142 research outputs found
Környezetükkel kölcsönható kvantumrendszerek: Dekoherencia és a kvantumos összefonódottság eltűnése = Open quantum systems: environment induced decoherence and entanglement degradation
A kutatás olyan fizikai rendszerek vizsgálatára irányult, amelyekben a kĂĽlönbözĹ‘ szabadsági fokok közötti kvantumos összefonĂłdottság kitĂĽntetett szerepet játszik. Megmutattuk, hogy kĂ©tállapotĂş atomok dipĂłl-csatolt láncában közeli atomok estĂ©ben ezek a fajta kvantumos korreláciĂłk a környezet hatásai ellenĂ©re növekedhetnek is. A tĂ©ma igen közel esik a kvantumos informáciĂłfeldolgozás gyakorlati megvalĂłsĂtásának a problĂ©májához, ezzel kapcsolatos az az eredmĂ©nyĂĽnk, hogy sĂkbeli fĂ©lvezetĹ‘ gyűrűk segĂtsĂ©gĂ©vel alapvetĹ‘ kvantumos kapuk realizálhatĂłk, illetve kĂ©t kimenet esetĂ©n a rendszer a Stern-Gerlach berendezĂ©s megfelelĹ‘jekĂ©nt alkalmas polarizált elektronok preparálására. SzintĂ©n a spin szabadsági fok határozza meg a nagy eredĹ‘ spinnel rendelkezĹ‘ molekulák (nanomágnesek) viselkedĂ©sĂ©t is. Ezzel a rendszerrel kapcsolatban megmutattuk, hogy a kĂsĂ©rletekben Ă©szlelt THz tartományba esĹ‘ sugárzás kibocsátási mechanizmusa a mĂ©zerek működĂ©sĂ©hez hasonlĂł. Emellett kiszámĂtottuk, hogy a kristályrács fononjai, mint a spin rendszer környezete, hogyan befolyásolják a hiszterĂ©zisgörbĂ©k lefutását. | The aim of the research was the investigation of the role of quantum mechanical entanglement in physical systems with several degrees of freedom. It has been shown that in a chain of dipole-coupled atoms, when the interatomic separation is small enough, spontaneous entanglement formation appears, despite the destructive effects of the environment. As a possible application, we demonstrated that mesoscopic rings made of semiconductor material can realize basic one-qubit logical gates. Considering molecular nanomagnets, where again the spin degree of freedom plays an important role, it was shown that the physical mechanism being responsible for the experimentally observed THz radiation is a maser effect. Furthermore, our calculations show that the influence of the phonon environment decreases the steps seen in the hysteresis curves
Laser induced distortion of band structure in solids: an analytic model
We consider a spatially periodic (cosine) potential as a model for a
crystalline solid that interacts with a harmonically oscillating external
electric field. This problem is periodic both in space and time and can be
solved analytically using the Kramers-Henneberger co-moving frame. By analyzing
the stability of the closely related Mathieu-type differential equation, the
electronic band structure can be obtained. We demonstrate that by changing the
field intensity, the width of the zero-field band gaps can be drastically
modified, including the special case when the external field causes the band
gaps to disappearComment: 8 pages, 3 figure
Oscillating spin-orbit interaction in two-dimensional superlattices: sharp transmission resonances and time-dependent spin polarized currents
We consider ballistic transport through a lateral, two-dimensional
superlattice with experimentally realizable, sinusoidally oscillating
Rashba-type spin-orbit interaction. The periodic structure of the rectangular
lattice produces a spin-dependent miniband structure for static SOI. Using
Floquet theory, transmission peaks are shown to appear in the mini-bandgaps as
a consequence of the additional, time-dependent SOI. A detailed analysis shows
that this effect is due to the generation of harmonics of the driving
frequency, via which e.g., resonances that cannot be excited in the case of
static SOI become available. Additionally, the transmitted current shows space
and time-dependent partial spin-polarization, in other words, polarization
waves propagate through the superlattice.Comment: 8 pages, 6 figure
Relation of the capital structure and profitability based on literature
The corporates are regularly forced to use different inner and external capital elements in their
life. Of Course, the companies will use that kind of sourc
es, which they are able to achieve a
value
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creation progress with, thereby contribute to the increment of the trade industry sector.
