Vortices and elementary excitations in dilute Bose-Einstein condensates

The condensation of weakly interacting alkali atom gases observed in 1995 is a manifestation of the macroscopic quantum phenomenon called Bose-Einstein condensation. In contrast to superconductivity and Helium superfluids, the dilute alkali atom condensates are directly observable using optical imaging techniques and they constitute highly controllable quantum systems. Hence, they offer an ideal test bench to investigate the applicability of quantum field theories and to predict the properties of the condensates starting from the first principles. Since the first experimental realizations of dilute Bose-Einstein condensates, a great interest has been focused on this field, resulting in numerous experimental studies concerning, for example, nonlinear dynamics and the elementary excitations of these many-particle quantum systems. The existence of topological phase singularities, quantized vortices, first predicted theoretically and finally observed experimentally in 1999, led to another series of pioneering theoretical and experimental studies. Several methods to create single-quantum vortices were realized already in 2001 but, however, the puzzle of creating multiply quantized vortices still remained. In this thesis, the so-called topological method to create two and four times quantized vortices is studied in detail. Since the instrumentation required for the experimental realization of the topological method had already been installed in many laboratories, the theoretical predictions of this thesis were experimentally verified shortly after their publication. Multiply quantized vortices are energetically unstable and tend to split into singly quantized vortices. However, the dissipation of energy in the condensates is minimal at low temperatures and, consequently, can lead to long lifetimes for the multiquantum vortices. Nevertheless, one prediction of this thesis is that the lifetimes of doubly quantized vortices are relatively short, owing to the known dynamical instability of the state. The splitting of the doubly quantized vortex has recently been observed experimentally. The stability of vortices in dilute Bose-Einstein condensates is intimately related to the excitation spectrum of the stationary vortex state. However, developing an accurate, computationally feasible finite-temperature quantum field theory remains an open problem. In this thesis, we study the elementary excitations of an irrotational condensate within a recently developed systematic second order perturbation theory. The collapse and revival of certain elementary excitations is discovered. The most recent studies of this thesis are devoted to stationary, but rotationally asymmetric vortex states. In weakly interacting condensates, a novel stationary state, a vortex tripole holding finite angular momentum is presented. Also the lowest elementary excitations of the so-called vortex dipole, tripole, and quadrupole states are studied.reviewe

Kvanttitietokoneet vihdoinkin lyöneet läpi

Kun ensimmäinen suprajohtavista sähköpiireistä valmistettu kvanttibitti eli kubitti demonstroitiin Japanissa vuonna 1999, siitä nousi valtava innostus kvanttilaskennan ja kvanttitietokoneiden rakentamiseen. Nyt maailmalla on jo näytetty, että kvanttitietokone voi suorittaa tietynlaisia laskutoimituksia nopeammin kuin nopeimmat supertietokoneet. Suomessa rakennetaan jopa kaupallisia kvanttitietokoneita, joista ensimmäinen on jo valmis ja toiminnassa Teknologian tutkimuskeskus VTT:n tiloissa Espoon Otaniemessä. Kvanttikoneista yritetään puristaa irti hyötyä miljardisijoitusten turvin

Development of backflow treatment in jet grouting

Jet grouting is a method that is used in situ to create soilcrete columns consisting of soil and cement. A negative aspect of the method is backflow slurry that is created in the process. In this study, new ways of reducing and treating the backflow were examined. The goal of the study was to find out new options for placement and utilization of the slurry and to examine and compare the possibilities to apply them in practice. This study explored the jet grouting method as well as the laws, ruling and current practice concerning the backflow treatment. Three new possible treatment options were being examined. Firstly, the study investigated the process of using the hardened backflow as a material for crushed concrete. Secondly, the equipment used in treating the slurry wall material was examined to find out if it is applicable in jet grouting method. Thirdly, the properties of the grout and the backflow were being measured to study the use of backflow as a material for waterproof underground walls. The study explained the requirements for the process of manufacturing and using of crushed concrete in terms of legislation. In addition, the study indicated that it may be possible to use the slurry wall equipment for recycling the material used in jet grouting. The empiric tests indicated that the jet grouting backflow can be applied as a material for waterproof underground walls when the ground properties are of certain type.Suihkuinjektointi on injektointimenetelmä, jolla voidaan valmistaa työmaaolosuhteissa maa-aineksesta ja sementistä koostuvia matalalujuuksisia betonipilareita. Suihkuinjektoinnin ohessa syntyvä paluuvirtausliete on yksi menetelmän haittapuolista. Tässä tutkimuksessa etsittiin uusia keinoja paluulietteen määrän vähentämiselle sekä sen jatkokäsittelylle. Tutkimuksen tavoitteena oli löytää lietteelle vaihtoehtoisia loppusijoituspaikkoja ja hyödyntämismenetelmiä sekä tutkia ja vertailla vaihtoehtojen soveltamismahdol1isuuksia. Tutkimuksessa perehdyttiin suihkuinjektointimenetelmään sekä paluuvirtauslietteen käsittelyyn liittyviin saannoksiin, ohjeisiin ja nykyisiin toimintatapoihin. Esiin nousseiden kehittämisvaihtoehtojen pohjalta tarkasteltiin kolmea mahdollista käsittelymenetelmää. Kirjallisuusosiossa tutkittiin sitoutuneen betonijätteen murskaamista ja jatkokäyttöä betonimurskeen materiaalina sekä bentoniittilietteen kierrättämiseen suunniteltujen laitteistojen soveltuvuutta suihkuinjektoinnissa syntyvän paluulietteen käsittelyyn. Empiirisessä osiossa tutkittiin sementtimassalle ja paluulietteelle suoritettavien mittausten avulla lietteen soveltumista vedenpitävän patoseinän materiaaliksi. Tutkimus selvitti, millä edellytyksillä injektointilietteen sitouttaminen ja murskaaminen sekä lopputuotteena saatavan betonimurskeen tuotteistaminen ja käyttö on lainsäädännön puitteissa mahdollista. Lisaksi tutkimus osoitti, että bentoniittilietteen kierrätyslaitteistot soveltuvat tietyin varauksin myös suihkuinjektointilietteen käsittelyyn. Suihkuinjektointi- ja injektointistandardeihin perustuvat empiiriset kokeet osoittivat, että paluuliete soveltuu patoseinämateriaaliksi tietyissä maaperäolosuhteissa

Bordism invariants of colored links and topologically protected tricolorings

We construct invariants of colored links using equivariant bordism groups of Conner and Floyd. We employ this bordism invariant to find the first examples of topological vortex knots, the knot structure of which is protected from decaying via topologically allowed local surgeries, i.e., by reconnections and strand crossings permitted by the topology of the vortex-supporting medium. Moreover, we show that, up to the aforementioned local surgeries, each tricolored link either decays into unlinked simple loops, or can be transformed into either a left-handed or a right-handed tricolored trefoil knot.Comment: 22 pages, 9 figures. Comments welcome