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Long-Distance Spin-Spin Coupling via Floating Gates

Luka Trifunovic, Oliver Dial, Mircea Trif, James R. Wootton, Rediet Abebe, Amir Yacoby, and Daniel Loss
Phys. Rev. X 2, 011006 – Published 26 January 2012
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Abstract

The electron spin is a natural two-level system that allows a qubit to be encoded. When localized in a gate-defined quantum dot, the electron spin provides a promising platform for a future functional quantum computer. The essential ingredient of any quantum computer is entanglement—for the case of electron-spin qubits considered here—commonly achieved via the exchange interaction. Nevertheless, there is an immense challenge as to how to scale the system up to include many qubits. In this paper, we propose a novel architecture of a large-scale quantum computer based on a realization of long-distance quantum gates between electron spins localized in quantum dots. The crucial ingredients of such a long-distance coupling are floating metallic gates that mediate electrostatic coupling over large distances. We show, both analytically and numerically, that distant electron spins in an array of quantum dots can be coupled selectively, with coupling strengths that are larger than the electron-spin decay and with switching times on the order of nanoseconds.

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  • Received 26 October 2011

DOI:https://doi.org/10.1103/PhysRevX.2.011006

This article is available under the terms of the Creative Commons Attribution 3.0 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

Synopsis

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Floating Gates

Published 26 January 2012

Semiconductor quantum dots connected by floating metallic gates point the way to a scalable quantum computer.

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Authors & Affiliations

Luka Trifunovic1, Oliver Dial2, Mircea Trif1,3, James R. Wootton1, Rediet Abebe2, Amir Yacoby2, and Daniel Loss1

  • 1Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
  • 2Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
  • 3Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA

Popular Summary

It was Richard Feynman who first proposed, in 1982, the far-reaching concept of a “quantum computer”—a device more powerful than digital computers that makes direct use of quantum mechanics to perform computational operations. The first proposals for the physical implementation of quantum computation appeared in the ’90s. Among those, the idea of using electron spins trapped in electrostatic semiconductor quantum dots as the building blocks of a quantum computer (the so-called spin qubits), put forward by Daniel Loss and David DiVincenzo in 1997, emerged as the most propitious one. The first decade of the new century saw a steady improvement in the decoherence time for spin qubits (the time over which the information carried by the spin qubits is lost) by 7 orders of magnitude, with the present state-of-the-art decoherence time being as large as 270μs!

Nevertheless, the implementation of the original Loss-DiVincenzo proposal posed a considerable technical challenge. It used quantum tunneling between qubits to enable their communication with each other, and thus required that the qubits be placed very close to each other. This requirement not only leaves little space for the placement of the vast amount of gates and wirings needed to define the electrostatic quantum dots, but also makes it challenging to control the local magnetic field needed for single-qubit operations. For these reasons, no system with more than a couple of spin qubits has been successfully implemented thus far. In the present work, we leap over this long-standing problem with an entirely different strategy of using metallic floating gates to couple together qubits that are separated over a long range.

This new strategy bestows the required space for gates and wirings and also dispenses with the demand for local manipulation of the magnetic field. We have analyzed it in great detail and shown, both analytically and numerically, that distant spins in an array of quantum dots can indeed be coupled, and coupled selectively. We have also shown how to implement in our design the CNOT operation, which can be used to entangle qubits, providing the set of operations required for universal quantum computation. That entanglement between the distant qubits, a purely quantum effect, can be achieved by classical objects—the metallic floating gates—is rather amazing, in our view.

Looking ahead, we strongly believe that the present work opens up new avenues to a future working quantum computer based on not one, but several, types of qubits.

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Vol. 2, Iss. 1 — January - March 2012

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