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
29
Jul
2024
Realization of high-fidelity perfect entangler between remote superconducting quantum processors
Building large-scale quantum computers from smaller modules offers a solution to many formidable scientific and engineering challenges. Nevertheless, engineering high-fidelity interconnects
between modules remains challenging. In recent years, quantum state transfer (QST) has provided a way to establish entanglement between two separately packaged quantum devices. However, QST is not a unitary gate, thus cannot be directly inserted into a quantum circuit, which is widely used in recent quantum computation studies. Here we report a demonstration of a direct CNOT gate realized by the cross resonance (CR) effect between two remotely packaged quantum devices connected by a microwave cable. We achieve a CNOT gate with fidelity as high as 99.15±0.02%. The quality of the CNOT gate is verified by cross-entropy benchmarking (XEB) and further confirmed by demonstrating Bell-inequality violation. This work provides a new method to realize remote two-qubit gates. Our method can be used not only to achieve distributed quantum computing but also to enrich the topology of superconducting quantum chips with jumper lines connecting distant qubits. This advancement gives superconducting qubits broader application prospects in the fields of quantum computing and quantum simulation.
26
Jul
2024
Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit
Superconducting qubits are one of the most promising physical systems for implementing a quantum computer. However, executing quantum algorithms of practical computational advantage
requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.890 GHz, an energy relaxation time with a median of 502 us and a maximum of (765 +/- 82.6) us, and an echo dephasing time with a median of 541 us and a maximum of (1057 +/- 138) us. We report details of our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.
Robust multi-mode superconducting qubit designed with evolutionary algorithms
Multi-mode superconducting circuits offer a promising platform for engineering robust systems for quantum computation. Previous studies have shown that single-mode devices cannot simultaneously
exhibit resilience against multiple decoherence sources due to conflicting protection requirements. In contrast, multi-mode systems offer increased flexibility and have proven capable of overcoming these fundamental limitations. Nevertheless, exploring multi-mode architectures is computationally demanding due to the exponential scaling of the Hilbert space dimension. Here, we present a multi-mode device designed using evolutionary optimization techniques, which have been shown to be effective for this computational task. The proposed device was optimized to feature an anharmonicity of a third of the qubit frequency and reduced energy dispersion caused by charge and magnetic flux fluctuations. It exhibits improvements over the fundamental errors limiting Transmon and Fluxonium coherence and manipulation, aiming for a balance between low depolarization error and fast manipulation; furthermore demonstrating robustness against fabrication errors, a major limitation in many proposed multi-mode devices. Overall, by striking a balance between coupling matrix elements and noise protection, we propose a device that paves the way towards finding proper characteristics for the construction of superconducting quantum processors.
Mitigating Losses of Superconducting Qubits Strongly Coupled to Defect Modes
The dominant contribution to the energy relaxation of state-of-the-art superconducting qubits is often attributed to their coupling to an ensemble of material defects which behave as
two-level systems. These defects have varying microscopic characteristics which result in a large range of observable defect properties such as resonant frequencies, coherence times and coupling rates to qubits g. Here, we investigate strategies to mitigate losses to the family of defects that strongly couple to qubits (g/2π≥ 0.5 MHz). Such strongly coupled defects are particularly detrimental to the coherence of qubits and to the fidelities of operations relying on frequency excursions, such as flux-activated two-qubit gates. To assess their impact, we perform swap spectroscopy on 92 frequency-tunable qubits and quantify the spectral density of these strongly coupled modes. We show that the frequency configuration of the defects is rearranged by warming up the sample to room temperature, whereas the total number of defects on a processor tends to remain constant. We then explore methods for fabricating qubits with a reduced number of strongly coupled defect modes by systematically measuring their spectral density for decreasing Josephson junction dimensions and for various surface cleaning methods. Our results provide insights into the properties of strongly coupled defect modes and show the benefits of minimizing Josephson junction dimensions to improve qubit properties.
25
Jul
2024
Cross-resonance control of an oscillator with a fluxonium ancilla
The conditional displacement (CD) gate between an oscillator and a discrete-variable ancilla plays a key role in quantum information processing tasks, such as enabling universal control
of the oscillator and longitudinal readout of the qubit. However, the gate is unprotected against the propagation of ancilla decay errors and hence not fault-tolerant. Here, we propose a CD gate scheme with fluxonium as the ancilla, which has been experimentally demonstrated to have a large noise bias and millisecond-level lifetimes. The proposed gate is applied cross-resonantly by modulating the external flux of the fluxonium at the frequency of the target oscillator, which requires minimal hardware overhead and does not increase sensitivity to decoherence mechanisms like dephasing. We further provide a perturbative description of the gate mechanism and identify the error budget. Additionally, we develop an approximate procedure for choosing device and gate parameters that optimizes gate performance. Following the procedure for multiple sets of fluxonium parameters from the literature, we numerically demonstrate CD gates with unitary fidelity exceeding 99.9% and gate times of hundreds of nanoseconds.
