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arXiv2026

A 75-mL intrinsically stable atom-filtered laser enabling deployable quantum devices

Numerous quantum devices require lasers strictly locked to atomic transitions. Atom-filtered lasers (AFLs) are considered a leading candidate for quantum device laser sources due to their ability to self-align to atomic transitions. However, the sub-GHz sharp transmission spectra of conventional atomic filters impose constraints on both laser miniaturization and output stability. Herein, we demonstrate a micro Faraday anomalous dispersion optical filter (μFADOF) operating within the extreme hyperfine Paschen-Back regime, generating a 7.5 GHz flat-top transmission window. By integrating this filter, the miniaturization bottleneck of the AFLs is overcome, achieving a 30-fold volume reduction to a compact package volume of 75 mL. Simultaneously, investigations into the optical self-feedback characteristics of the μFADOF reveal a stable operating regime, where the optical self-feedback of the atomic filter acts as a stabilizing mechanism, enabling the laser to achieve a power instability of 9 x 10^-6 at 1 s and 3.2 x 10^-5 at 8400 s. The optical frequency standard based on this 75-mL AFL achieves a further improvement in long-term frequency stability to 3 x 10^-13 at 10000 s and maintains turn-key operation under environmental shocks, highlighting the critical role of this laser in the development of deployable quantum devices.

quant-ph↗

arXiv2026

Sub-Doppler cooling of trapped ions using optical tweezers

We propose a sub-Doppler cooling scheme for trapped-ion crystals in Paul traps. The combination of a hollow tweezer and microwave drive creates an effective two-level system which is cooled through attractive Sisyphus cooling. For ${}^{171}\rm{Yb}^+$ ions, we numerically identify optimal parameters for one- and two-ion crystals, and investigate the cooling rate in ion crystals with up to three ions. Our cooling scheme utilizes a single ion addressed with a tweezer to achieve sub-Doppler cooling of the $N$ axial motional modes perpendicular to the tweezer propagation direction, does not rely on the Lamb-Dicke regime and uses off-resonant light. We reach $1/e$ cooling times for the center of mass mode of $τ_{\rm com}<2$ ms, and a steady-state excitation number of $\bar{n}_{\rm{com}}\approx 0.7$. Our scheme is well-suited as an intermediate stage between Doppler cooling and ground-state sideband cooling, and can be readily realized in trapped-ion platforms.

physics.atom-ph↗

arXiv2026

Microscopic theory of the collective optical response of dilute atomic clouds at finite temperature

At low temperature, collective light-induced dipole-dipole interactions are known to strongly reshape the optical response of atomic clouds, revealing the breakdown of the independent-scattering picture as the density increases. While thermal motion is generally believed to progressively suppress these interactions, the microscopic mechanisms behind this suppression remain largely unexplored. Here, we develop a microscopic theory of the optical response of a dilute atomic gas at finite temperature, explicitly accounting for atomic ballistic motion during the recurrent-scattering events associated with the collective corrections. Using a diagrammatic approach for scalar light, we derive the collective contribution to the optical permittivity and characterize its behavior across the full temperature range. We find that thermal corrections scale as $\sim T$ at low temperature, while recurrent-scattering contributions are suppressed as $\sim T^{-3/2}$ at high temperature. Our predictions are confirmed by extensive coupled-dipole simulations that explicitly account for atomic motion. These simulations also reveal the inaccuracy of the modified frozen-dipole approximation commonly used to treat thermal effects. Finally, extending our theory to vector light, we provide a complete picture of how the resonance shift continuously evolves from the collective Lamb shift at low temperatures to the classical Lorentz-Lorenz shift at high temperatures.

cond-mat.quant-gas↗

arXiv2013

A dual-isotope rubidium comagnetometer to search for anomalous long-range spin-mass (spin-gravity) couplings of the proton

The experimental concept of a search for a long-range coupling between rubidium (Rb) nuclear spins and the mass of the Earth is described. The experiment is based on simultaneous measurement of the spin precession frequencies for overlapping ensembles of Rb-85 and Rb-87 atoms contained within an evacuated, antirelaxation-coated vapor cell. Rubidium atoms are spin-polarized in the presence of an applied magnetic field by synchronous optical pumping with circularly polarized laser light. Spin precession is probed by measuring optical rotation of far-off-resonant, linearly polarized laser light. Simultaneous measurement of Rb-85 and Rb-87 spin precession frequencies enables suppression of magnetic-field-related systematic effects. The nuclear structure of the Rb isotopes makes the experiment particularly sensitive to anomalous spin-dependent interactions of the proton. Experimental sensitivity and a variety of systematic effects are discussed, and initial data are presented.

