. The cavity in our design is a Ramsey-type, E-bend one, which is the same as the conventional method in most cesium beam clocks. Requirements for the design are proposed based on the frequency shift associated with the cavity. The basic structure of the cavity is given by theoretical analysis and detailed dimensions are determined by means of electromagnetic field simulation with the help of commercial software. The cavity is manufactured and fabricated successfully. The preliminary test result of the cavity is given, which is in good agreement with the simulation. The resonant frequency is 6.835 GHz, equal to the clock transition frequency of 87Rb, and the loaded quality factor is 500. These values are adjustable with posts outside the cavity. Estimations on the Ramsey line width and several frequency shifts are made.

Ramsey-fringe shape in an alkali-metal vapor cell with buffer gas

In this paper we describe a few phenomena that may affect the linewidth and/or the line shape of Ramsey fringes observed in an alkali-metal vapor cell with buffer gas. Experiments are performed on a vapor-cell--microwave-cavity arrangement in which ${}^{87}\mathrm{Rb}$ atoms successively experience laser pumping preparation (through a ${D}_{2}$ line), microwave interrogation with a couple of coherent pulses, and clock signal detection by means of laser absorption. We show that the linewidth of the Ramsey central fringe is influenced by the Rabi pattern produced by a single microwave pulse. Moreover, cycle memory and atomic density effects can modify the shape of the Ramsey central fringe from the common sinusoidal behavior. A simple three-level model well reproduces the experimental observations.

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.

Cryogenic fountain development at NIST and INRIM: preliminary characterization

This paper describes the new twin laser-cooled Cs fountain primary frequency standards NIST-F2 and ITCsF2, and presents some of their design features. Most significant is a cryogenic microwave interrogation region which dramatically reduces the blackbody radiation shift. We also present a preliminary accuracy evaluation of IT-CsF2.

Switching atomic fountain clock microwave interrogation signal and high-resolution phase measurements

This paper focuses on the development of tools aiming to solve several problems related to the microwave interrogation signal in atomic fountains. We first consider the problem related to cycle synchronous phase transients caused by the sequential operation of the atomic fountain. To search for such systematic phase variations deeply buried in the microwave synthesizer phase noise, we have developed a novel triggered-phase transient analyzer capable of processing the microwave signal to extract the phase in a synchronous manner even in the presence of frequency modulation. With this device we check in vivo the LNE-SYRTE fountain's interrogation signals with a resolution approaching 1 microradian. In addition, using this device, we investigate an innovative approach to solve a second problem, namely, the shift caused by microwave leakage in the fountain. Our approach consists of switching off the fountain microwave interrogation signal when atoms are outside the microwave cavity. To do that, we have developed a switch that is almost free of phase transients and is thus able to eliminate the frequency shift caused by microwave leakage without inducing significant phase transients on the interrogation signal.

Temperature of laser-cooled /sup 171/Yb/sup +/ ions and application to a microwave frequency standard

Microwave frequency standards based on buffer gas-cooled /sup 171/Yb/sup +/ ions have demonstrated high stability but are limited in accuracy by the second-order Doppler shift caused by thermal motion. We have previously obtained near shot noise-limited Ramsey fringes with a laser-cooled ion cloud. Here, we present measurements confirming that the ion temperature remains <1 K throughout the microwave interrogation period for a Ramsey pulse separation of up to 10 s and longer. The potential stability of the ions as a frequency standard is better than /spl sigma//sub y/ (/spl tau/)=5/spl times/10/sup -14/ /spl tau//sup -1/2/, and estimates of the systematic offsets to the clock frequency and their uncertainties indicate that a total uncertainty of four parts in 10/sup 15/ or better is achievable.

Microwave interrogation of cesium atoms cooled in isotropic light

Describes here a configuration of an atomic clock using a source of cesium atoms cooled with isotropic light. This three-dimensional (3-D)-cooling scheme yields a number of atoms of nearly 10/sup 9/ with a temperature below 100 /spl mu/K. These values are suitable for metrological applications. We present the preliminary results obtained and give a first estimation of the frequency performances for this kind of atomic clock.

Frequency shifts in cesium beam clocks induced by microwave leakages

The anomalous sensitivity of an optically pumped cesium beam clock to microwave power and the unexpected frequency shifts observed are demonstrated to result from microwave leakages outside the arms of the Ramsey cavity. A new design of the cavity associated with a careful realisation provides very weak microwave leakages and negligible related frequency offsets. We have established a theoretical model that allows us to calculate the frequency shifts due to microwave field components propagating along the beam axis in regions which are free-field in an ideal Ramsey cavity. This results in first order Doppler effect shifts. The order of magnitude of the frequency shift can be predicted and agrees with the measured one when the amplitude of the leakage magnetic field is about a 1000 times smaller than the amplitude of the microwave interrogation field in the cavity.