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Microwave driven ion trap quantum computing

Periodic Reporting for period 1 - MicroQC (Microwave driven ion trap quantum computing)

Reporting period: 2018-10-01 to 2020-03-31

The construction of a large-scale trapped-ion quantum processor can be made decisively simpler by using the well-developed and compact microwave technology present already in today’s mobile phones and other devices. Microwave technology has tremendous simplification potential by condensing experimental effort from an optical table with several square meters of accurately aligned optical components down to an engineered conductor microstructure embedded into a chip surface and a few off-the-shelve microwave components. It allows execution of quantum gates by the application of a voltage to a microchip potentially replacing millions of laser beams and it can operate at room temperature or mild cooling. Thus, this technology can be the key enabling step for addressing the formidable challenge of a scalable quantum processor.

There are still enormous technical challenges in scaling ion trap (or any other) systems up to the millions of qubits required to implement full-scale quantum computation. The main objective of MicroQC is to demonstrate, through state-of-art quantum engineering, fast and fault-tolerant microwave two-qubit and multi-qubit gates and to design scalable technology components for multi-qubit quantum processors. These challenges are pursued in a combined effort of three experimental groups, including the pioneers in microwave quantum logic with static and oscillating magnetic gradients, and two theory groups.

WP1 targets crucial technological development of surface microtraps aiming to suppress the physical origins of decoherence and strategies to characterize and actively counteract residual decoherence. Theoretical research is focused on innovations to simplify the experimental requirements and methodologies. The objective of WP2 is, by using the new technologies in WP1, to realize multi-qubit gates featuring high speed and fidelity above the fault-tolerant threshold. The objective of WP3 is, by using the developments in WP1 and WP2, to achieve high-fidelity control of many-qubit systems and implement basic quantum algorithms. All partners will prepare a Roadmap to high-technology readiness levels specifically designed for microwave-driven ion traps.
The three experimental groups focused on the technological development of large-gradient multilayer surface-electrode traps. Figure 1 shows the optimized ion trap used at Hannover. They developed a technology that thermally anchors the relevant metal conductors directly to the substrate for efficient heat removal. They demonstrated that the microwave currents can be applied in continuous-wave regime rather than pulsed only, thereby allowing a factor of 10 more power dissipation. For the given duty cycle of gate operations gradients can be increased significantly for faster and more resilient gates. Towards the development of optimized connectivity for ion traps, Hannover started the development of through-substrate vias and developed a 3D coating process allowing the coating of side-walls of apertures in wafers. Based on these techniques, it will be possible to mount substrates on an interposer for mounting on standard sockets.

The Hannover and Siegen groups developed a new geometry for a surface-electrode trap with embedded current-carrying wires (CCW) supporting the realization of strong gradients. They also expanded their simulation capability for microwave near-field conductors to implement a new scheme which brings together both the static-gradient and the near-field oscillating-gradient approaches. The new trap chip has been designed in Siegen and is currently in production at Hannover. The new experimental setup features: a) an aluminum vacuum chamber that should allow achieving higher vacuum and has an advantage of being non-magnetic; b) an argon ion gun for in situ cleaning of the trap chip surface that should reduce the anomalous heating; c) mu-metal shielding for suppressing external magnetic-field noise; d) novel magnetic system designed to create static magnetic-field gradient of up to 200 T/m. The chip will have a multi-layer structure, which will allow placing ion trap electrodes and the microwave resonator at different layers, and will allow trapping two adjacent strings of ions with variable separation between them.

The Sussex group carried out work on microchips featuring permanent magnets and a new microchip generation featuring CCWs. The work on the permanent magnet setup includes their alignment to a microchip to achieve a magnetic-field gradient of 100 T/m. Before this project, the CCW microchips with copper wires embedded in a silicon substrate had been already developed. However, there had been still a few problems including topography after a polishing process, so called dishing, whose depth is more than a micrometer and kΩ-level shorts between CCWs and surrounding silicon. It was essential to fabricate flatter surface of CCWs for following fabrication steps for CCW-integrated ion trap chips. Figure 2 shows a wafer with uniform polishing rate without any scratch or remaining dielectric or metallic films on silicon after polishing and ion beam milling with no electrical failures detected.

Sussex conducted work on the roadmap for practical quantum computing with trapped ions. The cost of enabling connectivity in Noisy-Intermediate-Scale-Quantum devices is an important factor in determining computational power. An efficient ion routing algorithm has been created along with an appropriate error model, which can be used to estimate the achievable circuit depth and quantum volume.

The two theoretical groups focused on the development of new quantum control schemes adapted to the conditions at the three experimental groups. The Jerusalem group developed a new mixed dynamical decoupling scheme, which combines continuous and pulsed dynamical decoupling. The Sofia group developed new composite pulse sequences optimized for high-fidelity error-resilient quantum gates in microwave-driven trapped ions.
The Hannover group demonstrated their first two-qubit entangling gate with 98.2% fidelity and found motional-mode frequency changes to be the major factor to the observed gate infidelity. They developed amplitude pulse shaping as a means of making the resulting gates more resilient to this type of noise by two orders of magnitude. To overcome timing limits of single-qubit gates, they used resilient pulse sequences developed by the Sofia group resulting in a final fidelity of 99.7% no longer limited by motional mode instabilities. Further improvements are expected upon the introduction of dynamic decoupling.

The Siegen group implemented a novel two-qubit phase gate generated by pulsed dynamical decoupling. The novel sequence protects two qubits from decoherence while generating a tunable two-qubit phase gate with high fidelity. The fidelity of this gate is robust against motional excitation, variations of motional frequencies, and amplitude variations in the driving field. The gate speed scales better than quadratically with the field gradient, an important feature in novel high-gradient traps.

The Sussex group used a resilient two-qubit multitone Molmer-Sorensen gate, which employs additional fields closely resonant to higher-order sidebands. They experimentally showed that this entangling gate is more robust to motional decoherence, and trap frequency fluctuations. They also tested a new type of quantum gate based on spin-spin coupling with dressed states.
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Figure 2