Impurity and soliton dynamics in a Fermi gas with nearest-neighbor interactions

We study spinless fermions with repulsive nearest-neighbor interactions perturbed by an impurity particle or a local potential quench. Using the numerical time-evolving block decimation method and a simplified analytic model, we show that the pertubations create a soliton-antisoliton pair. If solitons are already present in the bath, the two excitations have a drastically different dynamics: The antisoliton does not annihilate with the solitons and is therefore confined close to its origin while the soliton excitation propagates. We discuss the consequences for experiments with ultracold gases.Comment: 12 pages, 16 figure

Conditions for waveguide decoupling in square-lattice photonic crystals

We study coupling and decoupling of parallel waveguides in two-dimensional square-lattice photonic crystals. We show that the waveguide coupling is prohibited at some wavelengths when there is an odd number of rows between the waveguides. In contrast, decoupling does not take place when there is even number of rows between the waveguides. Decoupling can be used to avoid cross talk between adjacent waveguides.Comment: 6 pages, 2 figure

Localization and diffusion in Ising-type quantum networks

We investigate the effect of phase randomness in Ising-type quantum networks. These networks model a large class of physical systems. They describe micro- and nanostructures or arrays of optical elements such as beam splitters (interferometers) or parameteric amplifiers. Most of these stuctures are promising candidates for quantum information processing networks. We demonstrate that such systems exhibit two very distinct types of behaviour. For certain network configurations (parameters), they show quantum localization similar to Anderson localization whereas classical stochastic behaviour is observed in other cases. We relate these findings to the standard theory of quantum localization.Comment: 12 page

Fermi condensates for dynamic imaging of electro-magnetic fields

Ultracold gases provide micrometer size atomic samples whose sensitivity to external fields may be exploited in sensor applications. Bose-Einstein condensates of atomic gases have been demonstrated to perform excellently as magnetic field sensors \cite{Wildermuth2005a} in atom chip \cite{Folman2002a,Fortagh2007a} experiments. As such, they offer a combination of resolution and sensitivity presently unattainable by other methods \cite{Wildermuth2006a}. Here we propose that condensates of Fermionic atoms can be used for non-invasive sensing of time-dependent and static magnetic and electric fields, by utilizing the tunable energy gap in the excitation spectrum as a frequency filter. Perturbations of the gas by the field create both collective excitations and quasiparticles. Excitation of quasiparticles requires the frequency of the perturbation to exceed the energy gap. Thus, by tuning the gap, the frequencies of the field may be selectively monitored from the amount of quasiparticles which is measurable for instance by RF-spectroscopy. We analyse the proposed method by calculating the density-density susceptibility, i.e. the dynamic structure factor, of the gas. We discuss the sensitivity and spatial resolution of the method which may, with advanced techniques for quasiparticle observation \cite{Schirotzek2008a}, be in the half a micron scale.Comment: 10 pages, 4 figure

Expansion dynamics in the one-dimensional Fermi-Hubbard model

Expansion dynamics of interacting fermions in a lattice are simulated within the one-dimensional (1D) Hubbard model, using the essentially exact time-evolving block decimation (TEBD) method. In particular, the expansion of an initial band-insulator state is considered. We analyze the simulation results based on the dynamics of a two-site two-particle system, the so-called Hubbard dimer. Our findings describe essential features of a recent experiment on the expansion of a Fermi gas in a two-dimensional lattice. We show that the Hubbard-dimer dynamics, combined with a two-fluid model for the paired and non-paired components of the gas, gives an efficient description of the full dynamics. This should be useful for describing dynamical phenomena of strongly interacting Fermions in a lattice in general.Comment: Fig. 9 changed, text + supplementary material revise

Noise correlations of the ultra-cold Fermi gas in an optical lattice

In this paper we study the density noise correlations of the two component Fermi gas in optical lattices. Three different type of phases, the BCS-state (Bardeen, Cooper, and Schieffer), the FFLO-state (Fulde, Ferrel, Larkin, and Ovchinnikov), and BP (breach pair) state, are considered. We show how these states differ in their noise correlations. The noise correlations are calculated not only at zero temperature, but also at non-zero temperatures paying particular attention to how much the finite temperature effects might complicate the detection of different phases. Since one-dimensional systems have been shown to be very promising candidates to observe FFLO states, we apply our results also to the computation of correlation signals in a one-dimensional lattice. We find that the density noise correlations reveal important information about the structure of the underlying order parameter as well as about the quasiparticle dispersions.Comment: 25 pages, 11 figures. Some figures are updated and text has been modifie

