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Andreev qubits for scalable quantum computation

Periodic Reporting for period 2 - AndQC (Andreev qubits for scalable quantum computation)

Reporting period: 2020-04-01 to 2021-09-30

Scalable quantum information processing has become one of the most sought-after disruptive technology, owing to the prospect of providing solutions exponentially faster to various mathematical problem classes, relevant in the field of cryptography, materials science, optimization problems and artificial intelligence. The building block of quantum computers, the quantum bit (qubit) is a microscopic two-level system which harnesses the laws of quantummechanics enabling a massive parallelism in comparison to its classical counterpart. Because of the vast opportunities of technical applications, several physical approaches are already being investigated in both the academic and industrial sector, such as superconducting qubits, spin qubits in semiconductor quantum dots, or trapped ion systems. However, thus far, no clear single platform has emerged with a well-defined technical roadmap towards scalability, therefore it is a timely challenge to investigate alternative, novel platforms for quantum computation.
The AndQC consortium set out to demonstrate the Andreev qubit as a scalable platform in hybrid nanodevices consisting of superconducting quantum circuits and semiconductor nanostructures. This platform is implemented by utilizing the discrete superconducting quasiparticle levels (Andreev levels) that appear in weak links between superconductors, and can be occupied by zero, one or two electrons. These Andreev levels therefore can host quantum bits, both based on their occupation number (zero or two) and by utilizing the spin degree of freedom of the single, localized electron. This configuration has the unprecedented functionality of coupling a single localized spin to the dissipationless supercurrent, and enables the so far experimentally unexplored scheme of fermionic quantum computation, which has the potential of efficiently simulating electron systems in complex molecules and novel materials. To achieve these scientific goals, the consortium combines the expertise of leading groups in the fields of nanodevice growth, quantum theory and experiment.
In the first one year of the research program, we made important steps towards reaching the final goal of the project, which is the demonstration of a novel scalable quantum computation platform. Our research in this period spanned the field of materials science, theoretical device modelling and quantum transport experiments.
Our material platform is based on semiconductor nanostructures with high quality superconducting contacts, and two leading geometries are investigated: semiconductor nanowires and planar semiconductor heterostructures. However, these devices must be of high purity in order to avoid the degradation of the quantum bit lifetime and noise properties. Our researchers used special, ultra-high vacuum techniques to grow these nanostructures, including chemical beam epitaxy, molecular beam epitaxy and in-situ junction forming. The new semiconductor structures are now distributed in the consortium where the quantum devices are being built and measured.
Next, in order to build a scalable qubit system, the source of the environmental noise and its potential countermeasures have to be understood as quantum bits are very sensitive to the noise and fluctuations in their environment. This was achieved by monitoring the qubit transitions in combination with a systematic device modeling, confirming the role of lattice vibrations, phonons, as a limiting factor for the qubit lifetime. We will use this information to design the next generation of Andreev quantum bits using the newly developed growth and processing techniques.
Finally, focusing on the novel quantum bit functionalities enabled by the Andreev levels, we worked on experimental protocols operating the Andreev spin qubit and the fermionic quantum bit.
Next, we consolidated the semiconductor materials platforms used for the project by characterizing their electronic transport properties. High purity devices exhibit ballistic transport, that is, the electrons flow in the channel without being scattered from crystalline defects or impurities. This low backscattering is needed to be able to tune the quantum bit transition frequency, which becomes particularly important in the context of multi-qubit devices. Ballistic transport is now demonstrated in InAs nanowires and InSb nanoflag devices, which underlines the recent progress in material growth. Our other focus is on the induced superconductivity in the semiconductor channel, which determines the frequency regime of the Andreev quantum bits. This induced superconductivity is demonstrated in both InAs nanowires and 2DEGs as well as InSb nanowires. Our experimental activities included the measurement of the quantum lifetimes. In particular, the decoherence time of the qubit is limited by gate charge noise, which can be mitigated by further improvement in the materials. Finally, in this reporting period, further theoretical work addressed the quasiparticle lifetimes of the quantum bits, which will in turn aid future experimental work.
Our work performed in the first year demonstrated the feasibility of the longer term objectives of the scientific work program, adding a radically new platform to quantum technologies, the scalable Andreev quantum bit. Combining semiconductor and superconductor nanostructures, our work complements the technological development of pure superconductor and semiconductor quantum bits.
Our goal is to combine the best of these two, thus far distinct worlds, in an effort to provide an alternative qubit platform for scalable, fault-tolerant quantum computers. This effort fits well in the European innovation ecosystem, which will benefit not only from the scientific knowledge built by the consortium, but also from our interactions with the leading suppliers of specialized electronic components, cryogenic systems and clean materials.
In the second year of the project, the growth, nanofabrication and measurement of the superconductor-semiconductor nanodevices continued, with state of the art results in device quality as well as the flexibility of the material platform to achieve full scalability. Thus far, our results are summarized in 45 scientific publications covering all aspects of the project.
Andreev qubit coupled to a superconducting resonator. Credit:University of Basel
InAs nanowire with in-situ defined junction. Credit: University of Copenhagen