Universal control of a bosonic mode via drive-activated native cubic interactions

Linear bosonic modes offer a hardware-efficient alternative for quantum information processing but require access to some nonlinearity for universal control. The lack of nonlinearity in photonics has led to encoded measurement-based quantum computing, which rely on linear operations but requires access to resourceful ('nonlinear') quantum states, such as cubic phase states. In contrast, superconducting microwave circuits offer engineerable nonlinearities but suffer from static Kerr nonlinearity. Here, we demonstrate universal control of a bosonic mode composed of a superconducting nonlinear asymmetric inductive element (SNAIL) resonator, enabled by native nonlinearities in the SNAIL element. We suppress static nonlinearities by operating the SNAIL in the vicinity of its Kerr-free point and dynamically activate nonlinearities up to third order by fast flux pulses. We experimentally realize a universal set of generalized squeezing operations, as well as the cubic phase gate, and exploit them to deterministically prepare a cubic phase state in 60 ns. Our results initiate the experimental field of universal continuous-variables quantum computing.Comment: 11 pages, 6 figures and supplementary material

Robust Preparation of Wigner-Negative States with Optimized SNAP-Displacement Sequences

Hosting nonclassical states of light in three-dimensional microwave cavities has emerged as a promising paradigm for continuous-variable quantum information processing. Here we experimentally demonstrate high-fidelity generation of a range of Wigner-negative states useful for quantum computation, such as Schrodinger-cat states, binomial states, Gottesman-Kitaev-Preskill states, as well as cubic phase states. The latter states have been long sought after in quantum optics and have never been achieved experimentally before. We use a sequence of interleaved selective number-dependent arbitrary phase (SNAP) gates and displacements. We optimize the state preparation in two steps. First we use a gradient-descent algorithm to optimize the parameters of the SNAP and displacement gates. Then we optimize the envelope of the pulses implementing the SNAP gates. Our results show that this way of creating highly nonclassical states in a harmonic oscillator is robust to fluctuations of the system parameters such as the qubit frequency and the dispersive shift

Galliumnitridi ohutkalvoresonaattorit mikroaaltotaajuuksille

Mechanical resonators are pushing into the quantum limit on mechanical motion detection making them ideal candidates for studying quantum behavior of macroscopic objects. In the quantum regime of measurements, the resonators must have very low losses, i.e. have a high Q factor. In this thesis, gallium nitride (GaN) is studied as a material for high quality resonators for quantum limited measurements. GaN is a wide band-gap semiconductor material with great electromechanical properties that make it excellent candidate for micro-electromechanical systems. The piezoelectric properties of GaN can be used for actuation and sensing of the resonator. The resonator can be excited in a thickness resonance mode where the frequency is defined by the thickness of the structure. This thesis describes the fabrication and microwave characterization of film bulk acoustic resonator (FBAR) structures, fabricated from GaN on Si wafers with an intermediate buffer layer of aluminium nitride. The thickness of the GaN membrane is a 500 nm and the AlN buffer layer is 300 nm thick. The thickness is selected to provide resonance frequency for microwave frequency region around 5 GHz. The eigenfrequencies and mode shapes of the resonators are acquired by numerical simulations based on finite-element method using COMSOL software. Tests of electrode geometry, electrode material, and electromechanical coupling are detailed. Q factors are obtained by means of equivalent circuit parameter extraction. The FBARs display mechanical quality factors of Q ~ 100 - 200, which falls short of the requirements for quantum measurements. The limiting factor is believed to be the electrode material and the material losses of GaN.Mekaanisia resonaattoreita voidaan mitata jo kvanttimaailman rajalla. Siksi ne sopivat kvanttimekaniikan tutkimiseen makroskooppisessa mittakaavassa. Tässä työssä tutkitaan galliumnitridiä (GaN) materiaalina korkealaatuisille resonaattoreille, joita voitaisiin käyttää kvanttitason mittauksissa. Galliumnitridi on puolijohdemateriaali, jolla on erinomaiset sähkömekaaniset ominaisuudet, mitkä tekevät siitä sopivan mekaanisiin mikrosysteemisovelluksiin. Galliumnitridin pietsosähköisiä ominaisuuksia voidaan käyttää resonaattorin värähtelyjen sekä tuottamiseen että mittaamiseen. Galliumnitridiresonaattori värähtelee paksuusvärähtelytilassa, jonka taajuudeen määrittää resonaattorin paksuus. Tämä työ kuvaa GaN ohutkalvoresonaattorien (FBAR) valmistuksen ja mikroaaltomittauksen. Resonaattorit ovat valmistettu piikiekon päälle kasvatetusta GaN:sta, joiden välissä on kerros alumiininitridiä. GaN kerroksen paksuus on 500 nm ja AlN kerroksen paksuus 300 nm. Paksuus tuottaa värähtelytaajuuden n. 5 GHz:n alueelle. Värähtelytaajuuksia ja niiden moodeja simuloitiin elementtimenetelmällä käyttäen COMSOL ohjemistoa. Työssä esitellään yksityiskohtaisesti elektrodigeometrian, elektrodimateriaalin ja sähkömekaanisen kytkennän tulokset. Q-arvot saatiin selville sähköisen piirimallin avulla, josta voitiin laskea Q-arvot. Q-arvot olivat luokkaa 100-200, mikä jää työlle asetetuista tavoitteista. Rajoittava tekijä on ilmeisesti elektrodimateriaali ja GaN:n materiaalihäviöt

