Ion Traps

Ion trap for quantum computing  | Credit: NQIT/Stuart Bebb
Ion trap for quantum computing | Credit: NQIT/Stuart Bebb
We designed and constructed a new ion trap with high optical access which will be used for initial experiments in interfacing trapped-ion 'memory' qubits using photonic ‘communication’ qubits. This is the basic element of the Q20:20 network architecture.

Two key achievements from our first year are:

  1. The demonstration of a laser-driven, quantum logic gate between two different species of trapped-ion qubit (see “Research Case Study: ‘Hybrid’ Quantum Logic Gate”)
  2. The achievement of two-qubit quantum logic gates driven by electronic (microwave) signals with world-leading precision.

Microwave Quantum Logic with Trapped Ions

Quantum logic gates, the fundamental information processing function of a quantum computer, can be done with many different types of quantum system – NQIT is working on several, including ion traps, superconducting qubits and NV centres in diamond.

Within ion traps, NQIT is working on two different technology approaches for controlling the ions to perform logic operations: lasers and microwaves. Microwave ion trap devices can either use near-field microwaves or far-field microwaves – both are being explored by NQIT researchers.

Currently, laser-controlled quantum logic is the most mature technology of any quantum computing platform, with 99.9% fidelity having been achieved for two-qubit gates [1]. Microwave-controlled trapped-ion quantum logic is slightly behind this, with near-field achieving 99.7%, as demonstrated in a recent paper by Harty et al. [2] and far-field 98.5%, as demonstrated in the recent paper by Weidt et al. [3]. NQIT researchers hold the world record in both types of microwave quantum logic with experiments using the near field approach carried out by NQIT researchers at Oxford University and the far-field approach being implemented by the NQIT team at the University of Sussex

The minimum threshold for fault-tolerant quantum computing lies at around 99%, so currently the far-field microwave approach would not quite be sufficient; however, the precision of microwave techniques has advanced rapidly in recent years, as the plot below illustrates:

Graph showing fidelity achieved by various approaches to quantum logic
The plot above shows the dramatic improvement in precision of 2-qubit quantum logic gates over the last 15 years, in two leading quantum computing candidate technologies, trapped ions and superconducting circuits. For trapped ions, logic gates have been demonstrated both with laser control (black line) and, more recently, electronic microwave control (red and purple lines). The two recent results that use microwave ion traps are the the red point marked "Oxford" and the purple point marked "Sussex" on the plot: one technique uses “near-field” microwaves and 43Ca+ ions (where the ions are confined on a “chip trap” close to a microwave waveguide), the other uses “far-field” microwaves and 171Yb+ (which are “broadcast” onto the ions using a microwave antenna).

NQIT researchers at Sussex expect to cross this threshold within the next few months. The approach using far-field microwaves allows for a fundamentally different approach for trapped-ion quantum computing that uses voltages to execute quantum logic gates, rather than lasers. This new approach is based on individually-controlled voltages applied to each logic gate location, analogous to a traditional transistor architecture within a classical computer processor. When implemented, it would allow a substantial reduction in the number of laser beams required.

In the long-term, microwaves might be the more practical solution for creating large-scale quantum computers because microwaves can be controlled using conventional electronics, whereas laser-controlled ion traps require multiple laser beams, which could require more complex fabrication and thus be more expensive.

References:

[1] “High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits” by C. J. Ballance, T. P. Harty, N. M. Linke, M. A. Sepiol, and D. M. Lucas (Phys. Rev. Lett. 117, 060504 (2016)) and "High-Fidelity Universal Gate Set for 9Be+ Ion Qubits" J. P. Gaebler, T. R. Tan, Y. Lin, Y. Wan, R. Bowler, A. C. Keith, S. Glancy, K. Coakley, E. Knill, D. Leibfried, and D. J. Wineland (Phys. Rev. Lett. 117, 060505, (2016))

[2] “High-Fidelity Trapped-Ion Quantum Logic Using Near-Field Microwaves”, by T. P. Harty, M. A. Sepiol, D. T. C. Allcock, C. J. Ballance, J. E. Tarlton, and D. M. Lucas (Phys. Rev. Lett. 117, 140501 (2016))

[3] “Trapped-ion quantum logic with global radiation fields”, by S. Weidt, J. Randall, S. C. Webster, K. Lake, A. E. Webb, I. Cohen, T. Navickas, B. Lekitsch, A. Retzker, and W. K. Hensinger (Phys. Rev. Lett. 117, 220501 (2016))