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arXiv:2108.11260 · Physical Review Applied

Engineering, Control, and Longitudinal Readout of Floquet Qubits

Anthony Gandon, Camille Le Calonnec, Ross Shillito, Alexandru Petrescu, Alexandre Blais

Published Jun 2, 2022• 26 citations
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Abstract

Time-dependent eigenstates of Hamiltonians offer a large degree of tunability, which can be leveraged to operate single- and two-qubit gates so that they stay dynamically protected from noise. However, the usual analytical techniques for describing these gates are limited for systems subject to drives of different frequencies. Thus the authors generalize the Floquet approach to the understanding of such systems, and provide protocols to robustly control and measure these complex systems.

DOI
10.1103/physrevapplied.17.064006

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Time-dependent eigenstates of Hamiltonians offer a large degree of tunability, which can be leveraged to operate single- and two-qubit gates so that they stay dynamically protected from noise. However, the usual analytical techniques for describing these gates are limited for systems subject to drives of different frequencies. Thus the authors generalize the Floquet approach to the understanding of such systems, and provide protocols to robustly control and measure these complex systems.
Quantum control and noise protection of a Floquet 0−π0-π0−π qubit
Time-periodic systems allow engineering new effective Hamiltonians from limited physical interactions. For example, the inverted position of the Kapitza pendulum emerges as a stable equilibrium with rapid drive of its pivot point. In this work, we propose the Kapitzonium\textit{Kapitzonium}Kapitzonium: a Floquet qubit that is the superconducting circuit analog of a mechanical Kapitza pendulum. Under periodic driving, the emerging qubit states are exponentially protected against bit and phase flips caused by dissipation, which is the primary source of decoherence of current qubits. However, we find that dissipation causes leakage out of the Floquet qubit subspace. We engineer a passive cooling scheme to stabilize the qubit subspace, which is crucial for high fidelity quantum control under dissipation. Furthermore, we introduce a hardware-efficient fluorescence-based method for qubit measurement and discuss the experimental implementation of the Floquet qubit. The proposed Kapitzonium is one of the simplest Floquet qubits that can be realized with current technology -- and it already has many intriguing features and capabilities. Our work provides the first steps to develop more complex Floquet quantum systems from the ground up to realize large-scale protected engineered dynamics.
Periodic driving is a powerful tool for engineering tunable many-body quantum states. Such Floquet engineering relies on the existence of a localized prethermal regime, a state which remains incompletely understood. We report experiments on a many-body Floquet system consisting of ultracold atoms in an optical lattice subjected to sign-changing amplitude modulation. A double-quench protocol enables measurement of an inverse participation ratio quantifying the degree of prethermal localization as a function of continuously tunable drive parameters and interactions. Following the time evolution of the driven system, we observe the sequential formation of two localized prethermal plateaux, interaction-driven delocalization, and strongly frequency-dependent dynamics of departure from the prethermal state. The experimental control and quantitative characterization of Floquet prethermalization opens new possibilities for dynamical quantum engineering.
Tunability of an artificial quantum system is crucial to its capability to process quantum information. However, tunability usually poses significant demand on the design and fabrication of a device. In this work, we demonstrate that Floquet engineering based on longitudinal driving provides distinct possibilities in enhancing the tunability of a quantum system without needing additional resources. In particular, we study a multilevel model based on gate-defined double quantum dots, where coherent interference occurs when the system is driven longitudinally. We develop an effective model to describe the driven dynamics of this multilevel system, and show that it is highly tunable via the driving field. We then illustrate the versatility and rich physics of a driven multilevel system by exploring phenomena such as driving modulation of resonances, adiabatic state transfer, and dark state. In the context of qubit control, we propose noise-resistant quantum gates based on adiabatic passage. The theoretical consideration we present here is rather general, and is in principle valid for other multilevel quantum systems.