arXiv subjects

Sven Herrmann

Publications and source records attributed to Sven Herrmann.

At least 55 records · Page 3Linked to original sources

arXiv2020

Quantum-Assisted Optical Interferometers: Instrument Requirements

It has been recently suggested that optical interferometers may not require a phase-stable optical link between the stations if instead sources of quantum-mechanically entangled pairs could be provided to them, enabling extra-long baselines and benefiting numerous topics in astrophysics and cosmology. We developed a new variation of this idea, proposing that photons from two different sources could be interfered at two decoupled stations, requiring only a slow classical connection between them. We show that this approach could allow high-precision measurements of the relative astrometry of the two sources, with a simple estimate giving angular resolution of $10 \ μ$as in a few hours' observation of two bright stars. We also give requirements on the instrument for these observations, in particular on its temporal and spectral resolution. Finally, we discuss possible technologies for the instrument implementation and first proof-of-principle experiments.

astro-ph.IM↗

arXiv2020

Twin-lattice atom interferometry

Inertial sensors based on cold atoms have great potential for navigation, geodesy, or fundamental physics. Similar to the Sagnac effect, their sensitivity increases with the space-time area enclosed by the interferometer. Here, we introduce twin-lattice atom interferometry exploiting Bose-Einstein condensates. Our method provides symmetric momentum transfer and large areas in palm-sized sensor heads with a performance similar to present meter-scale Sagnac devices.

quant-ph↗

arXiv2019

Evaporative cooling from an optical dipole trap in microgravity

In recent years, cold atoms could prove their scientific impact not only on ground but in microgravity environments such as the drop tower in Bremen, sounding rockets and parabolic flights. We investigate the preparation of cold atoms in an optical dipole trap, with an emphasis on evaporative cooling under microgravity. Up to $ 1\times10^{6} $ rubidium-87 atoms were optically trapped from a temporarily dark magneto optical trap during free fall in the droptower in Bremen. The efficiency of evaporation is determined to be equal with and without the effect of gravity. This is confirmed using numerical simulations that prove the dimension of evaporation to be three-dimensional in both cases due to the anharmonicity of optical potentials. These findings pave the way towards various experiments on ultra-cold atoms under microgravity and support other existing experiments based on atom chips but with plans for additional optical dipole traps such as the upcoming follow-up missions to current and past spaceborne experiments.

physics.atom-ph↗

arXiv2018

Test of the Gravitational Redshift with Galileo Satellites in an Eccentric Orbit

On August 22, 2014, the satellites GSAT-0201 and GSAT-0202 of the European GNSS Galileo were unintentionally launched into eccentric orbits. Unexpectedly, this has become a fortunate scientific opportunity since the onboard hydrogen masers allow for a sensitive test of the redshift predicted by the theory of general relativity. In the present Letter we describe an analysis of approximately three years of data from these satellites including three different clocks. For one of these we determine the test parameter quantifying a potential violation of the combined effects of the gravitational redshift and the relativistic Doppler shift. The uncertainty of our result is reduced by more than a factor 4 as compared to the values of Gravity Probe A obtained in 1976.

gr-qc↗

arXiv2018

The Athena WFI Science Products Module

The Science Products Module (SPM), a US contribution to the Athena Wide Field Imager, is a highly capable secondary CPU that performs special processing on the science data stream. The SPM will have access to both accepted X-ray events and those that were rejected by the on-board event recognition processing. It will include two software modules. The Transient Analysis Module will perform on-board processing of the science images to identify and characterize variability of the prime target and/or detection of serendipitous transient X-ray sources in the field of view. The Background Analysis Module will perform more sophisticated flagging of potential background events as well as improved background characterization, making use of data that are not telemetered to the ground, to provide improved background maps and spectra. We present the preliminary design of the SPM hardware as well as a brief overview of the software algorithms under development.

astro-ph.IM↗

arXiv2018

BOOST -- A Satellite Mission to Test Lorentz Invariance Using High-Performance Optical Frequency References

BOOST (BOOst Symmetry Test) is a proposed satellite mission to search for violations of Lorentz invariance by comparing two optical frequency references. One is based on a long-term stable optical resonator and the other on a hyperfine transition in molecular iodine. This mission will allow to determine several parameters of the standard model extension in the electron sector up to two orders of magnitude better than with the current best experiments. Here, we will give an overview of the mission, the science case and the payload.

