Cold ion beam in a storage ring as a platform for large-scale quantum computers and simulators: Challenges and directions for research and development

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Cold ion beam in a storage ring as a platform for large-scale quantum computers and simulators: Challenges and directions for research and development

T. Shaftan and Boris B. Blinov

Phys. Rev. Accel. Beams 24, 094701 – Published 13 September 2021

Abstract

The purpose of this paper is to evaluate the possibility of constructing a large-scale storage-ring-type ion-trap system capable of storing, cooling, and controlling a large number of ions as a platform for scalable quantum computing (QC) and quantum simulations. In such a trap, the ions form a crystalline beam moving along a circular path with a constant velocity determined by the frequency and intensity of the cooling lasers. In this paper, we consider a large leap forward in terms of the number of ions that serve as qubits in QC, from fewer than 100 available in state of the art linear ion-trap devices today to an order of $1 0^{5}$ crystallized ions in the storage-ring setup. This new trap design unifies two different concepts: the storage rings of charged particles and the linear ion traps used for QC and mass spectrometry. In this paper, we use the language of particle accelerators to discuss the ion state and dynamics. We outline the differences between the above concepts, analyze challenges of the large ring with a revolving chain of ions, and propose goals for the research and development required to enable future quantum computers with 1000 times more qubits than available today. The challenge of creating such a large-scale quantum system while maintaining the necessary coherence of the qubits and the high fidelity of quantum logic operations is significant. Performing analog quantum simulations may be an achievable initial goal for such a device. Quantum calculations and simulations of complex quantum systems will move forward both the fundamental science and the applied research. Nuclear and particle physics, many-body quantum systems, lattice gauge theories, and nuclear structure calculations are just a few examples in which a large-scale quantum simulation system will become a very powerful tool to move forward our understanding of nature.

Received 28 October 2020
Accepted 9 August 2021

DOI:https://doi.org/10.1103/PhysRevAccelBeams.24.094701

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Low-energy multiple-particle dynamics Quantum computation Quantum information processing

Accelerators & Beams

Authors & Affiliations

T. Shaftan¹ and Boris B. Blinov ²

¹National Synchrotron Light Source–II, Brookhaven National Laboratory, Upton, New York 11973, USA
²Department of Physics, University of Washington, Seattle, Washington 98195, USA

Article Text

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Issue

Vol. 24, Iss. 9 — September 2021

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Images

Figure 1
(a) Simplified diagram of the valence electron state in the ion and indication of the energy levels. The qubit transition (yellow arrow) is between the ground state $| g, 0 ⟩$ and the excited state $| e, 0 ⟩$ , while the sideband transitions are shown with red and blue arrows. (b) The machine geometry with the ion chain orbiting the ring.
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Figure 2
Doppler laser cooling beam arrangement. (a) Two focused, counter propagating laser beams (“Doppler laser 1” and “Doppler laser 2”) with different frequencies but equal intensities are applied tangentially to the ion orbit to cool the ion motion in the $z$ -direction, as well as to set the orbital velocity. An additional pair of counterpropagating laser beams (“Transverse Doppler laser”), shaped to have an elongated profile along the ion orbit direction, is applied perpendicular to the orbit to enable cooling of the x and y directions. (b) The relevant atomic energy levels and laser detunings in the lab frame (left) and the moving Ion frame (right). In the lab frame, the detuning $Δ_{1}$ of the “Doppler laser 1” is greater than the detuning $Δ_{2}$ of the “Doppler laser 2.” This creates a larger scattering force by the “Doppler laser 2,” so that the ions are forced to move in the direction of that laser until their velocity is such that the detunings of both lasers are the same ( $Δ$ ) in the Ion frame. Then the scattering forces due to both lasers are equal, and the ions move with a constant speed.
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Figure 3
Model of the quantum gate, illustrating the laser Gaussian beam transverse envelope in red, intensity distribution of the laser beam at the waist in brown, and positions of ions in the crystalline beam (blue), which is moving from bottom to top and traversing the laser beam in the interaction region. The laser waist is a half of the distance between the consecutive ions and the laser pulse length is a half of the time of flight of the ions through the same region. A single laser pulse interacts with a single ion setting the qubit’s state prior to execution of a cycle of quantum computation.
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Figure 4
Intensified charge-coupled device CCD camera images of trapped Ba ions in a University of Washington linear trap. Only the ${}^{138}{Ba}$ ions are resonant with the Doppler cooling laser, so only they are visible. ${}^{136}{Ba}$ ions appear as dark gaps in the chain. Top: A five-ion chain composed of five ${}^{138}{Ba}$ ions. Middle: A five-ion chain composed of four ${}^{138}{Ba}$ ions and one ${}^{136}{Ba}$ ion (second from the left). Bottom: A five-ion chain composed of three ${}^{138}{Ba}$ ions and two ${}^{136}{Ba}$ ions (second and fifth from the left).
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