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Superconducting qubits are considered as a promising platform for implementing a fault tolerant quantum computing. However, surface defects of superconductors and the substrate leading to qubit state decoherence and fluctuations in qubit parameters constitute a significant problem. The amount and type of defects depend both on the chip materials and fabrication procedure. In this work, transmons produced by two different methods of aluminum etching: wet etching in a solution of weak acids and dry etching using a chlorine-based plasma are experimentally studied. The relaxation and coherence times for dry-etched qubits are more than twice as long as those for wet-etched ones. Additionally, the analysis of time fluctuations of qubit frequencies and relaxation times, which is an effective method to identify the dominant dielectric loss mechanisms, indicates a significantly lower impact of two-level systems in the dry-etched qubits compared to the wet-etched ones., Comment: 7 pages, 6 figures

Building a scalable universal high-performance quantum processor is a formidable challenge. In particular, the problem of realizing fast high-perfomance two-qubit gates of high-fidelity remains needful. Here we propose a unitary cell for a scalable quantum processor consisting of two transmons and a tunable three-mode coupler allowing for a ZZ interaction control. We experimentally demonstrate the native CZ gate with the pulse duration of 60~ns achieving the two-qubit gate fidelity above 98%, limited mostly by qubit coherence time. Numerical simulations show that by optimizing the gate duration the fidelity can be pushed over 99.97%., Comment: 13 pages, 9 figures, 3 tables

Granular Aluminum is a superconductor known for more than eighty years, which recently found its application in qubits, microwave detectors and compact resonators, due to its high kinetic inductance, critical magnetic field and critical current. Here we report on the nonlinear dependence of granular Aluminum inductance on current, which hints towards parametric amplification of the microwave signal in granular Aluminum films. The phase shift of the microwave signal reached 4 radians at a frequency of 7 GHz, which makes it possible to estimate the nonlinearity of the system as 1.4 % and the potential gain of the order of 17 dB, Comment: 5 pages, 3 figures

Various techniques are available in order to obtain information on samples of a different nature in near-field scanning microwave microscopy (NSMM), with transmission-line resonator (TLR) techniques considered as the most advanced in terms of sensitivity and resolution. In this paper, we focus on development of a TLR-based NSMM supplied by a new source of useful signal: coplanar waveguide ground potentials imbalance. Electromagnetic modeling of the device and experimental scanning of two planar structures are conducted to examine the performance of the proposed technique. Both modeling and experimental results demonstrate the ability to distinguish symmetric inhomogeneity positions with respect to the central conductor of the coplanar waveguide by the phase of imbalance signal. The thin-film structure scanning procedure displays the possibility of proposed approach to resolve low-contrast structures with accuracy up to 98.7 %., Comment: 7 pages, 4 figures

Scalable quantum computers hold the promise to solve hard computational problems, such as prime factorization, combinatorial optimization, simulation of many-body physics, and quantum chemistry. While being key to understanding many real-world phenomena, simulation of non-conservative quantum dynamics presents a challenge for unitary quantum computation. In this work, we focus on simulating non-unitary parity-time symmetric systems, which exhibit a distinctive symmetry-breaking phase transition as well as other unique features that have no counterpart in closed systems. We show that a qutrit, a three-level quantum system, is capable of realizing this non-equilibrium phase transition. By using two physical platforms -- an array of trapped ions and a superconducting transmon -- and by controlling their three energy levels in a digital manner, we experimentally simulate the parity-time symmetry-breaking phase transition. Our results indicate the potential advantage of multi-level (qudit) processors in simulating physical effects, where additional accessible levels can play the role of a controlled environment., Comment: 20 pages, 10 figures

We have fabricated an array of subgap kinetic inductance detectors (SKIDs) made of granular aluminum ($T_c\sim$2~K) sensitive in the 80-90 GHz frequency band and operating at 300~mK. We measure a noise equivalent power of $1.3\times10^{-16}$~W/Hz$^{0.5}$ on average and $2.6\times10^{-17}$~W/Hz$^{0.5}$ at best, for an illuminating power of 50~fW per pixel. Even though the circuit design of SKIDs is identical to that of the kinetic inductance detectors (KIDs), the SKIDs operating principle is based on their sensitivity to subgap excitations. This detection scheme is advantageous because it avoids having to lower the operating temperature proportionally to the lowest detectable frequency. The SKIDs presented here are intrinsically selecting the 80-90 GHz frequency band, well below the superconducting spectral gap of the film, at approximately 180 GHz., Comment: Suggestions and comments are welcome

