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  • Technical Review
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Engineering of microfabricated ion traps and integration of advanced on-chip features

Nature Reviews Physics volume 2, pages 285–299 (2020)Cite this article

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Abstract

Atomic ions trapped in electromagnetic potentials have long been used for fundamental studies in quantum physics. Over the past two decades, trapped ions have been successfully used to implement technologies such as quantum computing, quantum simulation, atomic clocks, mass spectrometers and quantum sensors. Advanced fabrication techniques, taken from other established or emerging disciplines, are used to create new, reliable ion-trap devices aimed at large-scale integration and compatibility with commercial fabrication. This Technical Review covers the fundamentals of ion trapping before discussing the design of ion traps for the aforementioned applications. We overview the current microfabrication techniques and the various considerations behind the choice of materials and processes. Finally, we discuss current efforts to include advanced, on-chip features in next-generation ion traps.

Key points

  • Trapped atomic ions are a highly versatile tool for a wide variety of fields from fundamental physics to quantum technologies.

  • Ion traps use electric and magnetic fields to provide 3D confinement of ions in free space. Ion traps can be fabricated with greater ease by using a surface electrode structure integrated within a microchip.

  • The realization of several quantum technologies that use trapped ions requires the integration of advanced features such as optics and electronics into such a microchip, either within a monolithic structure or through multi-wafer stacking.

  • Development is in progress concerning the integration of multiple features, especially in terms of the compatibility of fabrication processes, chip modularity, functionality and exact specifications of the desired features.

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Fig. 1: Method of rotating the principal axis by angle θ using a six-wire surface trap design41.
Fig. 2: X-junction electrode geometry.
Fig. 3: Ion-trap chips.
Fig. 4: Advanced on-chip technology in an ion trap.

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Acknowledgements

This work is supported by the UK Engineering and Physical Sciences Research Council via the EPSRC Hub in Quantum Computing and Simulation (EP/T001062/1), the UK Quantum Technology hub for Networked Quantum Information Technologies (no. EP/M013243/1), the European Commission’s Horizon-2020 Flagship on Quantum Technologies Project no. 820314 (MicroQC), the US Army Research Office under contract no. W911NF-14-2-0106, the Fonds National de la Recherche Luxembourg (National Research Fund) Project Code 11615035 and the University of Sussex.

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  1. Sussex Centre for Quantum Technologies, Department of Physics and Astronomy, University of Sussex, Brighton, UK

    Zak David Romaszko, Seokjun Hong, Martin Siegele, Reuben Kahan Puddy, Foni Raphaël Lebrun-Gallagher, Sebastian Weidt & Winfried Karl Hensinger

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Glossary

RF nil

Also known as RF null. The minimum energy of the RF pseudopotential, which gives the ion’s position in an RF field.

Hessian matrix

Matrix of second-order partial derivatives, in this case derivatives of the total potential seen by the ion.

Fidelity

The reliability of a certain operation. For a fault-tolerant quantum computer, the fidelity of every single operation must be above the relevant fault-tolerant threshold (99%).

Node

An established fabrication method that uses fixed fabrication methods to create structures. These can be characterized by feature size, materials, voltages and many other aspects.

Pseudopotential

An effective potential that accounts for the ion’s motion in an oscillating electric field.

MEMS

Micro-electro-mechanical systems combine electrical and mechanical features in a fabricated device. The sizes of the features used often make the processing similar to ion traps.

CMOS

Complementary metal–oxide semiconductor is a material structure that is used to make digital logic circuits. This is a mature industry that produces highly reliable structures in high quantities.

Plasma-enhanced chemical vapour deposition

A method of depositing materials through chemical reaction of ionized gases.

Breakdown

The point at which two electrically isolated electrodes become shorted because of a large voltage, often damaging the electrodes in the process.

Sputtering

A method of depositing a variety of materials onto a surface using accelerated ions striking a target of the desired material.

Evaporation

A method of depositing metals onto a surface by evaporating a metal and allowing the resulting flux to coat the surface.

Reactive ion etching

Also known as plasma etching, a method that uses a plasma to directionally etch a material.

CCWs

Current-carrying wires are ion-trap-specific structures that are designed to use currents to generate magnetic fields.

Damascene process

A fabrication process in which a pattern is etched that is subsequently filled (such as with copper). It is then planarized using a chemical mechanical polish. The dual damascene process combines pattern and vertical connections into one fabrication process.

Collection efficiency

The percentage of collected photons that have been emitted by an object.

Numerical aperture

A value that characterizes the solid angle over which a sensor or light source is exposed to an object, in this case an ion. This is a dimensionless quantity.

Die

A cut-out piece of a larger, fabricated wafer. For integrated circuits, a die is typically packaged in epoxy afterwards, making the circuits incompatible with ultra-high-vacuum environments.

Ball-grid array

A method that uses raised metal balls/bumps to attach a die to a circuit.

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Romaszko, Z.D., Hong, S., Siegele, M. et al. Engineering of microfabricated ion traps and integration of advanced on-chip features. Nat Rev Phys 2, 285–299 (2020). https://doi.org/10.1038/s42254-020-0182-8

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