physics.atom-ph↗

arXiv2018

Sensitivity coefficients to variation of fundamental constants

Atoms and molecules can serve as sensitive probes of a possible variation of the fine structure constant $α$ and electron-to-proton mass ratio $μ$. Two types of sensitivity coefficients are often used to quantify and compare the sensitivity of different species to the variation of fundamental constants. The dimensionless coefficients $K$ are related to the fractional sensitivity, while dimensional factors $q$ are related to the absolute sensitivity. Here we discuss several common errors and misconceptions regarding these coefficients that frequently appear in the literature.

physics.atom-ph↗

arXiv2018

Measurement of the ratio between g-factors of the ground states of $^{87}$Rb and $^{85}$Rb

The ratio between the Landé g-factors of the $^{87}$Rb $F=2$ and $^{85}$Rb $F=3$ ground state hyperfine levels is experimentally measured to be $g_F(87)/g_F(85) = 1.4988586(1)$, consistent with previous measurements. The g-factor ratio is determined by comparing the Larmor frequencies of overlapping ensembles of $^{87}$Rb and $^{85}$Rb atoms contained within an evacuated, antirelaxation-coated vapor cell. The atomic spins are polarized via synchronous optical pumping and the Larmor frequencies are measured by off-resonant probing using optical rotation of linearly polarized light. The accuracy of this measurement of $g_F(87)/g_F(85)$ exceeds that of previous measurements by a factor of $\approx 50$ and is sensitive to effects related to quantum electrodynamics.

physics.atom-ph↗

arXiv2018

The photon-ion merged-beams experiment PIPE at PETRAIII - The first five years

The Photon-Ion Spectrometer at PETRA III - in short, PIPE - is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion-optics in combination with the record-high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon-ion merged-beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi-electron processes upon inner-shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions and of endohedral fullerene ions.

physics.atom-ph↗

arXiv2018

Physical implementations of quantum absorption refrigerators

Absorption refrigerators are autonomous thermal machines that harness the spontaneous flow of heat from a hot bath into the environment in order to perform cooling. Here we discuss quantum realizations of absorption refrigerators in two different settings: namely, cavity and circuit quantum electrodynamics. We first provide a unified description of these machines in terms of the concept of virtual temperature. Next, we describe the two different physical setups in detail and compare their properties and performance. We conclude with an outlook on future work and open questions in this field of research.

quant-ph↗

arXiv2012

Open Quantum Systems. An Introduction

We revise fundamental concepts in the dynamics of open quantum systems in the light of modern developments in the field. Our aim is to present a unified approach to the quantum evolution of open systems that incorporates the concepts and methods traditionally employed by different communities. We present in some detail the mathematical structure and the general properties of the dynamical maps underlying open system dynamics. We also discuss the microscopic derivation of dynamical equations, including both Markovian and non-Markovian evolutions.

quant-ph↗

arXiv2018

Model-independent determinations of the electron EDM and the role of diamagnetic atoms

We perform model-independent analyses extracting limits for the electric dipole moment of the electron and the P,T-odd scalar-pseudoscalar (S-PS) nucleon-electron coupling from the most recent measurements with atoms and molecules. The analysis using paramagnetic systems, only, is improved substantially by the inclusion of the recent measurement on HfF+ ions, but complicated by the fact that the corresponding constraints are largely aligned, owing to a general relation between the coefficients for the two contributions. Since this same relation does not hold in diamagnetic systems, it is possible to find atoms that provide essentially orthogonal constraints to those from para\-magnetic ones. However, the coefficients are suppressed in closed-shell systems and enhancements of P,T-odd effects are only prevalent in the presence of hyperfine interactions. We formulate the hyperfine-induced time-reversal-symmetry breaking S-PS nucleon-electron interaction in general atoms in a mixed perturbative and variational approach, based on electronic Dirac-wavefunctions including the effects of electron correlations. The method is applied to the Hg atom, yielding the first direct calculation of the coefficient of the S-PS nucleon-electron coupling in a diamagnetic system. This results in additionally improved model-independent limits for both the electron EDM and the nucleon-electron coupling from the global fit. Finally we employ this fit to provide indirect limits for several paramagnetic systems under investigation.

hep-ph↗

arXiv2014

The semiclassical propagator in fermionic Fock space

We present a rigorous derivation of a semiclassical propagator for anticommuting (fermionic) degrees of freedom, starting from an exact representation in terms of Grassmann variables. As a key feature of our approach the anticommuting variables are integrated out exactly, and an exact path integral representation of the fermionic propagator in terms of commuting variables is constructed. Since our approach is not based on auxiliary (Hubbard-Stratonovich) fields, it surpasses the calculation of fermionic determinants yielding a standard form $\int {\cal D}[ψ,ψ^{*}] {\rm e}^{i R[ψ,ψ^{*}]}$ with real actions for the propagator. These two features allow us to provide a rigorous definition of the classical limit of interacting fermionic fields and therefore to achieve the long-standing goal of a theoretically sound construction of a semiclassical van Vleck-Gutzwiller propagator in fermionic Fock space. As an application, we use our propagator to investigate how the different universality classes (orthogonal, unitary and symplectic) affect generic many-body interference effects in the transition probabilities between Fock states of interacting fermionic systems.