Akustisten ylivärähtelykvanttien kvanttimekaaninen hallinta

16.10.2020 18:00 – 22:00 via Zoom https://aalto.zoom.us/j/63078213234Mechanical resonators are harmonic oscillators which means that their energy eigenstates are linearly spaced. The highly linear response makes observing and accessing quantum features in mechanical modes difficult. However, in quantum acoustic systems, the controllable quantum properties of a superconducting qubit can be extended to mechanical resonators. Accessing the individual energy levels of the harmonic oscillator is possible through the superconducting qubit since superconducting qubits have a non-uniform energy level separation through the inclusion of Josephson junctions. This allows to benefit from the complementary functionalities of the different systems in order to create, detect, and control quantum states of mechanical motion. The quantum acoustic devices that are studied in this thesis are created by coupling a superconducting qubit to a high overtone bulk acoustic resonator (HBAR). HBAR systems are promising with their qubit-compatible microwave frequencies and long decay times. Their high mode density allows accessing multiple acoustic modes in a small form factor on-chip. The resonator forms an acoustic cavity where the acoustic waves propagate inside a solid-state material forming a standing wave. The piezoelectric properties of the resonator materials allow for actuation and readout, as well as great enhancement of the coupling strength between the qubit and the resonator. In this thesis, quantum mechanical effects in a mechanical resonator are explored down to the level of a single quantum. A new quantum bulk acoustic device design is introduced that combines a strong coupling to the mechanical resonator with a high qubit coherence. This allows to observe vacuum Rabi swaps between the qubit and an acoustic mode in order to prepare a single mechanical excitation in the resonator. Another technique uses sideband transitions that are induced by parametric modulation of the qubit energy in order to generate selective coupling to different acoustic modes inside the HBAR. The coupling strength is determined by the amplitude of the parametric modulation. This method is used to drive photon-assisted Rabi oscillations between the qubit and the selected acoustic mode. These techniques are used to increase the control of the bulk acoustic phonons, which is necessary for new technological applications that use mechanical resonators as quantum resources.Mekaaniset värähtelijät ovat harmonisia oskillaattoreita, joten niiden ominaisenergiat ovat jakautuneet tasaisesti. Niiden lineaarisen vasteen takia kvantti-ilmiöiden havainnointi ja hyödyntäminen mekaanisissa tiloissa on vaikeaa. Kvanttiakustisissa järjestelmissä suprajohtavan kvanttibitin, eli kubitin, hallittavat kvanttitilat voidaan kuitenkin laajentaa myös mekaanisiin värähtelijöihin. Harmonisen värähtelijän yksittäisiä energiatiloja voidaan tutkia suprajohtavan kubitin avulla, sillä suprajohtavissa kubiteissa on epäharmoniset energiatilat Josephson-liitosten ansiosta. Tällöin näiden eri järjestelmien eroavat ominaisuudet täydentävät toisiaan, jolloin voidaan luoda, mitata ja hallita mekaanisen värähtelyn kvanttitiloja. Tässä työssä tutkittavat kvanttiakustiset näytteet ovat valmistettu yhdistämällä suprajohtava kubitti ylivärähtelyresonaattoriin (HBAR). HBAR-resonaattorit ovat lupaavia tutkimuskohteita, sillä ne toimivat samoilla mikroaaltotaajuuksilla kuin suprajohtavat kubitit, ja niiden värähtelytiloilla on pitkä elinaika. Niillä on korkea värähtelytilatiheys, jonka ansiosta montaa eri tilaa voidaan tutkia yhdellä näytteellä. Värähtely on rajoitettu resonaattorimateriaalin sisälle, jossa akustiset aallot etenevät muodostaen seisovan aallon. Materiaalien pietsosähköisten ominaisuuksien ansiosta värähtelyjä voidaan aktivoida ja mitata sähköisesti, sekä värähtelyjen ja mittapiirin välisen kytkennän suuruus kasvaa. Tässä väitöskirjassa tutkitaan kvanttimekaanisia ilmiöitä mekaanisessa värähtelijässä yksittäisten energiakvanttien tarkkuudella. Työssä esitellään uudentyyppinen kvanttiakustinen näyte, joka yhdistää vahvan kytkennän ja hyvän kubitin elinajan. Tämän ansiosta voidaan havainnoida tyhjiöoskillaatioita kubitin ja akustisen tilan välillä, ja valmistaa yhden mekaanisen kvantin tila värähtelijään. Vaihtoehtoinen tekniikka on käyttää sivukaistasiirtymiä, joiden avulla voidaan valita kytkentä eri akustisiin tiloihin HBAR-resonaattorissa. Kytkennän voimakkuuden määrittää sivukaista-ajon amplitudi. Tällä keinolla voidaan ajaa stimuloituja oskillaatioita kubitin ja valitun värähtelytilan välillä. Näitä tekniikoita käytetään akustisten ylivärähtelykvanttien hallintaan, mikä on välttämätöntä, jotta mekaanisia resonaattoreita voidaan käyttää kvanttiresursseina

