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
25
Feb
2020
Universal fast flux control of a coherent, low-frequency qubit
The extit{heavy-fluxonium} circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the half-flux frustration
point. However, the suppressed charge matrix elements and low transition frequency have made it challenging to perform fast single-qubit gates using standard protocols. We report on new protocols for reset, fast coherent control, and readout, that allow high-quality operation of the qubit with a 14 MHz transition frequency, an order of magnitude lower in energy than the ambient thermal energy scale. We utilize higher levels of the fluxonium to initialize the qubit with 97\% fidelity, corresponding to cooling it to 190 μK. We realize high-fidelity control using a universal set of single-cycle flux gates, which are comprised of directly synthesizable fast pulses, while plasmon-assisted readout is used for measurements. On a qubit with T1,T2e∼~300~μs, we realize single-qubit gates in 20−60~ns with an average gate fidelity of 99.8% as characterized by randomized benchmarking.
Anneal-path correction in flux qubits
Quantum annealers require accurate control and optimized operation schemes to reduce noise levels, in order to eventually demonstrate a computational advantage over classical algorithms.
We study a high coherence four-junction capacitively shunted flux qubit (CSFQ), using dispersive measurements to extract system parameters and model the device. We confirm the multi-level structure of the circuit model of our CSFQ by annealing it through small spectral gaps and observing quantum signatures of energy level crossings. Josephson junction asymmetry inherent to the device causes a deleterious nonlinear cross-talk when annealing the qubit. We implement a nonlinear annealing path to correct the asymmetry in-situ, resulting in a 50% improvement in the qubit performance. Our results demonstrate a low-level quantum control scheme which enhances the success probability of a quantum annealer.
Landau-Zener-Stückelberg Interferometry in dissipative Circuit Quantum Electrodynamics
We study Landau-Zener-Stückelberg (LZS) interferometry in a cQED architecture under effects of dissipation. To be specific, we consider a superconducting qubit driven by a dc+ac signal
and coupled to a transmission line resonator, but our results are valid for general qubit-resonators devices. To take the environment into account, we assume that the resonator is coupled to an ohmic quantum bath. The Floquet-Born-Markov master equation is numerically solved to obtain the dynamics of the system for arbitrary amplitude of the drive and different time scales. We unveil important differences in the resonant patterns between the Strong Coupling and Ultra Strong Coupling regimes in the qubit-resonator interaction, which are mainly due to the magnitude of photonic gaps in the energy spectrum of the system. We identify in the LZS patterns the contribution of the qubit gap and the photonic gaps, showing that for large driving amplitudes the patterns present a weaving structure due to the combined intercrossing of the different gaps contributions.
21
Feb
2020
Characterizing and optimizing qubit coherence based on SQUID geometry
The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically
study flux noise amplitudes in more than 50 flux qubits with varied SQUID geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.
18
Feb
2020
High-contrast ZZ interaction using multi-type superconducting qubits
For building a scalable quantum processor with superconducting qubits, the ZZ interaction is of great concert because of relevant for implementing two-qubit gates, and the close contact
between gate infidelity and its residual. Two-qubit gates with fidelity beyond fault-tolerant thresholds have been demonstrated using the ZZ interaction. However, as the performance of quantum processor improves, the residual static-ZZ can also become a performance-limiting factor for quantum gate operations and quantum error correction. Here, we introduce a scalable superconducting architecture for addressing this challenge. We demonstrate that by coupling two superconducting qubits with opposite-sign anharmonicities together, high-contrast ZZ interaction can be realized in this architecture. Thus, we can control ZZ interaction with high on/off ratio for implementing two-qubit CZ gate, or suppress it during the two-qubit gate operations using XY interaction (e.g. iSWAP). Meanwhile, the ZZ crosstalk related to neighboring spectator qubits can also be heavily suppressed in fixed coupled multi-qubit systems. This architecture could provide a promising way towards scalable superconducting quantum processor with high gate fidelity and low qubit crosstalk.