gr-qc↗

arXiv2016

Miniaturized lab system for future cold atom experiments in microgravity

We present the technical realization of a compact system for performing experiments with cold $^{87}{\text{Rb}}$ and $^{39}{\text{K}}$ atoms in microgravity in the future. The whole system fits into a capsule to be used in the drop tower Bremen. One of the advantages of a microgravity environment is long time evolution of atomic clouds which yields higher sensitivities in atom interferometer measurements. We give a full description of the system containing an experimental chamber with ultra-high vacuum conditions, miniaturized laser systems, a high-power thulium-doped fiber laser, the electronics and the power management. In a two-stage magneto-optical trap atoms should be cooled to the low $μ$K regime. The thulium-doped fiber laser will create an optical dipole trap which will allow further cooling to sub-$μ$K temperatures. The presented system fulfills the demanding requirements on size and power management for cold atom experiments on a microgravity platform, especially with respect to the use of an optical dipole trap. A first test in microgravity, including the creation of a cold Rb ensemble, shows the functionality of the system.

physics.atom-ph↗

arXiv2015

Macroscopic quantum resonators (MAQRO): 2015 Update

Do the laws of quantum physics still hold for macroscopic objects - this is at the heart of Schrödinger's cat paradox - or do gravitation or yet unknown effects set a limit for massive particles? What is the fundamental relation between quantum physics and gravity? Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times and the quality of vacuum and microgravity. The proposed mission MAQRO may overcome these limitations and allow addressing those fundamental questions. MAQRO harnesses recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal of MAQRO is to probe the vastly unexplored "quantum-classical" transition for increasingly massive objects, testing the predictions of quantum theory for truly macroscopic objects in a size and mass regime unachievable in ground-based experiments. The hardware for the mission will largely be based on available space technology. Here, we present the MAQRO proposal submitted in response to the (M4) Cosmic Vision call of the European Space Agency for a medium-size mission opportunity with a possible launch in 2025.

quant-ph↗

arXiv2015

Nonlinear X-ray Compton Scattering

X-ray scattering is a weak linear probe of matter. It is primarily sensitive to the position of electrons and their momentum distribution. Elastic X-ray scattering forms the basis of atomic structural determination while inelastic Compton scattering is often used as a spectroscopic probe of both single-particle excitations and collective modes. X-ray free-electron lasers (XFELs) are unique tools for studying matter on its natural time and length scales due to their bright and coherent ultrashort pulses. However, in the focus of an XFEL the assumption of a weak linear probe breaks down, and nonlinear light-matter interactions can become ubiquitous. The field can be sufficiently high that even non-resonant multiphoton interactions at hard X-rays wavelengths become relevant. Here we report the observation of one of the most fundamental nonlinear X-ray-matter interactions, the simultaneous Compton scattering of two identical photons producing a single photon at nearly twice the photon energy. We measure scattered photons with an energy near 18 keV generated from solid beryllium irradiated by 8.8-9.75 keV XFEL pulses. The intensity in the X-ray focus reaches up to 4x20 W/cm2, which corresponds to a peak electric field two orders of magnitude higher than the atomic unit of field-strength and within four orders of magnitude of the quantum electrodynamic critical field. The observed signal scales quadratically in intensity and is emitted into a non-dipolar pattern, consistent with the simultaneous two-photon scattering from free electrons. However, the energy of the generated photons shows an anomalously large redshift only present at high intensities. This indicates that the instantaneous high-intensity scattering effectively interacts with a different electron momentum distribution than linear Compton scattering, with implications for the study of atomic-scale structure and dynamics of matter

physics.optics↗

arXiv2015

Optimal realisations of two-dimensional, totally-decomposable metrics

A realisation of a metric $d$ on a finite set $X$ is a weighted graph $(G,w)$ whose vertex set contains $X$ such that the shortest-path distance between elements of $X$ considered as vertices in $G$ is equal to $d$. Such a realisation $(G,w)$ is called optimal if the sum of its edge weights is minimal over all such realisations. Optimal realisations always exist, although it is NP-hard to compute them in general, and they have applications in areas such as phylogenetics, electrical networks and internet tomography. In [Adv. in Math. 53, 1984, 321-402] A.~Dress showed that the optimal realisations of a metric $d$ are closely related to a certain polytopal complex that can be canonically associated to $d$ called its tight-span. Moreover, he conjectured that the (weighted) graph consisting of the zero- and one-dimensional faces of the tight-span of $d$ must always contain an optimal realisation as a homeomorphic subgraph. In this paper, we prove that this conjecture does indeed hold for a certain class of metrics, namely the class of totally"=decomposable metrics whose tight-span has dimension two. As a corollary, it follows that the minimum Manhattan network problem is a special case of finding optimal realisations of two-dimensional totally-decomposable metrics.

