arXiv subjects

Fritz Riehle

Publications and source records attributed to Fritz Riehle.

18 recordsLinked to original sources

arXiv2026

Lunar Silicon Cavity

The Moon's permanently shadowed regions (PSRs) are among the coldest places in the Solar System and are expected to become key landing sites for upcoming international space agency missions. Their proximity to peaks of perpetual solar power and potential resource richness makes them prime candidates for lunar exploration and future Moon bases. Here we propose to deploy a passive, ultrastable optical resonator in these regions that will enable laser systems with unprecedented phase-coherence. The unique physical environment of lunar PSRs greatly benefits the construction of a cryogenic monolithic silicon cavity that exhibits low $10^{-18}$ thermal noise-limited stability and coherence time exceeding 1 minute, more than a decade better than the current best terrestrial system. Such a stable laser will form an enabling infrastructure for quantum technology in space to serve many applications, including establishing a lunar time standard, building long-baseline optical interferometry, distribution of stable optical signals across networks of satellites, testing general relativity and gravitational physics, and forming the backbone for space-based quantum networks.

quant-ph↗

arXiv2026

Laser stabilized to a room temperature cavity with AlGaAs coatings reaching $4.2 \times 10^{-17}$ fractional frequency instability

We present a laser system referenced to a room-temperature ultrastable cavity employing crystalline AlGaAs coatings. We demonstrate a fractional frequency instability of $4.2 \times 10^{-17}$, which is one of the lowest for room temperature systems and surpasses the limit imposed by Brownian noise if dielectric coatings were employed. For the first time in a room temperature system we identified the spontaneous fluctuations of the coating birefringence as a leading contribution to frequency instability. At optimized conditions we achieve an ultrastable cavity with an eigenfrequency that is highly immune to power fluctuations. As acceleration noise is the main noise contribution, we demonstrated that a feed-forward method can reduce the influence of accelerations on the cavity-stabilized laser frequency by a factor of four.

physics.optics↗

arXiv2026

Photo-birefringent effects in crystalline AlGaAs mirror coatings

High-reflective crystalline $GaAs/Al_{0.92}Ga_{0.08}As$ coatings show reduced Brownian noise compared to conventional dielectric coatings. However, several ultra stable laser systems observed additional noise sources that hinder the realization of the expected improvements in frequency stability. These additional noise sources are related to the birefringence of the coatings and its modification by intracavity light. The origin of the birefringence is not yet well understood and its modification via illumination remains unexplained. Here we present an extensive study on the steady-state and transient modification of the birefringence by intracavity light and by uniform illumination at various wavelengths using an optical cavity at room temperature. We find a unified description that suggests a primary two-photon process for photon energies below the bandgap of GaAs, or a single-photon process at higher energies. Adding external illumination allows to reduce noise induced by laser power fluctuations by balancing the photo-thermal-optic response of the mirrors and the photo-birefringent effect at a more favorable low intracavity power.

physics.optics↗

arXiv2025

Frequency stability of $2.5\times10^{-17}$ in a Si cavity with AlGaAs crystalline mirrors

Developments in ultrastable lasers have fueled remarkable advances in optical frequency metrology and quantum science. A key ingredient in further improving laser frequency stability is the use of low-noise mirror materials such as AlGaAs crystalline coatings. However, excess noise observed with these coatings limits the performance of cryogenic silicon cavities with AlGaAs mirrors to similar levels achieved with conventional dielectric coatings. With a new pair of crystalline coated mirrors in a 6-cm-long cryogenic silicon cavity operated at 17 K, we demonstrate a clear advantage of crystalline coatings over dielectric coatings. The achieved fractional frequency stability of $2.5 \times 10^{-17}$ at 10 s is four times better than expected for dielectric mirrors and corresponds to more than tenfold reduction in the coating mechanical loss factor. We also combine two silicon cavities to demonstrate optical frequency averaging for enhanced stability. In addition, we present a long-term frequency drift record of four cryogenic silicon cavities measured over several years. These results open up realistic prospects for cavity-stabilized lasers with $10^{-18}$ fractional stability, as well as an all-optical timescale with continuously operating optical local oscillators.