physics.chem-ph↗

arXiv2011

Laser ablation loading of a radiofrequency ion trap

The production of ions via laser ablation for the loading of radiofrequency (RF) ion traps is investigated using a nitrogen laser with a maximum pulse energy of 0.17 mJ and a peak intensity of about 250 MW/cm^2. A time-of-flight mass spectrometer is used to measure the ion yield and the distribution of the charge states. Singly charged ions of elements that are presently considered for the use in optical clocks or quantum logic applications could be produced from metallic samples at a rate of the order of magnitude 10^5 ions per pulse. A linear Paul trap was loaded with Th+ ions produced by laser ablation. An overall ion production and trapping efficiency of 10^-7 to 10^-6 was attained. For ions injected individually, a dependence of the capture probability on the phase of the RF field has been predicted. In the experiment this was not observed, presumably because of collective effects within the ablation plume.

physics.atom-ph↗

arXiv2012

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high power Yb:fiber laser, a high finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75 μm is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e., the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4 {\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous oxide at atmospheric pressure are also presented, and a line shape model is developed to simulate the experimental data.

physics.optics↗

arXiv2013

Experimental study of 199Hg spin anti-relaxation coatings

We report on a comparison of spin relaxation rates in a $^{199}$Hg magnetometer using different wall coatings. A compact mercury magnetometer was built for this purpose. Glass cells coated with fluorinated materials show longer spin coherence times than if coated with their hydrogenated homologues. The longest spin relaxation time of the mercury vapor was measured with a fluorinated paraffin wall coating.

physics.ins-det↗

arXiv2013

Sub-Micron Positioning of Trapped Ions with Respect to the Absolute Center of a Standing Wave Cavity Field

We demonstrate that it is possible, with sub-micron precision, to locate the absolute center of a Fabry-Pérot resonator oriented along the rf-field-free axis of a linear Paul trap through the application of two simultaneously resonating optical fields. In particular, we apply a probe field, which is near-resonant with an electronic transition of trapped ions, simultaneously with an off-resonant strong field acting as a periodic AC Stark-shifting potential. Through the resulting spatially modulated fluorescence signal we can find the cavity center of an 11.7 mm-long symmetric Fabry- Pérot cavity with a precision of $\pm$135 nm, which is smaller than the periodicity of the individual standing wave fields. This can e.g. be used to position the minimum of the axial trap potential with respect to the center of the cavity at any location along the cavity mode.

physics.atom-ph↗

arXiv2013

Simple vibration insensitive cavity for laser stabilization at the 10^{-16} level

We present the design and realization of two reference cavities for ultra-stable lasers addressing narrow transitions in mixed-species (In+ / Yb+) Coulomb crystals. With a simple setup, we achieve a fractional frequency instability close to the thermal noise limit of a 12 cm long cavity, reaching 4.7 * 10^{-16} at 10 s with a linear drift of 53 mHz/s. We discuss the individual instability contributions and show that in a setup with a lower thermal noise floor and vibration sensitivity, an instability of 1 * 10^{-16} can be reached. To achieve this, we implement a vibration insensitive design for a 30 cm long cavity mounted horizontally and conduct first tests that show a sensitivity of 1.8 * 10^{-11} / ms^{-2} to vertical accelerations. This is about a factor of 20 less than the value observed for the short cavity. Mechanical tolerances and ways to further reduce the sensitivity are discussed.

physics.optics↗

arXiv2013

Entangling quantum gate in trapped ions via Rydberg blockade

We present a theoretical analysis of the implementation of an entangling quantum gate between two trapped Ca$^+$ ions which is based on the dipolar interaction among ionic Rydberg states. In trapped ions the Rydberg excitation dynamics is usually strongly affected by mechanical forces due to the strong couplings between electronic and vibrational degrees of freedom in inhomogeneous electric fields. We demonstrate that this harmful effect can be overcome by using dressed states that emerge from the microwave coupling of nearby Rydberg states. At the same time these dressed states exhibit long range dipolar interactions which we use to implement a controlled adiabatic phase gate. Our study highlights a route towards a trapped ion quantum processor in which quantum gates are realized independently of the vibrational modes.

quant-ph↗

arXiv2013

Trapping of ultra cold atoms in a 3He/4He dilution refrigerator

We describe the preparation of ultra cold atomic clouds in a dilution refrigerator. The closed cycle 3He/4He cryostat was custom made to provide optical access for laser cooling, optical manipulation and detection of atoms. We show that the cryostat meets the requirements for cold atom experiments, specifically in terms of operating a magneto-optical trap, magnetic traps and magnetic transport under ultra high vacuum conditions. The presented system is a step towards the creation of a quantum hybrid system combining ultra cold atoms and solid state quantum devices.

physics.atom-ph↗