math.CO↗

arXiv2014

Computing the blocks of a quasi-median graph

Quasi-median graphs are a tool commonly used by evolutionary biologists to visualise the evolution of molecular sequences. As with any graph, a quasi-median graph can contain cut vertices, that is, vertices whose removal disconnect the graph. These vertices induce a decomposition of the graph into blocks, that is, maximal subgraphs which do not contain any cut vertices. Here we show that the special structure of quasi-median graphs can be used to compute their blocks without having to compute the whole graph. In particular we present an algorithm that, for a collection of $n$ aligned sequences of length $m$, can compute the blocks of the associated quasi-median graph together with the information required to correctly connect these blocks together in run time $\mathcal O(n^2m^2)$, independent of the size of the sequence alphabet. Our primary motivation for presenting this algorithm is the fact that the quasi-median graph associated to a sequence alignment must contain all most parsimonious trees for the alignment, and therefore precomputing the blocks of the graph has the potential to help speed up any method for computing such trees.

math.CO↗

arXiv2014

Searching for Realizations of Finite Metric Spaces in Tight Spans

An important problem that commonly arises in areas such as internet traffic-flow analysis, phylogenetics and electrical circuit design, is to find a representation of any given metric $D$ on a finite set by an edge-weighted graph, such that the total edge length of the graph is minimum over all such graphs. Such a graph is called an optimal realization and finding such realizations is known to be NP-hard. Recently Varone presented a heuristic greedy algorithm for computing optimal realizations. Here we present an alternative heuristic that exploits the relationship between realizations of the metric $D$ and its so-called tight span $T_D$. The tight span $T_D$ is a canonical polytopal complex that can be associated to $D$, and our approach explores parts of $T_D$ for realizations in a way that is similar to the classical simplex algorithm. We also provide computational results illustrating the performance of our approach for different types of metrics, including $l_1$-distances and two-decomposable metrics for which it is provably possible to find optimal realizations in their tight spans.

math.MG↗

arXiv2014

Design of a dual species atom interferometer for space

Atom interferometers have a multitude of proposed applications in space including precise measurements of the Earth's gravitational field, in navigation & ranging, and in fundamental physics such as tests of the weak equivalence principle (WEP) and gravitational wave detection. While atom interferometers are realized routinely in ground-based laboratories, current efforts aim at the development of a space compatible design optimized with respect to dimensions, weight, power consumption, mechanical robustness and radiation hardness. In this paper, we present a design of a high-sensitivity differential dual species $^{85}$Rb/$^{87}$Rb atom interferometer for space, including physics package, laser system, electronics and software. The physics package comprises the atom source consisting of dispensers and a 2D magneto-optical trap (MOT), the science chamber with a 3D-MOT, a magnetic trap based on an atom chip and an optical dipole trap (ODT) used for Bose-Einstein condensate (BEC) creation and interferometry, the detection unit, the vacuum system for $10^{-11}$ mbar ultra-high vacuum generation, and the high-suppression factor magnetic shielding as well as the thermal control system. The laser system is based on a hybrid approach using fiber-based telecom components and high-power laser diode technology and includes all laser sources for 2D-MOT, 3D-MOT, ODT, interferometry and detection. Manipulation and switching of the laser beams is carried out on an optical bench using Zerodur bonding technology. The instrument consists of 9 units with an overall mass of 221 kg, an average power consumption of 608 W (819 W peak), and a volume of 470 liters which would well fit on a satellite to be launched with a Soyuz rocket, as system studies have shown.

