Periodic Reporting for period 4 - QUESS (Quantum Environment Engineering for Steered Systems)
Reporting period: 2021-07-01 to 2021-12-31
The superconducting quantum computer has recently reached quantum supremacy, the 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 circuits. 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. This promotes a concept of a quantum simulator for open quantum systems, for which the basic building blocks are experimentally demonstrated.
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.
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 circuits. 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. This promotes a concept of a quantum simulator for open quantum systems, for which the basic building blocks are experimentally demonstrated.
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.
We have successfully completed the project. Namely, we have built high-quality co-planar waveguide resonators with the quality factor up to a million and used them as a basis to implement long-lived superconducting Xmon qubits with lifetime up to 0.05 ms. We have also modelled the tunable environments in three 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 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 quality factor of a resonator leading to roughly three orders of magnitude tuning and a new discovery of a tunable Lamb shift that was published in the journal Nature Physics in 2019. We also have theoretically modelled the quantum driving 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 and are going to experimentally demonstrate this in the future. 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. We have also carried out simulations of a qubit coupled to a tunable-environment resonator and designed samples for forthcoming experiments. We also recently demonstrated a fast and accurate bolometer that can be used in the future for qubit readout [Nature (2020)] and a low-noise millikelvin microwave source meeting the specifications needed to drive superconducting qubits [Nature Electronics (2021)]. There have also been some adjustments to the plan. For example, when working on the single-shot measurement scheme for superconducting qubits, we found out a new faster way to measure the qubit state [Physical Review Letters (2019)]. Consequently, we applied the prioritization plan to complete the tasks with the highest priority with the remaining time and resources. We published 36 peer-reviewed papers related to this project and importantly many pioneering papers that are at the heart of the objectives. We have given hundreds of media interviews related to these works and disseminated then in tens of invited talks in scientific conferences.
We have published 36 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 it can substantially cool down superconducting resonators and change the dissipation that it induces on the resonators by many orders of magnitude with on/off switching time of a few nanoseconds. We also used this device to heat up resonators, in the case of which they can operate as tunable sources of incoherent microwave radiation and used this source as a calibration tool for cryogenic amplifications chains. Furthermore, we used the QCR to observe for the first time the Lamb shift in engineered quantum systems, an unexpected but scientifically important result. We also implemented for the first time a tunable heat sink. In addition, 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. Importantly, we have developed a new method to readout qubits. This project paves the way for fast unconditional reset of superconducting qubits in many-qubit quantum processors and for the studies of the interplay between dissipation and driving of quantum systems in general.