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Quantum Information and Measurement (QIM)

Microwave-driven high-fidelity quantum logic with 43Ca+

R Hanley, Jochen. Wolf, Clemens Löschnauer, Marius Weber, Joseph F. Goodwin, T. P. Harty, Andrew M. Steane, David. M. Lucas

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We present the design and initial characterisation of a next-generation surface-electrode ion-trap designed for room-temperature or cryogenic operation, that will aim to improve both the fidelity and speed achieved in microwave-driven quantum gates.
DOI
10.1364/qim.2019.s4b.4

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•Jan 1, 2019•8 citations
Abstract We report the design, fabrication, and characterization of a cryogenic ion trap system for the implementation of quantum logic driven by near-field microwaves. The trap incorporates an on-chip microwave resonator with an electrode geometry designed to null the microwave field component that couples directly to the qubit, while giving a large field gradient for driving entangling logic gates. We map the microwave field using a single 43Ca+ ion, and measure the ion trapping lifetime and motional mode heating rates for one and two ions.
The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis reports experimental progress in both of these areas. In the early part of the thesis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trapping of 40Ca+ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two decrease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in 40Ca+ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.
Microwave-driven quantum logic gates in trapped-ion systems offer a scalable and laser-free alternative to optical control, with the potential for robust integration into surface-electrode trap architectures. In this work, we present a systematic design guideline for planar ion traps optimized for fast two-qubit microwave gates using chip-integrated conductors. We investigate two electrode configurations, one employing a single microwave line for driving σ transitions, and another with two symmetric lines for π transitions. Through finite-element simulations, we analyze ion height, magnetic field gradients, heating effects, and gate durations under realistic cryogenic conditions. Our results show that both configurations can achieve two-qubit gate times in the order of 10 μs for Be+9 and Ca+40 ions.
Microwave-driven trapped ion logic gates offer a promising avenue for advancing beyond laser-based logic operations. In future microwave-based operations, however, the joule heat produced by large microwave currents flowing through narrow microwave electrodes would potentially hinder improvements in gate speed and fidelity. Moreover, scalability, particularly in cryogenic trapped ion systems, is impeded by the excessive joule heat. To address these challenges, we present a novel approach: superconducting surface trap chips that integrate high- Q microwave resonators with large current capacities. Utilizing sub-ampere microwave currents in superconducting Nb resonators, we generate substantial magnetic field gradients with significantly reduced losses compared to conventional metal chips. By harnessing the high Q factors of superconducting resonators, we propose a power-efficient two-qubit gate scheme capable of achieving a sub-milliwatt external microwave input power at a gate Rabi frequency of 1 kHz.
We describe the design, fabrication, and testing of a surface-electrode ion trap, which incorporates microwave waveguides, resonators, and coupling elements for the manipulation of trapped ion qubits using near-field microwaves. The trap is optimised to give a large microwave field gradient to allow state-dependent manipulation of the ions' motional degrees of freedom, the key to multi-qubit entanglement. The microwave field near the centre of the trap is characterised by driving hyperfine transitions in a single laser-cooled 43Ca+ ion.
Towards the goal of a large-scale quantum computer based on trapped ions, near-field microwaves represent a promising approach to perform the techni- cally challenging key operation of a two-qubit entangling gate. In this thesis we present the first microwave-driven two-qubit gate in 9Be+ ions employing a first-order field-independent qubit transition and a scalable surface-electrode ion trap at room temperature. We test the quality of the gate operation by producing a maximally entangled state and measuring the resulting state preparation fidelity in a reduced tomography procedure. For the best two- qubit gate achieved in the system we find this fidelity to be F = 98.2 ± 1.2 %. Following a comprehensive error analysis based on numerical simulations and experimentally determined input parameters, we identify current infidelity con- tributions of the apparatus. Here, the natural error source of the microwave near-field approach, namely fluctuating AC Zeeman shifts, could be reduced to the 10^−4 level due to the optimized design of the employed microwave con- ductor. As we find that the three largest errors can all be reduced upon purely technical improvements, higher fidelities are feasible in the future. Besides the gate realization, the thesis also comprises the initial characterization of the employed ion trap as well as the design and construction of a Raman laser at ∼ 313 nm which is utilized to perform near ground state cooling of radial modes of a single- and two-ion crystal. Here, ensuing heating rate measure- ments on a single-ion’s radial modes show good agreement with the electric- field noise spectral density expected from literature given the chosen mode frequency and nearest ion-to-electrode distance of 70 µm. Finally, we present the operation of a first multi-layer ion trap whose electrode layout features a 3-dimensional microwave conductor intended for performing microwave-driven two-qubit gates. Following a characterization of the resulting near-field pat- tern using a single ion as a local field probe, we find the multi-layer conductor to have significantly better field properties when compared to an equivalent single-layer design. Given these promising results, future work will focus on the integration of similar microwave circuitry in a multi-zone trap array as envisioned by the QCCD architecture.