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/□ 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 4×10^{3}  μΩ 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_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼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 nonequilibrium QPs.

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 RQ = 6.4 kΩ. 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 R 2 up to 30 µs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors. arXiv:1809.10646v1 [cond-mat.supr-con]

Granular aluminum (grAl) is a promising high kinetic inductance material for detectors, amplifiers, and qubits. Here we model the grAl structure, consisting of pure aluminum grains separated by thin aluminum oxide barriers, as a network of Josephson junctions, and we calculate the dispersion relation and nonlinearity (self-Kerr and cross-Kerr coefficients). To experimentally study the electrodynamics of grAl thin films, we measure microwave resonators with open-boundary conditions and test the theoretical predictions in two limits. For low frequencies, we use standard microwave reflection measurements in a low-loss environment. The measured low-frequency modes are in agreement with our dispersion relation model, and we observe self-Kerr coefficients within an order of magnitude from our calculation starting from the grAl microstructure. Using a high-frequency setup, we measure the plasma frequency of the film around 70 GHz, in agreement with the analytical prediction.

As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor thirty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.

We present a planar qubit design based on a superconducting circuit that we call concentric transmon. While employing a straightforward fabrication process using Al evaporation and lift-off lithography, we observe qubit lifetimes and coherence times in the order of 10 µs. We systematically characterize loss channels such as incoherent dielectric loss, Purcell decay and radiative losses. The implementation of a gradiometric SQUID loop allows for a fast tuning of the qubit transition frequency and therefore for full tomographic control of the quantum circuit. Due to the large loop size, the presented qubit architecture features a strongly increased magnetic dipole moment as compared to conventional transmon designs. This renders the concentric transmon a promising candidate to establish a site-selective passive directẐ coupling between neighboring qubits, being a pending quest in the field of quantum simulation.Quantum bits based on superconducting circuits are leading candidates for constituting the basic building block of a prospective quantum computer. A common element of all superconducting qubits is the Josephson junction. The nonlinearity of Josephson junctions generates an anharmonic energy spectrum in which the two lowest energy states can be used as the computational basis 1,2 . Over the last decade there has been a two order of magnitude increase in coherence times of superconducting qubits. This tremendous improvement allowed for demonstration of several major milestones in the pursuit of scalable quantum computing, such as the control and entanglement of multiple qubits 3,4 . Further increases in coherence times will eventually allow for building a fault tolerant quantum computer with a reasonable overhead in terms of error correction, as well as implementing novel quantum simulation schemes by accessing wider experimental parameter ranges 5 . While superconducting qubits embedded in a 3D cavity 6 have shown coherence times in excess of 100 µs 7 , this approach may impose some constraint on the scalability of quantum circuits. Since the Josephson junction itself does not limit qubit coherence 6 , comparably long lifetimes can also be achieved in a planar geometry by careful circuit engineering.In this paper, we present the design and characterization of a superconducting quantum circuit comprising a concentric transmon qubit 8 , schematically depicted in Fig. 1(a). The two capacitor pads forming the transmon's a) Electronic mail: jochen.braumueller@kit.edu. large shunt capacitance are implemented by a central disk island and a concentrically surrounding ring. The two islands are interconnected by two Josephson junctions forming a gradiometric SQUID. A 50 Ω impedance matched on-chip flux bias line located next to the qubit allows for fast flux tuning of the qubit frequency due to the imposed asymmetry. This guarantees high experimental flexibility and enables full tomographic control. The gradiometric flux loop design reduces the sensitivity to external uniform magnetic fields and thus to external flux ...

Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are 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.Superconducting circuits play a central role in a variety of research and application areas, such as solid state quantum optics 1 , metrology 2,3 , and low temperature detectors 4,5 . In particular, the field of superconducting qubits has grown impressively during the last decade 6,7 . In these devices quantum states can live for up to tens of microseconds, while gate times can be as short as tens of nanoseconds [8][9][10][11] . Nevertheless, coherence times need to be further improved by orders of magnitude in order to be able to perform quantum error correction 12,13 with an affordable hardware overhead.One of the main sources of decoherence in superconducting devices at millikelvin temperatures are out of equilibrium quasiparticles (QPs) [14][15][16][17][18][19][20][21][22] , which can be viewed as broken Cooper pairs (CPs). Quasiparticles can be particularly damaging in high kinetic inductance circuits [23][24][25][26][27] , which are a promising avenue for protected qubits 28 and hybrid superconductingsemiconducting devices [29][30][31] . Proposed mechanisms for CP breaking include stray infrared radiation 32,33 , direct microwave drive 34,35 , and high energy phonons in the device substrate created by environmental or cosmic radioactivity [36][37][38] . The latter is particularly damaging because it gives rise to correlated QP bursts in multiple devices on the same chip 36,39 , possibly resulting in a) Both authors contributed equally b) Electronic

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 non-linearity, 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 25 aW/ √ Hz at α ≈ 0.9, which results from a trade-off between the onset of non-linearity and a non-monotonic dependence of the noise spectral density vs. resistivity.

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 nondegenerate 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 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 GHz.