December 11, 2016

A few Magnets with controllable voltages could replace millions of lasers for large scale trapped ion quantum computers

Ion trap quantum computers hold and manipulate charged particles, called ions, to encode information. But to make a processor that works faster than a classical computer would require millions of such traps, each controlled with its own precisely aligned laser – making it extremely complicated.

Now, Winfried Hensinger at the University of Sussex in the UK and his colleagues have replaced the millions of lasers with some static magnets and a handful of electromagnetic fields. “Our invention has led to a radical simplification of the engineering required, which means we are now able to construct a large-scale device,” he says.

In their scheme, each ion is trapped by four permanent magnets, with a controllable voltage across the trap. The entire device is bathed in a set of tuned microwave and radio-frequency electromagnetic fields. Tweaking the voltage shifts the ions to a different position in the magnetic field, changing their state.

The simple method is where voltages are applied to a quantum computer microchip (without having to align laser beams) – to the same effect as the lasers. Professor Winfried Hensinger and his team also succeeded in demonstrating the core building block of this new method with an impressively low error rate at their quantum computing facility at Sussex.

Professor Hensinger said: “This development is a game changer for quantum computing making it accessible for industrial and government use. We will construct a large-scale quantum computer at Sussex making full use of this exciting new technology.”

Quantum computers may revolutionize society in a similar way as the emergence of classical computers. Dr Seb Weidt, part of the Ion Quantum Technology Group said: “Developing this step-changing new technology has been a great adventure and it is absolutely amazing observing it actually work in the laboratory.”

Scaling up will mean creating magnetic fields that vary in strength over relatively short distances. This a significant engineering challenge, says Mukherjee. Then there’s the challenge of handling waste heat, which becomes more problematic as the processor gets bigger. “As with any architecture, you need low heating rates,” he says.

Schematic of the linear Paul trap (yellow) fitted with four permanent magnets (blue), arranged to create a strong magnetic field gradient along the trap axis.

Physical Review Letters - Trapped-Ion Quantum Logic with Global Radiation Fields


Trapped ions are a promising tool for building a large-scale quantum computer. However, the number of required radiation fields for the realization of quantum gates in any proposed ion-based architecture scales with the number of ions within the quantum computer, posing a major obstacle when imagining a device with millions of ions. Here, we present a fundamentally different approach for trapped-ion quantum computing where this detrimental scaling vanishes. The method is based on individually controlled voltages applied to each logic gate location to facilitate the actual gate operation analogous to a traditional transistor architecture within a classical computer processor. To demonstrate the key principle of this approach we implement a versatile quantum gate method based on long-wavelength radiation and use this method to generate a maximally entangled state of two quantum engineered clock qubits with fidelity 0.985(12). This quantum gate also constitutes a simple-to-implement tool for quantum metrology, sensing, and simulation.

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