I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
27
Jul
2022
Non-Hermitian topological phases in traveling-wave parametric amplifiers
We study the phenomena of topological amplification in one-dimensional traveling-wave parametric amplifiers. We find two phases of topological amplification, both with directional transport
and exponential gain with the number of sites, and one of them featuring squeezing. We also find a topologically trivial phase with zero-energy modes which produces amplification but lacks topological protection. We characterize the resilience to disorder of the different phases, their stability, gain and noise-to-signal ratio. Finally, we discuss their experimental implementation with state-of-the-art techniques.
Directional Josephson traveling-wave parametric amplifier via non-Hermitian topology
Low-noise microwave amplification is crucial for detecting weak signals in quantum technologies and radio astronomy. An ideal device must amplify a broad range of frequencies while
adding minimal noise, and be directional, so that it favors the observer’s direction while protecting the source from its environment. Current amplifiers do not satisfy all these requirements, severely limiting the scalability of superconducting quantum devices. Here, we demonstrate the feasibility of building a near-ideal quantum amplifier using a homogeneous Josephson junction array and the non-trivial topology of its dynamics. Our design relies on breaking time-reversal symmetry via a non-local parametric drive, which induces directional amplification in a way similar to edge states in topological insulators. The system then acquires unprecedented amplifying properties, such as a gain growing exponentially with system size, exponential suppression of back-wards noise, and topological protection against disorder. We show that these features allow a state-of-the-art superconducting device to manifest near-quantum-limited directional amplification with a gain largely surpassing 20 dB and -30 dB of reverse attenuation over a large bandwidth of GHz. This opens the door for integrating near-ideal and compact pre-amplifiers on the same chip as quantum processors.
26
Jul
2022
Mitigation of quasiparticle loss in superconducting qubits by phonon scattering
Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently
uncorrelated in space and time. In superconducting qubits this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high energy phonons in the substrate. These phonons can break Cooper-pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential mitigation technique is to place large volumes of normal or superconducting metal on the device, capable of reducing the phonon energy to below the superconducting gap of the qubits. To investigate the effectiveness of this method we fabricate a quantum device with four nominally identical nanowire-based transmon qubits. On the device, half of the niobium-titanium-nitride ground plane is replaced with aluminum (Al), which has a significantly lower superconducting gap. We deterministically inject high energy phonons into the substrate by voltage biasing a galvanically isolated Josephson junction. In the presence of the low gap material, we find a factor of 2-5 less degradation in the injection-dependent qubit lifetimes, and observe that undesired excited qubit state population is mitigated by a similar factor. We furthermore turn the Al normal with a magnetic field, finding no change in the phonon-protection. This suggests that the efficacy of the protection in our device is not limited by the size of the superconducting gap in the Al ground plane. Our results provide a promising foundation for protecting superconducting qubit processors against correlated errors from ionizing radiation.
Ultra low noise readout with travelling wave parametric amplifiers: the DARTWARS project
The DARTWARS project has the goal of developing high-performing innovative travelling wave parametric amplifiers with high gain, large bandwidth, high saturation power, and nearly quantum-limited
noise. The target frequency region for its applications is 5 – 10 GHz, with an expected noise temperature of about 600 mK. The development follows two different approaches, one based on Josephson junctions and one based on kinetic inductance of superconductors. This contribution mainly focuses on the Josephson travelling wave parametric amplifier, presenting its design, preliminary measurements and the test of homogeneity of arrays of Josephson junctions.
Electromagnetic Simulation and Microwave Circuit Approach of Heat Transport in Superconducting Qubits
The study of quantum heat transport in superconducting circuits is significant for further understanding the connection between quantum mechanics and thermodynamics, and for possible
applications for quantum information. The first experimental realisations of devices demonstrating photonic heat transport mediated by a qubit have already been designed and measured. Motivated by the analysis of such experimental results, and for future experimental designs, we numerically evaluate the photonic heat transport of qubit-resonator devices in the linear circuit regime through electromagnetic simulations using Sonnet software, and compare with microwave circuit theory. We show that the method is a powerful tool to calculate heat transport and predict unwanted parasitic resonances and background.
