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Physical Review A

High-fidelity quantum memory with floating electrons coupled to superconducting circuits

Ji-kun Xie, Rong-teng Cao, Ya-long Ren, Sheng-li Ma, Ren Zhang, Fuli Li

Published Nov 18, 2024• 1 citation
Publisher

Abstract

Hybridizing different degrees of freedom or physical platforms potentially offers various advantages in building scalable quantum architectures. We study the hybrid quantum system of a single electron trapped on a solid neon surface, where the motional states of an electron interact with microwave photons in an on-chip superconducting cavity. By applying a nonuniform magnetic field with a gradient along the vibrational axis, the spin-orbit coupling of the electron's spin degree of freedom and its lateral motion can be engineered. We show that the electron's spin states with ultralong coherence time as a quantum memory can be explored to store the quantum information of a superconducting qubit. As an extension, we also propose to implement high-quality quantum state transfer between the spin states of two electrons trapped in spatially separated superconducting cavities. The present work may have practical applications for realizing floating-electron-based hybrid quantum computation.

DOI
10.1103/physreva.110.052607

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Hybridizing different degrees of freedom or physical platforms potentially offers various advantages in building scalable quantum architectures. We study the hybrid quantum system of a single electron trapped on a solid neon surface, where the motional states of an electron interact with microwave photons in an on-chip superconducting cavity. By applying a nonuniform magnetic field with a gradient along the vibrational axis, the spin-orbit coupling of the electron's spin degree of freedom and its lateral motion can be engineered. We show that the electron's spin states with ultralong coherence time as a quantum memory can be explored to store the quantum information of a superconducting qubit. As an extension, we also propose to implement high-quality quantum state transfer between the spin states of two electrons trapped in spatially separated superconducting cavities. The present work may have practical applications for realizing floating-electron-based hybrid quantum computation.
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We propose a hybrid architecture in which an on-chip high finesse superconducting cavity is coupled to the lateral motion and spin state of a single electron trapped on the surface of superfluid helium. We estimate the motional coherence times to exceed 15 μs, while energy will be coherently exchanged with the cavity photons in less than 10 ns for charge states and faster than 1 μs for spin states, making the system attractive for quantum information processing and strong coupling cavity quantum electrodynamics experiments. The cavity is used for nondestructive readout and as a quantum bus mediating interactions between distant electrons or an electron and a superconducting qubit.
Electrons floating in vacuum provide a clean platform for quantum information processing owing to their isolation from material defects. In particular, electrons on solid neon have emerged as a promising qubit platform because of their potentially long coherence times. Here, toward spin-qubit realization, we couple a single electron on solid neon to a magnetic-field-compatible superconducting NbTiN nanowire resonator. We realize a charge qubit and demonstrate microwave readout and coherent control, with Rabi frequencies up to 76 MHz, an order of magnitude larger than in previous studies. Under strong driving, we observe a qubit frequency shift from nonlinear interactions with the intense microwave field. Deterministic electron trapping at an intended position remains challenging due to solid neon surface roughness; we characterize the electron's position from its differential coupling to distinct electrodes. Although not trapped at an intended position, our estimates indicate that spin-qubit demonstrations remain feasible.
The promise of quantum computing has driven a persistent quest for new qubit platforms with long coherence, fast operation, and large scalability. Electrons, ubiquitous elementary particles of nonzero charge, spin, and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits via either motional or spin states depends critically on their material environment. Here we report our experimental realization of a new qubit platform based upon isolated single electrons trapped on an ultraclean solid neon surface in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, we achieve strong coupling between the motional states of a single electron and microwave photons in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are used to measure the energy relaxation time T1T_1T1​ of 15 μ15~\mu15 μs and phase coherence time T2T_2T2​ over 200 ns, indicating that the electron-on-solid-neon qubit already performs near the state of the art as a charge qubit.
The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer.Circuit quantum electrodynamics allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom.Here, we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator.The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath.Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model.The coupling strength of the ensemble to the cavity is found to be ≈1 MHz per electron, indicating the feasibility of achieving single electron strong coupling.
Electron-on-neon (eNe) qubits have recently emerged as a compelling platform for quantum computing, which combines the vacuum isolation advantages of trapped-ion qubits with the scalability of superconducting circuits. In this system, electrons are trapped in vacuum above a solid neon film deposited on superconducting microwave resonators, where they exhibit strong coupling to the resonators, coherence times of ~0.1 ms, and single-qubit gate fidelities exceeding 99.97%. A central challenge, however, is the spontaneous binding of electrons to neon surface bumps. These bumps, originating from substrate roughness, vary in size: electrons on bumps of suitable sizes within the resonator can couple to microwave photons and function as qubits, whereas those on unfavorable bumps remain inactive yet contribute to background charge noise. Moreover, both the bump landscape and the sites where electrons bind differ from run to run, leading to variable qubit characteristics that hinder scalability. To address this challenging issue, we present an on-chip magnetic-levitation architecture in which arrays of solid-neon microparticles are suspended above the processor chip to act as electron carriers. This design eliminates substrate effects while retaining strong qubit-resonator coupling and supporting inter-qubit connectivity. Our analysis further shows that the qubit transition frequency can be tuned across the gigahertz range and its anharmonicity can reach ~0.8 GHz by tuning the resonator bias voltage. Together, these features suggest a promising pathway toward robust, reproducible, and scalable eNe-based quantum computing.