physics.ins-det↗

arXiv2014

Atom Interferometry in Space: Thermal Management and Magnetic Shielding

Atom interferometry is an exciting tool to probe fundamental physics. It is considered especially apt to test the universality of free fall by using two different sorts of atoms. The increasing sensitivity required for this kind of experiment sets severe requirements on its environments, instrument control, and systematic effects. This can partially be mitigated by going to space as was proposed, for example, in the Spacetime Explorer and Quantum Equivalence Principle Space Test (STE-QUEST) mission. However, the requirements on the instrument are still very challenging. For example, the specifications of the STE-QUEST mission imply that the Feshbach coils of the atom interferometer are allowed to change their radius only by about 260 nm or 2.6E-4% due to thermal expansion although they consume an average power of 22 W. Also Earth's magnetic field has to be suppressed by a factor of 10E5. We show in this article that with the right design such thermal and magnetic requirements can indeed be met and that these are not an impediment for the exciting physics possible with atom interferometers in space.

physics.ins-det↗

arXiv2012

Dressians, Tropical Grassmannians, and Their Rays

The Dressian Dr(k,n) parametrizes all tropical linear spaces, and it carries a natural fan structure as a subfan of the secondaryfan of the hypersimplex Δ(k,n). We explore the combinatorics of the rays of Dr(k,n), that is, the most degenerate tropical planes, for arbitrary k and n. This is related to a new rigidity concept for configurations of n-k points in the tropical (k-1)-torus. Additional conditions are given for k=3. On the way, we compute the entire fan Dr(3,8).

math.CO↗

arXiv2012

Recognizing Treelike k-Dissimilarities

A k-dissimilarity D on a finite set X, |X| >= k, is a map from the set of size k subsets of X to the real numbers. Such maps naturally arise from edge-weighted trees T with leaf-set X: Given a subset Y of X of size k, D(Y) is defined to be the total length of the smallest subtree of T with leaf-set Y . In case k = 2, it is well-known that 2-dissimilarities arising in this way can be characterized by the so-called "4-point condition". However, in case k > 2 Pachter and Speyer recently posed the following question: Given an arbitrary k-dissimilarity, how do we test whether this map comes from a tree? In this paper, we provide an answer to this question, showing that for k >= 3 a k-dissimilarity on a set X arises from a tree if and only if its restriction to every 2k-element subset of X arises from some tree, and that 2k is the least possible subset size to ensure that this is the case. As a corollary, we show that there exists a polynomial-time algorithm to determine when a k-dissimilarity arises from a tree. We also give a 6-point condition for determining when a 3-dissimilarity arises from a tree, that is similar to the aforementioned 4-point condition.

math.MG↗

arXiv2012

The Split Decomposition of a k-Dissimilarity Map

A k-dissimilarity map on a finite set X is a function D : X \choose k \rightarrow R assigning a real value to each subset of X with cardinality k, k \geq 2. Such functions, also sometimes known as k-way dissimilarities, k-way distances, or k-semimetrics, are of interest in many areas of mathematics, computer science and classification theory, especially 2-dissimilarity maps (or distances) which are a generalisation of metrics. In this paper, we show how regular subdivisions of the kth hypersimplex can be used to obtain a canonical decomposition of a k-dissimilarity map into the sum of simpler k-dissimilarity maps arising from bipartitions or splits of X. In the special case k = 2, this is nothing other than the well-known split decomposition of a distance due to Bandelt and Dress [Adv. Math. 92 (1992), 47-105], a decomposition that is commonly to construct phylogenetic trees and networks. Furthermore, we characterise those sets of splits that may occur in the resulting decompositions of k-dissimilarity maps. As a corollary, we also give a new proof of a theorem of Pachter and Speyer [Appl. Math. Lett. 17 (2004), 615-621] for recovering k-dissimilarity maps from trees.

math.CO↗

arXiv2012

Trees, Tight-Spans and Point Configuration

Tight-spans of metrics were first introduced by Isbell in 1964 and rediscovered and studied by others, most notably by Dress, who gave them this name. Subsequently, it was found that tight-spans could be defined for more general maps, such as directed metrics and distances, and more recently for diversities. In this paper, we show that all of these tight-spans as well as some related constructions can be defined in terms of point configurations. This provides a useful way in which to study these objects in a unified and systematic way. We also show that by using point configurations we can recover results concerning one-dimensional tight-spans for all of the maps we consider, as well as extend these and other results to more general maps such as symmetric and unsymmetric maps.

math.CO↗