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 $10~\mathrm{GHz}$., Comment: P.W. and I.T. contributed equally. 9 pages, 5 figures and appendices

We report on a detailed study of the optical response and $T_c-\rho$ phase diagram ($T_c$ being the superconducting critical temperature and $\rho$ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The $T_c-\rho$ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion and the superconducting gap $\Delta$. We provide a direct evidence for two different types of well resolved sub-gap absorptions, at $\omega_1\simeq\Delta$ and at $\Delta\lesssim\omega_2\lesssim2\Delta$ (decreasing with increasing resistivity).

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 $\alpha$. However, in circuits with $\alpha$ close to unity, the low supercurrent density reduces the maximum number of readout photons before bifurcation due to self-Kerr non-linearity, therefore setting a bound for the maximum $\alpha$ 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 $25\; \text{aW}/\sqrt{\text{Hz}}$ at $\alpha \approx 0.9$, which results from a tradeoff between the onset of non-linearity and a non-monotonic dependence of the noise spectral density vs. resistivity.

Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, of about 100 Ohm, and small anharmonicity, which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum $R_Q = 6.4$ k$\Omega$. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here we present a fluxonium qubit design using a granular aluminum (grAl) superinductor strip. Granular aluminum is a particularly attractive material, as it self-assembles into an effective junction array with a remarkably high kinetic inductance, and its fabrication can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time $T_2^R$ up to 30 $\mu$s illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors., Comment: L.G. and M.S. contributed equally. 4 pages, 3 figures, and supplemental material

The introduction of crystalline defects or dopants can give rise to so-called "dirty superconductors", characterized by reduced coherence length and quasiparticle mean free path. In particular, granular superconductors such as Granular Aluminum (GrAl), consisting of remarkably uniform grains connected by Josephson contacts have attracted interest since the sixties thanks to their rich phase diagram and practical advantages, like increased critical temperature, critical field, and kinetic inductance. Here we report the measurement and modeling of circuit quantum electrodynamics properties of GrAl microwave resonators in a wide frequency range, up to the spectral superconducting gap. Interestingly, we observe self-Kerr coefficients ranging from $10^{-2}$ Hz to $10^5$ Hz, within an order of magnitude from analytic calculations based on GrAl microstructure. This amenable nonlinearity, combined with the relatively high quality factors in the $10^5$ range, open new avenues for applications in quantum information processing and kinetic inductance detectors., Comment: 7 pages, 4 figures, supplementary information

Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (GrAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH$/\Box$ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from GrAl with a room temperature resistivity of $4 \times 10^3\,\mu\Omega\cdot$cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of $Q_{\mathrm{i}} = 10^5$ in the single photon regime, and we demonstrate that non-equilibrium quasiparticles (QP) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every $\sim 20$ s. The current level of coherence of GrAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of non-equilibrium QPs., Comment: 5 pages, 4 figures, supplementary material

We present the design of an inductively shunted transmon qubit with flux-tunable coupling to an embedded harmonic mode. This circuit construction offers the possibility to flux-choose between pure transverse and pure longitudinal coupling, that is coupling to the $\sigma_x$ or $\sigma_z$ degree of freedom of the qubit. While transverse coupling is the coupling type that is most commonly used for superconducting qubits, the inherently different longitudinal coupling has some remarkable advantages both for readout and for the scalability of a circuit. Being able to choose between both kinds of coupling in the same circuit provides the flexibility to use one for coupling to the next qubit and one for readout, or vice versa. We provide a detailed analysis of the system's behavior using realistic parameters, along with a proposal for the physical implementation of a prototype device., Comment: 14 pages, 14 figures

We present an argon ion beam milling process to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of 50$\,\mathrm{m}\Omega \cdot \mu \mathrm{m}^2$ at a frequency of 4.5 GHz. Resonators for which only $6\%$ of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic field, showed quality factors above $10^6$ in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing., Comment: 4 pages, 4 figures, supplementary material