According to many literatures, the income
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generating’s ability of the enterprises can be affected
by the major financial’s
decisions, following the inner and external financing opportunities,
strategics, forms and principles, and the capital structure of all these. Of course, we can make
differences among the lifecycles of the corporate’s life. The small and medium sized enter
prises
need to have capital for the fluently running, but it is, that from where and how it is available.
The capital structure theories helps us to understand this wide range of consequences. There are
also two differencies between the creator’s outlook o
f the traditional and the new capital
structure theories. Modigliani
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Miller theorems are classified into the new capital structure
theories, where there are no taxes originally, but the effects of the taxes are already taken into
count by the modified theo
ries
Fénnyel manipulált összefonódott atomi állapotok és kvantumos kapuk = Quantum gates and entagled atomic states manipulated by light
1. Ă“riás spinű molekulák (nanomágnesek) állapotának idĹ‘ben változĂł mezĹ‘vel valĂł kölcsönhatásában megmutattuk, hogyan lehet ezeket egy kĂvánt spinállapotba hozni. A dekoherencia figyelembevĂ©telĂ©vel magyarázatot adtunk a kĂsĂ©rleti hiszterĂ©zisgörbĂ©k alakjára Ă©s ĂĽregbe zárt nanomágnesek esetĂ©n a mĂ©zerszerű működĂ©sre. 2. FĂ©lvezetĹ‘ gyűrűben mozgĂł elektron spinjĂ©re alapozva egyqubites kvantumkapuk megvalĂłsĂtását javasoltuk. Egy, vagy nĂ©hány gyűrűvel a Stern-Gerlach kĂsĂ©rlet spintronikai változatának elvĂ©t dolgoztuk ki. Az eszközzel a tĂ©rbeli Ă©s a spin szabadsági fokok között összefonĂłdás hozhatĂł lĂ©tre. A szennyezĂ©sek Ă©s a vĂ©ges hĹ‘mĂ©rsĂ©klet miatti dekoherencia hatására számos gyűrűbĹ‘l állĂł hálĂłzat esetĂ©n a mĂ©ret növekedĂ©sĂ©vel egy ilyen rendszer távolabb kerĂĽl az ideális viselkedĂ©stĹ‘l, de a kiaknázhatĂł kvantumos tulajdonságok nem az alkotĂłelemek számával arányosan, hanem annál lassabban tűnnek el. Periodikusan változĂł spin-pálya kölcsönhatás esetĂ©n a gyűrűben a spin Rabi oszcilláciĂłja lĂ©p fel, ami vĂ©ges hĹ‘mĂ©rsĂ©kleten Ă©s vĂ©letlen szĂłrási folyamatok jelenlĂ©tĂ©ben is megmarad. Az oszcillálĂł spin-pálya kölcsönhatás a Raman szĂłráshoz hasonlĂł stokesi Ă©s anti-stokesi csĂşcsokat hoz lĂ©tre a vezetĹ‘kĂ©pessĂ©gben. 3. Megmutattuk, hogy N qubitbĹ‘l, illetve általánosabban K dimenziĂłs rĂ©szekbĹ‘l állĂł kvantumrendszer szimmetrikus alterĂ©ben a nem összefonĂłdott állapotok pontosan megegyeznek a koherens állapotokkal, Ă©s a szimmetrikus altĂ©rre ortogonális állapotok mindegyike összefonĂłdott. | 1. We have shown how giant spin molecules (nanomagnets) can be brought into specific quantum states with time varying magnetic fields . We explained the experimentally observed hysteresis curves and a maser-like operation for molecules in a cavity as caused by decoherence. 2. We proposed how to realize one-qubit quantum gates based on the electron spin in a semiconductor ring. Applying one or few such rings the spintronic analogue of the Stern-Gerlach experiment can be realized, and the device can entangle the spatial and spin degrees of freedom. Increasing the size of networks consisting of such rings decoherence caused by impurities and by finite temperature reduces the ideal properties of the system, but the quantum features to be exploited diminish promisingly slow. A periodic change of the strength of the spin-orbit interaction leads to Rabi oscillations of the spin, which persists also at finite temperatures and in the presence of random scattering events. The oscillating spin-orbit interaction creates Raman type Stokes and anti-Stokes peaks in the conductivity. 3. In an N qubit system, or more generally in an N-partite system consisting of K dimensional parts, the class of nonentangled states in the symmetric subspace is identical with the class of the coherent states. The subspace orthogonal to the symmetric one contains only entangled states
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