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Microwave driven ion trap quantum computing

Periodic Reporting for period 2 - MicroQC (Microwave driven ion trap quantum computing)

Reporting period: 2020-04-01 to 2022-06-30

The construction of a large-scale trapped-ion quantum information 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. Thus, this technology can be the key enabling step for addressing the formidable challenge of a scalable quantum processor. This technology 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.

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 and simulation. 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 that apply these techniques in multi-qubit quantum processors. The successful accomplishment of these objectives, in a combined effort by five leading groups in this field can make large-scale quantum computation and simulation with microwave-controlled microfabricated ion traps possible.

WP1 targets crucial technological development of surface microtraps to be used in the research described in the next WPs. Moreover, technical approaches to suppress the physical origins of decoherence and strategies to characterize and actively counteract residual decoherence are developed. The objective of WP2 is, by using the technological developments in WP1, to realize microwave multi-qubit gates in surface microtraps, featuring high speed and fidelity above the fault-tolerant threshold. The objective of WP3 is, by taking advantage of the technological and theoretical developments in WP1 and WP2, to demonstrate high-fidelity control of many-qubit systems and implement basic quantum algorithms. The knowledge, insight and experience gained at this point is condensed in a Roadmap to high-technology readiness levels specifically designed for microwave-based quantum information processing with trapped ions.
The partners focused on the technological development of large-gradient multilayer surface-electrode traps and the implementation of high-fidelity quantum gates and quantum algorithms with them. LUH and USIEGEN developed a new geometry for a surface-electrode trap with embedded current-carrying wires (CCW) supporting the realization of strong gradients of up to 200 T/m. They also expanded their near-field 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. In particular, LUH has been able to establish a lower bound of 13.6 (8.1) s on the coherence time of their field-independent qubit in the new setup without any shielding, refocusing pulses or dynamic decoupling.

In order to realize the qubit-qubit coupling based on the static magnetic field gradient, a novel approach based on a quasi-static permanent magnet has been developed and patented by USIEGEN. It is advantageous compared to both a permanent magnet as its field can be tuned and switched off, and to a system based on current-carrying electrodes as there are no running currents that create magnetic field noise. They demonstrated a gradient of 116 T/m while the field in the minimum is only 15 G. Applying adaptive frequency correction (resulting in a factor of 50 increase in frequency precision) together with using dynamical decoupling led to an increase of the two-qubit CNOT fidelity from 0.79(4) to 0.97(6).

The easiest way to raise quantum gate fidelities and gate speeds in a microwave driven ion trap is to increase the magnetic field gradient. Early experiments at UOS were carried out using a macroscopic ion trap setup including permanent magnets giving rise to a magnetic field gradient of 24 T/m. They have successfully developed a new generation of ion microchips with integrated CCWs. The microchips can obtain a magnetic field gradient of 145 T/m, contain X-junction ion trap arrays, and feature low resistance integrated current carrying wires. They have successfully trapped ions using these ion chips demonstrating lifetimes of 7-16 hours. The expected two-qubit gate fidelity is over 99.9% (previously 98.5% with 24 T/m).

Finally, UOS created a Roadmap of microwave quantum computation to high-technology readiness level. 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. They aim the roadmap for a large scale quantum computer to cater for a qubit number of 10^6 along with a base error rate of 10^-4 as a useful starting point as resource specification for a universal transport-based quantum computer.
LUH has systematically characterized and generalized pulse-shaped multi-qubit gates. In the first period, LUH demonstrated their first two-qubit entangling gate with 98.2% fidelity and found motional-mode frequency changes to be the major contributor 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 TCPA resulting in a final fidelity of 99.7%. In the second period, further improvements in collaboration with TCPA have been made, using numerical optimization of the pulse shapes, which resulted in a speedup of the gate by a factor of 3, while maintaining its high fidelity.

USIEGEN reported the implementation of the perceptron quantum gate in an ion-trap quantum computer. A perceptron’s target qubit changes its state depending on the interactions with several qubits. The target qubit displays a tunable sigmoid switching behavior becoming a universal approximator when nested with other perceptrons. They also used two successive perceptron quantum gates to implement a XNOR gate, where the perceptron qubit changes its state only when the parity of two input qubits is even. This bodes well for future efforts to implement machine learning and deep learning algorithms on trapped-ion quantum computers, which yield significant speed-ups. This method could be used to reduce gate-count overhead in quantum algorithms.

UOS have worked on developing a full toolbox of ion transport operation on our X-junction ion trap array, which are required for the implementation of quantum algorithms within a transport-based quantum processor. This approach is alternative and superior to the photonic based approaches, as it makes use of electric field links between quantum computing modules instead of photonic interconnects. Following the successful construction of this two-module quantum computer prototype, UOS achieved the demonstration of ion transport between two quantum computing modules at a transfer rate of 2424 s^−1 with ion transfer fidelity of 99.999993%, both of which are currently the state of the art.

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