physics.optics↗

arXiv2025

Einstein's basement - A kinematic sector complementing special relativity

We revisit the concept of particles as it is used in special relativity. The presented model treats the energy-momentum relation of relativistic particles as the upper branch of a generalized energy-momentum relation of quasi particles. These particles emerge from a forbidden crossing between the constant energy of a massive particle in rest and the linear energy momentum relation of a massless particle. The lower branch, a regime dubbed as Einstein's basement, gives rise to particles with different kinematics that is analyzed in the low-velocity limit. Allowing for gravitational interaction between those particles we find both attraction and repulsion, depending on their velocity with respect to an absolute space. This absolute frame is only relevant for Einstein's basement and does not affect the relativistic dynamics of regular matter as long as no coupling between the branches is considered, while weak coupling induces local Lorentz violations. We finally discuss whether our approach can be used to model phenomena of dark matter and dark energy.

gr-qc↗

arXiv2023

Excess noise and photo-induced effects in highly reflective crystalline mirror coatings

Thermodynamically induced length fluctuations of high-reflectivity mirror coatings put a fundamental limit on sensitivity and stability of precision optical interferometers like gravitational wave detectors and ultra-stable lasers. The main contribution - Brownian thermal noise - is related to the mechanical loss of the coating material. Owing to their low mechanical losses, Al\textsubscript{0.92}Ga\textsubscript{0.08}As/GaAs crystalline mirror coatings are expected to reduce this limit. At room temperature they have demonstrated lower Brownian thermal noise than with conventional amorphous coatings. %However, no detailed study on the noise constituents from these coatings in optical interferometers has been conducted. We present a detailed study on the spatial and temporal noise properties of such coatings by using them in two independent cryogenic silicon optical Fabry-Perot resonators operated at 4 K, 16 K and 124 K. We confirm the expected low Brownian thermal noise, but also discover two new noise sources that exceed the Brownian noise: birefringent noise that can be canceled via polarization averaging and global excess noise (10 dB above Brownian noise). These new noise contributions are a barrier to improving ultra-stable lasers and the related performance of atomic clocks, and potentially limit the sensitivity of third-generation gravitational wave detectors. Hence, they must be considered carefully in precision interferometry experiments using similar coatings based on semiconductor materials.

physics.optics↗

arXiv2022

Frequency stability of cryogenic silicon cavities with semiconductor crystalline coatings

State-of-the-art optical oscillators employing cryogenic reference cavities are limited in performance by the Brownian thermal noise associated with the mechanical dissipation of the mirror coatings. Recently, crystalline Al$_{1-x}$Ga$_{x}$As/GaAs coatings have emerged as a promising candidate for improved coating thermal noise. We present measurements of the frequency noise of two fully crystalline cryogenic reference cavities with Al$_{0.92}$Ga$_{0.08}$As/GaAs optical coatings. We report on previously unmeasured birefringent noise associated with anti-correlated frequency fluctuations between the polarization modes of the crystalline coatings, and identify variables that affect its magnitude. Comparing the birefringent noise between the two cryogenic reference cavities reveals a phenomenological set of scalings with intracavity power and mode area. We implement an interrogation scheme that cancels this noise by simultaneous probing of both polarization modes. The residual noise remaining after this cancellation is larger than both cavities thermal noise limits, but still lower than the instabilities previously measured on equivalent resonators with dielectric coatings. Though the source of these novel noise mechanisms is unclear, we demonstrate that crystalline coatings can provide stability and sensitivity competitive with resonators employing dielectric coatings.

physics.optics↗

arXiv2020

Thermal noise and mechanical loss of SiO$_2$/Ta$_2$O$_5$ optical coatings at cryogenic temperatures

Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise from room temperature to 4 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the corresponding mechanical loss for SiO$_2$/Ta$_2$O$_5$ coatings, which are compared with previously reported values.

