Probing individual split Cooper-pairs using the spin qubit toolkit

A superconductor is a natural source of spin-entangled spatially separated electron pairs. Although the first Cooper-pair splitter devices have been realized recently, an experimental confirmation of the spin state and the entanglement of the emitted electron pairs is lacking up to now. In this paper a method is proposed to confirm the spin-singlet character of individual split Cooper pairs. Two quantum dots (QDs), each of them holding one spin-prepared electron, serve as the detector of the spin state of a single Cooper pair that is forced to split when it tunnels out from the superconductor to the QDs. The number of charges on the QDs, measured at the end of the procedure, carries information on the spin state of the extracted Cooper pair. The method relies on the experimentally established toolkit of QD-based spin qubits: resonant spin manipulation, Pauli blockade, and charge measurement.Comment: 15 pages, 7 figure

Elektron transzport molekuláris nanostruktúrákban = Electron transport in molecular nanostructures

Az OTKA K76010 kutatói pályázat keretében nanokontaktusok viselkedését vizsgáltuk, melyekben az áram egyetlen atomon vagy egyetlen molekulán keresztül folyik. A kutatás keretében kifejlesztettünk egy molekulák kontrollált adagolására alkalmas mérőrendszert, mely egyedi molekulák kontaktálását teszi lehetővé folyékony hélium hőmérsékleten. Szupravezető subgap módszerrel karakterizáltunk különböző anyagból készült atomi méretű kontaktusokat és elméleti csoportokkal együttműködve részt vettünk egy vezetőképesség hisztogramok szimulálására alkalmas módszer fejlesztésében, melynek megbízhatóságát a subgap módszer segítségével egyedi vezetési csatornák szintjén ellenőriztük. Kifejlesztettünk egy új statisztikai módszert, melynek segítségével atomi és molekuláris kontaktusokban kialakuló konfigurációk közötti korrelációk vizsgálhatók. Ezen módszer segítségével számos atomi és molekuláris nanovezeték viselkedését sikerült a szokásos vezetőképesség hisztogram technikánál részletesebben feltérképezni. | Within the OTKA K76010 project nano-scale junctions were investigated, in which the current flows along single atoms or molecules. A novel measurement setup was developed which is capable of a controlled dosing of molecules to a liquid helium temperature break junction device so that single molecules can be contacted in a well controlled manner. With the help of superconducting subgap structure measurements various metal atomic sized contacts were characterized. In collaboration with theory groups a novel method was developed to simulate conductance histograms. With the help of subgap structure measurements the reliability of this method was tested on the level of individual channel transmission eigenvalues. Beside, we have introduced a novel statistical analysis method based on the two-dimensional cross-correlation histogram analysis of conductance traces. This method provides new information about the relation of different junction configurations that occur during the formation and evolution of metal and single-molecule junctions. The correlation method was successfully applied to study the behavior a several atomic and single-molecule structures beyond conductance histograms

Large voltage-tunable spin valve based on a double quantum dot

We study the spin-dependent transport properties of a spin valve based on a double quantum dot. Each quantum dot is assumed to be strongly coupled to its own ferromagnetic lead, while the coupling between the dots is relatively weak. The current flowing through the system is determined within the perturbation theory in the hopping between the dots, whereas the spectrum of a quantum dot-ferromagnetic lead subsystem is determined by means of the numerical renormalization group method. The spin-dependent charge fluctuations between ferromagnets and quantum dots generate an effective exchange field, which splits the double dot levels. Such field can be controlled, separately for each quantum dot, by the gate voltages or by changing the magnetic configuration of external leads. We demonstrate that the considered double quantum dot spin valve setup exhibits enhanced magnetoresistive properties, including both normal and inverse tunnel magnetoresistance. We also show that this system allows for the generation of highly spin-polarized currents, which can be controlled by purely electrical means. The considered double quantum dot with ferromagnetic contacts can thus serve as an efficient voltage-tunable spin valve characterized by high output parameters.Comment: 12 pages, 8 figure

Nanovezetékeken alapuló áramkörök gyártása és kvantum effektusainak elektron transzport vizsgálata = Fabrication and Electron Transport Study of Nanowire based Quantum Devices

