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
15
Dez
2023
Plasma-enhanced atomic layer deposition of titanium nitride for superconducting devices
This study presents a comprehensive investigation into the exceptional superconducting attributes of titanium nitride (TiN) achieved through plasma-enhanced atomic layer deposition
(PEALD) on both planar and intricate three-dimensional (3D) structures. We introduced an additional substrate biasing cycle to densify the film and remove ligand residues, augmenting the properties while minimizing impurities. While reactive-sputtered TiN films exhibit high quality, our technique ensures superior uniformity by consistently maintaining a desired sheet resistance greater than 95 percent across a 6inch wafer, a critical aspect for fabricating extensive arrays of superconducting devices and optimizing wafer yield. Moreover, our films demonstrate exceptional similarity to conventional reactive-sputtered films, consistently reaching a critical temperature (Tc) of 4.35 K with a thickness of around 40 nm. This marks a notable achievement compared to previously reported ALD-based superconducting TiN. Using the same process as for planar films, we obtained Tc for aspect ratios (ARs) ranging from 2 to 40, observing a Tc of approximately 2 K for ARs between 2 and 10.5. We elucidate the mechanisms contributing to the limitations and degradation of superconducting properties over these aggressive 3D structures. Our results seamlessly align with both current and next-generation superconducting technologies, meeting stringent criteria for thin-film constraints, large-scale deposition, conformality, 3D integration schemes, and yield optimization.
Dynamical Casimir cooling in circuit QED systems
A transmission line coupled to an externally driven superconducting quantum interference device (SQUID) can exhibit the Dynamical Casimir Effect (DCE). Employing this setup, we quantize
the SQUID degrees of freedom and show that it gives rise to a three-body interaction Hamiltonian with the cavity modes. By considering only two interacting modes from the cavities we show that the device can function as an autonomous cooler where the SQUID can be used as a work source to cool down the cavity modes. Moreover, this setup allows for coupling to all modes existing inside the cavities, and we show that by adding two other extra modes to the interaction with the SQUID the cooling effect can be enhanced.
14
Dez
2023
SPulseGen: Succinct pulse generator architecture maximizing gate fidelity for superconducting quantum computers
This paper proposes a cost-effective architecture for an RF pulse generator for superconducting qubits. Most existing works use arbitrary waveform generators (AWGs) that require both
a large amount of high-bandwidth memories and high-performance analog circuits to achieve the highest gate fidelity with an optimized RF pulse waveform. The proposed pulse generator architecture significantly simplifies both the generator circuit and the waveform of the RF pulse to a cost-aware square pulses. This architecture eliminates the requirement for power- and cost-intensive AWG, a major obstacle in realizing scalable quantum computers. Additionally, this paper proposes a process to optimize pulse waveforms to maximize fidelity of gate operations for single and multiple qubits. Quantum dynamics simulation of transmon qubits, wherein the state of system evolves with time, demonstrates that our pulse generator can achieve practically the same gate fidelity as ideal RF pulses, while substantially reducing the performance requirements of memory and analog circuits.
13
Dez
2023
Toolbox for nonreciprocal dispersive models in circuit QED
We provide a systematic method for constructing effective dispersive Lindblad master equations to describe weakly-anharmonic superconducting circuits coupled by a generic dissipationless
nonreciprocal linear system, with effective coupling parameters and decay rates written in terms of the immittance parameters characterizing the coupler. This article extends the foundational work of Solgun et al. (2019) for linear reciprocal couplers described by an impedance response. Here, we expand the existing toolbox to incorporate nonreciprocal elements, account for direct stray coupling between immittance ports, circumvent potential singularities, and include dissipative interactions arising from interaction with a common bath. We illustrate the use of our results with a circuit of weakly-anharmonic Josephson junctions coupled to a multiport nonreciprocal environment and a dissipative port. The results obtained here can be used for the design of complex superconducting quantum processors with non-trivial routing of quantum information, as well as analog quantum simulators of condensed matter systems.
12
Dez
2023
Multiplexed control scheme for scalable quantum information processing with superconducting qubits
The advancement of scalable quantum information processing relies on the accurate and parallel manipulation of a vast number of qubits, potentially reaching into the millions. Superconducting
qubits, traditionally controlled through individual circuitry, currently face a formidable scalability challenge due to the excessive use of wires. This challenge is nearing a critical point where it might soon surpass the capacities of on-chip routing, I/O packaging, testing platforms, and economically feasible solutions. Here we introduce a multiplexed control scheme that efficiently utilizes shared control lines for operating multiple qubits and couplers. By integrating quantum hardware-software co-design, our approach utilizes advanced techniques like frequency multiplexing and individual tuning. This enables simultaneous and independent execution of single- and two-qubit gates with significantly simplified wiring. This scheme has the potential to diminish the number of control lines by one to two orders of magnitude in the near future, thereby substantially enhancing the scalability of superconducting quantum processors.