physics.ins-det↗

arXiv2019

Demonstration of a time scale based on a stable optical carrier

We demonstrate a time scale based on a phase stable optical carrier that accumulates an estimated time error of $48\pm94$ ps over 34 days of operation. This all-optical time scale is formed with a cryogenic silicon cavity exhibiting improved long-term stability and an accurate $^{87}$Sr lattice clock. We show that this new time scale architecture outperforms existing microwave time scales, even when they are steered to optical frequency standards. Our analysis indicates that this time scale is capable of reaching a stability below $1\times10^{-17}$ after a few months of averaging, making timekeeping at the $10^{-18}$ level a realistic prospect.

physics.atom-ph↗

arXiv2018

Crystalline optical cavity at 4 K with thermal noise limited instability and ultralow drift

Crystalline optical cavities are the foundation of today's state-of-the-art ultrastable lasers. Building on our previous silicon cavity effort, we now achieve the fundamental thermal noise-limited stability for a 6 cm long silicon cavity cooled to 4 Kelvin, reaching $6.5\times10^{-17}$ from 0.8 to 80 seconds. We also report for the first time a clear linear dependence of the cavity frequency drift on the incident optical power. The lowest fractional frequency drift of $-3\times10^{-19}$/s is attained at a transmitted power of 40 nW, with an extrapolated drift approaching zero in the absence of optical power. These demonstrations provide a promising direction to reach a new performance domain for stable lasers, with stability better than $1\times10^{-17}$ and fractional linear drift below $1\times10^{-19}$/s.

physics.ins-det↗

arXiv2017

Ground-state properties of Ca$_2$ from narrow line two-color photoassociation

By two-color photoassociation of $^{40}$Ca four weakly bound vibrational levels in the Ca$_2$ \Xpot ground state potential were measured, using highly spin-forbidden transitions to intermediate states of the coupled system $^3Π_{u}$ and $^3Σ^+ _{u}$ near the ${^3P_1}$+${^1S_0}$ asymptote. From the observed binding energies, including the least bound state, the long range dispersion coefficients $\mathrm{C}_6, \mathrm{C}_8,\mathrm{C}_{10}$ and a precise value for the s-wave scattering length of 308.5(50)~$a_0$ were derived. From mass scaling we also calculated the corresponding scattering length for other natural isotopes. From the Autler-Townes splitting of the spectra, the molecular Rabi frequency has been determined as function of the laser intensity for one bound-bound transition. The observed value for the Rabi-frequency is in good agreement with calculated transition moments based on the derived potentials, assuming a dipole moment being independent of internuclear separation for the atomic pair model.

physics.atom-ph↗

arXiv2015

Nonlinear Zeeman effect in photoassociation spectra of $^{40}$Ca near the $^3$P$_1$+$^1$S$_0$ asymptote

We present calculations of the Zeeman effect of narrow photoassociation lines of $^{40}$Ca near the $^3$P$_1$ + $^1$S$_0$ asymptote. Using a coupled-channel model we find a nonlinear Zeeman effect that even at low fields of a few mT amounts to several kHz. With this model we analyze previous measurements and give corrected long range dispersion coefficients of the $^3Π_{u}$ and $^3Σ^+ _{u}$ states.

physics.atom-ph↗

arXiv2015

Towards a Re-definition of the Second Based on Optical Atomic Clocks

The rapid increase in accuracy and stability of optical atomic clocks compared to the caesium atomic clock as primary standard of time and frequency asks for a future re-definition of the second in the International System of Units (SI). The status of the optical clocks based on either single ions in radio-frequency traps or on neutral atoms stored in an optical lattice is described with special emphasis of the current work at the Physikalisch-Technische Bundesanstalt (PTB). Besides the development and operation of different optical clocks with estimated fractional uncertainties in the 10^-18 range, the supporting work on ultra-stable lasers as core elements and the means to compare remote optical clocks via transportable standards, optical fibers, or transportable clocks is reported. Finally, the conditions, methods and next steps are discussed that are the prerequisites for a future re-definition of the second.

physics.atom-ph↗

arXiv2014

Ultra-stable laser with average fractional frequency drift rate below $5\times10^{-19}/\mathrm{s}$

