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
10
Nov
2022
Chiral coupling between a ferromagnetic magnon to a superconducting qubit
Chiral coupling at the single-quantum level promises to be a remarkable potential for quantum information processing. Here we propose to achieve a chiral interaction between a magnon
mode in a ferromagnetic sphere and a superconducting qubit mediated by a one-dimensional coupled-cavity array. When the qubit is coupled to two lattice sites of the array and each one is encoded with a tunable phase, we can acquire a directional qubit-magnon interaction via the quantum interference effect. This work opens up a new route to construct chiral devices, which are expected to become a building block in quantum magnonic networks.
02
Nov
2022
Optimizing for periodicity: a model-independent approach to flux crosstalk calibration for superconducting circuits
Flux tunability is an important engineering resource for superconducting circuits. Large-scale quantum computers based on flux-tunable superconducting circuits face the problem of flux
crosstalk, which needs to be accurately calibrated to realize high-fidelity quantum operations. Typical calibration methods either assume that circuit elements can be effectively decoupled and simple models can be applied, or require a large amount of data. Such methods become ineffective as the system size increases and circuit interactions become stronger. Here we propose a new method for calibrating flux crosstalk, which is independent of the underlying circuit model. Using the fundamental property that superconducting circuits respond periodically to external fluxes, crosstalk calibration of N flux channels can be treated as N independent optimization problems, with the objective functions being the periodicity of a measured signal depending on the compensation parameters. We demonstrate this method on a small-scale quantum annealing circuit based on superconducting flux qubits, achieving comparable accuracy with previous methods. We also show that the objective function usually has a nearly convex landscape, allowing efficient optimization.
28
Okt
2022
Robust cryogenic matched low-pass coaxial filters for quantum computing applications
Electromagnetic noise is one of the key external factors decreasing superconducting qubits coherence. Matched coaxial filters can prevent microwave and IR photons negative influence
on superconducting quantum circuits. Here, we report on design and fabrication route of matched low-pass coaxial filters for noise-sensitive measurements at milliKelvin temperatures. A robust transmission coefficient with designed linear absorption (-1dB/GHz) and ultralow reflection losses less than -20 dB up to 20 GHz is achieved. We present a mathematical model for evaluating and predicting filters transmission parameters depending on their dimensions. It is experimentally approved on two filters prototypes different lengths with compound of Cu powder and Stycast commercial resin demonstrating excellent matching. The presented design and assembly route are universal for various compounds and provide high repeatability of geometrical and microwave characteristics. Finally, we demonstrate three filters with almost equal reflection and transmission characteristics in the range from 0 to 20 GHz, which is quite useful to control multiple channel superconducting quantum circuits.
27
Okt
2022
Scaling up Superconducting Quantum Computers with Cryogenic RF-photonics
Today’s hundred-qubit quantum computers require a dramatic scale up to millions of qubits to become practical for solving real-world problems. Although a variety of qubit technologies
have been demonstrated, scalability remains a major hurdle. Superconducting (SC) qubits are one of the most mature and promising technologies to overcome this challenge. However, these qubits reside in a millikelvin cryogenic dilution fridge, isolating them from thermal and electrical noise. They are controlled by a rack-full of external electronics through extremely complex wiring and cables. Although thousands of qubits can be fabricated on a single chip and cooled down to millikelvin temperatures, scaling up the control and readout electronics remains an elusive goal. This is mainly due to the limited available cooling power in cryogenic systems constraining the wiring capacity and cabling heat load management.
In this paper, we focus on scaling up the number of XY-control lines by using cryogenic RF-photonic links. This is one of the major roadblocks to build a thousand qubit superconducting QC. We will first review and study the challenges of state-of-the-art proposed approaches, including cryogenic CMOS and deep-cryogenic photonic methods, to scale up the control interface for SC qubit systems. We will discuss their limitations due to the active power dissipation and passive heat leakage in detail. By analytically modeling the noise sources and thermal budget limits, we will show that our solution can achieve a scale up to a thousand of qubits. Our proposed method can be seamlessly implemented using advanced silicon photonic processes, and the number of required optical fibers can be further reduced by using wavelength division multiplexing (WDM).
An integrated microwave-to-optics interface for scalable quantum computing
Microwave-to-optics transduction is emerging as a vital technology for scaling quantum computers and quantum networks. To establish useful entanglement links between qubit processing
units, several key conditions have to be simultaneously met: the transducer must add less than a single quantum of input referred noise and operate with high-efficiency, as well as large bandwidth and high repetition rate. Here we present a new design for an integrated transducer based on a planar superconducting resonator coupled to a silicon photonic cavity through a mechanical oscillator made of lithium niobate on silicon. We experimentally demonstrate its unique performance and potential for simultaneously realizing all of the above conditions, measuring added noise that is limited to a few photons, transduction efficiencies as high as 0.9%, with a bandwidth of 14.8 MHz and a repetition rate of up to 100 kHz. Our device couples directly to a 50-Ohm transmission line and can easily be scaled to a large number of transducers on a single chip, paving the way for distributed quantum computing.
