Periodic Reporting for period 4 - QBusSi (Optomechanical quantum bus for spins in silicon)
Reporting period: 2024-09-01 to 2025-08-31
By many accounts, quantum computing will be one of the defining technologies of this century. A general-purpose quantum computer could significantly speed-up various calculation intensive tasks ranging from drug research to optimization of logistics routes with immediate societal benefits. Many different materials are studied as the basis on which a quantum computer could be build. In this project, we will advance the use of silicon, the material that is already ubiquitous around us, as a quantum platform. As silicon has already been the material underpinning the modern information technology revolution, using it would be of advantageous to the diffusion of quantum technologies as they could then leverage the existing infrastructure of silicon electronics and photonics. The ultimate long-term goal of the project is enabling a not-too-distant future where silicon chips encompassing quantum enabled sensors and/or quantum computing processors are widely available and only require push-of-a-button coolers and laser light to operate.
The project advances a specific physical implementation of qubits, donor spin states in silicon, and specifically explores new ways to connect these quantum systems to light. The spin of donor atoms, impurities placed intentionally inside a silicon crystal, are now known to be excellent qubits with some of the longest single qubit coherence times demonstrated in solid state. This is a significant advantage for both quantum sensing and quantum information applications. However, at the moment the application potential of silicon donor qubits is hindered by two related obstacles: current readout techniques require nanoelectric connections, below 1 K temperatures and high magnetic fields, and - most importantly - there are no scalable methods to couple multiple qubits.
To solve this, the project developed miniature mechanical and optical devices on silicon chips. These “optomechanical” devices can vibrate and interact with light, acting as bridges between different forms of energy. The long-term idea is to make these vibrations connect the donor spins to both light signals and to each other, enabling quantum information to move between solid-state qubits and to optical communication channels. Such technology could form the basis of scalable silicon quantum computers.
The project advances a specific physical implementation of qubits, donor spin states in silicon, and specifically explores new ways to connect these quantum systems to light. The spin of donor atoms, impurities placed intentionally inside a silicon crystal, are now known to be excellent qubits with some of the longest single qubit coherence times demonstrated in solid state. This is a significant advantage for both quantum sensing and quantum information applications. However, at the moment the application potential of silicon donor qubits is hindered by two related obstacles: current readout techniques require nanoelectric connections, below 1 K temperatures and high magnetic fields, and - most importantly - there are no scalable methods to couple multiple qubits.
To solve this, the project developed miniature mechanical and optical devices on silicon chips. These “optomechanical” devices can vibrate and interact with light, acting as bridges between different forms of energy. The long-term idea is to make these vibrations connect the donor spins to both light signals and to each other, enabling quantum information to move between solid-state qubits and to optical communication channels. Such technology could form the basis of scalable silicon quantum computers.
From the start of the project, the research focused on creating and testing tiny silicon structures that could link the quantum spins of donor atoms to mechanical motion and light. The team designed, fabricated, and characterized advanced “optomechanical” devices—microscopic silicon beams with built-in optical cavities that react to mechanical vibrations. These devices form the physical platform for connecting the quantum and optical worlds on a chip.
The first stage of the work concentrated on understanding how donor implantation affects the mechanical and optical properties of the devices. This work led to the publication Shakespeare et al., Materials for Quantum Technology (2021) and provided essential guidance for later designs. In parallel, the project built up a strong research team, new cryogenic infrastructure, and international collaborations.
When early measurements did not show the expected quantum coupling signal, the team pursued research avenues to pinpoint the reason for this. One major research avenue was the development of an optical method to read out donor spin states using donor-bound excitons using microfabricated electrodes. First results on this were published as Loippo et al., Physical Review Materials (2023). This technique will provide a new way to monitor the spins independently from their mechanical effects, once the integration to SOI materials is solved.
In parallel, the project achieved significant progress in optomechanical control methods, including a single-laser feedback cooling scheme (Kumar et al., arXiv 2022) and an improved homodyne interferometer for precise optical measurements (la Gala et al., Physical Review Applied 2023). Another important result was the detailed study of photothermal effects, showing that they do not need to limit the operation of the devices (Shakespeare et al., Optics Express 2024).
While the direct observation of spin–mechanical coupling remains a future goal, the project achieved significant progress in technology development, experimental methods, and theoretical understanding. The results of the latter are now in arxiv (Lyyra et al., arXiv 2025 and Hijano et al., arXiv 2025).
The new infrastructure and expertise established through the project have already enabled follow-up research using improved device designs and stronger magnetic coupling schemes. These ongoing efforts continue to build on the project’s foundation, moving closer to the original goals of the project that will in the long-run contribute to practical silicon-based quantum technologies.