Cryogenic single-crystal optical cavities have the potential to provide highest dimensional stability. We have investigated the long-term performance of an ultra-stable laser system which is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a primary caesium fountain standard and to the $^{87}\mathrm{Sr}$ optical lattice clock at PTB. With fractional frequency instabilities of $σ_y(τ)\leq2\times10^{-16}$ for averaging times of $τ=60\mathrm{~s}$ to $1000\mathrm{~s}$ and $σ_y(1 \mathrm{d})\leq 2\times10^{-15}$ the stability of this laser, without any aid from an atomic reference, surpasses the best microwave standards for short averaging times and is competitive with the best hydrogen masers for longer times of one day. The comparison of modeled thermal response of the cavity with measured data indicates a fractional frequency drift below $5\times 10^{-19}/\mathrm{s}$, which we do not expect to be a fundamental limit.

physics.optics↗

arXiv2013

High accuracy photoassociation of $^{40}$Ca near the ${^3P_1}+{^1S_0}$ asymptote and its Zeeman effect

We report on the first measurement of narrow photoassociation lines of $^{40}$Ca near the ${^3P_1}+{^1S_0}$ asymptote related to the molecular states $^3Π_u$ and $^3Σ^+_u$. The highly accurate binding energies and Zeeman splittings are well described by a coupled channel theoretical model, confirming theoretical predictions of long-range coefficients. Our analysis shows that only the inclusion of both energies and Zeeman splittings provides an accurate description of the long-range interaction potentials.

physics.atom-ph↗

arXiv2010

Tackling the blackbody shift in a strontium optical lattice clock

A major obstacle for optical clocks is the frequency shift due to black body radiation. We discuss how one can tackle this problem in an optical lattice clock; in our case 87-Sr: firstly, by a measurement of the dc Stark shift of the clock transition and, secondly, by interrogating the atoms in a cryogenic environment. Both approaches rely on transporting ultracold atoms over several cm within a probe cycle. We evaluate this approach of mechanically moving the optical lattice and conclude that it is feasible to transport the atoms over 50 mm within 300 ms. With this transport a dc Stark shift measurement will allow to reduce the contribution of the blackbody radiation to the fractional uncertainty below 2 * 10^-17 at room temperature by improving the shift coefficient known only from atomic structure calculations up to now. We propose a cryogenic environment at 77 K that will reduce this contribution to few parts in 10^-18.

physics.atom-ph↗

arXiv2009

Bose-Einstein condensation of alkaline earth atoms: $^{40}${Ca}

We have achieved Bose-Einstein condensation of $^{40}$Ca, the first for an alkaline earth element. The influence of elastic and inelastic collisions associated with the large ground state s-wave scattering length of $^{40}$Ca was measured. From these findings, an optimized loading and cooling scheme was developed that allowed us to condense about $2 \cdot 10^4$ atoms after laser cooling in a two-stage magneto-optical trap and subsequent forced evaporation in a crossed dipole trap within less than 3 s. The condensation of an alkaline earth element opens novel opportunities for precision measurements on the narrow intercombination lines as well as investigations of molecular states at the $^1$S--$^3$P asymptotes.

cond-mat.quant-gas↗

arXiv2004

Wavelength dependent ac-Stark shift of the 1S0 - 3P1 transition at 657 nm in Ca

We have measured the ac-Stark shift of the 4s2 1S0 - 4s4p 3P1 line in 40Ca for perturbing laser wavelengths between 780 nm and 1064 nm with a time domain Ramsey-Borde atom interferometer. We found a zero crossing of the shift for the mS = 0 - mP = 0 transition and σpolarized perturbation at 800.8(22) nm. The data was analyzed by a model deriving the energy shift from known transition wavelengths and strengths. To fit our data, we adjusted the Einstein A coefficients of the 4s3d 3D - 4s4p 3P and 4s5s 3S - 4s4p 3P fine structure multiplets. With these we can predict vanishing ac-Stark shifts for the 1S0 m = 0 - 3P1 m = 1 transition and σ- light at 983(12) nm and at 735.5(20) nm for the transition to the 3P0 level.

physics.atom-ph↗