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arXiv:1911.04635 · Applied Physics Letters

A long-lived capacitively shunted flux qubit embedded in a 3D cavity

Leonid V. Abdurakhimov, Imran Mahboob, Hiraku Toida, Kousuke Kakuyanagi, Shiro Saito

Published Dec 23, 2019• 43 citations
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Abstract

We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.

DOI
10.1063/1.5136262

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We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.
Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T1_11​ = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.
Abstract Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T 1 = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.
We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20 μs range and a pure dephasing time comprised between 3 and 10 μs. This significant improvement over previous works opens the way to the coherent coupling of a flux qubit to individual spins.
This article reviews a recent experiment that demonstrated a new hybrid type of tunable superconducting qubit consisting of a two-dimensional (2D) superconducting capacitively shunted flux qubit coupled to a 3D microwave cavity.Such a 3D hybrid flux qubit has extremely long relaxation times, which are comparable to or exceed the best values reported for 2D flux qubits.This new design of frequency-tunable qubits is uniquely suited for applications in bosonic quantum computing, which exploits multiphoton states of 3D microwave cavities.
A flux qubit can have a relatively long decoherence time at the degeneracy point, but away from this point the decoherence time is greatly reduced by dephasing. This limits the practical applications of flux qubits. Here we propose a new qubit design modified from the commonly used flux qubit by introducing an additional capacitor shunted in parallel to the smaller Josephson junction (JJ) in the loop. Our results show that the effects of noise can be considerably suppressed, particularly away from the degeneracy point, by both reducing the coupling energy of the JJ and increasing the shunt capacitance. This shunt capacitance provides a novel way to improve the qubit.
A flux qubit can have a relatively long decoherence time at the degeneracy point, but away from this point the decoherence time is greatly reduced by dephasing. This limits the practical applications of flux qubits. Here we propose a qubit design modified from the commonly used flux qubit by introducing an additional capacitor shunted in parallel to the smaller Josephson junction (JJ) in the loop. Our results show that the effects of noise can be considerably suppressed, particularly away from the degeneracy point, by both reducing the coupling energy of the JJ and increasing the shunt capacitance. This shunt capacitance provides a novel way to improve the qubit.