IST Austria Thesis

A qubit, a unit of quantum information, is essentially any quantum mechanical two-level system which can be coherently controlled. Still, to be used for computation, it has to fulfill criteria. Qubits, regardless of the system in which they are realized, suffer from decoherence. This leads to loss of the information stored in the qubit. The upper bound of the time scale on which decoherence happens is set by the spin relaxation time. In this thesis I studied a two-level system consisting of a Zeeman-split hole spin confined in a quantum dot formed in a Ge hut wire. Such Ge hut wires have emerged as a promising material system for the realization of spin qubits, due to the combination of two significant properties: long spin coherence time as expected for group IV semiconductors due to the low hyperfine interaction and a strong valence band spin-orbit coupling. Here, I present how to fabricate quantum dot devices suitable for electrical transport measurements. Coupled quantum dot devices allowed the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot. By integrating the charge sensor into a radio-frequency reflectometry setup, I performed for the first time single-shot readout measurements of hole spins and extracted the hole spin relaxation times in Ge hut wires

Fast Hole Tunneling Times in Germanium Hut Wires Probed by Single-Shot Reflectometry

Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times. Here we focus on such heavy-hole states confined in Germanium hut wires. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot, with charge-transfer signals as high as 0.3e. By integrating the T-like structure into a radio-frequency reflectometry setup, single-shot measurements allowing the extraction of hole tunneling times are performed. The extracted tunneling times of less than 10$\mu$s are attributed to the small effective mass of Ge heavy-hole states and pave the way towards projective spin readout measurements

Single-shot readout of hole spins in Ge

The strong atomistic spin orbit coupling of holes makes single-shot spin readout measurements difficult because it reduces the spin lifetimes. By integrating the charge sensor into a high bandwidth radio-frequency reflectometry setup we were able to demonstrate single-shot readout of a germanium quantum dot hole spin and measure the spin lifetime. Hole spin relaxation times of about 90 $\mu$s at 500\,mT are reported. By analysing separately the spin-to-charge conversion and charge readout fidelities insight into the processes limiting the visibilities of hole spins has been obtained. The analyses suggest that very high hole visibilities are feasible at realistic experimental conditions underlying the potential of hole spins for the realization of viable qubit devices

Ge hole spin qubit

Holes confined in quantum dots have gained considerable interest in the past few years due to their potential as spin qubits. Here we demonstrate double quantum dot devices in Ge hut wires. Low temperature transport measurements reveal Pauli spin blockade. We demonstrate electric-dipole spin resonance by applying a radio frequency electric field to one of the electrodes defining the double quantum dot. Next, we induce coherent hole spin oscillations by varying the duration of the microwave burst. Rabi oscillations with frequencies reaching 140MHz are observed. Finally, Ramsey experiments reveal dephasing times of 130ns. The reported results emphasize the potential of Ge as a platform for fast and scalable hole spin qubit devices

Zero field splitting of heavy-hole states in quantum dots

Using inelastic cotunneling spectroscopy we observe a zero field splitting within the spin triplet manifold of Ge hut wire quantum dots. The states with spin ±1 in the confinement direction are energetically favored by up to 55 μeV compared to the spin 0 triplet state because of the strong spin–orbit coupling. The reported effect should be observable in a broad class of strongly confined hole quantum-dot systems and might need to be considered when operating hole spin qubits

Heavy hole states in Germanium hut wires

Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so far unexplored type of nanostructure. Low temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function which is of heavy hole character. A light hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors. However, this tiny light hole contribution does not influence the spin lifetimes, which are expected to be very long, even in non isotopically purified samples

Single SiGe Quantum Dot Emission Deterministically Enhanced in a High-Q Photonic Crystal Resonator

We report the resonantly enhanced radiative emission from a single SiGe quantum dot (QD), which is deterministically embedded into a bichromatic photonic crystal resonator (PhCR) at the position of its largest modal electric field by a scalable method. By optimizing our molecular beam epitaxy (MBE) growth technique, we were able to reduce the amount of Ge within the whole resonator to obtain an absolute minimum of exactly one QD, accurately positioned by lithographic methods relative to the PhCR, and an otherwise flat, a few monolayer thin, Ge wetting layer (WL). With this method, record quality (Q) factors for QD-loaded PhCRs up to $Q\sim 10^5$ are achieved. A comparison with control PhCRs on samples containing a WL but no QDs is presented, as well as a detailed analysis of the dependence of the resonator-coupled emission on temperature, excitation intensity, and emission decay after pulsed excitation. Our findings undoubtedly confirm a single QD in the center of the resonator as a potentially novel photon source in the telecom spectral range

Self-controlled growth of highly uniform Ge/Si hut wires for scalable qubit devices

Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite towards the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, we report on the growth of Ge wires on pre-patterned Si (001) substrates with controllable position, distance, length and structure. This is achieved by a novel growth process which uses a SiGe strain-relaxation template and can be generalized to other material combinations. Transport measurements show an electrically tunable spin-orbit coupling, with a spin-orbit length similar to that of III-V materials. Also, capacitive coupling between closely spaced wires is observed, which underlines their potential as a host for implementing two qubit gates. The reported results open a path towards scalable qubit devices with Si compatibility

