Ion trap quantum computing

The main focus of the ABaQuS project is developing tools and techniques to realise high-precision quantum control of long chains of ¹³³Ba⁺ ions. The most natural way to scale up a trapped ion quantum system is by simply adding more ions to the same (linear) chain. The challenge is then to maintain the operation fidelity at a level that allows one to do non-trivial computations. This is a complex task for the following reasons. First, the micrometre-scale ion-ion separation makes individual qubit addressing very difficult. In addition, the reduced spectral gap between neighbouring motional modes makes it increasingly challenging to implement high-fidelity entangling gates. Through the right choice of physical system and ion trap device, and by utilising state-of-the-art laser technology and advanced classical control techniques, we aim at building a high-performance quantum device.

¹³³Ba⁺ is a synthetic nucleus whose level structure is favourable for trapped ion quantum computing in several ways. All electronic transitions used to manipulate the ions are in the visible part of the spectrum, hence we can use commercial state-of-the-art laser and fibre components. Moreover, the nuclear spin of ¹³³Ba⁺ is ½ and therefore it has hyperfine structure and offers a range of potential ‘clock’ qubis with very long coherence times. In particular, there is a clock qubit in the ground S_1/2 state (this is our main qubit), and a clock qubit in the metastable D_5/2 state that is connected to the ground state qubit via a pair of clock transitions. The small nuclear spin also implies a simple level structure that allows us to trivially implement fundamental operations, such as state preparation and readout, with high fidelity. The combination of unique aspects of the ¹³³Ba⁺ level structure opens up an interesting playground for implementing many novel qubit control techniques, including partial projective measurements, mid-circuit measurements and qubit hiding schemes.

The ions are confined in a monolithic microfabricated 3D trap that is designed and manufactured by the National Physical Laboratory in the UK. The 3D architecture allows us to create a very deep trapping potential such that the ions are very well isolated from the environment. The segmented structure of the trap provides a high degree of control of the trapping potential. This is crucial for being able to reconfigure the ion crystal, split up the ion chain and shuttle ions between different experiment zones in the trap.

As of May 2021 the system is reliably trapping naturally abundant ¹³⁸Ba⁺ ions that are used to characterise various aspects of the hardware performance. When this is complete, we will begin work with ¹³³Ba⁺. In parallel with the experimental work in the lab, there are several theory projects underway aimed at simulating and optimising multi-qubit couplings in large ion registers.

Research directions

• Pushing the limits of laser and optics technology to realise low-noise high-precision individual quantum control of the ions
• Developing theoretical numerical simulation tools to understand qubit interactions in large registers
• Investigating novel qubit control techniques, including, but not limited to, partial projective measurements, mid-circuit measurements, qubit hiding schemes
• Understanding of, and designing, quantum algorithms for intermediate-scale trapped-ion devices

People

Current

• Dr. Chris Ballance (PI)
• Dr. Fabian Pokorny (Postdoc)
• Ana Sotirova (Graduate student)
• Jamie Leppard (Graduate student)
• Andres Vazquez-Brennan (Graduate student)

Former

• Lee Peleg (Visiting graduate student)
• Peter Jones (Masters student)
• Ross Jenkinson (Masters student)