NTT technical review

A Long-lived Tunable Qubit for Bosonic Quantum Computing

Leonid Victorovich Abdurakhimov, Imran Mahboob, Hiraku Toida, Kosuke Kakuyanagi, Shiro Saito

Published May 1, 2021• 2 citations

Abstract

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.

DOI: 10.53829/ntr202105fa3

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NTT technical review

A Long-lived Tunable Qubit for Bosonic Quantum Computing

Leonid Victorovich Abdurakhimov, Imran Mahboob, Hiraku Toida, Kosuke Kakuyanagi, Shiro Saito

Published May 1, 2021• 2 citations

Publisher

Abstract

DOI: 10.53829/ntr202105fa3

Related papers

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6

with a

Z

-fidelity of

82\%

. We also find that this enhancement is not limited by fundamental constraints.

^4

He. By measuring spectroscopic and coherence properties we find that the cavity, the qubit, and their coupling are all modified by the presence of the dielectric superfluid, which we analyze within the framework of circuit quantum electrodynamics (cQED). At temperatures relevant to quantum computing experiments, the energy relaxation time of the qubit is not significantly changed by the presence of the superfluid, while the pure dephasing time modestly increases, which we attribute to improved thermalization via the superfluid.

_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 construct a series of graphene-based superconducting quantum circuits and integrate them into 3D cavities. For a single-qubit device, we demonstrate flux-tunable qubit transition, with a measured

T_1

\approx

48 ns and a lower bound estimate of

T_2^\ast

\approx

17.63 ns. By coupling the device to cavities with different resonant frequencies, we access multiple qubit-cavity coupling regimes, enabling the observation of vacuum Rabi splitting and flux-dependent spectral linewidths. In a two-qubit device consisting of a SQUID and a single junction, power-dependent measurements reveal a two-stage dispersive shift. By flux-tuning the cavity frequency at different readout powers, we attribute the first shift to the fixed-qubit and the second to the SQUID-qubit, indicating successful coupling between the two circuits and a single cavity mode. Our study demonstrates the flexible coupling achievable between 2D-material-based superconducting circuits and 3D cavities, and paves the way toward constructing multi-qubit 3D transmon devices from 2D materials.