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Maximising the power of digital quantum simulators requires careful optimisation of experimental resources. We conclusively demonstrate a direct connection between quantum chaos and the breakdown of Trotterisation, exhibiting surprisingly universal behaviour across diverse simulator systems [arXiv:2110.11113 (2021)].

Non-equilibrium quasiparticles can deteriorate the performance of superconducting qubits by reducing their coherence. We are investigating a source of quasiparticles that has been too long neglected, namely radioactivity: cosmic rays, environmental radioactivity and contaminants in the materials can all generate phonons of energy sufficient to break Cooper pairs and thus increase the number of quasiparticles. In this contribution, we describe the status of the project and its perspectives.

High kinetic inductance materials constitute a valuable resource for superconducting quantum circuits and hybrid architectures. Superconducting granular aluminum (grAl) reaches kinetic sheet inductances in the nH/$\square$ range, with proven applicability in superconducting quantum bits and microwave detectors. Here we show that the single photon internal quality factor $Q_{\mathrm{i}}$ of grAl microwave resonators exceeds $10^5$ in magnetic fields up to 1T, aligned in-plane to the grAl films. Small perpendicular magnetic fields, in the range of 0.5mT, enhance $Q_{\mathrm{i}}$ by approximately 15%, possibly due to the introduction of quasiparticle traps in the form of fluxons. Further increasing the perpendicular field deteriorates the resonators' quality factor. These results open the door for the use of high kinetic inductance grAl structures in circuit quantum electrodynamics and hybrid architectures with magnetic field requirements., Comment: Resolved typos and added data availability statement

Determining the state of a qubit on a timescale much shorter than its relaxation time is an essential requirement for quantum information processing. With the aid of a new type of non-degenerate parametric amplifier, we demonstrate the continuous detection of quantum jumps of a transmon qubit with 90% fidelity in state discrimination. Entirely fabricated with standard two-step optical lithography techniques, this type of parametric amplifier consists of a dispersion engineered Josephson junction (JJ) array. By using long arrays, containing $10^3$ JJs, we can obtain amplification at multiple eigenmodes with frequencies below $10~\mathrm{GHz}$, which is the typical range for qubit readout. Moreover, by introducing a moderate flux tunability of each mode, employing superconducting quantum interference device (SQUID) junctions, a single amplifier device could potentially cover the entire frequency band between 1 and $10~\mathrm{GHz}$., P.W. and I.T. contributed equally. 9 pages, 5 figures and appendices

Microwave kinetic inductance detectors (MKIDs) are thin-film, cryogenic, superconducting resonators. Incident Cooper pair-breaking radiation increases their kinetic inductance, thereby measurably lowering their resonant frequency. For a given resonant frequency, the highest MKID responsivity is obtained by maximizing the kinetic inductance fraction α. However, in circuits with α close to unity, the low supercurrent density reduces the maximum number of readout photons before bifurcation due to self-Kerr nonlinearity, therefore setting a bound for the maximum α before the noise-equivalent power (NEP) starts to increase. By fabricating granular aluminum MKIDs with different resistivities, we effectively sweep their kinetic inductance from tens to several hundreds of pH per square. We find a NEP minimum in the range of 30aW/√Hz at α≈0.9, which results from a trade-off between the onset of nonlinearity and a nonmonotonic dependence of the noise spectral density versus resistivity.

Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits and one that is particularly detrimental in devices with high kinetic inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.

Submitted by Daniele Fonseca (daniele@ct.ufrj.br) on 2020-05-07T12:33:29Z No. of bitstreams: 1 CesarCunhaPacheco.pdf: 14280316 bytes, checksum: 45820be2edc284194ff73adca8d8b360 (MD5) Made available in DSpace on 2020-05-07T12:33:29Z (GMT). No. of bitstreams: 1 CesarCunhaPacheco.pdf: 14280316 bytes, checksum: 45820be2edc284194ff73adca8d8b360 (MD5) Previous issue date: 2018-03 O tratamento de tumores através de hipertermia tem apresentado, ao longo dos anos, resultados promissores. Porém, o sucesso desta abordagem está profundamente associado à medição adequada da temperatura no interior do tecido biológico em tratamento. O avanço das técnicas de medição de temperatura via ressonância magnética representa uma alternativa para profissionais da área de saúde. Entretanto, algumas limitações ainda podem ser encontradas, como a dificuldade de se medir a temperatura absoluta no tecido — ficando o operador sujeito às incertezas presentes em importantes parâmetros fisiológicos, provenientes de diversos fatores, incluindo diferenças intrínsecas entre pacientes. Este trabalho busca desenvolver um algoritmo computacional capaz de, ao obter as medições do equipamento de ressonância magnética, calcular a temperatura no interior do tecido, através da atuação conjunta de modelos matemáticos de transferência de calor e de técnicas de quantificação de incertezas. De modo geral, espera-se que este algoritmo ajude profissionais da área de saúde a obter medições de temperatura mais confiáveis com maior facilidade, permitindo que o espectro de possibilidades do tratamento por hipertermia seja ampliado. Promising advances have been achieved in the field of hyperthermia treatment for tumors in the recent years; however, a successful treatment in this particular area is profoundly associated with reliable techniques for quantifying the temperature distribution in the biological tissue being treated. The advancement in thermometry techniques using magnetic resonance presents itself as an alternative for health professionals to perform this task; however, some limitations are still present, such as the difficulty in measuring absolute temperature in tissue — limiting the physician to measure only the local temperature rise; and the uncertainties in important physiological parameters, from different factors, including intrinsic differences between patients. This work seeks to develop a computer algorithm capable of, while obtaining measurements from the magnetic resonance device, calculating the temperature within the tissue through the joint application of mathematical models for heat conduction and uncertainty quantification techniques. It is expected that this algorithm help health professionals to obtain more reliable information, allowing the broadening of the range of possibilities for hyperthermia treatment.