Accelerating quantum algorithms using microwave-driven multi-qubit logic

Aims and Objectives

We aim to accelerate trapped-ion quantum algorithms by using the native advantages of microwave-driven hyperfine qubits to replace slow shuttling and re-cooling operations by fast, high-fidelity addressing in a multi-qubit register, combined with global operations. We also propose to use a new "omg"-type scheme to enable efficient, same-species cooling without the need for a secondary ion species.

We have recently set new benchmarks for speed and fidelity of addressed single-qubit gates driven by microwaves [Leu2023], as well as achieving an order-of-magnitude speed-up in the two-qubit gate [Weber2023]. With these gate speeds approaching those of typical laser gates, the bottleneck in quantum CCD processing speeds becomes shuttling and sideband cooling operations [Moses2023].

Here we aim to leverage the high-performance of microwave operations to reduce the reliance on shuttling/cooling, to enable acceleration of algorithms by one to two orders of magnitude. Novelty of the research methodology:

Proof-of-principle experiments will be carried out in our existing flow cryostat system. We will co-develop a next-generation system based around a closed-cycle cryostat, which will allow more rapid testing of new trap designs; we will use this to trial a new trap design incorporating integrated addressing capability for both single- and two-qubit gates.

EPSRC Research Areas:

This project falls within the EPSRC Quantum technology/ ICT(q.computing) research area' where Quantum technology is one of the themes or research areas listed on this website. Collaborators / companies none at present Research plan

The principal technical tasks of this project are described below, together with their expected timescales and completion milestones. 1- Flow cryostat experiments Task 1.1 - System upgrades

Description - Laser infrastructure upgrades will support the development of a same-species sympathetic coolant, and consists of extensions to existing 729nm and 854nm laser systems. We will require AOM breadboards and beam delivery optics for both wavelengths and an additional cavity to which we will lock the 854nm laser. Testing of these upgrades will be dominated by the spectroscopy of the 729 nm quadrupole transitions in the yet unexplored intermediate to strong field regime at 288 Gauss. Installation of a "Stinger" helium recycling system will be carried out in parallel.

Expected duration - 6 months Milestone - Cryogenic system functional and new laser beams installed Task 1.2 - Same-species sympathetic cooling

Description - Demonstrate operation of a same-species two-ion 43Ca+ crystal with logic and coolant ions (scheme described above). Expected duration - 6 months

Milestone - sympathetic cooling of one motional mode of a two-ion crystal close to its ground state while maintaining errors on logic qubit below 10-3. Task 1.3 - Development of 3-ion manipulation techniques

Description - Develop three-ion routines based on existing two-ion implementations, such as loading, state-preparation, shuttling-readout and ion addressing in view of operating two logical ions and one sympathetic coolant or three logical ions. Expected duration - 6 months

Milestone - Addressed single-qubit gate RB on a chain of 3 ions with errors <10^(-4), where the induced ion motion from crystal-twisting causes entangling gate errors <10^(-3). Task 1.4 - Randomised benchmarking (RB) of 2-qubit gate with a sympathetic coolant

Description - Initial tests will be carried out in a two-ion crystal without single-ion addressing, resorting to "subspace" RB. We will then interleave single ion addressing with two qubit gates to implement "full" RB. Finally, we will add a third ion to implement RB with interleaved sympathetic cooling.

Expected duration - 6 months

Milestone - Demonstrate an elimination of errors induced by the gate-to-gate increase of phonon occupation through sympathetic cooling, without deg