Charge-noise tolerant exchange gates of singlet-triplet qubits in asymmetric double quantum dots

In the semi-conductor double quantum dot singlet-triplet qubit architecture, the decoherence caused by the qubit's charge environment poses a serious obstacle in the way towards large scale quantum computing. The effects of the charge decoherence can be mitigated by operating the qubit in the so called sweet spot regions where it is insensitive to electrical noise. In this paper, we propose singlet-triplet qubits based on two quantum dots of different sizes. Such asymmetric double dot systems allow the implementation of exchange gates with controllable exchange splitting $J$ operated in the doubly occupied charge region of the larger dot, where the qubit has high resilience to charge noise. In the larger dot, $J$ can be quenched to a value smaller than the intra-dot tunneling using magnetic fields, while the smaller dot and its larger splitting can be used in the projective readout of the qubit

Validity of the single-particle description and charge noise resilience for multielectron quantum dots

We construct an optimal set of single-particle states for few-electron quantum dots (QDs) using the method of natural orbitals (NOs). The NOs include also the effects of the Coulomb repulsion between electrons. We find that they agree well with the noniteracting orbitals for GaAs QDs of realistic parameters, while the Coulomb interactions only rescale the radius of the NOs compared to the noninteracting case. We use NOs to show that four-electron QDs are less susceptible to charge noise than their two-electron counterparts.Comment: 11+ pages, 5 figure

Non-adiabatic charge state transitions in singlet-triplet qubits

Peer reviewe

Responses of the atmospheric concentration of radon-222 to the vertical mixing and spatial transportation

Radon-222 (Rn-222) has traditionally been used as an atmospheric tracer for studying air masses and planetary boundary-layer evolution. However, there are various factors that influence its atmospheric concentration. Therefore, we investigated the variability of the atmospheric radon concentration in response to the vertical air mixing and spatial transport in a boreal forest environment in northern Europe. Long-term Rn-222 data collected at the SMEAR II station in southern Finland during 2000-2006 were analysed along with meteorological data, mixing layer height retrievals and air-mass back trajectory information. The daily mean atmospheric radon concentration followed a log-normal distribution within the range <0.1-11 Bq m(-3), with the geometric mean of 2.5 Bq m(-3) and a geometric standard deviation of 1.7 Bq m(3). In spring, summer, autumn and winter, the daily mean concentrations were 1.7, 2.7, 2.8 and 2.7 Bq m(-3), respectively. The low, spring radon concentration was especially attributed to the joint effect of enhanced vertical mixing due to the increasing solar irradiance and inhibited local emissions due to snow thawing. The lowest atmospheric radon concentration was observed with northwesterly winds and high radon concentrations with southeasterly winds, which were associated with the marine and continental origins of air masses, respectively. The atmospheric radon concentration was in general inversely proportional to the mixing layer height. However, the ambient temperature and small-scale turbulent mixing were observed to disturb this relationship. The evolution of turbulence within the mixing layer was expected to be a key explanation for the delay in the response of the atmospheric radon concentration to the changes in the mixing layer thickness. Radon is a valuable naturally-occurring tracer for studying boundary layer mixing processes and transport patterns, especially when the mixing layer is fully developed. However, complementing information, provided by understanding the variability of the atmospheric radon concentration, is of high necessity to be taken into consideration for realistically interpreting the evolution of air masses or planetary boundary layer.Peer reviewe

Patterns in airborne pollen and other primary biological aerosol particles (PBAP), and their contribution to aerosol mass and number in a boreal forest

We studied variation in concentrations of airborne pollen and other particles of biological origin in a boreal forest in Finland during 2003–2004. The highest concentrations of pollen were observed in late spring and early summer, whereas the peak concentrations of other particles of biological origin (including e.g. fungal spores) occurred in August–September. Although the patterns in concentrations in 2003 and 2004 were similar, the concentration levels were significantly different between the years. The contribution of pollen and other particles of biological origin led to an increase in the measured particulate matter (PM) mass during the pollen season (mass of pollen and other particles of biological origin 5.9 and 0.4 μg m–3, respectively, in respect to PMtotal mass of 9.9 μg m–3) but the effect on total particle number was negligible. The other particles of biological origin constituted the largest fraction of measured primary biological aerosol particle (PBAP) numbers (~99%), whereas pollen showed a higher relative mass fraction (~97%) of PBAP. These results underline the important contribution of PBAP to coarse atmospheric particle mass providing up to 65% of the total mass during the peak pollen season

Kvanttilaskenta käyttäen kahden elektronin spini-tiloja puolijohdekvanttipisteissä