Interfacing planar superconducting qubits with high overtone bulk acoustic phonons

Mechanical resonators are a promising way for interfacing qubits in order to realize hybrid quantum systems that offer great possibilities for applications. Mechanical systems can have very long energy lifetimes, and they can be further interfaced to other systems. Moreover, integration of a mechanical oscillator with qubits creates a potential platform for the exploration of quantum physics in macroscopic mechanical degrees of freedom. The utilization of high overtone bulk acoustic resonators coupled to superconducting qubits is an intriguing platform towards these goals. These resonators exhibit a combination of high-frequency and high-quality factors. They can reach their quantum ground state at dilution refrigeration temperatures and they can be strongly coupled to superconducting qubits via their piezoelectric effect. In this paper, we demonstrate our system where bulk acoustic phonons of a high overtone resonator are coupled to a transmon qubit in a planar circuit architecture. We show that the bulk acoustic phonons are interacting with the qubit in a simple design architecture at the quantum level, representing further progress towards the quantum control of mechanical motion.Peer reviewe

Multiphonon Transitions in a Quantum Electromechanical System

| openaire: EC/H2020/732894/EU//HOT Funding Information: We acknowledge the facilities and technical support of Otaniemi research infrastructure for Micro and Nanotechnologies (OtaNano) that is part of the European Microkelvin Platform. This work is supported by the Academy of Finland (Contracts No. 307757 and No. 312057), by the European Research Council (615755-CAVITYQPD), by the Wihuri Foundation, and by the Aalto Centre for Quantum Engineering. The work is performed as part of the Academy of Finland Centre of Excellence program (Project No. 336810). We acknowledge funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 732894 (FETPRO HOT). Publisher Copyright: © 2022 American Physical Society.Studies of micromechanical and acoustic modes in the quantum regime have shed light on quantum properties of massive objects. Integrating these systems into superconducting circuits shows great promise for applications as quantum memory elements, bosonic codes, or in frequency conversion. To this end, investigation of nonclassical properties of acoustic degrees of freedom is critical also for applications. Here, we investigate a strongly driven system consisting of a transmon qubit interacting with a high-overtone bulk acoustic resonator. We observe multiphonon transitions, which enable mapping the energy landscape in the coupled system. At a high driving amplitude comparable to the qubit-oscillator coupling, we observe a shift of the multiphonon spectral lines, reminiscent of Stark shift, which is explained using a simple model. Our work thus also provides insight in multiquanta transitions in other qubit-oscillator systems, not limited to acoustics or circuit quantum electrodynamics.Peer reviewe

Landau-Zener-Stückelberg Interference in a Multimode Electromechanical System in the Quantum Regime

| openaire: EC/H2020/732894/EU//HOTThe studies of mechanical resonators in the quantum regime not only provide insight into the fundamental nature of quantum mechanics of massive objects, but also introduce promising platforms for novel hybrid quantum technologies. Here we demonstrate a configurable interaction between a superconducting qubit and many acoustic modes in the quantum regime. Specifically, we show how consecutive Landau-Zener-Stückelberg (LZS) tunneling type of transitions, which take place when a system is tuned through an avoided crossing of the coupled energy levels, interfere in a multimode system. The work progresses experimental LZS interference to cover a new class of systems where the coupled levels are those of a quantum two-level system interacting with a multitude of mechanical oscillators. The work opens up applications in controlling multiple acoustic modes via parametric modulation.Peer reviewe

Sideband Control of a Multimode Quantum Bulk Acoustic System

| openaire: EC/H2020/615755/EU//CAVITYQPD | openaire: EC/H2020/732894/EU//HOTMultimode bulk acoustic systems show promise for use in superconducting quantum computation. They can serve as a medium-term memory storage, with exceptional coherence times being demonstrated, and they exhibit a mode density that is physically highly compact. Here, we experimentally demonstrate accessing individual acoustic modes without being hindered by the uniform frequency spacing of the modes. We use sideband control where a low-frequency modulation is applied to the transmon qubit energy. The amplitude of the modulation defines the qubit-acoustic mode coupling and its frequency detunes the acoustic sidebands, therefore selectively enabling the switching on or off of the interaction, and it allows for full control of the individual modes.Peer reviewe