Towards a scalable trapped ion logical qubit

Introduction: Trapped ion qubits achieve excellent coherence times and gate fidelities, well below the threshold for fault tolerant quantum error correction. A key challenge now is to scale ion quantum processors to the large... [ view full abstract ]

Introduction:

Trapped ion qubits achieve excellent coherence times and gate fidelities, well below the threshold for fault tolerant quantum error correction. A key challenge now is to scale ion quantum processors to the large number of qubits required for error corrected algorithms to run. We present progress on the implementation of a high fidelity logical qubit in an ion qubit architecture designed for true scalability, based on robust microwave-driven gates and physical shuttling of ion qubits.

Methods & Results:

We show an X-junction surface ion trap, made using standard silicon microfabrication techniques. The 4-arm geometry permits dedicated zones, optimised for ion loading, memory, entangling interactions and readout. The two dimensional trap geometry is also scalable in a way that will facilitate a highly connected array of qubits, and therefore support topological error correction schemes such as the surface code.

Qubits are defined in the internal hyperfine states of the 171Yb+ ion. We utilise different qubit representations within the 4 natural spin states defined by the electron and nuclear spins, dressing the states to engineer an artificial clock-qubit less sensitive to environmental noise.

Entanglement is generated by modulating the coulomb interaction between ion pairs in a strong magnetic field gradient. Driving the spin-motion entangling (Molmer-Sorenen) gate directly with global microwave and RF fields rather than using locally focussed laser-driven Raman transitions relives much of the complexity in laser stabilisation, but requires a large magnetic field gradient [1]. We demonstrate improved fidelity with a new type of Molmer-Sorensen gate [2], showing it is robust to imperfect calibration of gate parameters, and also suppresses infidelity due to motional heating.

We also outline plans and progress toward implementing a fully protected logical qubit in the surface code. This will rely on controlled shuttling sequences interleaved with quantum logic, ion loading and readout operations.

Discussion:

Trapped ion qubits are long-lived, high fidelity and are mobile. The integration of global control fields with micro-fabricated silicon surface ion traps offers a promising and scalable pathway toward fault-tolerant scalable computing [3]. The implementation of a single robust logical qubit is a major milestone along this route.

 [1] S. Weidt, et al. Trapped-ion quantum logic with global radiation fields. Phys. Rev. Lett., 117:220501, 2016.

[2] A.  E.  Webb, et al. Resilient entangling gates for trapped ions. arXiv:1805.07351 [quant-ph], 2018.

[3] Lekitsch, B. et al. Blueprint for a microwave trapped ion quantum computer. Science Advances, 3(2), 2017.