Az anyagtudomány fejlődésének köszönhetően számos nanoméretű objektum létrehozására nyílik mód napjainkban, ilyenek például a félvezető nanopálcák (NW). Kis méretük miatt ezek a NW-k nagyon ígéretesek kvantum elektronikai célokra. Kvantum elektronika a modern szilárdtestfizika dinamikusan fejlődő területe, melynek fő célja elektromos áramkörök kvantum mechanikai szabadsági fokainak kontrollálása és kiolvasása, mint amilyen egy mesterséges atomba (QD) zárt elektron spinje. Jelen projektben nanoméretű áramkörök készítésére és kvantum effektusainak alacsony hőmérsékleti vizsgálatára alkalmas infrastruktúrát építettünk ki. Vizsgáltuk InAs NW-kból és szupravezető (S) ill. ferromágneses (F) elektródák kombinációjából készített áramköröket. InAs NW-ban kialakított dupla kvantum dotot szupravezetőhöz csatolva megmutattuk, hogy Cooper- párokat lehet térben szeparálni, ami megnyitja az utat mobil elektronokból álló Einstein Podolsky Rosen párok keltésére és összefonódottságuk tanulmányozására. Másrészről, ha F vezetéket kapcsolunk InAs NW-ban kialakított QD-hoz, akkor a ferromágnesség lokális kicserélődési téret kelt a mesterséges atomon. Megmutattuk, hogy ez a tér erősen függ a QD töltés állapotától. Sőt a tér még előjelet is válthat ugyanazon töltés állapotban, lehetővé téve az alap állapot spinjének megfordítását kapu feszültség segítségével. Megmutattuk, hogy egy ilyen F-QD rendszer hatékony spin áram erősítőként tud funkcionálni. | Recent development in material science allowed the synthesis of various nanoscale objects, like semiconductor nanowires (NW). Due to their small size, these NWs are very attractive for quantum electronic purpose. Quantum electronics is a dynamically progressing field of modern solid state physics, where the central goal is to manipulate and read out quantum mechanical degrees of freedom of circuits, like the spin of an electron trapped into a quantum dot (QD). In the present project the fabrication and transport characterization infrastructure have been established, which allows the production of nanocircuits and exploring their quantum effects in cryogenic measurements. We have investigated various InAs NW based electric circuits, where superconducting (S) and ferromagnetic (F) leads were attached to the wire. Coupling S lead to NW based double QD system, we have demonstrated, that Cooper pairs can be separated, which opens the way for Einstein Podolsky Rosen pair generation and entanglement analysis of mobile electrons. On the other hand attaching F lead to a QD, ferromagnetism penetrates into the dot inducing a local exchange field. We have shown that this exchange field strongly depends on the charge state of the QD. Furthermore it can even change sign for the same state, which allows the spin reversal of the ground state of the QD by gate voltage. We have shown that such an F- quantum dot system act as an efficient spin current amplifier

Signatures of single quantum dots in graphene nanoribbons within the quantum Hall regime

We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantum dots that are formed in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system`s signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that are formed between the Landau levels, suggesting a way to create gate-defined quantum dots that can be accessed with quantum Hall edge states

Strong nonlocal tuning of the current-phase relation of a quantum dot based Andreev molecule

Multiple systems hosting Andreev molecular states have been proposed and studied, consisting of closely spaced Josephson junctions modeled as ballistic channels. We show that replacing the ballistic channels in the weak link of the Josephson junctions with quantum dots (QD), leads to a very exciting, rich phase diagram. It shows a strong nonlocal Josephson effect: as one junction is tuned the current-phase relation of the other junction is modified. This architecture hosts $0 - \pi$ transitions and shows a tunable anomalous phase-shift $\phi_0$ , nonlocally controlled in both cases, without relying on spin-orbit interaction or Zeeman fields. In addition significant superconducting diode effect can also be observed. The presented non-local current-phase relation can be used as a signature of the formation of an Andreev molecular state, as well as to introduce new ways to tune quantum architectures