The first stage of the work concentrated on understanding how donor implantation affects the mechanical and optical properties of the devices. This work led to the publication Shakespeare et al., Materials for Quantum Technology (2021) and provided essential guidance for later designs. In parallel, the project built up a strong research team, new cryogenic infrastructure, and international collaborations.
When early measurements did not show the expected quantum coupling signal, the team pursued research avenues to pinpoint the reason for this. One major research avenue was the development of an optical method to read out donor spin states using donor-bound excitons using microfabricated electrodes. First results on this were published as Loippo et al., Physical Review Materials (2023). This technique will provide a new way to monitor the spins independently from their mechanical effects, once the integration to SOI materials is solved.
In parallel, the project achieved significant progress in optomechanical control methods, including a single-laser feedback cooling scheme (Kumar et al., arXiv 2022) and an improved homodyne interferometer for precise optical measurements (la Gala et al., Physical Review Applied 2023). Another important result was the detailed study of photothermal effects, showing that they do not need to limit the operation of the devices (Shakespeare et al., Optics Express 2024).
While the direct observation of spin–mechanical coupling remains a future goal, the project achieved significant progress in technology development, experimental methods, and theoretical understanding. The results of the latter are now in arxiv (Lyyra et al., arXiv 2025 and Hijano et al., arXiv 2025).
The new infrastructure and expertise established through the project have already enabled follow-up research using improved device designs and stronger magnetic coupling schemes. These ongoing efforts continue to build on the project’s foundation, moving closer to the original goals of the project that will in the long-run contribute to practical silicon-based quantum technologies.
The project has advanced the state of the art in several aspects of silicon-based quantum technology. Donor spin qubits in silicon are recognized for their excellent coherence but lack scalable methods for coupling to other quantum systems or for optical readout. This project introduced and experimentally pursued a new hybrid approach combining donor spins with optomechanical devices—mechanical resonators that can interact both with microwave fields and with light.
This concept represents a step beyond existing architectures. By designing silicon photonic crystal cavities with mechanical modes and embedding donor atoms directly into these structures, the project has created a viable physical platform for spin–mechanical coupling on a chip. The investigations of a possible local optical readout method for donor spins (Loippo et al., Phys. Rev. Materials 2023) and advances in precision optomechanical control (Kumar et al., arXiv 2022; la Gala et al., Phys. Rev. Applied 2023) have broadened the experimental toolkit for quantum control in silicon.
In addition, detailed studies on ion implantation damage (Shakespeare et al., Materials for Quantum Technology 2021) and photothermal effects (Shakespeare et al., Optics Express 2024) have provided crucial insights that guide both our own and the wider community’s device fabrication strategies. The combination of theoretical modelling and numerical simulations has resulted in two ongoing manuscripts (Lyyra et al., arXiv 2025; Hijano et al., arXiv 2025) that deepen understanding of the coupling dynamics and design optimization.
Soon after the project, the team expects to finalize the next-generation devices incorporating magnetic field gradient coupling, which theory predicts to yield orders of magnitude stronger interaction between donor spins and mechanical motion. These developments, together with the established cryogenic optomechanics laboratory and active international collaborations, place the project consortium at the forefront of efforts to realize scalable, silicon-compatible quantum interfaces linking spins, phonons, and photons.
This concept represents a step beyond existing architectures. By designing silicon photonic crystal cavities with mechanical modes and embedding donor atoms directly into these structures, the project has created a viable physical platform for spin–mechanical coupling on a chip. The investigations of a possible local optical readout method for donor spins (Loippo et al., Phys. Rev. Materials 2023) and advances in precision optomechanical control (Kumar et al., arXiv 2022; la Gala et al., Phys. Rev. Applied 2023) have broadened the experimental toolkit for quantum control in silicon.
In addition, detailed studies on ion implantation damage (Shakespeare et al., Materials for Quantum Technology 2021) and photothermal effects (Shakespeare et al., Optics Express 2024) have provided crucial insights that guide both our own and the wider community’s device fabrication strategies. The combination of theoretical modelling and numerical simulations has resulted in two ongoing manuscripts (Lyyra et al., arXiv 2025; Hijano et al., arXiv 2025) that deepen understanding of the coupling dynamics and design optimization.
Soon after the project, the team expects to finalize the next-generation devices incorporating magnetic field gradient coupling, which theory predicts to yield orders of magnitude stronger interaction between donor spins and mechanical motion. These developments, together with the established cryogenic optomechanics laboratory and active international collaborations, place the project consortium at the forefront of efforts to realize scalable, silicon-compatible quantum interfaces linking spins, phonons, and photons.