11
Dez
2023
Coherent control of a few-channel hole type gatemon qubit
Gatemon qubits are the electrically tunable cousins of superconducting transmon qubits. In this work, we demonstrate the full coherent control of a gatemon qubit based on hole carriers
in a Ge/Si core/shell nanowire, with the longest coherence times in group IV material gatemons to date. The key to these results is a high-quality Josephson junction obtained in a straightforward and reproducible annealing technique. We demonstrate that the transport through the narrow junctions is dominated by only two quantum channels, with transparencies up to unity. This novel qubit platform holds great promise for quantum information applications, not only because it incorporates technologically relevant materials, but also because it provides new opportunities, like an ultrastrong spin-orbit coupling in the few-channel regime of Josephson junctions.
Optimizing Resonator Frequency Stability in Flip-Chip Architectures: A Novel Experimental Design Approach
In multi-qubit superconducting systems utilizing flip-chip technology, achieving high accuracy in resonator frequencies is of paramount importance, particularly when multiple resonators
share a common Purcell filter with restricted bandwidth. Nevertheless, variations in inter-chip spacing can considerably influence these frequencies. To tackle this issue, we present and experimentally validate the effectiveness of a resonator design. In our design, we etch portions of the metal on the bottom chip that faces the resonator structure on the top chip. This enhanced design substantially improves frequency stability by a factor of over 3.5 compared to the non-optimized design, as evaluated by the root mean square error of a linear fitting of the observed frequency distribution, which is intended to be linear. This advancement is crucial for successful scale-up and achievement of high-fidelity quantum operations.
10
Dez
2023
Quasiparticle dynamics in a superconducting qubit irradiated by a localized infrared source
A known source of decoherence in superconducting qubits is the presence of broken Cooper pairs, or quasiparticles. These can be generated by high-energy radiation, either present in
the environment or purposefully introduced, as in the case of some hybrid quantum devices. Here, we systematically study the properties of a transmon qubit under illumination by focused infrared radiation with various powers, durations, and spatial locations. Despite the high energy of incident photons, our observations agree well with a model of low-energy quasiparticle dynamics dominated by trapping. This technique can be used for understanding and potentially mitigating the effects of high-energy radiation on superconducting circuits with a variety of geometries and materials.
08
Dez
2023
Lecture Notes on Quantum Electrical Circuits
During the last 30 years, stimulated by the quest to build superconducting quantum processors, a theory of quantum electrical circuits has emerged and this theory goes under the name
of circuit quantum electrodynamics or circuit-QED. The goal of the theory is to provide a quantum description of the most relevant degrees of freedom. The central objects to be derived and studied are the Lagrangian and the Hamiltonian governing these degrees of freedom. Central concepts in classical network theory such as impedance and scattering matrices can be used to obtain the Hamiltonian and Lagrangian description for the lossless (linear) part of the circuits. Methods of analysis, both classical and quantum, can also be developed for nonreciprocal circuits. These lecture notes aim at giving a pedagogical overview of this subject for theoretically-oriented Master or PhD students in physics and electrical engineering, as well as Master and PhD students who work on experimental superconducting quantum devices and wish to learn more theory.
07
Dez
2023
Universal readout error mitigation scheme characterized on superconducting qubits
Quantum technologies rely heavily on accurate control and reliable readout of quantum systems. Current experiments are limited by numerous sources of noise that can only be partially
captured by simple analytical models and additional characterization of the noise sources is required. We test the ability of readout error mitigation to correct realistic noise found in systems composed of quantum two-level objects (qubits). To probe the limit of such methods, we designed a universal readout error mitigation protocol based on quantum state tomography (QST), which estimates the density matrix of a quantum system, and quantum detector tomography (QDT), which characterizes the measurement procedure. By treating readout error mitigation in the context of state tomography the method becomes largely device-, architecture-, noise source-, and quantum state-independent. We implement this method on a superconducting qubit and benchmark the increase in reconstruction fidelity for QST. We characterize the performance of the method by varying important noise sources, such as suboptimal readout signal amplification, insufficient resonator photon population, off-resonant qubit drive, and effectively shortened T1 and T2 decay times. As a result, we identified noise sources for which readout error mitigation worked well, and observed decreases in readout infidelity by a factor of up to 30.