24
Jul
2024
Systematic study of High EJ/EC transmon qudits up to d=12
Qudits provide a resource-efficient alternative to qubits for quantum information processing. The multilevel nature of the transmon, with its individually resolvable transition frequencies,makes it an attractive platform for superconducting circuit-based qudits. In this work, we systematically analyze the trade-offs associated with encoding high-dimensional quantum information in fixed-frequency transmons. Designing high EJ/EC ratios of up to 325, we observe up to 12 levels (d=12) on a single transmon. Despite the decreased anharmonicity, we demonstrate process infidelities ef<3×10−3 for qubit-like operations in each adjacent-level qubit subspace in the lowest 10 levels. Furthermore, we achieve a 10-state readout assignment fidelity of 93.8% with the assistance of deep neural network classification of a multi-tone dispersive measurement. We find that the Hahn echo time T2E for the higher levels is close to the limit of T1 decay, primarily limited by bosonic enhancement. We verify the recently introduced Josephson harmonics model, finding that it yields better predictions for the transition frequencies and charge dispersion. Finally, we show strong ZZ-like coupling between the higher energy levels in a two-transmon system. Our high-fidelity control and readout methods, in combination with our comprehensive characterization of the transmon model, suggest that the high-EJ/EC transmon is a powerful tool for exploring excited states in circuit quantum electrodynamics.[/expand]
23
Jul
2024
A high-efficiency plug-and-play superconducting qubit network
Modular architectures are a promising approach to scale quantum devices to the point of fault tolerance and utility. Modularity is particularly appealing for superconducting qubits,
as monolithically manufactured devices are limited in both system size and quality. Constructing complex quantum systems as networks of interchangeable modules can overcome this challenge through `Lego-like‘ assembly, reconfiguration, and expansion, in a spirit similar to modern classical computers. First prototypical superconducting quantum device networks have been demonstrated. Interfaces that simultaneously permit interchangeability and high-fidelity operations remain a crucial challenge, however. Here, we demonstrate a high-efficiency interconnect based on a detachable cable between superconducting qubit devices. We overcome the inevitable loss in a detachable connection through a fast pump scheme, enabling inter-module SWAP efficiencies at the 99%-level in less than 100 ns. We use this scheme to generate high-fidelity entanglement and operate a distributed logical dual-rail qubit. At the observed ~1% error rate, operations through the interconnect are at the threshold for fault-tolerance. These results introduce a modular architecture for scaling quantum processors with reconfigurable and expandable networks.
Pure kinetic inductance coupling for cQED with flux qubits
We demonstrate a qubit-readout architecture where the dispersive coupling is entirely mediated by a kinetic inductance. This allows us to engineer the dispersive shift of the readout
resonator independent of the qubit and resonator capacitances. We validate the pure kinetic coupling concept and demonstrate various generalized flux qubit regimes from plasmon to fluxon, with dispersive shifts ranging from 60 kHz to 2 MHz at the half-flux quantum sweet spot. We achieve readout performances comparable to conventional architectures with quantum state preparation fidelities of 99.7 % and 92.7 % for the ground and excited states, respectively, and below 0.1 % leakage to non-computational states.
22
Jul
2024
24 days-stable CNOT-gate on fluxonium qubits with over 99.9% fidelity
Fluxonium qubit is a promising building block for quantum information processing due to its long coherence time and strong anharmonicity. In this paper, we realize a 60 ns direct CNOT-gate
on two inductively-coupled fluxonium qubits using selective darkening approach, resulting in a gate fidelity as high as 99.94%. The fidelity remains above 99.9% for 24 days without any recalibration between randomized benchmarking measurements. Compared with the 99.96% fidelity of a 60 ns identity gate, our data brings the investigation of the non-decoherence-related errors during gate operations down to 2×10−4. The present result adds a simple and robust two-qubit gate into the still relatively small family of „the beyond three nines“ demonstrations on superconducting qubits.
Verifying the analogy between transversely coupled spin-1/2 systems and inductively-coupled fluxoniums
We report a detailed characterization of two inductively coupled superconducting fluxonium qubits for implementing high-fidelity cross-resonance gates. Our circuit stands out because
it behaves very closely to the case of two transversely coupled spin-1/2 systems. In particular, the generally unwanted static ZZ-term due to the non-computational transitions is nearly absent despite a strong qubit-qubit hybridization. Spectroscopy of the non-computational transitions reveals a spurious LC-mode arising from the combination of the coupling inductance and the capacitive links between the terminals of the two qubit circuits. Such a mode has a minor effect on our specific device, but it must be carefully considered for optimizing future designs.