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

24
Jan
2023

# Conditional not displacement: fast multi-oscillator control with a single qubit

Bosonic encoding is an approach for quantum information processing, promising lower hardware overhead by encoding in the many levels of a harmonic oscillator. Scaling to multiple modes

requires them to be decoupled for independent control, yet strongly coupled for fast interaction. How to perform fast and efficient universal control on multiple modes remains an open problem. We develop a control method that enables fast multi-mode generation and control of bosonic qubits which are weakly coupled to a single ancilla qubit. The weak coupling allows for excellent independent control, despite the weak ancilla coupling our method allows for fast control. We demonstrate our control by using a superconducting transmon qubit coupled to a multi-mode superconducting cavity. We create both entangled and separate cat-states in different modes of a multi-mode cavity, showing the individual and coupled control of the modes. We show that the operation time is not limited by the inverse of the dispersive coupling rate, which is the typical timescale, and we exceed it in practice by almost 2 orders of magnitude. Our scheme allows for multi-mode bosonic codes as well as more efficient scaling of hardware.

19
Jan
2023

# Radiation-induced secondary emissions in solid-state devices as a possible contribution to quasiparticle poisoning of superconducting circuits

This report estimates the potential for secondary emission processes induced by ionizing radiation to result in the generation of quasiparticles in superconducting circuits. These estimates

are based on evaluation of data collected from a small superconducting detector and a fluorescence measurement of typical read-out circuit board materials. Specifically, we study cosmic ray muons interacting with substrate or mechanical support materials present within the vicinity of superconducting circuits. We evaluate the potential for secondary emission, such as scintillation and/or fluorescence, from these nearby materials to occur at sufficient energy (wavelength) and rate (photon flux) to ultimately lead to the breaking of superconducting Copper pairs (i.e., production of quasiparticles). This evaluation leads to a conclusion that material fluorescence in the vicinity of superconducting circuits is a potential contributor to undesirable elevated quasiparticle populations. A co-design approach evaluating superconducting circuit design and the material environment within the immediate vicinity of the circuit would prove beneficial for mitigating undesired environmentally-induced influences on superconducting device performance, such as in direct detection dark matter sensors or quantum computing bits (qubits).

18
Jan
2023

# Disentangling Losses in Tantalum Superconducting Circuits

Superconducting qubits are a leading system for realizing large scale quantum processors, but overall gate fidelities suffer from coherence times limited by microwave dielectric loss.

Recently discovered tantalum-based qubits exhibit record lifetimes exceeding 0.3 ms. Here we perform systematic, detailed measurements of superconducting tantalum resonators in order to disentangle sources of loss that limit state-of-the-art tantalum devices. By studying the dependence of loss on temperature, microwave photon number, and device geometry, we quantify materials-related losses and observe that the losses are dominated by several types of saturable two level systems (TLSs), with evidence that both surface and bulk related TLSs contribute to loss. Moreover, we show that surface TLSs can be altered with chemical processing. With four different surface conditions, we quantitatively extract the linear absorption associated with different surface TLS sources. Finally, we quantify the impact of the chemical processing at single photon powers, the relevant conditions for qubit device performance. In this regime we measure resonators with internal quality factors ranging from 5 to 15 x 10^6, comparable to the best qubits reported. In these devices the surface and bulk TLS contributions to loss are comparable, showing that systematic improvements in materials on both fronts will be necessary to improve qubit coherence further.

# Evolution of 1/f Flux Noise in Superconducting Qubits with Weak Magnetic Fields

The microscopic origin of 1/f magnetic flux noise in superconducting circuits has remained an open question for several decades despite extensive experimental and theoretical investigation.

Recent progress in superconducting devices for quantum information has highlighted the need to mitigate sources of qubit decoherence, driving a renewed interest in understanding the underlying noise mechanism(s). Though a consensus has emerged attributing flux noise to surface spins, their identity and interaction mechanisms remain unclear, prompting further study. Here we apply weak in-plane magnetic fields to a capacitively-shunted flux qubit (where the Zeeman splitting of surface spins lies below the device temperature) and study the flux-noise-limited qubit dephasing, revealing previously unexplored trends that may shed light on the dynamics behind the emergent 1/f noise. Notably, we observe an enhancement (suppression) of the spin-echo (Ramsey) pure dephasing time in fields up to B=100 G. With direct noise spectroscopy, we further observe a transition from a 1/f to approximately Lorentzian frequency dependence below 10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field. We suggest that these trends are qualitatively consistent with an increase of spin cluster sizes with magnetic field. These results should help to inform a complete microscopic theory of 1/f flux noise in superconducting circuits.