A quantum computer would exploit the phenomena of quantum superposition and entanglement in its functioning and with them offer pathways to solving problems that are too hard or complex to even the best classical computers built today. The implementation of a large-scale working quantum computer could bring about a change in our society rivaling the one started by the digital computer. However, the field is still in its infancy and there are many theoretical and practical issues needing to be solved before large-scale quantum computing can become reality.  In digital computers, data is stored in bits. The quantum equivalent is called a qubit (quantum bit) and it is basically a quantum mechanical two-level system that can be in a superposition of its two basis states. There are many different proposals for implementing qubits, but one of the most promising ones is to encode the qubit using electron spins trapped in semiconductor quantum dots. Singlet-triplet qubits are spin qubits where the two-electron spin eigenstates are used as the qubit's basis. The required one and two-qubit operations have already been demonstrated experimentally by several research groups around the world in this qubit architecture. The most severe factor limiting the implementation of larger systems of qubits is decoherence. The qubits are not isolated systems, they interact with their environment, which can lead to the loss of quantum information.  Few-electron systems can be simulated accurately using first principle methods that become too taxing when the particle number increases. The topic of this thesis is the simulation of quantum dot singlet-triplet qubit systems using accurate exact diagonalization based methods. The emphasis is on the realistic description of qubit operations, both single-qubit ones and those involving the interaction between neighboring qubits. The decoherence effects are also discussed alongside with certain proposals to alleviate their effects.Kvanttitietokone hyödyntäisi kvantti-ilmiöitä, kuten superpositiota ja lomittumista ja mahdollistaisi tiettyjen nykyisille klassisille tietokoneille vaikeiden ongelmien ratkaisemisen. Suuren mittakaavan kvanttilaskenta saattaisi muuttaa yhteiskuntaamme yhtä perustavalla tavalla kuin digitaalisen tietokoneen keksiminen 1900-luvulla. Toimivaan ison mittakaavan kvanttitietokoneeseen on kuitenkin yhä matkaa, ja on useita teoreettisia ja käytännöllisiä ongelmia, jotka täytyy ratkaista ennen kuin tästä tulee todellisuutta.  Klassisessa tietokoneessa data tallennetaan bitteihin. Bitin kvanttivastinetta kutsutaan kubitiksi. Se on kvanttimekaaninen kaksitasosysteemi, joka voi olla ominaistilojensa superpositiossa. Kubitti voidaan realisoida monessa erilaisessa systeemissä. Yksi lupaavimmista ehdotuksista on käyttää puolijohdekvanttipisteisiin vangittujen elektronien spinejä kubitin pohjana. Jos kubitin tilat muodostuvat kahden elektronin spin-ominaistiloista, kubittia kutsutaan singletti-tripletti kubitiksi. Universaaliin kvanttilaskentaan tarvittavat yksi- ja kaksikubittioperaatiot on jo toteutettu kokeellisesti singletti-tripletti kubiteilla. Suuren mittakaavan kubittisysteemien implementoimista estävät tällä hetkellä ennen kaikkea dekoherenssi-ilmiöt. Kubitit eivät ole eristettyjä systeemejä; ne vuorovaikuttavat ympäristönsä kanssa, mikä johtaa kvantti-informaation katoamiseen.  Muutaman elektronin systeemejä voidaan mallintaa tarkoilla yksittäisten elektronien tasolla toimivilla malleilla. Monet tällaiset mallit ovat liian kompleksisia laskennallisesti suurempien systeemien simuloimiseen, mutta soveltuvat hyvin esimerkiksi muutaman singletti-tripletti kubitin tapaukseen. Tämän väitöskirjan aihe kvanttipiste singletti-tripletti kubittien mallintaminen käyttäen tarkkoja ns. exact diagonalization-metodeita. Painopiste on yksi- ja monikubittioperaatioiden tarkassa kuvaamisessa. Myös dekoherenssi-ilmiöitä ja tiettyjä ratkaisuja niiden helpottamiseksi käsitellään

Grafeenipohjaiset fotodetektorit ja Seebeckin ilmiö

Graphene is a very rapidly rising star among nanomaterials. It has great potential in both terms of commercial applications, and understanding of fundamental physics. Graphene's unique properties make it a promising new material in nano-electronics. It has been proposed that it could one day replace silicon in semiconductor technology. Among the numerous future applications are graphene based photodetectors. In this field, graphene could offer fundamentally different applications compared to the traditional photodetectors based on the IV and III-V semiconductors. The main subject of this Master's thesis is the photothermoelectric effect (or the Seebeck effect) in graphene. It is considered as one of the main mechanisms in the generation of photocurrents in graphene. The photoelectic effect is also briefly discussed in terms of optical transition rates. This Master's thesis is a computational study. The modeling of graphene's electronic states is done with the tight-binding approximation. The photocurrents are simulated using the Landauer-Büttiker transport formalism and the Green's function method for the computation of the transmission probabilities. The conductances and Seebeck coefficients of various graphene based systems are computed. The obtained computational results are compared to existing computational and experimental studies.Grafeeni on nouseva tähti nanomateriaalien joukossa. Sillä on suuri potentiaali sekä sovelluksien että fysiikan ilmiöiden ymmärtämisen kannalta. Grafeenin ainutlaatuiset sähköiset ominaisuudet tekevät siitä lupaavan materiaalin nanoelektroniikassa. Pidetään mahdollisena, että grafeeni voisi tulevaisuudessa haastaa piin puolijohdeteknologiassa. Eräs lukuisista mahdollisista sovelluksista grafeenille on grafeenipohjaiset fotodetektorit. Grafeeni voisi mahdollistaa aivan uudenlaisten fotodetektorien valmistamisen. Tämän diplomityön pääaihe on valoindusoitu lämpösähköinen ilmiö grafeenissa. Sitä pidetään yhtenä päämekanismeista, joiden avulla grafeenin valoindusoidut sähkövirrat syntyvät. Myös valosähköistä ilmiötä käsitellään lyhyesti liittyen optisiin transitiotodennäköisyyksiin. Tämä diplomityö on luonteeltaan laskennallinen. Grafeenin elektronirakenne lasketaan käyttäen tightbinding metodia. Sähkövirtoja mallinnetaan Landauer-Büttiker kuljetusformalismin ja Greenin funktioiden avulla. Laskemme useiden grafeenipohjaisten systeemien konduktanssit ja Seebeckkertoimet. Saatuja laskennallisia tuloksia verrataan aikaisempaan laskennalliseen ja kokeelliseen tutkimukseen