Journal Article

FZJ-2019-02013

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Toward a silicon-based quantum computer

Schreiber, L. R. ; Bluhm, H. (Corresponding author)FZJ*

2018
Moses King Cambridge, Mass.

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Please use a persistent id in citations: doi:10.1126/science.aar6209

Abstract: Quantum computing could enable exponential speedups for certain classes of problems by exploiting superposition and entanglement in the manipulation of quantum bits (qubits). The leading quantum systems that can be used include trapped ions, superconducting qubits, and spins in semiconductors. The latter are considered particularly promising for scaling to very large numbers of qubits. On page 439 of this issue, Zajac et al. (1) demonstrate a quantum operation involving two qubits in silicon (Si), which is a major step for the field of semiconductor qubits. Together with easier-to-achieve manipulation of single qubits, these operations represent the basic steps of any quantum algorithm.The coupling between the two qubits is achieved through the so-called exchange interaction, which results from coupling of the two electrons through a tunnel barrier. This barrier can be controlled by changing the voltage on the central gate. The authors further use microwave excitation to implement the desired operation. In an external magnetic field, spins that are not aligned with the field precess around it like an asymmetrically suspended top. If an excitation (a microwave signal applied to one of the gates, which translates to a magnetic microwave field in the inhomogeneous stray field of a micromagnet) has the same frequency as the precession, it is possible to rotate the spin direction, for example, from parallel to antiparallel with the field.This technique, commonly used to control individual qubits, enables two-qubit operation via the dependence of the precession rate of one spin on the state of the other because of the exchange interaction. Whether single-qubit or two-qubit operation is executed can be selected by the choice of tunnel coupling and microwave frequency. For the two-qubit operation, they are set such that if the so-called control spin is aligned with the external field, the other so-called target spin is on resonance with the microwave signal (and off resonance if anti-aligned). Whether the target spin is inverted depends on the state of the control spin, so this system functions as a controlled NOT (CNOT) gate.Zajac et al. used a device consisting of a layer of Si that was strained by being grown between two layers of SiGe (Si/SiGe), which confine electrons to the Si layer. Additional lateral confinement was provided by electrostatic gates fabricated on top of the structure, which were arranged such that two electrons can be captured (see the figure). The spin of each of those electrons encoded one qubit. Similar qubits have been realized in GaAs/AlGaAs heterostructures, but in that material system, the interaction with unavoidable nuclear spins is a major complication that impedes highly accurate qubit operation (2). Only 4.7% of the nuclei carry spin in Si with a natural composition, and that fraction can be further reduced with isotopic purification. This approach has recently led to record-setting coherence times over which quantum states could be preserved (3). However, the controlled confinement of single electrons in Si has been a major challenge because of disorder in the material. Petta and co-workers (4) made important progress on sample quality and design, culminating in an array of nine quantum dots, which could in principle host nine qubits.

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Contributing Institute(s):

1. JARA Institut Quanteninformation (PGI-11)

Research Program(s):

1. 144 - Controlling Collective States (POF3-144) (POF3-144)

Appears in the scientific report 2019

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