Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically-polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled DFB laser we achieve magnetic field sensitivity of 7 fT/$\sqrt{% \mathrm{Hz}}$ with a miniature $5\times5\times5$ mm Rb vapor cell.

Spin-Exchange-Relaxation-Free Magnetometry Using Elliptically-Polarized Light

Ultralight axion-like particles are well-motivated dark matter candidates introduced by theories beyond the standard model of particle physics. However, directly constraining their parameter space with laboratory experiments usually yields weaker limits than indirect approaches relying on astrophysical observations. Here we report the search for axion-like particles with a quantum sensor in the mass range of 8.3–744.0 feV. The sensor makes use of hyperpolarized long-lived nuclear spins as a pre-amplifier that effectively enhances a coherently oscillating axion-like dark matter field by a factor of more than 100. Using these spin-based amplifiers, we achieve an ultrahigh magnetic sensitivity of 18 fT Hz–(1/2), which exceeds the performance of state-of-the-art nuclear spin magnetometers. Our experiment constrains the parameter space describing the coupling of axion-like particles to nucleons over the aforementioned mass range, namely, at 67.5 feV reaching 2.9 × 10−9 GeV−1, improving on previous laboratory constraints by at least five orders of magnitude. Our measurements also constrain the quadratic interaction between axion-like particles and nucleons as well as interactions between dark photons and nucleons, exceeding bounds from astrophysical observations. A search for axion-like dark matter with a quantum sensor that enhances potential signals is reported. This work constrains the parameter space of different interactions between nucleons and axion-like particles and between nucleons and dark photons.

Search for axion-like dark matter with spin-based amplifiers

Following optical excitations’ life span from creation to decay into photons is crucial in understanding materials photophysics. Macroscopically, this is studied using optical techniques, such as photoluminescence excitation spectroscopy. However, excitation and emission pathways can vary at nanometer scales, preventing direct access, as no characterization technique has the relevant spatial, spectral, and time resolution. Here, using combined electron spectroscopies, we explore excitations’ creation and decay in two representative optical materials: plasmonic nanoparticles and luminescent two-dimensional layers. The analysis of the energy lost by an exciting electron that is coincident in time with a visible-ultraviolet photon unveils the decay pathways from excitation toward light emission. This is demonstrated for phase-locked (coherent) interactions (localized surface plasmons) and non–phase-locked ones (point defect excited states). The developed cathodoluminescence excitation spectroscopy images energy transfer pathways at the nanometer scale, widening the available toolset to explore nanoscale materials.

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Cathodoluminescence excitation spectroscopy: Nanoscale imaging of excitation pathways

Spin-exchange collisions among alkali-metal atoms affect the coherence time and gyromagnetic ratio of atoms by redistributing the ground-state population. We theoretically study the interactions between magnetic fields and atoms in the presence of spin-exchange collisions. Using the modified Bloch equations derived from the density-matrix equation in a low spin-polarization limit, we obtain explicit formulas describing the role of spin-exchange collisions in the interactions between atoms and magnetic fields, especially the nonresonant radio-frequency fields, which are widely used to manipulate atoms. The results derived from these equations agree well with density-matrix simulations and previous experimental results, and provide a simple and precise way to quantitatively analyze the effect of spin-exchange collisions on atoms. Our paper is helpful to gain physical insight into the spin-exchange collisions and optimize the performance of atom-based precision measurement instruments.

Atomic spin-exchange collisions in magnetic fields

With the emerging trend of big data and internet of things, silicon (Si) photonics technology has been developed and applied for high‐bandwidth data transmission. Contributed by the advantages of Si including high refractive index, low loss, significant thermal‐optical effect, and CMOS compatibility, various functional photonic devices have been demonstrated. One exception is the laser source, which is limited by the indirect bandgap of Si. Researchers have come up with different ways to make lasers on Si. This review presents the development progress of integrated laser sources on Si, with the focus on the wavelength regimes for communication. These lasers are categorized based on their gain media, including III–V semiconductor‐based laser, germanium/germanium tin‐based laser, Raman‐based laser, and rare‐earth‐doped laser. The laser performance and characteristics are summarized in tables for comparison. The key results are discussed. Future perspectives on the development of integrated lasers on Si have also been provided.

