Periodic Reporting for period 4 - QUCUBE (3D integration technology for silicon spin qubits)
Période du rapport: 2023-08-01 au 2025-07-31
Originally conceived to describe the microscopic world of atoms and elementary particles, the theory of quantum mechanics has eventually served to predict macroscopic phenomena, e.g. the electrical and optical properties of semiconductors, resulting in a wide range of technological applications that have changed our way of living. Foundational properties like quantum superposition and entanglement, however, have remained essentially unexploited. Their use may allow achieving computational powers inaccessible to classical digital computers, opening unprecedented opportunities. In a quantum computer, the elementary bits of information are encoded onto two-level quantum systems called qubits. Since qubits interact with the uncontrolled degrees of freedom of their environment, the evolution of their quantum states can become quickly unpredictable, leading to a reduced qubit fidelity. In topological quantum computing schemes, e.g. the surface code, the reduced fidelity is compensated by using decoherence-free logical qubits consisting of a large number (at least thousands) of entangled physical qubits. As a result, a useful quantum processor should host millions of qubits. Although dauntingly large, this number is still small as compared to the number of transistors in a modern silicon microprocessor.
QuCube leverages industrial-level silicon technology to realize a scalable quantum processor where the elementary bits of quantum information are encoded in the spin degrees of freedom of semiconductor quantum dots.
QuCube leverages industrial-level silicon technology to realize a scalable quantum processor where the elementary bits of quantum information are encoded in the spin degrees of freedom of semiconductor quantum dots.
Following the consolidation of a trans-institutional research team of around 50 people (staff engineers, researchers, technicians, PhD students and post-docs), research efforts have focused on developing the building blocks of a scalable quantum processor based on semiconductor spin qubits. In work-package (WP) 1, devoted to device fabrication and basic characterization, priority was given to the development of a new gate stack consisting of two overlapping gate layers, with the goal to increase the electrical tunability of our MOS quantum dots. In WP 2, we worked on the study and improvement of qubit basic functionalities (initialisation, control, readout) and performance (coherence, control speed, etc.). Using rf reflectometry techniques we achieved high-fidelity spin readout on microsecond time scale. In the case of hole spin qubits, the improved device quality enabled us to access the few-hole regime with unprecedented agreement between measured and simulated spin properties. We revealed the existence of operational sweet spots with hole-spin coherence times as high as 88 microseconds, i.e. well beyond the state-of-the-art. We also showed that the sweet-spot condition can be established while simultaneously maximising the qubit control efficiency resulting in an optimal operation regime yielding high qubit fidelity. In WP3, we explored the possibility of coupling hole spin qubits to microwave photons in superconducting resonators. We observed a very strong spin-photon coupling exceeding 300 MHz, which opens great prospects for using microwave resonators to perform non-demolition spin readout or to couple hole spin qubits far away from each other. As an alternative approach to long-range coupling, we also investigated the use of electron shuttling using GaAs multi-dot devices as a test bench. Finally, we worked on the development of cryogenic electronics to be integrated with quantum processors through specially designed interposers embedding superconducting interconnections.
Silicon spin qubits can potentially leverage the large-scale integration capabilities of silicon technology. So far, experimental demonstrations have involved only up to a few coupled qubits, which were used to demonstrate elementary quantum operations. QuCube was conceived to address the scalability of silicon spin qubits in order take them to a pre-industrial maturity level with the ultimate objective to demonstrate an operational quantum processor based on a two-dimensional array of spin qubits with tunable, nearest-neighbor exchange interactions. Part of WP3 research initiatives were also devoted to the exploration of alternative schemes for long-range qubit coupling, leveraging either microwave photons or electron shuttling.