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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - QUESS (Quantum Environment Engineering for Steered Systems)

Teaser

Summary

The superconducting quantum computer has very recently reached the theoretical thresholds for fault-tolerant universal quantum computing and an quantum annealer based on superconducting quantum bits, qubits, is already commercially available. However, several fundamental questions on the way to efficient large-scale quantum computing have to be answered: qubit initialization, extreme gate accuracy, and quantum-level power consumption.

This action, QUESS, aims for a breakthrough in the realization and control of dissipative environments for quantum devices. Based on novel concepts for normal-metal components integrated with superconducting quantum nanoelectronics, we experimentally realize in-situ-tunable low-temperature environments for superconducting qubits. These environments can be used to precisely reset qubits at will, thus providing an ideal initialization scheme for the quantum computer. The environment can also be well decoupled from the qubit to allow for coherent quantum computing. Utilizing this base technology, we find fundamental quantum-mechanical limitations to the accuracy and power consumption in quantum control, and realize optimal strategies to achieve these limits in practice. Finally, we build a concept of a universal quantum simulator for non-Markovian open quantum systems and experimentally realize its basic building blocks.

This action provides key missing ingredients in realizing efficient large-scale quantum computers ultimately leading to a quantum technological revolution, with envisioned practical applications in materials and drug design, energy harvesting, artificial intelligence, telecommunications, and internet of things. Furthermore, this action opens fruitful horizons for tunable environments in quantum technology beyond the superconducting quantum computer, for applications of quantum-limited control, for quantum annealing, and for simulators of non-Markovian open quantum systems.

Work performed

We have successfully completed the first 1.5-year period of the DoA. Namely, we have built high-quality co-planar waveguide resonators (Task 1.1) with Q up to a million and used them as a basis to implement ling-lived superconducting Xmon qubits (Task 2.1) with lifetime up to 0.05 ms. We have also modelled the tunable environments (Task 1.2) in two peer-reviewed papers published. We also published a seminar paper titled Quantum-circuit refrigerator (QCR) where a voltage-tunable environment for quantum electric devices was discovered. Furthermore, we implemented a controllable heat sink using a flux-tunable resonator (Tasks 1.3 and 1.4) and published a paper on this. The heat sink achieved two orders of magnitude of lifetime tuning. In addition, we used the QCR to tune the Q factor of a resonator (Task 1.4) leading to roughly three orders of magnitude tuning and a new discovery of a tunable Lamb shift. We have submitted a journal paper on these results. We also have theoretically modelled the quantum driving (Task 2.2) and published an important paper titled Energy-efficient quantum computing. We successfully found a fundamental lower bound for the energy content of a qubit control pulse with a given tolerable gate error. We have also implemented so-called random benchmarking protocol to benchmark the quantum gate fidelity which is a needed to characterize the planned implementation quantum gates using low-energy pulses (Task 2.3). In conclusion, all tasks for the reporting period have either finished or are under way as planned. Although no task has been failed, there are some minor adjustments which are discussed in Sec. 2.3. We have already published more than ten peer-reviewed papers related to this project and importantly many pioneering papers that are at the heart of the objectives.

Final results

We have published more than ten peer-reviewed journal papers within the scope of this project, and hence have gone beyond the state of the art in many ways. Most importantly, we discovered a quantum-circuit refrigerator which is the first stand-alone device that can induce on-demand dissipation on quantum electric systems. This refrigerator is controlled by a bias voltage and we have observed that is can substantially cool down superconducting resonators and change the dissipation that it induced on the resonators by many orders of magnitude. We also used this device to heat up resonators, in the case of which they can operate as tunable sources of incoherent microwave radiation. Furthermore, we implemented for the first time a tunable heat sink. We also managed to theoretically find optimal quantum sates for qubit driving pulses and derive a lower bound for the energy needed to implement a quantum gate with a given fidelity. Thus everything looks good for our next series of experiments where we implement the refrigerators with superconducting qubits and show fast qubit reset and the interplay between dissipation and driving.