High-Coherence Hybrid Superconducting Qubit

Abstract

We measure the coherence of a new superconducting qubit, the low-impedance flux qubit, finding T * 2 ∼ T1 ∼ 1.5µs. It is a three-junction flux qubit, but the ratio of junction critical currents is chosen to make the qubit's potential have a single well form. The low impedance of its large shunting capacitance protects it from decoherence. This qubit has a moderate anharmonicity, whose sign is reversed compared with all other popular qubit designs. The qubit is capacitively coupled to a high-Q resonator in a λ/2 configuration, which permits the qubit's state to be read out dispersively. PACS numbers:While there have been many successful superconducting qubit types, their large diversity suggests that the optimal qubit will be a hybrid combining favorable features of all: the tunability of the flux qubit [1][2][3], the simplicity, robustness and low impedance of the phase qubit [4][5][6] and the high coherence and compatibility with high-Q superconducting resonators of the transmon [7,8]. We have built such a hybrid, related to a suggested design of You et al. [9]. Our capacitively shunted flux qubit begins as a traditional three-junction loop[1], but is made to have low impedance by virtue of a large capacitive shunt (C s = 100fF) of the small junction. This new superconducting qubit is as coherent as the best currently reported; we measure T * 2 ∼ T 1 ∼ 1.5µs. Since the key to this qubit is the large shunting capacitance C s and therefore its low effective impedance L J /C s , we will call it the low-impedance flux qubit ( Z flux qubit). As Fig. 1(a) shows, the shunt capacitor is realized using a simple, reliable single-level interdigitated structure. We choose the ratio of the small and large junction critical currents I 0 to be around α = 0.3. For this α the qubit potential has only one minimum (see Eq. (3) below), and the qubit shows only a weak dependence of the qubit frequency ω 01 on applied flux Φ. As for the original flux qubit, a "sweet spot" exists at which the qubit is to first order insensitive to Φ, giving rise to long dephasing times, but even away from this degeneracy point our frequency sensitivity is about a factor of 30 smaller than in the traditional flux qubit. Our flux sensitivity is comparable to that of the phase qubit (∂ω 01 /∂Φ ∼ 30 GHz/Φ 0 ) which permits tunability without completely destroying phase coherence, despite the presence of significant flux noise amplitude on the order of S Φ = 1 − 2µΦ 0 / √ Hz. Modeling indicates that our qubit at the sweet spot still has appreciable anharmonicity, with |ω 12 − ω 01 |/2π in the neighborhood of several 100 MHz (or about 2 − 10% of the qubit resonance frequency, depending on α), but interestingly, with ω 12 > ω 01 , the opposite of any important qubit except the flux qubit. Such anharmonicity leads to a situation where all of the lowest energy levels for a two-qubit system would be those of the computational manifold |0 and |1 , which will facilitate coupled qubit experiments. The reduced impedance of this qubit has several advantages. Qubits...

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