Hardware-Encoding Grid States in a Non-Reciprocal Superconducting Circuit
We present a circuit design composed of a non-reciprocal device and Josephson junctions whose ground space is doubly degenerate and the ground states are approximate codewords of the
Gottesman-Kitaev-Preskill (GKP) code. We determine the low-energy dynamics of the circuit by working out the equivalence of this system to the problem of a single electron confined in a two-dimensional plane and under the effect of strong magnetic field and of a periodic potential. We find that the circuit is naturally protected against the common noise channels in superconducting circuits, such as charge and flux noise, implying that it can be used for passive quantum error correction. We also propose realistic design parameters for an experimental realization and we describe possible protocols to perform logical one- and two-qubit gates, state preparation and readout.
17
Feb
2020
Leakage detection for a transmon-based surface code
Leakage outside of the qubit computational subspace, present in many leading experimental platforms, constitutes a threatening error for quantum error correction (QEC) for qubits. We
develop a leakage-detection scheme via Hidden Markov models (HMMs) for transmon-based implementations of the surface code. By performing realistic density-matrix simulations of the distance-3 surface code (Surface-17), we observe that leakage is sharply projected and leads to an increase in the surface-code defect probability of neighboring stabilizers. Together with the analog readout of the ancilla qubits, this increase enables the accurate detection of the time and location of leakage. We restore the logical error rate below the memory break-even point by post-selecting out leakage, discarding about 47% of the data. Leakage detection via HMMs opens the prospect for near-term QEC demonstrations, targeted leakage reduction and leakage-aware decoding and is applicable to other experimental platforms.
16
Feb
2020
Pulse-qubit interaction in a superconducting circuit under frictively dissipative environment
Microwave pulses are used ubiquitously to control and measure qubits fabricated on superconducting circuits. Due to continual environmental coupling, the qubits undergo decoherence
both when it is free and during its interaction with the microwave pulse. As quantum logic gates are executed through pulse-qubit interaction, we study theoretically the decoherence-induced effects during the interaction, especially the variations of the pulse, under a dissipative environment with linear spectral distribution. We find that a transmissible pulse of finite width adopts an asymmetric multi-hump shape, due to the imbalanced pumping and emitting rates of the qubit during inversion when the environment is present. The pulse shape reduces to a solitonic pulse at vanishing dissipation and a pulse train at strong dissipation. We give detailed analysis of the environmental origin from both the perspectives of envelope and phase of the propagating pulse.
05
Feb
2020
Breaking the trade-off between fast control and long lifetime of a superconducting qubit
The rapid development in designs and fabrication techniques of superconducting qubits has helped making coherence times of qubits longer. In the near future, however, the radiative
decay of a qubit into its control line will be a fundamental limitation, imposing a trade-off between fast control and long lifetime of the qubit. In this work, we successfully break this trade-off by strongly coupling another superconducting qubit along the control line. This second qubit, which we call a Josephson quantum filter~(JQF), prevents the qubit from emitting microwave photons and thus suppresses its relaxation, while faithfully transmitting large-amplitude control microwave pulses due to the saturation of the quantum filter, enabling fast qubit control. We observe an improvement of the qubit relaxation time without a reduction of the Rabi frequency. This device could potentially help in the realization of a large-scale superconducting quantum information processor in terms of the heating of the qubit environments and the crosstalk between qubits.
Causality test on Cherenkov effect in circuit QED
We investigate the Cherenkov radiation triggered by qubit acceleration simulated by superconducting circuit. By analyzing the radiation probability, we confirm the existence of Cherenkov
speed threshold, implying that simulating superluminal qubit motion is possible for such a scenario. A question immediately arises: Is such motion compatible with the causality principle? To address the question, we perform a causality test on the simulating system based on the recently developed notion of temporal quantum correlations, pseudo-density matrix and temporal quantum steering. The results suggest that single-mode approximation breaks down even when the system is restricted in weak coupling regime.