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
02
Dez
2021
Optimizing frequency allocation for fixed-frequency superconducting quantum processors
Fixed-frequency superconducting quantum processors are one of the most mature quantum computing architectures with high-coherence qubits and low-complexity controls. However, high-fidelity
multi-qubit gates pose tight requirements on individual qubit frequencies in these processors and their fabrication suffers from the large dispersion in the fabrication of Josephson junctions. It is inefficient to make a large number of processors because degeneracy in frequencies can degrade the processors‘ quality. In this article, we propose an optimization scheme based on mixed-integer programming to maximize the fabrication yield of quantum processors. We study traditional qubit and qutrit (three-level) architectures with cross-resonance interaction processors. We compare these architectures to a differential AC-Stark shift based on entanglement gates and show that our approach greatly improves the fabrication yield and also increases the scalability of these devices. Our approach is general and can be adapted to problems where one must avoid specific frequency collisions.
01
Dez
2021
Entanglement between charge qubit states and coherent states of nanomechanical resonator generated by AC Josephson effect
We considered a nanoelectromechanical system consisting of a movable Cooper-pair box qubit which is subject to an electrostatic field, and coupled to the two bulk superconductors via
tunneling processes. We suggest that qubit dynamics is related to the one of a quantum oscillator and demonstrate that a bias voltage applied between superconductors generates states represented by the entanglement of qubit states and coherent states of the oscillator if certain resonant conditions are fulfilled. It is shown that a structure of this entanglement may be controlled by the bias voltage in a way that gives rise to the entanglement incorporating so-called cat-states – the superposition of coherent states. We characterize the formation and development of such states analyzing the entropy of entanglement and corresponding Wigner function. The experimentally feasible detection of the effect by measuring the average current is also considered.
30
Nov
2021
Design of the Ultra-Low Noise Amplifier for Quantum Applications
The present article mainly emphasizes the design of a low-noise amplifier that can be used for quantum applications. For this reason, the design circuit specifically concentrates on
the noise figure and its improvement to be used in quantum applications at which the noise added due to the circuit designed should be strongly limited. If the designed low noise amplifier could have quantum-associated applications, its noise temperature should be around 0.4 K, in which the designed circuit is comparable with the Josephson Junction amplifier. Although this task seems to be highly challenging, this work focuses on engineering the circuit, minimizing the mismatch and reflection coefficients in the circuit, and enhancing the circuit transconductance to improve the noise figure in the circuit as efficiently as possible. The results indicated the possibility of reaching the noise figure around 0.008 dB in the circuit operating at 10 K. Additionally, the circuit is analyzed via quantum mechanical analysis, through which some important quantities, such as noise figure, is theoretically derived. In fact, the derived relationship using quantum theory reveals that on which quantities the design should focus in order to optimize the noise figure. Thus, merging quantum theory and engineering the approach contributed to designing a highly efficient circuit for strongly minimizing the noise figure.
Engineering Strong Beamsplitter Interaction between Bosonic Modes via Quantum Optimal Control Theory
In continuous-variable quantum computing with qubits encoded in the infinite-dimensional Hilbert space of bosonic modes, it is a difficult task to realize strong and on-demand interactions
between the qubits. One option is to engineer a beamsplitter interaction for photons in two superconducting cavities by driving an intermediate superconducting circuit with two continuous-wave drives, as demonstrated in a recent experiment. Here, we show how quantum optimal control theory (OCT) can be used in a systematic way to improve the beamsplitter interaction between the two cavities. We find that replacing the two-tone protocol by a three-tone protocol accelerates the effective beamsplitter rate between the two cavities. The third tone’s amplitude and frequency are determined by gradient-free optimization and make use of cavity-transmon sideband couplings. We show how to further improve the three-tone protocol via gradient-based optimization while keeping the optimized drives experimentally feasible. Our work exemplifies how to use OCT to systematically improve practical protocols in quantum information applications.
29
Nov
2021
Quantum dynamics of disordered arrays of interacting superconducting qubits: signatures of quantum collective states
We study theoretically the collective quantum dynamics occurring in various interacting superconducting qubits arrays (SQAs) in the presence of a spread of individual qubit frequencies.
The interaction is provided by mutual inductive coupling between adjacent qubits (short-range Ising interaction) or inductive coupling to a low-dissipative resonator (long-range exchange interaction). In the absence of interaction the Fourier transform of temporal correlation function of the total polarization (z-projection of the total spin), i.e. the dynamic susceptibility C(ω), demonstrates a set of sharp small magnitude resonances corresponding to the transitions of individual superconducting qubits. We show that even a weak interaction between qubits can overcome the disorder with a simultaneous formation of the collective excited states. This collective behavior manifests itself by a single large resonance in C(ω). In the presence of a weak non-resonant microwave photon field in the low-dissipative resonator, the positions of dominant resonances depend on the number of photons, i.e. the collective ac Stark effect. Coupling of an SQA to the transmission line allows a straightforward experimental access of the collective states in microwave transmission experiments and, at the same time, to employ SQAs as sensitive single-photon detectors.
26
Nov
2021
Fluxonium: an alternative qubit platform for high-fidelity operations
Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple
milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit-coherence, fast frequency-tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, for the first time within the realm of superconducting qubits, reveals an approach toward fault-tolerant quantum computing that is alternative and competitive to the transmon system.
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.