Portable atomic clocks are essential in a wide variety of applications, most notably in the operation of global navigation satellite systems. Existing portable atomic clocks utilizing microwave-based interrogation schemes are now routinely eclipsed by the next generation of atomic frequency standards based on optical interrogation. While optical frequency standards demonstrate greatly improved frequency stability, they have only recently reached a level of technical maturity required to demonstrate this improved performance outside of well curated laboratory environments. Here, we demonstrate a fully autonomous and portable optical frequency standard based on an efficient dual-color excitation of the 5S_1/2→5D_5/2 two-photon transition in ^87Rb. The standard utilizes a combination of robust, highly developed commercial-off-the-shelf telecommunications technologies and a fully integrated portable optical frequency comb, providing the optical and microwave outputs vital for interfacing with existing electronic systems and infrastructure. The system demonstrates a fractional frequency stability of 1.9×10^-13 at 1s of integration time, reaching 3.5×10^-15 at 8000s of integration time without the need for drift removal. This portable demonstrator unit marks a significant achievement in the development of Rb optical atomic frequency standards, and for the deployment of optical atomic frequency standards outside of the laboratory.

We present a low Size, Weight and Power (SWaP), low-complexity, ytterbium magneto-optical trap (MOT). We demonstrate trapping of 1.4 × 10^6 ^171Yb atoms on the |^1S_0, F = 1/2⟩↔ |^1P_1, F' = 3/2⟩ transition directly from a hot thermal beam. We explore the effect of trap detuning and oven temperature on trap number, density, loading rate and sample temperature. The low SWaP and low-complexity design presents a realistic pathway towards portable ytterbium MOTs, allowing cold atom ytterbium systems to escape the confines of the laboratory and perform precision measurements in field environments.

Rubidium two-photon frequency standards are emerging as powerful contenders for compact, durable devices with exceptional stability. To date, the field has focused on single-color excitation; here, we demonstrate the key advantages of a two-color excitation of a two-photon optical frequency standard based on the 5S1/2 -> 5D5/2 transition of rubidium-87 using driving fields at 780 and 776 nm. Using the 5P3/2 intermediate state to resonantly enhance the transition, we show that frequency stabilities comparable to rubidium single-color two-photon frequency standards can be attained, notably with approximately tenfold lower optical power and tenfold lower rubidium vapor density. Optimization of the detuning from the 5P3/2 intermediate state and of the optical powers of the driving lasers has a dramatic effect on the frequency stability, achieving the best short-term stability of any two-photon rubidium frequency standard to date: 6 x 10-14 at r = 1 s. We demonstrate that this level of performance is compatible with a compact geometry by fully self-referencing the frequency standard using an integrated fiber frequency comb to simultaneously stabilize the 780-nm laser's detuning from the 5P3/2 intermediate state, producing a frequency-stable microwave output. A comprehensive noise characterization underpins our observations of this two-color frequency standard, explaining the measured stability and showing that this frequency standard is initially shot-noise limited before becoming limited by light shifts in the long term. This work represents a major advance toward a low size, weight, and power frequency standard based on two-color excitation.

We present a metrological technique to characterize the thermodynamic state (temperature, pressure, and density) of gaseous molecules in a single spectroscopic measurement. We demonstrate the accuracy and precision of our approach by using a low-pressure (number density approximately 1.76 x10(18) molecules/cm(3)) acetylene gas in a sealed glass cell over a temperature range of -70(degrees)C up to +60(degrees)C. Under these experimental conditions the sample is in the gaseous state, displaying a behavior very close to that of an ideal gas. This allows us to predict the thermodynamic state with high accuracy, and thus rigorously test the performance of our approach. Our spectroscopic measurement uses a tunable wide-band optical frequency comb to acquire the full ro-vibrationally broadened acetylene spectrum between 1512 and 1538 nm with approximately 80-kHz spectral resolution. Through nonlinear fitting of this entire spectrum we obtain high accuracy ( <1%) and high-precision (approximately 0.1%) estimates of the temperature, pressure, and density of the gas across the full examined temperature range. This combination of broadband probing and ultra-high-resolution spectrum ensures that we obtain reliable and precise measurements on the thermodynamic state.

We experimentally investigate near-infrared to telecommunications frequency conversion via a diamond four-wave mixing scheme in rubidium vapor contained within a hollow-core photonic-crystal fiber. The strong light-atom interaction in the fiber results in lower pump power requirements and higher conversion efficiency than can be achieved under equivalent conditions in a rubidium vapor cell. We also observe non-intuitive pump and signal frequency dependence of the four-wave mixing efficiency in the fiber due to the large nonlinearities present in the system. These results indicate the potential for hollow-core fibers to provide a scalable solution for quantum information network infrastructure, with additional modelling required for a full understanding of the extreme atom-light interaction effects present.

