Resonaattorin nollaaminen käyttäen monikanavaista ajoa

Superconducting circuits constitue a promising platform for physically realizing a quantum computer. One of the most fundamental operations of a quantum computer is to measure the state of its quantum bits (qubits). The measurement process should be as fast as possible, in order to minimize errors in the computation caused by noise. Conventionally, the state of a qubit in a superconducting circuit is determined by coupling the qubit to a readout resonator and measuring how the state of the resonator changes when a readout pulse is applied to it. Recently, a method was proposed that makes it possible to read out the state of a qubit faster by applying pulses to both the resonator and the qubit simultaneously. In this thesis, we study whether this so-called multichannel technique could be used to unconditionally drive the readout resonator back to its initial state after the readout has been finished. Using numerical simulations, we find that compared to just waiting passively, the time it takes to initialize the resonator can be reduced by up to 86%. We also implement a part of the initialization protocol experimentally and find the results encouraging for the future implementation of the full protocol.Suprajohtavat virtapiirit ovat lupaava alusta kvanttitietokoneen toteuttamiselle. Yksi kvanttitietokoneen keskeisimmistä operaatioista on sen kvanttibittien eli kubittien tilan mittaaminen. Mittausprosessin tulee olla mahdollisimman nopea, jotta kohinan aiheuttamat virheet laskennassa olisivat vähäisiä. Tyypillisesti suprajohtavan kubitin tila määritetään kytkemällä kubitti värähtelijäpiiriin, ja tutkimalla kuinka tämän värähtelijän tila muuttuu mittauspulssin vaikutuksesta. Äskettäin julkaistiin menetelmä, jolla kubitin tila voidaan määrittää aiempaa nopeammin lähettämällä mittauspulssit samanaikaisesti sekä värähtelijään että kubittiin. Tässä diplomityössä tutkitaan, kuinka tätä niin kutsuttua monikanavamenetelmää voidaan soveltaa siihen, että värähtelijä ajetaan takaisin sen alkuperäiseen tilaan mittausprosessin päätteeksi. Numeeristen simulaatioiden perusteella todetaan, että aikaa, joka värähtelijältä kuluu alkutilaan palaamiseen, on mahdollista lyhentää 86% verrattuna siihen, että alkutilaan palautumista odotetaan passiivisesti. Tämän lisäksi värähtelijän nollausta tutkittiin kokeellisesti. Teknisistä vaikeuksista johtuen palautusprosessi saatiin toteutettua ainoastaan osittain, mutta tulokset viittaavat siihen, että palautuksen nopeutuminen koko prosessissa on mahdollista saavuttaa myös kokeellisesti

Data for "Single-Shot Readout of a Superconducting Qubit Using a Thermal Detector"

<p>Data and code used to generate the plots in the publication "Single-Shot Readout of a Superconducting Qubit Using a Thermal Detector".</p>See the README.md file for additional instructions

Compact inductor-capacitor resonators at sub-gigahertz frequencies

| openaire: EC/HE/101053801/EU//ConceptQ | openaire: EC/HE/101113946/EU//OpenSuperQPlus100Compact inductor-capacitor (LC) resonators, in contrast to coplanar waveguide (CPW) resonators, have a simple lumped-element circuit representation but usually call for sophisticated finite-element method (FEM) simulations for an accurate modeling. Here we present a simple analytical model for a family of coplanar LC resonators where the electrical properties are directly obtained from the circuit geometry with a satisfying accuracy. Our experimental results on ten high-internal-quality-factor resonators (Qi≳2×105), with frequencies ranging from 300MHz to 1GHz, show an excellent consistency with both the derived analytical model and detailed FEM simulations. These results showcase the ability to design sub-gigahertz resonators with less than 2% deviation in the resonance frequency, which has immediate applications, for example, in the implementation of ultrasensitive cryogenic detectors. The achieved compact resonator size of the order of a square millimeter indicates a feasible way to integrate hundreds of microwave resonators on a single chip for realizing photonic lattices.Peer reviewe

Qubit Measurement by Multichannel Driving

| openaire: EC/H2020/681311/EU//QUESS | openaire: EC/H2020/795159/EU//NEQCWe theoretically propose and experimentally implement a method of measuring a qubit by driving it close to the frequency of a dispersively coupled bosonic mode. The separation of the bosonic states corresponding to different qubit states begins essentially immediately at maximum rate, leading to a speedup in the measurement protocol. Also the bosonic mode can be simultaneously driven to optimize measurement speed and fidelity. We experimentally test this measurement protocol using a superconducting qubit coupled to a resonator mode. For a certain measurement time, we observe that the conventional dispersive readout yields close to 100% higher average measurement error than our protocol. Finally, we use an additional resonator drive to leave the resonator state to vacuum if the qubit is in the ground state during the measurement protocol. This suggests that the proposed measurement technique may become useful in unconditionally resetting the resonator to a vacuum state after the measurement pulse.Peer reviewe

Nanobolometer with ultralow noise equivalent power

| openaire: EC/H2020/681311/EU//QUESS | openaire: EC/H2020/727305/EU//SNABO | openaire: EC/H2020/766853/EU//EFINED | openaire: EC/H2020/820505/EU//QMiCSSince the introduction of bolometers more than a century ago, they have been used in various applications ranging from chemical sensors, consumer electronics, and security to particle physics and astronomy. However, faster bolometers with lower noise are of great interest from the fundamental point of view and to find new use-cases for this versatile concept. We demonstrate a nanobolometer that exhibits roughly an order of magnitude lower noise equivalent power, 20zW/root Hzp, than previously reported for any bolometer. Importantly, it is more than an order of magnitude faster than other low-noise bolometers, with a time constant of 30 mu s at 60zW/root Hzp. These results suggest a calorimetric energy resolution of 0.3 zJ = h x 0.4 THz with a time constant of 30 mu s. Further development of this nanobolometer may render it a promising candidate for future applications requiring extremely low noise and high speed such as those in quantum technology and terahertz photon counting.Peer reviewe