Broadband SNAIL parametric amplifier with microstrip impedance transformer
Josephson parametric amplifiers have emerged as a promising platform for quantum information processing and squeezed quantum states generation. Travelling wave and impedance-matched
parametric amplifiers provide broad bandwidth for high-fidelity single-shot readout of multiple qubit superconducting circuits. Here, we present a quantum-limited 3-wave-mixing parametric amplifier based on superconducting nonlinear asymmetric inductive elements (SNAILs), whose useful bandwidth is enhanced with an on-chip two-section impedance-matching circuit based on microstrip transmission lines. The amplifier dynamic range is increased using an array of sixty-seven SNAILs with 268 Josephson junctions, forming a nonlinear quarter-wave resonator. Operating in a current-pumped mode, we experimentally demonstrate an average gain of 17dB across 300MHz bandwidth, along with an average saturation power of −100dBm, which can go as high as −97dBm with quantum-limited noise performance. Moreover, the amplifier can be fabricated using a simple technology with just a one e-beam lithography step. Its central frequency is tuned over a several hundred megahertz, which in turn broadens the effective operational bandwidth to around 1.5GHz.
Improving Josephson junction reproducibility for superconducting quantum circuits: junction area fluctuation
Josephson superconducting qubits and parametric amplifiers are prominent examples of superconducting quantum circuits that have shown rapid progress in recent years. With the growing
complexity of such devices, the requirements for reproducibility of their electrical properties across a chip have become stricter. Thus, the critical current Ic variation of the Josephson junction, as the most important electrical parameter, needs to be minimized. Critical current, in turn, is related to normal-state resistance the Ambegaokar-Baratoff formula, which can be measured at room temperature. Here, we focus on the dominant source of Josephson junction critical current non-uniformity junction area variation. We optimized Josephson junctions fabrication process and demonstrate resistance variation of 9.8−4.4% and 4.8−2.3% across 22×22 mm2 and 5×10 mm2 chip areas, respectively. For a wide range of junction areas from 0.008 μm2 to 0.12 μm2 we ensure a small linewidth standard deviation of 4 nm measured over 4500 junctions with linear dimensions from 80 to 680 nm. The developed process was tested on superconducting highly coherent transmon qubits (T1>100μs) and a nonlinear asymmetric inductive element parametric amplifier.
26
Okt
2022
Theory of strong down-conversion in multi-mode cavity and circuit QED
We revisit the superstrong coupling regime of multi-mode cavity quantum electrodynamics (QED), defined to occur when the frequency of vacuum Rabi oscillations between the qubit and
the nearest cavity mode exceeds the cavity’s free spectral range. A novel prediction is made that the cavity’s linear spectrum, measured in the vanishing power limit, can acquire an intricate fine structure associated with the qubit-induced cascades of coherent single-photon down-conversion processes. This many-body effect is hard to capture by a brute-force numerics and it is sensitive to the light-matter coupling parameters both in the infra-red and the ultra-violet limits. We focused at the example case of a superconducting fluxonium qubit coupled to a long transmission line section. The conversion rate in such a circuit QED setup can readily exceed a few MHz, which is plenty to overcome the usual decoherence processes. Analytical calculations were made possible by an unconventional gauge choice, in which the qubit circuit interacts with radiation via the flux/charge variable in the low-/high-frequency limits, respectively. Our prediction of the fine spectral structure lays the foundation for the „strong down-conversion“ regime in quantum optics, in which a single photon excited in a non-linear medium spontaneously down-converts faster than it is absorbed.
25
Okt
2022
Emergent macroscopic bistability induced by a single superconducting qubit
The photon blockade breakdown in a continuously driven cavity QED system has been proposed as a prime example for a first-order driven-dissipative quantum phase transition. But the
predicted scaling from a microscopic system – dominated by quantum fluctuations – to a macroscopic one – characterized by stable phases – and the associated exponents and phase diagram have not been observed so far. In this work we couple a single transmon qubit with a fixed coupling strength g to an in-situ bandwidth κ tuneable superconducting cavity to controllably approach this thermodynamic limit. Even though the system remains microscopic, we observe its behavior to become more and more macroscopic as a function of g/κ. For the highest realized g/κ≈287 the system switches with a characteristic dwell time as high as 6 seconds between a bright coherent state with ≈8×103 intra-cavity photons and the vacuum state with equal probability. This exceeds the microscopic time scales by six orders of magnitude and approaches the near perfect hysteresis expected between two macroscopic attractors in the thermodynamic limit. These findings and interpretation are qualitatively supported by semi-classical theory and large-scale Quantum-Jump Monte Carlo simulations. Besides shedding more light on driven-dissipative physics in the limit of strong light-matter coupling, this system might also find applications in quantum sensing and metrology.
23
Okt
2022
A microwave scattering spectral method to detect the nanomechanical vibrations embedded in a superconducting qubit
Nanomechanical resonators (NMRs), as the quantum mechanical sensing probers, have played the important roles for various high-precision quantum measurements. Differing from the previous
emission spectral probes (i.e., the NMR modified the atomic emission), in this paper we propose an alternative approach, i.e., by probing the scattering spectra of the quantum mechanical prober coupled to the driving microwaves, to characterize the physical features of the NMR embedded in a rf-SQUID based superconducting qubit. It is shown that, from the observed specifical frequency points in the spectra, i.e., either the dips or the peaks, the vibrational features (i.e., they are classical vibration or quantum mechanical one) and the physical parameters (typically such as the vibrational frequency and displacements) of the NMR can be determined effectively. The proposal is feasible with the current technique and should be useful to design the desired NMRs for various quantum metrological applications.