Atomic clocks are at the leading edge of accuracy and precision and are essential for synchronisation of distributed critical infrastructure, position, navigation and timing, and scientific applications. There has been significant improvements in the performance of atomic clocks with the shift from microwave to optical frequency transitions. However, this performance increase has come at the cost of size, complexity and fragility, which has confined optical clocks to laboratories. Here we report on a recent international collaboration where three emerging optical clocks, each based on different operating principles, were trialled at sea. These clocks demonstrated world-class performance and reliability by providing stable frequency outputs in optical, microwave and radio-frequency domains over three weeks of unsupervised naval operation.

We present an experimental realization of a noise-free and high-bandwidth quantum memory scheme using a rubidium vapor that is confined within the hollow core of a photonic crystal fiber. We achieve the same internal efficiencies as similar free-space experiments (30%) for 4.5-ns-long optical pulses but with a 100-fold reduction in the control-field power required. Modeling indicates that this efficiency could be improved to 88% with higher control powers and the implementation of techniques such as light-induced atomic desorption to increase the optical depth. The compactness, robustness, and low drive power of this approach lends itself to direct integration into large-scale fiber-based quantum processors

We demonstrate massively parallel spectroscopic measurements of 12C2H2 using an optical frequency comb. This allows for the rapid and simultaneous estimation of self-broadening and self-shifting of more than 50 optical transitions between 1512 and 1538 nm. The use of a temperature-controlled sealed gas cell allows us to measure both pressure- and temperature-mediated broadening and shifting. We present the results for the pressure-mediated self-broadening and self-shifting coefficients for 59 optical lines that make up the v1 + v3 combination band and a selection of hot bands. Our ability to measure the broadening of numerous transitions allows for the confirmation of prior work that shows that there is no measurable vibrational dependence across all acetylene bands, despite the strong dependence of the broadening coefficient on the rotational number. We also present an extensive measurement of the temperature dependence of the self-broadening for each of these 59 lines. This work shows the revolutionary power afforded by the frequency combs for rapid generation of large datasets related to thermodynamic variations of the key spectroscopic parameters of important gases.

Precision timekeeping is fundamental to modern technologies such as Global Navigation Satellite Systems (GNSS), communication networks, financial transactions, and power grid management. Over the past 50 years, microwave atomic clocks have been the standard for timing precision. The new generation of optical atomic clocks have demonstrated orders of magnitude better performance, and are now transitioning from research to practical applications. We provide a web resource that tracks the performance of these optical atomic clocks, measured in terms of their Allan deviation at various integration times, against their SWaP requirements via an interactive plot. The most current data and additional resources are available online, providing a continuously updated reference for those interested in precision timing.

It is a common requirement to lock the frequency of a laser to an optical cavity resonance in experimental physics research. While the Pound-Drever-Hall technique is among the most popular methods, it requires the use of auxiliary phase modulators, which increases the complexity and cost. Moreover, the laser-locking noise caused by unwanted residual amplitude modulation within the modulator often limits the achievable locking stability. In this work, we demonstrate an alternative approach that negates the need for any external modulators, and inherently mitigates the residual amplitude noise. We directly apply fast electro-optic modulation to a monolithic lithium niobate microresonator to efficiently generate laser sidebands at frequencies much higher than the optical resonance bandwidth. Such a modulation allows us to generate low-noise laser-locking signals after appropriate photodetection and demodulation. Our noise characterization and frequency instability measurement show that the direct modulation technique renders a locking instability that is one order of magnitude better than that of the conventional Pound-Drever-Hall technique. The residual locking instability is also numerically analyzed and experimentally verified.

We have developed a direct frequency comb spectroscopy instrument, which we have tested on Saccharomyces cerevisiae (baker’s yeast) by measuring its CO2 output and production rate as we varied the environmental conditions, including the amount and type of feed sugar, the temperature, and the amount of yeast. By feeding isotopically-enhanced sugar to the yeast, we demonstrate the capability of our device to differentiate between two isotopologues of CO2, with a concentration measurement precision of 260 ppm for 12C16O2 and 175 ppm for 13C16O2. We also demonstrate the ability of our spectrometer to measure the proportion of carbon in the feed sugar converted to CO2, and estimate the amount incorporated into the yeast biomass.

