Periodic Reporting for period 1 - Q-Line (Line defects as building blocks of a defect-based quantum computer)
Reporting period: 2021-04-01 to 2023-03-31
A quantum computer (QC) is a device that exploits quantum behavior to solve a computational problem that cannot be tackled, or would take too long to solve, in a classical computer. In order to build a functioning QC, several physical systems have been proposed to be used as platforms for quantum bits or “qubits”, e.g. photons, trapped atoms/ions, nuclear spins in molecules immersed in liquid solutions and point defects in solids. The latter system is advantageous from the point of view of scalability since integrated quantum devices could, in principle, be built by means of adapted fabrication techniques developed in the semiconductor industry. Nevertheless, it remains challenging to position the point defects in a deterministic array and to integrate them into large networks. The scientific aim of Q-Line is to carry out a theoretical assessment of the potential use of line defects (dislocations) as a “quantum bus”, able to both create a deterministic pattern of relevant point defects and to connect them by means of localized phonons. Until now, dislocations have only been considered as detrimental for the correct functioning of QC. Therefore, Q-Line opens a completely new area of research, aligned with the quantum technologies flagship of the European Commission and will help putting Europe at the forefront of the development of quantum technologies.
Based on our state-of-the-art atomistic simulations, we propose that, in order to have potential for quantum applications, dislocations should be undissociated screws and be electrically inactive. Such conditions are satisfied in cubic silicon carbide (3C-SiC). Our results show that the undissociated screw dislocation in this material is able to attract defect-based qubits into its core. As a consequence, it would allow the creation of a one-dimension array of qubits along its line direction. Furthermore, we show that the strain field induced by this specific dislocation type is able to modulate the electronic properties of the qubit located in its core, without itself being electrically active. For the specific case of the neutral divacancy in 3C-SiC, know to have real potential as qubit, our results show that these modulations result in the loss of its potential as a qubit. However, these same modulations could transform defects with no potential as qubits when located in bulk, into promising options when located inside the core of the screw dislocations. Altogether our findings represent a paradigm shift within quantum technologies, as they point out that dislocations can be used as active building blocks of future defect-based quantum computers.
Based on our state-of-the-art atomistic simulations, we propose that, in order to have potential for quantum applications, dislocations should be undissociated screws and be electrically inactive. Such conditions are satisfied in cubic silicon carbide (3C-SiC). Our results show that the undissociated screw dislocation in this material is able to attract defect-based qubits into its core. As a consequence, it would allow the creation of a one-dimension array of qubits along its line direction. Furthermore, we show that the strain field induced by this specific dislocation type is able to modulate the electronic properties of the qubit located in its core, without itself being electrically active. For the specific case of the neutral divacancy in 3C-SiC, know to have real potential as qubit, our results show that these modulations result in the loss of its potential as a qubit. However, these same modulations could transform defects with no potential as qubits when located in bulk, into promising options when located inside the core of the screw dislocations. Altogether our findings represent a paradigm shift within quantum technologies, as they point out that dislocations can be used as active building blocks of future defect-based quantum computers.
In order to assess the potential use of dislocations for quantum technologies, we focused on the most promising materials as of today: silicon, diamond and silicon carbide. Unfortunately, experiments and simulations (including our own calculations) show that undissociated screw dislocations in silicon and diamond are found to be unstable and/or electrically active. However, in the case of 3C-SiC, this specific type of dislocation is stable and electrically inactive. After setting up the corresponding simulations, we calculated the energetic cost of creating (formation energy) the divacancy in 3C-SiC when sitting at all atomic sites located within a 10 Ångstrom radius from the core of the screw dislocation. These results allowed us to concluded that there is a strong attraction between the divacancies and the screw dislocations. Furthermore, the obtained formation energies allowed us to analyze the stability of the neutral state of the divacancy, which is the relevant one for quantum applications. Our results in this regard show that, when compared to the isolated case, the neutral divacancy located inside the screw dislocation core has a much smaller stability region. For completeness, we analyzed the effect of the dislocation strain field on the defect states induced by the neutral divacancies. On this matter, we conclude that the neutral divacancy located inside the core of the screw dislocation in 3C-SiC becomes non-magnetic. Altogether, these findings mean that the quantum potential divacancy in 3C-SiC is lost when interacting with the studied dislocation. Nevertheless, we found that the screw dislocation does not induce any deep state by itself and it allows a defect-based qubit sitting in its core to have bound states. Thus, our second conclusion is that new defect-based qubits, perhaps useless in bulk, could become paramagnetic when located near screw dislocations in 3C-SiC.
These results and analysis have been shared with the scientific community via participation in internationally recognized conferences: SpinQubit5 and QUANTUMatter 2023. Furthermore, the corresponding paper will be submitted to physical review letters and it is currently available as pre-print in the Arxiv repository.
These results and analysis have been shared with the scientific community via participation in internationally recognized conferences: SpinQubit5 and QUANTUMatter 2023. Furthermore, the corresponding paper will be submitted to physical review letters and it is currently available as pre-print in the Arxiv repository.
We carried out the first assessment of the potential of undissociated dislocations in technologically relevant materials. Ours is a state-of-the-art theoretical study and we aim at firing up the interest on dislocations within the quantum community. Furthermore, there is consensus on the fact that the definitive physical system to build qubits may not be even known. Perhaps using dislocations could be a route towards building a large-scale solid-state quantum computer and our research puts Europe at the forefront in this potentially fruitful idea.