Integrated Lasers on Silicon at Communication Wavelength: A Progress Review

Erbium-doped fiber amplifiers revolutionized long-haul optical communications and laser technology. Erbium ions could provide a basis for efficient optical amplification in photonic integrated circuits but their use remains impractical as a result of insufficient output power. We demonstrate a photonic integrated circuit–based erbium amplifier reaching 145 milliwatts of output power and more than 30 decibels of small-signal gain—on par with commercial fiber amplifiers and surpassing state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We apply ion implantation to ultralow–loss silicon nitride (Si3N4) photonic integrated circuits, which are able to increase the soliton microcomb output power by 100 times, achieving power requirements for low-noise photonic microwave generation and wavelength-division multiplexing optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of various fiber-based devices such as high–pulse-energy femtosecond mode-locked lasers. Description On-chip optical amplification The success of long-haul optical communications and our information society is largely due to the invention of the erbium-doped fiber amplifier. Because the need for faster chips is expected to see a shift from electronics- to photonics-based technologies, erbium ions could form the basis for amplification in optical integrated circuits. Liu et al. used an ultra-low-loss silicon nitride photonic integrated circuit with a waveguide length up to 0.5 meters and erbium ion implantation to fabricate an erbium-doped waveguide amplifier on a compact photonic chip (see the Perspective by Kim). Operating in the continuous-wave regime and providing large optical gain in the telecommunication bands, the results are promising for device applications. —ISO An erbium-doped optical amplifier is fabricated on a silicon-nitride-based optical platform.

A photonic integrated circuit–based erbium-doped amplifier

Quantum information, communication, and sensing rely on the generation and control of quantum correlations in complementary degrees of freedom. Free electrons coupled to photonics promise novel hybrid quantum technologies, although single-particle correlations and entanglement have yet to be shown. In this work, we demonstrate the preparation of electron-photon pair states using the phase-matched interaction of free electrons with the evanescent vacuum field of a photonic chip–based optical microresonator. Spontaneous inelastic scattering produces intracavity photons coincident with energy-shifted electrons, which we employ for noise-suppressed optical mode imaging. This parametric pair-state preparation will underpin the future development of free-electron quantum optics, providing a route to quantum-enhanced imaging, electron-photon entanglement, and heralded single-electron and Fock-state photon sources. Description Generating electron-photon pairs The interaction of electron beams with cavities and resonant structures represents a universal scheme for generating electromagnetic radiation. Feist et al. fabricated structures with phase-matched interactions between free electrons and the vacuum cavity field of a photonic chip-based microresonator. As the electrons passed near the resonator, coupling between them and the vacuum field resulted in the spontaneous generation of photons within the cavity. Because the electron-photon pairs are correlated, they should be a useful source for the development of free-electron quantum optics providing enhanced imaging and sensing capabilities. —ISO The parametric generation of electron-photon pair states enables heralded single-particle sources.

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Cavity-mediated electron-photon pairs

We report an all-optical atomic vector magnetometer using dual Bell-Bloom optical pumping beams in a Rb vapor cell. This vector magnetometer consists of two orthogonal optical pumping beams, with amplitude modulations at $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$ Larmor frequencies, respectively. We simultaneously detect atomic signals excited by these two pumping beams using a single probe beam in the third direction, and extract the field orientation information using the phase delays between the modulated atomic signals and the driving beams. By adding a Herriott cavity inside the vapor cell, we improve the magnetometer sensitivity. We study the performance of this vector magnetometer in a magnetic field ranging from 100 mG to 500 mG, and demonstrate a field angle sensitivity better than 10 $\ensuremath{\mu}\mathrm{rad}/{\mathrm{Hz}}^{1/2}$ above 10 Hz.

Herriott-cavity-assisted all-optical atomic vector magnetometer

We study the interactions between oscillating nonresonant rf fields and atoms with strong spin-exchange collisions in the presence of a weak dc magnetic field. We find that the atomic Larmor precession frequency shows a new functional form to the rf field parameters when the spin-exchange collision rate is tuned. In the weak rf field amplitude regime, a strong modification of atomic Larmor frequency appears when the spin-exchange rate is comparable to the rf field frequency. This effect has been neglected before due to its narrow observation window. We compare the experimental results with density matrix calculations, and explain the data by an underdamped oscillator model. When the rf field amplitude is large, there is a minimum atomic gyromagnetic ratio point due to the rf photon dressing, and we find that strong spin-exchange interactions modify the position of such a point.

Interactions between nonresonant rf fields and atoms with strong spin-exchange collisions

We demonstrate photonic integrated erbium-implanted Si<inf>3</inf>N<inf>4</inf> amplifiers that enable record-high 26 dB on-chip net gain and saturation output power exceeding 25 mW, providing an active Si<inf>3</inf>N<inf>4</inf> nanophotonic platform for integrated lasers, LiDARs and microwave photonics.

Zheru Qiu

Papers

A photonic integrated circuit–based erbium-doped amplifier

Cavity-mediated electron-photon pairs

Herriott-cavity-assisted all-optical atomic vector magnetometer

Interactions between nonresonant rf fields and atoms with strong spin-exchange collisions

Photonic integrated erbium-doped silicon nitride amplifiers with intense net gain