Photon-noise-tolerant dispersive readout of a superconducting qubit using a nonlinear Purcell filter

Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout. Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesired dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we fabricated is shown to be enhanced by a factor of three relative to a device with a linear filter. The measurement rate is enhanced by another factor of three by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a QND fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.Comment: 13 pages, 9 figure

Ympäristön synnyttämät pysyvät Rabi-oskillaatiot kubitissa

Decoherence is a quantum-mechanical phenomenon severely limiting quantum information processing and storage capability in quantum computers. Decoherence refers to mechanisms that cause transfer of information from a quantum system to its bath. Typically, in theoretical and experimental considerations of different systems containing superconducting qubits, the coupling strength between a qubit and its bath has been assumed to be constant in time. In this case, many standard results in the dynamics of two-level systems follow. For example, due to decoherence, driven Rabi oscillations in a qubit have a characteristic decay time. In addition, the oscillations may take place at any phase. However, by considering time-dependent coupling to the bath, these phenomena may be absent. In this thesis, we demonstrate with numerical simulations of a time-dependent Lindblad master equation persistent Rabi oscillations in a qubit driven by a coherent drive tone. The persistent Rabi oscillations are induced by modulating the coupling of the qubit to the bath with a sinusoidal temporal shape. We observe that the temporal modulation enforces a limit cycle on the evolution of the dressed qubit state with a fixed phase relative to the phase of the sinusoidal modulation regardless of the initial state of the system. Through a synchronization-like phenomenon, the frequency of the Rabi oscillation adapts to match the frequency of the modulation. We determine the effect of different system parameters on the limit cycle and study how the amplitude of the persistent oscillation depends on the coupling modulation parameters. We also observe a similar effect in the freely evolving qubit precession. In addition, the qubit acts as a non-linear mixer, resulting in different linear combinations of the original modulation waveform appearing in the bath-induced Rabi oscillations. We also introduce a design for a physical system where our simulations can be verified experimentally.Dekoherenssi on kvanttimekaaninen ilmiö, joka rajoittaa kvantti-informaation prosessointia ja säilöntää kvanttitietokoneissa. Dekoherenssi viittaa mekanismeihin, jotka aiheuttavat tiedon siirtymistä suljetusta kvanttisysteemistä kylpyyn. Kubitteja sisältävien systeemien tarkastelu on perinteisesti rajoittunut tapauksiin, joissa kytkentä kvanttibitin ja kylvyn välillä on ajassa vakio. Näissä tapauksissa useat hyvintunnetut tulokset kaksitasosysteemien dynamiikassa seuraavat. Dekoherenssin takia kubitin Rabi-oskillaatiolla on efektiivinen elinaika. Lisäksi Rabi-oskillaatiolla voi olla mikä vaihe tahansa. Tarkasteltaessa ajasta riippuvaa kytkentää kubitin ja kylvyn välillä nämä ilmiöt eivät kuitenkaan välttämättä esiinny. Tässä diplomityössä osoitamme aikariippuvan Lindbladin mestariyhtälöllä tehtyjen numeeristen simulaatioiden avulla, että aikariippuva kytkentä kubitin ja kylvyn välillä johtaa pysyviin Rabi-oskillaatioihin kubitissa. Havaitsemme, että aikamodulaatio kytkennässä pakottaa kubitin tilan kehityksen tietylle suljetulle radalle kubitin tila-avaruudessa. Tämän oskillaation vaihe määräytyy aikamoduloidun kytkennän mukaan riippumatta kubitin alkuperäisestä tilasta. Synkronisoinnin kaltaisen ilmiön vuoksi myös pysyvien oskillaatioiden taajuus muuttuu vastaamaan kytkennän modulaation taajuutta. Määritämme työssä myös eri ajon ja kytkennän parametrien vaikutuksen pysyvien oskillaatioden muotoon ja amplitudiin. Lisäksi havaitsemme samankaltaisen ilmiön kubitin vapaassa aika-kehityksessä tapahtuvassa kubitin oskillaatiossa. Kubitti toimii myös periaatteessa epä-lineaarisena mikserinä, minkä vuoksi pysyvissä Rabi-oskillaatioissa esiintyy komponentteja modulaatiotaajuuden monikertojen kohdalla. Työssä esittelemme myös laitteiston simulaatioiden kokeellista toistamista varten

Photon-Noise-Tolerant Dispersive Readout of a Superconducting Qubit Using a Nonlinear Purcell Filter

Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout. Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesirable dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we have fabricated is shown to be enhanced by a factor of 3 relative to a device with a linear filter. The measurement rate is enhanced by another factor of 3 by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a quantum nondemolition fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit

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