17
Jan
2023

# Dissipation and Dephasing of Interacting Photons in Transmon Arrays

Transmon arrays are one of the most promising platforms for quantum information science. Despite being often considered simply as qubits, transmons are inherently quantum mechanical

multilevel systems. Being experimentally controllable with high fidelity, the higher excited states beyond the qubit subspace provide an important resource for hardware-efficient many-body quantum simulations, quantum error correction, and quantum information protocols. Alas, dissipation and dephasing phenomena generated by couplings to various uncontrollable environments yield a practical limiting factor to their utilization. To quantify this in detail, we present here the primary consequences of single-transmon dissipation and dephasing to the many-body dynamics of transmon arrays. We use analytical methods from perturbation theory and quantum trajectory approach together with numerical simulations, and deliberately consider the full Hilbert space including the higher excited states. The three main non-unitary processes are many-body decoherence, many-body dissipation, and heating/cooling transitions between different anharmonicity manifolds. Of these, the many-body decoherence — being proportional to the squared distance between the many-body Fock states — gives the strictest limit for observing effective unitary dynamics. Considering experimentally relevant parameters, including also the inevitable site-to-site disorder, our results show that the state-of-the-art transmon arrays should be ready for the task of demonstrating coherent many-body dynamics using the higher excited states. However, the wider utilization of transmons for ternary-and-beyond quantum computing calls for improving their coherence properties.

16
Jan
2023

# Circle fit optimization for resonator quality factor measurements: point redistribution for maximal accuracy

The control of material loss mechansims is playing an increasingly important role for improving coherence times in superconducting quantum devices. Such material losses can be characterized

through the measurement of planar superconducting resonators, which reflect losses through the resonance’s quality factor Ql. The resonance quality factor consists of both internal (material) losses as well as coupling losses when resonance photons escape back into the measurement circuit. The combined losses are then described as Q−1l=Q−1c+Q−1i, where Qc and Qi reflect the coupling and internal quality factors of the resonator, respectively. To separate the relative contributions of Qi and Qc to Ql, diameter-correcting circle fits use algebraic or geometric means to fit the resonance signal on the complex plane. However, such circle fits can produce varied results, so to address this issue, we use a combination of simulation and experiment to determine the reliability of a fitting algorithm across a wide range of quality factor values from Qi≪Qc to Qc≪Qi. In addition, we develop a novel measurement protocol that can not only reduce fitting errors by factors ≳2 but also mitigates the influence of the measurement background on the fit results. This technique can be generalized for other resonance systems beyond superconducting resonators.

14
Jan
2023

# Quantum entanglement generation on magnons assisted with microwave cavities coupled to a superconducting qubit

We present protocols to generate quantum entanglement on nonlocal magnons in hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave cavities and a superconducting (SC)

qubit. In the schemes, the YIGs are coupled to respective microwave cavities in resonant way, and the SC qubit is placed at the center of the cavities, which interacts with the cavities simultaneously. By exchanging the virtual photon, the cavities can indirectly interact in the far-detuning regime. Detailed protocols are presented to establish entanglement for two, three and arbitrary N magnons with reasonable fidelities.

13
Jan
2023

# Single Flux Quantum-Based Digital Control of Superconducting Qubits in a Multi-Chip Module

The single flux quantum (SFQ) digital superconducting logic family has been proposed for the scalable control of next-generation superconducting qubit arrays. In the initial implementation,

SFQ-based gate fidelity was limited by quasiparticle (QP) poisoning induced by the dissipative on-chip SFQ driver circuit. In this work, we introduce a multi-chip module architecture to suppress phonon-mediated QP poisoning. Here, the SFQ elements and qubits are fabricated on separate chips that are joined with In bump bonds. We use interleaved randomized benchmarking to characterize the fidelity of SFQ-based gates, and we demonstrate an error per Clifford gate of 1.2(1)%, an order-of-magnitude reduction over the gate error achieved in the initial realization of SFQ-based qubit control. We use purity benchmarking to quantify the contribution of incoherent error at 0.96(2)%; we attribute this error to photon-mediated QP poisoning mediated by the resonant mm-wave antenna modes of the qubit and SFQ-qubit coupler. We anticipate that a straightforward redesign of the SFQ driver circuit to limit the bandwidth of the SFQ pulses will eliminate this source of infidelity, allowing SFQ-based gates with fidelity approaching theoretical limits, namely 99.9% for resonant sequences and 99.99% for more complex pulse sequences involving variable pulse-to-pulse separation.

11
Jan
2023

# Performance Analysis of Superconductor-constriction-Superconductor Transmon Qubits

This work presents a computational analysis of a superconducting transmon qubit design, in which the superconductor-insulator-superconductor (SIS) Josephson junction is replaced by

a co-planar, superconductor-constriction-superconductor (ScS) junction. For short junctions having a Kulik-Omelyanchuk current-phase relationship, we find that the ScS transmon has an improved charge dispersion compared to the SIS transmon, with a tradeoff of 50% smaller anharmonicity. These calculations provide a framework for estimating the superconductor material properties and junction dimensions needed to provide proper ScS transmon operation at typical gigahertz frequencies.

10
Jan
2023

# Synchronization of Josephson junction in series array

Multi-qubit quantum processors coupled to networking provides the state-of-the-art quantum computing platform. However, each qubit has unique eigenfrequency even though fabricated in

the same process. To continue quantum gate operations besides the detection and correction of errors it is required that the qubits must be synchronized in the same frequency. This study uses Kuramoto model which is a link between statistical mean-field technique and non-linear dynamics to synchronize the qubits applying small noise in the system. This noise could be any externally applied noise function or just noise from the difference of frequencies of qubits. The Kuramoto model tunes the coupled oscillators adjusting the coupling strength between the oscillators to evolve from the state of incoherence to the synchronized state.