Dietary fiber intake by diet and dietary supplements and constipation in pregnancy

Trudnoća je posebno stanje koje podrazumijeva značajan broj anatomskih i fizioloških promjena žene u kojemu svi organski sustavi poput kardiovaskularnog sustava, respiratornog sustava, gastrointestinalnog sustava, bubrežnog i urinarnog sustava te metabolizma podliježu promjenama. Procijenjeno je da otprilike 11 % do 38 % žena tijekom trudnoće ima problema sa konstipacijom uzrokovano promjenama unutar gastrointestinalnog trakta. Prehrambena vlakna su neprobavljivi biljni polisaharidi koji se nalaze u velikim količinama u voću, povrću, kruhu i žitaricama, mahunarkama te orašastim plodovima i sjemenkama, a obzirom da potiču kretanje hrane kroz probavni sustav, oni uvelike doprinose normalizaciji stolice, što u konačnici smanjuje pojavu konstipacije. Glavni cilj ovog rada bio je ispitati navike unosa prehrambenih vlakana kod trudnica i učestalost stolice te utvrditi postoji li pozitivna korelaciju između navedenih parametra. Studijom je analiziran uzorak od 110 trudnica. Podaci su se prikupljali putem anonimne ankete koja je obuhvaćala pitanja o njihovim dnevnim unosima prehrambenih vlakana po kategorijama te učestalosti stolice posljednjih tjedan dana. Rezultati su ukazali na blagu negativnu korelaciju između unosa prehrambenih vlakana i povećanja učestalosti stolice.Pregnancy is a special condition that implies a significant number of anatomical and physiological changes in a woman in which all organ systems such as the cardiovascular system, respiratory system, gastrointestinal system, renal and urinary system, and metabolism are subject to changes. It is estimated that approximately 11% to 38% of women experience constipation problems during pregnancy caused by applications within the gastrointestinal tract. Dietary fibers are indigestible plant polysaccharides that are found in large quantities in fruits, vegetables, bread and cereals, legumes and nuts and seeds, and since they stimulate the movement of food through the digestive system, they greatly contribute to the normalization of stool by absorbing liquid, which ultimately reduces occurrence of constipation. The main goal of this study was to examine the dietary fiber intake habits of pregnant women and the frequency of their stools, and to determine whether there is a positive correlation between the mentioned parameters. The study analyzed a sample of 110 pregnant women. Data was collected through an anonymous survey that included questions about their daily dietary fiber intake by category and stool frequency in the last week. The results indicated a slight negative correlation between dietary fiber intake and increased stool frequency

Dietary fiber intake by diet and dietary supplements and constipation in pregnancy

Trudnoća je posebno stanje koje podrazumijeva značajan broj anatomskih i fizioloških promjena žene u kojemu svi organski sustavi poput kardiovaskularnog sustava, respiratornog sustava, gastrointestinalnog sustava, bubrežnog i urinarnog sustava te metabolizma podliježu promjenama. Procijenjeno je da otprilike 11 % do 38 % žena tijekom trudnoće ima problema sa konstipacijom uzrokovano promjenama unutar gastrointestinalnog trakta. Prehrambena vlakna su neprobavljivi biljni polisaharidi koji se nalaze u velikim količinama u voću, povrću, kruhu i žitaricama, mahunarkama te orašastim plodovima i sjemenkama, a obzirom da potiču kretanje hrane kroz probavni sustav, oni uvelike doprinose normalizaciji stolice, što u konačnici smanjuje pojavu konstipacije. Glavni cilj ovog rada bio je ispitati navike unosa prehrambenih vlakana kod trudnica i učestalost stolice te utvrditi postoji li pozitivna korelaciju između navedenih parametra. Studijom je analiziran uzorak od 110 trudnica. Podaci su se prikupljali putem anonimne ankete koja je obuhvaćala pitanja o njihovim dnevnim unosima prehrambenih vlakana po kategorijama te učestalosti stolice posljednjih tjedan dana. Rezultati su ukazali na blagu negativnu korelaciju između unosa prehrambenih vlakana i povećanja učestalosti stolice.Pregnancy is a special condition that implies a significant number of anatomical and physiological changes in a woman in which all organ systems such as the cardiovascular system, respiratory system, gastrointestinal system, renal and urinary system, and metabolism are subject to changes. It is estimated that approximately 11% to 38% of women experience constipation problems during pregnancy caused by applications within the gastrointestinal tract. Dietary fibers are indigestible plant polysaccharides that are found in large quantities in fruits, vegetables, bread and cereals, legumes and nuts and seeds, and since they stimulate the movement of food through the digestive system, they greatly contribute to the normalization of stool by absorbing liquid, which ultimately reduces occurrence of constipation. The main goal of this study was to examine the dietary fiber intake habits of pregnant women and the frequency of their stools, and to determine whether there is a positive correlation between the mentioned parameters. The study analyzed a sample of 110 pregnant women. Data was collected through an anonymous survey that included questions about their daily dietary fiber intake by category and stool frequency in the last week. The results indicated a slight negative correlation between dietary fiber intake and increased stool frequency