Calibration of Drive Non-Linearity for Arbitrary-Angle Single-Qubit Gates Using Error Amplification

The ability to execute high-fidelity operations is crucial to scaling up quantum devices to large numbers of qubits. However, signal distortions originating from non-linear components in the control lines can limit the performance of single-qubit gates. In this work, we use a measurement based on error amplification to characterize and correct the small single-qubit rotation errors originating from the non-linear scaling of the qubit drive rate with the amplitude of the programmed pulse. With our hardware, and for a 15-ns pulse, the rotation angles deviate by up to several degrees from a linear model. Using purity benchmarking, we find that control errors reach $2\times 10^{-4}$, which accounts for half of the total gate error. Using cross-entropy benchmarking, we demonstrate arbitrary-angle single-qubit gates with coherence-limited errors of $2\times 10^{-4}$ and leakage below $6\times 10^{-5}$. While the exact magnitude of these errors is specific to our setup, the presented method is applicable to any source of non-linearity. Our work shows that the non-linearity of qubit drive line components imposes a limit on the fidelity of single-qubit gates, independent of improvements in coherence times, circuit design, or leakage mitigation when not corrected for

Trajectoires Quantiques avec Canaux de Décohérence Incompatibles

In contrast with its classical version, a quantum measurement necessarily disturbs thestate of the system. The projective measurement of a spin-1/2 in one direction maximallyrandomizes the outcome of a following measurement along a perpendicular direction.In this thesis, we discuss experiments on superconducting circuits that allow us toinvestigate this measurement back-action. In particular, we measure the dynamics ofa superconducting qubit whose three Bloch x, y and z components are simultaneouslyrecorded.Two recent techniques are used to make these simultaneous recordings. The x andy components are obtained by measuring the two quadratures of the fluorescence fieldemitted by the qubit. Conversely, the z component is accessed by probing an offresonantcavity dispersively coupled to the qubit. The frequency of the cavity dependson the energy of the qubit and the strength of this last measurement can be tunedfrom weak to strong in situ by varying the power of the probe. These observations areenabled by recent advances in ultra-low noise microwave amplification using Josephsoncircuits. This thesis details all these techniques, both theoretically and experimentally,and presents various unpublished additional results.In the presence of the simultaneous measurements, we show that the state of thesystem diffuses inside the sphere of Bloch by following a random walk whose steps obeythe laws of the backaction of incompatible measurements. The associated quantumtrajectories follow a variety of dynamics ranging from diffusion to Zeno blockade. Theirpeculiar dynamics highlights the non-trivial interplay between the back-action of thetwo incompatible measurements. By conditioning the records to the outcome of a finalprojective measurement, we also measure the weak values of the components of thequbit state and demonstrate that they exceed the mean extremal values. The thesisdiscusses in detail the statistics of the obtained trajectories.Au contraire de sa version classique, une mesure quantique perturbe nécessairementl’état du système. Ainsi, la mesure projective d’un spin-1/2 selon une direction rendparfaitement aléatoire le résultat d’une mesure successive de la composante du mêmespin le long d’un axe orthogonal. Dans cette thèse, nous discutons des expériencesbasées sur les circuits supraconducteurs qui permettent de mettre en évidence cetteaction en retour de la mesure. Nous mesurons en particulier la dynamique d’un qubitsupraconducteur dont on révèle simultanément les trois composantes de Bloch x, y etz.Deux techniques récentes sont utilisées pour réaliser ces enregistrements simultanés.Les composantes x et y sont obtenues par la mesure des deux quadratures du champ defluorescence émis par le qubit. La composante z est quant à elle obtenue en sondant unecavité non résonante couplée de manière dispersive au qubit. La fréquence de la cavitédépend de l’énergie du qubit et la force de cette dernière mesure peut être ajustée in situen faisant varier la puissance de la sonde. Ces observations sont rendues possibles grâceaux avancées récentes dans l’amplification ultrabas bruit des signaux micro-onde grâceaux circuits Josephson. Cette thèse détaille toutes ces techniques à la fois théoriquementet expérimentalement et présente différents résultats annexes inédits.En présence des mesures simultanées, nous montrons que l’état du système diffuse àl’intérieur de la sphère de Bloch en suivant une marche aléatoire dont les pas obéissentaux lois de l’action en retour de mesures incompatibles. Les trajectoires quantiquesassociées ont des dynamiques allant du régime diffusif au régime de blocage de Zénonsoulignant l’interaction non-triviale des actions en retours des deux mesures incompatibleseffectuées. En conditionnant les enregistrements aux résultats d’une mesureprojective finale, nous mesurons également les valeurs faibles des composantes de notrequbit et démontrons qu’elles dépassent les valeurs extrémales moyennes. La thèse discuteen détail de la statistique des trajectoires obtenues

Quantum adiabatic elimination at arbitrary order for photon number measurement

International audienceAdiabatic elimination is a perturbative model reduction technique based on timescale separation and often used to simplify the description of composite quantum systems. We here analyze a quantum experiment where the perturbative expansion can be carried out to arbitrary order, such that: (i) we can formulate in the end an exact reduced model in quantum form; (ii) as the series provides accuracy for ever larger parameter values, we can discard any condition on the timescale separation, thereby analyzing the intermediate regime where the actual experiment is performing best; (iii) we can clarify the role of some gauge degrees of freedom in this model reduction technique

Supplementary Videos for "Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing"

<div>Quantum trajectories of a transmon qubit while simultaneously monitoring its energy relaxation and dephasing decoherence channels.</div><div><br></div>The experiment was carried out for 30 experimental configurations and each time with 3 detector configurations corresponding to one or two of the abovementioned measurement channels

Topological power pumping in quantum circuits

In this article, we develop a description of topological pumps as slow classical dynamical variables coupled by a quantum system. We discuss the cases of quantum Hall pumps, Thouless pumps, and the more recent Floquet pumps based frequency converters. This last case corresponds to a quantum topological coupling between classical modes described by action-angle variables on which we focus. We propose a realization of such a topological coupler based on a superconducting qutrit suitably driven by three modulated drives. A detailed experimental protocol allowing to measure the quantized topological power transfer between the different modes of a superconducting circuit is discussed

Topological power pumping in quantum circuits

In this article, we develop a description of topological pumps as slow classical dynamical variables coupled by a quantum system. We discuss the cases of quantum Hall pumps, Thouless pumps, and the more recent Floquet pumps based frequency converters. This last case corresponds to a quantum topological coupling between classical modes described by action-angle variables on which we focus. We propose a realization of such a topological coupler based on a superconducting qutrit suitably driven by three modulated drives. A detailed experimental protocol allowing to measure the quantized topological power transfer between the different modes of a superconducting circuit is discussed

Topological power pumping in quantum circuits

In this article, we develop a description of topological pumps as slow classical dynamical variables coupled by a quantum system. We discuss the cases of quantum Hall pumps, Thouless pumps, and the more recent Floquet pumps based frequency converters. This last case corresponds to a quantum topological coupling between classical modes described by action-angle variables on which we focus. We propose a realization of such a topological coupler based on a superconducting qutrit suitably driven by three modulated drives. A detailed experimental protocol allowing to measure the quantized topological power transfer between the different modes of a superconducting circuit is discussed