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
26
Nov
2021
Scalable method for eliminating residual ZZ interaction between superconducting qubits
Unwanted ZZ interaction is a quantum-mechanical crosstalk phenomenon which correlates qubit dynamics and is ubiquitous in superconducting qubit system. It adversely affects the quality
of quantum operations and can be detrimental in scalable quantum information processing. Here we propose and experimentally demonstrate a practically extensible approach for complete cancellation of residual ZZ interaction between fixed-frequency transmon qubits, which are known for long coherence and simple control. We apply to the intermediate coupler that connects the qubits a weak microwave drive at a properly chosen frequency in order to noninvasively induce ac Stark shift for ZZ cancellation. We verify the cancellation performance by measuring vanishing two-qubit entangling phases and ZZ correlations. In addition, we implement randomized benchmarking experiment to extract the idling gate fidelity which shows good agreement with the coherence limit, demonstrating the effectiveness of ZZ cancellation. Our method allows independent addressability of each qubit-qubit connection, and is applicable to both non-tunable and tunable coupler, promising better compatibility with future large-scale quantum processors.
Mapping Surface Code to Superconducting Quantum Processors
In this paper, we formally describe the three challenges of mapping surface code on superconducting devices, and present a comprehensive synthesis framework to overcome these challenges.
The proposed framework consists of three optimizations. First, we adopt a geometrical method to allocate data qubits which ensures the existence of shallow syndrome extraction circuit. The proposed data qubit layout optimization reduces the overhead of syndrome extraction and serves as a good initial point for following optimizations. Second, we only use bridge qubits enclosed by data qubits and reduce the number of bridge qubits by merging short path between data qubits. The proposed bridge qubit optimization reduces the probability of bridge qubit conflicts and further minimizes the syndrome extraction overhead. Third, we propose an efficient heuristic to schedule syndrome extractions. Based on the proposed data qubit allocation, we devise a good initial schedule of syndrome extractions and further refine this schedule to minimize the total time needed by a complete surface code error detection cycle. Our experiments on mainsstream superconducting quantum architectures have demonstrated the efficiency of the proposed framework.
24
Nov
2021
Direct implementation of a perceptron in superconducting circuit quantum hardware
The utility of classical neural networks as universal approximators suggests that their quantum analogues could play an important role in quantum generalizations of machine-learning
methods. Inspired by the proposal in [Torrontegui and García-Ripoll 2019 EPL 125 30004], we demonstrate a superconducting qubit implementation of an adiabatic controlled gate, which generalizes the action of a classical perceptron as the basic building block of a quantum neural network. We show full control over the steepness of the perceptron activation function, the input weight and the bias by tuning the adiabatic gate length, the coupling between the qubits and the frequency of the applied drive, respectively. In its general form, the gate realizes a multi-qubit entangling operation in a single step, whose decomposition into single- and two-qubit gates would require a number of gates that is exponential in the number of qubits. Its demonstrated direct implementation as perceptron in quantum hardware may therefore lead to more powerful quantum neural networks when combined with suitable additional standard gates.
Distinguishing multi-spin interactions from lower-order effects
Multi-spin interactions can be engineered with artificial quantum spins. However, it is challenging to verify such interactions experimentally. Here we describe two methods to characterize
the n-local coupling of n spins. First, we analyze the variation of the transition energy of the static system as a function of local spin fields. Standard measurement techniques are employed to distinguish n-local interactions between up to five spins from lower-order contributions in the presence of noise and spurious fields and couplings. Second, we show a detection technique that relies on time dependent driving of the coupling term. Generalizations to larger system sizes are analyzed for both static and dynamic detection methods, and we find that the dynamic method is asymptotically optimal when increasing the system size. The proposed methods enable robust exploration of multi-spin interactions across a broad range of both coupling strengths and qubit modalities.
