A central challenge in building a scalable quantum computer with superconducting qubits is simultaneously achieving long coherence times and fast two-qubit gates. Currently, fast (tunable) gates are obtained at the expense of coherence time. The authors propose a ``flatsonium'' qubit, based on the highly nonlinear fluxonium qubit, that yields both coherence and tunability---jackpot. Dephasing due to global flux noise is reduced by engineering flux-insensitive sweet spots in the qubit spectrum at the frequencies of interest, while leakage errors are eliminated by the typically large anharmonicity of its energy spectrum.
A central challenge in building a scalable quantum computer with superconducting qubits is simultaneously achieving long coherence times and fast two-qubit gates. Currently, fast (tunable) gates are obtained at the expense of coherence time. The authors propose a ``flatsonium'' qubit, based on the highly nonlinear fluxonium qubit, that yields both coherence and tunability---jackpot. Dephasing due to global flux noise is reduced by engineering flux-insensitive sweet spots in the qubit spectrum at the frequencies of interest, while leakage errors are eliminated by the typically large anharmonicity of its energy spectrum.
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, within the realm of superconducting qubits, reveals an alternative qubit platform that is competitive with the transmon system.
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T 2 > 100 μs (T 2 > 400 μs in one device).Moreover, given the state-of-the-art values of the surface loss tangent and the 1=f flux-noise amplitude, the coherence time can be further improved beyond 1 ms.Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit-over 10 2 here-must be minimized for best qubit coherence.We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and relaxation times in excess of 40μs at its flux-insensitive point. Qubit relaxation times T1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T2≈85μs, approximately the 2T1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary qubits based on transverse qubit-resonator interaction.
Quantum computers utilize qubits to store and process quantum information. In superconducting quantum computers, qubits are implemented as quantum superconducting resonant circuits. The circuits are operated only at the two energy states, which form the computational basis for the qubit. To suppress leakage to uncomputational states, superconducting qubits are designed to be anharmonic oscillators, which is achieved using one or more Josephson junctions, a nonlinear superconducting element. One of the main challenges in developing quantum computers is minimizing the decoherence caused by environmental noise. Decoherence is characterized by two coherence times, T1 for depolarization processes and T2 for dephasing. This thesis reviews and investigates the decoherence properties of superconducting qubits. The main goal of the thesis is to analyze the tradeoff between anharmonicity and dephasing in a qubit unimon. Recently developed unimon incorporates a single Josephson junction shunted by a linear inductor and a capacitor. Unimon is tunable by external magnetic flux, and at the half flux quantum bias, the Josephson energy is partially canceled by the inductive energy, allowing unimon to have relatively high anharmonicity while remaining fully protected against low-frequency charge noise. In addition, at the sweet spot with respect to the magnetic flux, unimon becomes immune to first-order perturbations in the flux. The sweet spot, however, is relatively narrow, making unimon susceptible to dephasing through the quadratic coupling to the flux noise. In the first chapter of this thesis, we present a comprehensive look into the basic theory of superconducting qubits, starting with two-state quantum systems, followed by superconductivity and superconducting circuit elements, and finally combining these two by introducing circuit quantum electrodynamics (cQED), a framework for building superconducting qubits. We follow with a theoretical discussion of decoherence in two-state quantum systems, described by the Bloch-Redfield formalism. We continue the discussion by estimating decoherence using perturbation theory, with special care put into the dephasing due to the low-frequency 1/f noise. Finally, we review the theoretical model of unimon, which is used in the numerical analysis. As a main result of this thesis, we suggest a design parameter regime for unimon, which gives the best ratio between anharmonicity and T2.
This thesis reports recent achievements toward scalable quantum computing with fluxonium, a superconducting artificial atom with rich energy spectrum and selection rules similar to those found in natural atoms. We show how such spectral properties can be harnessed to protect the qubit from energy relaxation and dephasing. At half-integer flux quantum bias, we show that fluxonium’s |0〉→ |1〉qubit transition has high coherence by design, with T1, T2≈500 μs in one device, the highest reported in superconducting circuits so far. Yet, the qubit exhibits the same level of addressability found in more conventional superconducting qubits (Tgate<50 ns). In addition, a controlled-Z gate can be implemented by sending a short2π-pulse at a frequency near the |1〉→|2〉transition of the target qubit. Preliminary results suggest that this gate can be used to entangle two fluxonium qubits with high fidelity. We also discuss experimental techniques employed to characterize the qubits, and present a perspective on future fluxonium-based quantum technologies.
Scaling up superconducting quantum processors with optimized performance requires a sufficient flexibility in the choice of operating points for single and two qubit gates to maximize their fidelity and cope with imperfections. Flux control is an efficient technique to manipulate the parameters of tunable qubits, in particular to activate entangling gates. At flux sensitive points of operation, the ubiquitous presence of 1/f flux noise however gives rise to dephasing by inducing fluctuations of the qubit frequency. We show how two-tone modulation of the flux bias, a bichromatic modulation, gives rise to a continuum of dynamical sweet spots where dephasing due to slow flux noise is suppressed to first order for a wide range of time-averaged qubit frequencies. The qubits can be operated at these dynamical sweet spots to realize protected entangling gates and to avoid collisions with two-level-system defects.