Periodic Reporting for period 3 - QUCUBE (3D integration technology for silicon spin qubits)
Período documentado: 2022-02-01 hasta 2023-07-31
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 quantum processor containing hundreds of spin qubits confined to a two-dimensional array of electrostatically defined silicon quantum dots. To face the challenge of addressing the qubits individually, we use a three-dimensional architecture purposely designed to accommodate, on separated planes, the charge sensing devices necessary for qubit readout, and the metal gate lines for the electrical control and measurement. The gate lines are operated according to a multiplexing principle, enabling a scalable wiring layout. We aim at implementing fault-tolerant logical qubits and performing quantum simulations of complex Hamiltonians.
QuCube leverages industrial-level silicon technology to realize a quantum processor containing hundreds of spin qubits confined to a two-dimensional array of electrostatically defined silicon quantum dots. To face the challenge of addressing the qubits individually, we use a three-dimensional architecture purposely designed to accommodate, on separated planes, the charge sensing devices necessary for qubit readout, and the metal gate lines for the electrical control and measurement. The gate lines are operated according to a multiplexing principle, enabling a scalable wiring layout. We aim at implementing fault-tolerant logical qubits and performing quantum simulations of complex Hamiltonians.
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, priority was given to the development of a new gate stack consisting of two overlapping gate layers, with the goal is to increase the electrical tunability of our MOS quantum dots. Once fully validated in a 1D-like silicon-nanowire geometry, the new gate modules will be applied to 2D quantum-dot arrays, whose development has been carried out in parallel. In WP 2, we have worked on the study and improvement of single-qubit performance. Using rf reflectometry techniques we achieved high-fidelity spin readout on a time scale of a few microseconds. In the case of hole-type spin qubits, the improved device quality enabled us to access the few-hole regime with unprecedented agreement between measured and simulated spin properties. We also revealed the existence of operational sweet spots where the hole spin coherence time reaches 88 microseconds, i.e. well beyond the state-of-the-art. In WP3, we have begun to explore the possibility of coupling hole spin qubits to microwave photons in superconducting resonators. The first experiment revealed a spin-photon coupling strength beyond 300 MHz, opening great prospects for using microwave photons to couple hole spin qubits far away from each other. As an alternative approach to long-range coupling, we have also investigated the use of electron shuttling using GaAs multi-dot devices as a test bench. Finally, we have completed the first generation of interposers to be used for the interconnections between qubit chip and cryogenic control electronics.
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 aims at addressing the scalability of silicon spin qubits in order take them to a pre-industrial maturity level. Our ultimate objective is to fabricate an operational quantum processor consisting of a two-dimensional array with at least one hundred spin qubits coupled through tunable, nearest- neighbor exchange interactions. We shall explore the possibility to obtain a logical qubit with extended quantum coherence and evaluate possible roadblocks and solutions for large-scale integration.