High quality superconducting Nb co-planar resonators on sapphire substrate
We present measurements and simulations of superconducting Nb co-planar waveguide resonators on sapphire substrate down to millikelvin temperature range with different readout powers.
In the high temperature regime, we demonstrate that the Nb film residual surface resistance is comparable to that observed in the ultra-high quality, bulk Nb 3D superconducting radio frequency cavities while the resonator quality is dominated by the BCS thermally excited quasiparticles. At low temperature both the resonator quality factor and frequency can be well explained using the two-level system models. Through the energy participation ratio simulations, we find that the two-level system loss tangent is ∼10−2, which agrees quite well with similar studies performed on the Nb 3D cavities.
Characterization of Nb films for superconducting qubits using phase boundary measurements
Continued advances in superconducting qubit performance require more detailed understandings of the many sources of decoherence. Within these devices, two-level systems arise due to
defects, interfaces, and grain boundaries, and are thought to be a major source of qubit decoherence at millikelvin temperatures. In addition to Al, Nb is a commonly used metalization layer for superconducting qubits. Consequently, a significant effort is required to develop and qualify processes that mitigate defects in Nb films. As the fabrication of complete superconducting qubits and their characterization at millikelvin temperatures is a time and resource intensive process, it is desirable to have measurement tools that can rapidly characterize the properties of films and evaluate different treatments. Here we show that measurements of the variation of the superconducting critical temperature Tc with an applied external magnetic field H (of the phase boundary Tc−H) performed with very high resolution show features that are directly correlated with the structure of the Nb films. In combination with x-ray diffraction measurements, we show that one can even distinguish variations quality and crystal orientation of the grains in a Nb film by small but reproducible changes in the measured superconducting phase boundary.
25
Jul
2022
Stress-induced omega phase transition in Nb thin films for superconducting qubits
We report the observation of omega phase formation in Nb thin films deposited by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits using transmission electron
microscopy (TEM). We hypothesize that this phase transformation to the omega phase with hexagonal structure from bcc phase as well as the formation of {111}<112> mechanical twins is induced by internal stress in the Nb thin films. In terms of lateral dimensions, the size of the omega phase of Nb range from 10 to 100 nm, which is comparable to the coherence length of Nb (~40 nm). In terms of overall volume fraction, ~1 vol.% of the Nb grains exhibit this omega phase. We also find that the omega phase in Nb is not observed in large grain Nb samples, suggesting that the phase transition can be suppressed through reducing the grain boundary density, which may serve as a source of strain and dislocations in this system. The current finding may indicate that the Nb thin film is prone to the omega phase transition due to the internal stress in the Nb thin film. We conclude by discussing effects of the omega phase on the superconducting properties of Nb thin films and discussing pathways to mitigate their formation.
23
Jul
2022
Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits
Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport,
magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films show outstanding superconducting transition temperature of Tc=9.35 K and a fairly clean superconducting gap, along with superfluid density enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat conductance leading to thermo-magnetic instabilities. These may present an issue for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.
22
Jul
2022
Measurement driven quantum clock implemented with a superconducting qubit
We demonstrate a quantum clock, near zero temperature, driven in part by entropy reduction through measurement, and necessarily subject to quantum noise. The experimental setup is a
superconducting transmon qubit dispersively coupled to an open co-planar resonator. The cavity and qubit are driven by coherent fields and the cavity output is monitored with a quantum noise-limited amplifier. When the continuous measurement is weak, it induces sustained coherent oscillations (with fluctuating period) in the conditional moments. Strong continuous measurement leads to an incoherent cycle of quantum jumps. Both regimes constitute a clock with a signal extracted from the observed measurement current. This signal is analysed to demonstrate the relation between clock period noise and dissipated power for measurement driven quantum clocks. We show that a good clock requires high rates of energy dissipation and entropy generation.