Digital control of a superconducting qubit using a Josephson pulse generator at 3 K
Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energy-efficient, stable, and reproducible qubit control and readout electronics. In comparisonto traditional semiconductor control electronics (TSCE) located at room temperature, the signals generated by Josephson junction (JJ) based rf sources benefit from small device sizes, low power dissipation, intrinsic calibration, superior reproducibility, and insensitivity to ambient fluctuations. Previous experiments to co-locate qubits and JJ-based control electronics resulted in quasiparticle poisoning of the qubit; degrading the qubit’s coherence and lifetime. In this paper, we digitally control a 0.01~K transmon qubit with pulses from a Josephson pulse generator (JPG) located at the 3~K stage of a dilution refrigerator. We directly compare the qubit lifetime T1, coherence time T∗2, and thermal occupation Pth when the qubit is controlled by the JPG circuit versus the TSCE setup. We find agreement to within the daily fluctuations on ±0.5 μs and ±2 μs for T1 and T∗2, respectively, and agreement to within the 1\% error for Pth. Additionally, we perform randomized benchmarking to measure an average JPG gate error of 2.1×10−2. In combination with a small device size (<25~mm2) and low on-chip power dissipation (≪100 μW), these results are an important step towards demonstrating the viability of using JJ-based control electronics located at temperature stages higher than the mixing chamber stage in highly-scaled superconducting quantum information systems.[/expand]
23
Nov
2021
Quantum Dot-Based Parametric Amplifiers
Josephson parametric amplifiers (JPAs) approaching quantum-limited noise performance have been instrumental in enabling high fidelity readout of superconducting qubits and, recently,
semiconductor quantum dots (QDs). We propose that the quantum capacitance arising in electronic two-level systems (the dual of Josephson inductance) can provide an alternative dissipation-less non-linear element for parametric amplification. We experimentally demonstrate phase-sensitive parametric amplification using a QD-reservoir electron transition in a CMOS nanowire split-gate transistor embedded in a 1.8~GHz superconducting lumped-element microwave cavity, achieving parametric gains of -3 to +3 dB, limited by Sisyphus dissipation. Using a semi-classical model, we find an optimised design within current technological capabilities could achieve gains and bandwidths comparable to JPAs, while providing complementary specifications with respect to integration in semiconductor platforms or operation at higher magnetic fields.
Microscopic Theory of Magnetic Disorder-Induced Decoherence in Superconducting Nb Films
The performance of superconducting qubits is orders of magnitude below what is expected from theoretical estimates based on the loss tangents of the constituent bulk materials. This
has been attributed to the presence of uncontrolled surface oxides formed during fabrication which can introduce defects and impurities that create decoherence channels. Here, we develop an ab initio Shiba theory to investigate the microscopic origin of magnetic-induced decoherence in niobium thin film superconductors and the formation of native oxides. Our ab initio calculations encompass the roles of structural disorder, stoichiometry, and strain on the formation of decoherence-inducing local spin moments. With parameters derived from these first-principles calculations we develop an effective quasi-classical model of magnetic-induced losses in the superconductor. We identify d-channel losses (associated with oxygen vacancies) as especially parasitic, resulting in a residual zero temperature surface impedance. This work provides a route to connecting atomic scale properties of superconducting materials and macroscopic decoherence channels affecting quantum systems.
Quantum simulation of parity-time symmetry breaking with a superconducting quantum processor
The observation of genuine quantum effects in systems governed by non-Hermitian Hamiltonians has been an outstanding challenge in the field. Here we simulate the evolution under such
Hamiltonians in the quantum regime on a superconducting quantum processor by using a dilation procedure involving an ancillary qubit. We observe the parity-time ()-symmetry breaking phase transition at the exceptional points, obtain the critical exponent, and show that this transition is associated with a loss of state distinguishability. In a two-qubit setting, we show that the entanglement can be modified by local operations.
Elucidating the local atomic and electronic structure of amorphous oxidized superconducting niobium films
Qubits made from superconducting materials are a mature platform for quantum information science application such as quantum computing. However, materials-based losses are now a limiting
factor in reaching the coherence times needed for applications. In particular, knowledge of the atomistic structure and properties of the circuit materials is needed to identify, understand, and mitigate materials-based decoherence channels. In this work we characterize the atomic structure of the native oxide film formed on Nb resonators by comparing fluctuation electron microscopy experiments to density functional theory calculations, finding that an amorphous layer consistent with an Nb2O5 stoichiometry. Comparing X-ray absorption measurements at the Oxygen K edge with first-principles calculations, we find evidence of d-type magnetic impurities in our sample, known to cause impedance in proximal superconductors. This work identifies the structural and chemical composition of the oxide layer grown on Nb superconductors, and shows that soft X-ray absorption can fingerprint magnetic impurities in these superconducting systems.
18
Nov
2021
Emission of photon multiplets by a dc-biased superconducting circuit
We observe the emission of bunches of k⩾1 photons by a circuit made of a microwave resonator in series with a voltage-biased tunable Josephson junction. The bunches are emitted at
specific values Vk of the bias voltage, for which each Cooper pair tunneling across the junction creates exactly k photons in the resonator. The latter is a micro-fabricated spiral coil which resonates and leaks photons at 4.4~GHz in a measurement line. Its characteristic impedance of 1.97~kΩ is high enough to reach a strong junction-resonator coupling and a bright emission of the k-photon bunches. We show that a RWA treatment of the system accounts quantitatively for the observed radiation intensity, from k=1 to 6, and over three orders of magnitude when varying the Josephson energy EJ. We also measure the second order correlation function of the radiated microwave to determine its Fano factor Fk, which in the low EJ limit, confirms with Fk=k the emission of k photon bunches. At larger EJ, a more complex behavior is observed in quantitative agreement with numerical simulations.