Sensing minute temperature fluctuations is of key importance to many cutting-edge industrial and scientific applications. While recent developments in microphotonic thermometers have achieved unmatched sensitivity, the wide adoption of these sensors is hindered by their limited dynamic range as well as the complex design. Here, we present a coupled-resonance-based scheme that utilizes a single-laser-driven optical microresonator to enable self-referenced thermometry with unprecedented simplicity. The thermometry complexity is further reduced by applying nonadiabatic modulation to the coupled resonances, which not only eliminates the need for external modulation but also yields a record-setting few-nanokelvin resolution. To highlight the applicability and the versatility of the developed thermometers, state-of-the-art microresonator applications including laser frequency stabilization and robust nonlinear frequency conversion are demonstrated over a wide range of operating temperatures. Being compatible with existing photonic integrated circuit building blocks, this easy-to-use, high-performance thermometry approach may be implemented extensively in compact optical and photonic systems.

A gain-switched-laser frequency comb is self-injection-locked to an electrooptic microresonator. The comb exhibits narrowed linewidth, suppressed frequency instability, and agile frequency control ability.

We have developed a direct frequency comb spectroscopy instrument, which we have tested on Saccharomyces cerevisiae (baker’s yeast) by measuring its CO 2 output and production rate as we varied the environmental conditions, including the amount and type of feed sugar, the temperature, and the amount of yeast. By feeding isotopically-enhanced sugar to the yeast, we demonstrate the capability of our device to differentiate between two isotopologues of CO 2 , with a concentration measurement precision of 260 ppm for 12 C 16 O 2 and 175 ppm for 13 C 16 O 2 . We also demonstrate the ability of our spectrometer to measure the proportion of carbon in the feed sugar converted to CO 2 , and estimate the amount incorporated into the yeast biomass.

We present a self-referenced thermometry scheme based on a lithium niobate microresonator. Leveraging coupling between orthogonally polarized modes, dual-resonant state is obtained with a single pumping laser, enabling temperature sensing with a record-setting few-nanokelvin resolution.

We report progress on development of a next generation timing reference based on an optical transition in rubidium coupled with a compact optical frequency comb to deliver a high stability RF reference signal. Engineering models that indicate the entire system including all optical components and control electronics could occupy a volume of 25 L, have a weight of 17 kg and consume 100 W of power. Such a device would be useful in GPS denied or contested environments.

Atomic clocks provide a reproducible basis for our understanding of time and frequency. Recent demonstrations of compact optical clocks, employing thermal atomic beams, have achieved short-term fractional frequency instabilities in the 10^-16, competitive with the best international frequency standards available. However, a serious challenge inherent in compact clocks is the necessarily smaller optical beams, which results in rapid variation in interrogating wavefronts. This can cause inhomogeneous excitation of the thermal beam leading to long term drifts in the output frequency. Here we develop a model for Ramsey-Bordé interferometery using optical fields with curved wavefronts and simulate the ^40Ca beam clock experiment described in [Olson et al., Phys. Rev. Lett. 123, 073202 (2019)]. Olson et al.'s results had shown surprising and unexplained behaviour in the response of the atoms in the interrogation. Our model predicts signals consistent with experimental data and can account for the significant sensitivity to laser geometry that was reported. We find the signal-to-noise ratio is maximised when the laser is uncollimated at the interrogation zones to minimise inhomogeneity, and also identify an optimal waist size determined by both laser inhomogeneity and the velocity distribution of the atomic beam. We investigate the shifts and stability of the clock frequency, showing that the Gouy phase is the primary source of frequency variations arising from laser geometry.

Magnetic field imaging is a valuable resource for signal source localization and characterization. This work reports an optically pumped magnetometer (OPM) based on the free-induction-decay (FID) protocol, that implements microfabricated cesium (Cs) vapor cell technology to visualize the magnetic field distributions resulting from various magnetic sources placed close to the cell. The slow diffusion of Cs atoms in the presence of a nitrogen (N 2 ) buffer gas enables spatially independent measurements to be made within the same vapor cell by translating a 175 μ m diameter probe beam over the sensing area. For example, the OPM was used to record temporal and spatial information to reconstruct magnetic field distributions in one and two dimensions. The optimal magnetometer sensitivity was estimated to be 0.43 pT/Hz within a Nyquist limited bandwidth of 500 Hz. Furthermore, the sensor’s dynamic range exceeds the Earth’s field of approximately 50 μ T, which provides a framework for magnetic field imaging in unshielded environments.

Gain-switched semiconductor laser technology provides a simple and low-cost method to generate optical frequency combs. However, the spectral coverage of these compact comb sources has been limited to the near-infrared range. Here, we combine a gain-switched laser comb with a continuous-wave translation laser within a periodically poled lithium niobate microresonator and demonstrate efficient and broadband sum-frequency conversion, spectrally translating the near-infrared comb to the visible domain. The broadband nature of the nonlinear conversion arises from a chirping of the domain inversion grating period along the microresonator circumference. We also validate the coherence of the visible-wavelength comb teeth which underlines the general applicability of this spectral translation approach.