Critical relaxation with overdamped quasiparticles in open quantum systems

We study the late-time relaxation following a quench in a driven-dissipative quantum many-body system. We consider the open Dicke model, describing the infinite-range interactions between $N$ atoms and a single, lossy electromagnetic mode. We show that the dynamical phase transition at a critical atom-light coupling is characterized by the interplay between reservoir-driven and intrinsic relaxation processes. Above the critical coupling, small fluctuations in the occupation of the dominant quasiparticle-mode start to grow in time while the quasiparticle lifetime remains finite due to losses. Near the critical interaction strength we observe a crossover between exponential and power-law $1/\tau$ relaxation, the latter driven by collisions between quasiparticles. For a quench exactly to the critical coupling, the power-law relaxation extends to infinite times, but the finite lifetime of quasiparticles prevents ageing to appear. We predict our results to be accessible to quench experiments with ultracold bosons in optical resonators.Comment: 3+4 Figure

Non-equilibrium diagrammatic approach to strongly interacting photons

We develop a non-equilibrium field-theoretical approach based on a systematic diagrammatic expansion for strongly interacting photons in optically dense atomic media. We consider the case where the characteristic photon-propagation range $L_P$ is much larger than the interatomic spacing $a$ and where the density of atomic excitations is low enough to neglect saturation effects. In the highly polarizable medium the photons experience nonlinearities through the interactions they inherit from the atoms. If the atom-atom interaction range $L_E$ is also large compared to $a$, we show that the subclass of diagrams describing scattering processes with momentum transfer between photons is suppressed by a factor $a/L_E$. We are then able to perform a self-consistent resummation of a specific (Hartree-like) diagram subclass and obtain quantitative results in the highly non-perturbative regime of large single-atom cooperativity. Here we find important, conceptually new collective phenomena emerging due to the dissipative nature of the interactions, which even give rise to novel phase transitions. The robustness of these is investigated by inclusion of the leading corrections in $a/L_E$. We consider specific applications to photons propagating under EIT conditions along waveguides near atomic arrays as well as within Rydberg ensembles.Comment: 72 pages, 36 figure

Concurrence of dynamical phase transitions at finite temperature in the fully connected transverse-field Ising model

We construct the finite-temperature dynamical phase diagram of the fully connected transverse-field Ising model from the vantage point of two disparate concepts of dynamical criticality. An analytical derivation of the classical dynamics and exact diagonalization simulations are used to study the dynamics after a quantum quench in the system prepared in a thermal equilibrium state. The different dynamical phases characterized by the type of non-analyticities that emerge in an appropriately defined Loschmidt-echo return rate directly correspond to the dynamical phases determined by the spontaneous breaking of $\mathbb{Z}_2$ symmetry in the long-time steady state. The dynamical phase diagram is qualitatively different depending on whether the initial thermal state is ferromagnetic or paramagnetic. Whereas the former leads to a dynamical phase diagram that can be directly related to its equilibrium counterpart, the latter gives rise to a divergent dynamical critical temperature at vanishing final transverse-field strength.Comment: journal article, 15 pages, 12 figures. Final versio

Spontaneous particle-hole symmetry breaking of correlated fermions on the Lieb lattice

We study spinless fermions with nearest-neighbor repulsive interactions ($t$-$V$ model) on the two-dimensional three-band Lieb lattice. At half-filling, the free electronic band structure consists of a flat band at zero energy and a single cone with linear dispersion. The flat band is expected to be unstable upon inclusion of electronic correlations, and a natural channel is charge order. However, due to the three-orbital unit cell, commensurate charge order implies an imbalance of electron and hole densities and therefore doping away from half-filling. Our numerical results show that below a finite-temperature Ising transition a charge density wave with one electron and two holes per unit cell and its partner under particle-hole transformation are spontaneously generated. Our calculations are based on recent advances in auxiliary-field and continuous-time quantum Monte Carlo simulations that allow sign-free simulations of spinless fermions at half-filling. It is argued that particle-hole symmetry breaking provides a route to access levels of finite doping, without introducing a sign problem.Comment: 9 pages, 6 figures, added data for strong Coulomb repulsion and classical Ising-limi

