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
18
Okt
2023
Embedding networks for ideal performance of a travelling-wave parametric amplifier
We investigate the required embedding networks to enable ideal performance for a high-gain travelling-wave parametric amplifier (TWPA) based on three-wave mixing (3WM). By embedding
the TWPA in a network of superconducting diplexers and hybrid couplers, the amplifier can deliver a high stable gain with near-quantum-limited noise performance, with suppressed gain ripples, while eliminating the reflections of the signal, the idler and the pump as well as the transmission of all unwanted tones. We demonstrate a configuration where the amplifier can isolate. We call this technique Wideband Idler Filtering (WIF). The theory is supported by simulations that predict over 20 dB gain in the band 4-8 GHz with 10 dB isolation for a single amplifier and 30 dB isolation for two cascaded amplifiers. We demonstrate how the WIF-TWPAs can be used to construct switchable isolators with over 40 dB isolation over the full band 4-8 GHz. We also propose an alternative design where the WIF can be implemented without diplexers. Finally we show how, with small modifications, the technique can be implemented for four-wave mixing (4WM) TWPAs as well.
Native two-qubit gates in fixed-coupling, fixed-frequency transmons beyond cross-resonance interaction
Fixed-frequency superconducting qubits demonstrate remarkable success as platforms for stable and scalable quantum computing. Cross-resonance gates have been the workhorse of fixed-coupling,
fixed-frequency superconducting processors, leveraging the entanglement generated by driving one qubit resonantly with a neighbor’s frequency to achieve high-fidelity, universal CNOTs. Here, we use on-resonant and off-resonant microwave drives to go beyond cross-resonance, realizing natively interesting two-qubit gates that are not equivalent to CNOTs. In particular, we implement and benchmark native ISWAP, SWAP, ISWAP‾‾‾‾‾‾‾√, and BSWAP gates. Furthermore, we apply these techniques for an efficient construction of the B-gate: a perfect entangler from which any two-qubit gate can be reached in only two applications. We show these native two-qubit gates are better than their counterparts compiled from cross-resonance gates. We elucidate the resonance conditions required to drive each two-qubit gate and provide a novel frame tracking technique to implement them in Qiskit.
17
Okt
2023
Speeding up qubit control with bipolar single-flux-quantum pulse sequences
The development of quantum computers based on superconductors requires the improvement of the qubit state control approach aimed at the increase of the hardware energy efficiency. A
promising solution to this problem is the use of superconducting digital circuits operating with single-flux-quantum (SFQ) pulses, moving the qubit control system into the cold chamber. However, the qubit gate time under SFQ control is still longer than under conventional microwave driving. Here we introduce the bipolar SFQ pulse control based on ternary pulse sequences. We also develop a robust optimization algorithm for finding a sequence structure that minimizes the leakage of the transmon qubit state from the computational subspace. We show that the appropriate sequence can be found for arbitrary system parameters from the practical range. The proposed bipolar SFQ control reduces a single qubit gate time by halve compared to nowadays unipolar SFQ technique, while maintaining the gate fidelity over 99.99%.
12
Okt
2023
QRAM architectures using superconducting cavities
Quantum random access memory (QRAM) is a common architecture resource for algorithms with many proposed applications, including quantum chemistry, windowed quantum arithmetic, unstructured
search, machine learning, and quantum cryptography. Here we propose two bucket-brigade QRAM architectures based on high-coherence superconducting resonators, which differ in their realizations of the conditional-routing operations. In the first, we directly construct controlled- () operations, while in the second we utilize the properties of giant-unidirectional emitters (GUEs). For both architectures we analyze single-rail and dual-rail implementations of a bosonic qubit. In the single-rail encoding we can detect first-order ancilla errors, while the dual-rail encoding additionally allows for the detection of photon losses. For parameter regimes of interest the post-selected infidelity of a QRAM query in a dual-rail architecture is nearly an order of magnitude below that of a corresponding query in a single-rail architecture. These findings suggest that dual-rail encodings are particularly attractive as architectures for QRAM devices in the era before fault tolerance.
10
Okt
2023
Unraveling the role of disorderness in superconducting materials on qubit coherence
Introducing disorderness in the superconducting materials has been considered promising to enhance the electromagnetic impedance and realize noise-resilient superconducting qubits.
Despite a number of pioneering implementations, the understanding of the correlation between the material disorderness and the qubit coherence is still developing. Here, we demonstrate the first and a systematic characterization of fluxonium qubits with the superinductors made from titanium-aluminum-nitride with varied disorderness. From qubit noise spectroscopy, the flux noise and the dielectric loss are extracted as a measure of the coherence properties. Our results reveal that the 1/f flux noise dominates the qubit decoherence around the flux-frustration point, strongly correlated with the material disorderness; while the dielectric loss remains low under a wide range of material properties. From the flux-noise amplitudes, the areal density (σ) of the phenomenological spin defects and material disorderness are found to be approximately correlated by σ∝ρ3xx, or effectively (kFl)−3. This work has provided new insights on the origin of decoherence channels within superconductors, and could serve as a useful guideline for material design and optimization.
Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits
We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270μs, corresponding to Q = 5 million, and a highest observed value
of 501μs. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.
09
Okt
2023
High-fidelity optical readout of a superconducting qubit using a scalable piezo-optomechanical transducer
Superconducting quantum processors have made significant progress in size and computing potential. As a result, the practical cryogenic limitations of operating large numbers of superconductingqubits are becoming a bottleneck for further scaling. Due to the low thermal conductivity and the dense optical multiplexing capacity of telecommunications fiber, converting qubit signal processing to the optical domain using microwave-to-optics transduction would significantly relax the strain on cryogenic space and thermal budgets. Here, we demonstrate high-fidelity multi-shot optical readout through an optical fiber of a superconducting transmon qubit connected via a coaxial cable to a fully integrated piezo-optomechanical transducer. Using a demolition readout technique, we achieve a multi-shot readout fidelity of >99% at 6 μW of optical power transmitted into the cryostat with as few as 200 averages, without the use of a quantum-limited amplifier. With improved frequency matching between the transducer and the qubit readout resonator, we anticipate that single-shot optical readout is achievable. Due to the small footprint (<0.15mm2) and the modular fiber-based architecture, this device platform has the potential to scale towards use with thousands of qubits. Our results illustrate the potential of piezo-optomechanical transduction for low-dissipation operation of large quantum processors.[/expand]
08
Okt
2023
Quantum Sensing with superconducting qubits for Fundamental Physics
Quantum Sensing is a rapidly expanding research field that finds one of its applications in Fundamental Physics, as the search for Dark Matter. Recent developments in the fabrication
of superconducting qubits are contributing to driving progress in Quantum Sensing. Such devices have already been successfully applied in detecting few-GHz single photons via Quantum Non-Demolition measurement (QND). This technique allows us to detect the presence of the same photon multiple times without absorbing it, with remarkable sensitivity improvements and dark count rate suppression in experiments based on high-precision microwave photon detection, such as Axions and Dark Photons search experiments. In this context, the INFN Qub-IT project goal is to realize an itinerant single-photon counter based on superconducting qubits that will exploit QND. The simulation step is fundamental for optimizing the design before manufacturing and finally characterizing the fabricated chip in a cryogenic environment. In this study we present Qub-IT’s status towards the characterization of its first superconducting transmon qubit devices, illustrating their design and simulation.
06
Okt
2023
Observation of collapse and revival in a superconducting atomic frequency comb
Recent advancements in superconducting circuits have enabled the experimental study of collective behavior of precisely controlled intermediate-scale ensembles of qubits. In this work,
we demonstrate an atomic frequency comb formed by individual artificial atoms strongly coupled to a single resonator mode. We observe periodic microwave pulses that originate from a single coherent excitation dynamically interacting with the multi-qubit ensemble. We show that this revival dynamics emerges as a consequence of the constructive and periodic rephasing of the five superconducting qubits forming the vacuum Rabi split comb. In the future, similar devices could be used as a memory with in-situ tunable storage time or as an on-chip periodic pulse generator with non-classical photon statistics.
05
Okt
2023
Phonon engineering of atomic-scale defects in superconducting quantum circuits
Noise within solid-state systems at low temperatures, where many of the degrees of freedom of the host material are frozen out, can typically be traced back to material defects that
support low-energy excitations. These defects can take a wide variety of microscopic forms, and for amorphous materials are broadly described using generic models such as the tunneling two-level systems (TLS) model. Although the details of TLS, and their impact on the low-temperature behavior of materials have been studied since the 1970s, these states have recently taken on further relevance in the field of quantum computing, where the limits to the coherence of superconducting microwave quantum circuits are dominated by TLS. Efforts to mitigate the impact of TLS have thus far focused on circuit design, material selection, and material surface treatment. In this work, we take a new approach that seeks to directly modify the properties of TLS through nanoscale-engineering. This is achieved by periodically structuring the host material, forming an acoustic bandgap that suppresses all microwave-frequency phonons in a GHz-wide frequency band around the operating frequency of a transmon qubit superconducting quantum circuit. For embedded TLS that are strongly coupled to the electric qubit, we measure a pronounced increase in relaxation time by two orders of magnitude when the TLS transition frequency lies within the acoustic bandgap, with the longest T1 time exceeding 5 milliseconds. Our work paves the way for in-depth investigation and coherent control of TLS, which is essential for deepening our understanding of noise in amorphous materials and advancing solid-state quantum devices.