Finite temperature phase diagram of a polarized Fermi gas in an optical lattice

We present phase diagrams for a polarized Fermi gas in an optical lattice as a function of temperature, polarization, and lattice filling factor. We consider the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma or breached pair (BP), and BCS phases, and the normal state and phase separation. We show that the FFLO phase appears in a considerable portion of the phase diagram. The diagrams have two critical points of different nature. We show how various phases leave clear signatures to momentum distributions of the atoms which can be observed after time of flight expansion.Comment: Journal versio

Suomen ja Venäjän välisen rajan ylittävän liikkumisen ajallinen tarkastelu:mobiliteettien hallinnan muutokset vuosina 2020–2022

Tiivistelmä. Maailma on 2020-luvulla muuttunut entistä sulkeutuneempaan suuntaan. Ensin COVID-19, eli tutummin koronapandemia, levisi ympäri maailman, ja pian sen jälkeen Euroopassa alkoi tuhoisin sota sitten toisen maailmansodan. Rajat ovat lisääntyneet kovaa vauhtia, kuten rajojen ylittäminenkin ja rajavalvonta. Ihanteellinen valtion raja päästää lävitseen halutut mobiliteetit ja pysäyttää ei-halutut. Matkailijoihin ja työperäiseen maahanmuuttoon suhtaudutaan yleensä myönteisesti, sillä niillä on positiivinen vaikutus aluetalouteen. Myös kansainvälinen työkokemus ja vaihto-opiskelu katsotaan eduksi työmarkkinoilla. Vastaavasti taudit ja terroristit toimivat vahvana retoriikkana rajavalvonnan tehostamiselle. Suomen ja Venäjän välinen raja on luonteeltaan erilainen kuin esimerkiksi Suomen ja Ruotsin välinen raja, sillä edeltävä on samalla myös EU:n (ja nyt myös NATO:n) ulkoraja. Tässä tutkielmassa on tarkasteltu valtioneuvoston tiedotteisiin tehdyn sisällönanalyysin kautta Suomen ja Venäjän välisen rajat ylittävän mobiliteetin kontrollin kehitystä vuosina 2020–2022. Työn tarkastelujakson aikana Suomen ja Venäjän välinen raja on reagoinut kahteen lyhyessä ajassa tapahtuneeseen suureen tapahtumaketjuun: koronapandemiaan ja Ukrainassa käytävää Venäjän hyökkäyssotaan. Tutkielmassa pyritään vastaamaan kahteen kysymykseen: Millä tavoin mobiliteetin kontrollointi kehittyy tarkastelujakson aikana ja millaisiin mobiliteetteihin rajojen pysäyttävä voima kohdistuu. Tulosten perusteella Suomen rajavalvontaa on tehostettu lyhyessä ajassa ja rajat ylittäviin mobiliteetteihin kohdistuva kontrolli valtion toimesta on kasvanut. Suomi pyrkii EU:n mukana varautumaan kasvavaan hybridivaikuttamiseen kovalla rajapolitiikalla, mutta samalla pyritään sujuvoittamaan toivottuja mobiliteetteja. Teknologisen kehityksen myötä rajanylittäjien uhkatasot pyritään selvittämään ennakoivasti ja ei-toivotut maahantulijat pyritään käännyttämään jo rajalla. Samaan aikaan turvallisuustoimet, biopoliittinen hallinta ja vihapuhe kohdistuvat myös ihmisiin, jotka pyrkivät hakemaan turvapaikkaa Euroopasta

Coexistence of pairing gaps in three-component Fermi gases

We study a three-component superfluid Fermi gas in a spherically symmetric harmonic trap using the Bogoliubov-deGennes method. We predict a coexistence phase in which two pairing field order parameters are simultaneously nonzero, in stark contrast to studies performed for trapped gases using local density approximation. We also discuss the role of atom number conservation in the context of a homogeneous system.Comment: Text revised, added two figures and three reference