(Natural) Science and Technique in Medicine: Teaching Competences along with Research Activities

[EN] While factual knowledge is more and more present in digital format anywhereand anytime, (higher) education needs to extend its scope to supporting thedevelopment of personal skills and competences.The teaching and learning project “(Natural) Science and Technique inMedicine – SciTecMed” is closely related to recent research in theintersectional field of natural science, technique and medicine. Local expertsfrom the natural science and medical faculties engage in various teachingformats that are open for students of various majors. Together, studentsexperience the idea of interdisciplinary collaboration, discuss from theirindividual perspectives and learn to learn from each other and to instruct eachother. By the given context of the research activities, students earn insightsinto the scientific process and the usage of appropriate (digital) tools, enhancetheir corresponding skills and have the chance to take part into the scientificactivities. We describe the concept of the project, potential obstacles, student’sinterests as well as the syndetic benefits for both sides – education ANDresearch.Lang, J.; Repp, H. (2020). (Natural) Science and Technique in Medicine: Teaching Competences along with Research Activities. En 6th International Conference on Higher Education Advances (HEAd'20). Editorial Universitat Politècnica de València. (30-05-2020):1297-1304. https://doi.org/10.4995/HEAd20.2020.11256OCS1297130430-05-202

Fast logarithmic Fourier-Laplace transform of nonintegrable functions

We present an efficient and very flexible numerical fast Fourier-Laplace transform, that extends the logarithmic Fourier transform (LFT) introduced by Haines and Jones [Geophys. J. Int. 92(1):171 (1988)] for functions varying over many scales to nonintegrable functions. In particular, these include cases of the asymptotic form $f(\nu\to0)\sim\nu^a$ and $f(|\nu|\to\infty)\sim\nu^b$ with arbitrary real $a>b$. Furthermore, we prove that the numerical transform converges exponentially fast in the number of data points, provided that the function is analytic in a cone $|\Im{\nu}|<\theta|\Re{\nu}|$ with a finite opening angle $\theta$ around the real axis and satisfies $|f(\nu)f(1/\nu)|<\nu^c$ as $\nu\to 0$ with a positive constant $c$, which is the case for the class of functions with power-law tails. Based on these properties we derive ideal transformation parameters and discuss how the logarithmic Fourier transform can be applied to convolutions. The ability of the logarithmic Fourier transform to perform these operations on multiscale (non-integrable) functions with power-law tails with exponentially small errors makes it the method of choice for many physical applications, which we demonstrate on typical examples. These include benchmarks against known analytical results inaccessible to other numerical methods, as well as physical models near criticality.Comment: 14 pages, 8 figure

Profitiert der Feldhase vom ökologischen Landbau?

European brown hare numbers have dramatically declined in arable land throughout Europe. Loss of food abundance and cover due to mechanisation and intensification of agriculture are suggested to be the main reasons for this decline. Organic farming should sustain higher hare densities because of better habitat quality and higher food abundance. In our study hare densities estimated during spotlight counts increased from eight hares per km² (1998) up to 55 hares (2008) per km² after conversion of the study site from conventional to organic farming. Reasons for this increase in hare density may be higher abundance of year-round forage and cover as shown by preliminary results of radio-tracking data. Organic farming sustains good habitat quality for European hares and enhances their densities. Conservation strategies should therefore promote organic farming as a management tool

Randomisation of Pulse Phases for Unambiguous and Robust Quantum Sensing

We develop theoretically and demonstrate experimentally a universal dynamical decoupling method for robust quantum sensing with unambiguous signal identification. Our method uses randomisation of control pulses to suppress simultaneously two types of errors in the measured spectra that would otherwise lead to false signal identification. These are spurious responses due to finite-width $\pi$ pulses, as well as signal distortion caused by $\pi$ pulse imperfections. For the cases of nanoscale nuclear spin sensing and AC magnetometry, we benchmark the performance of the protocol with a single nitrogen vacancy centre in diamond against widely used non-randomised pulse sequences. Our method is general and can be combined with existing multipulse quantum sensing sequences to enhance their performance