Diamond Chiplets Boost Photonic Quantum Computing Integration

A new cross waveguide design for diamond-based colour centre chiplets has been presented by Alessio Miranda and colleagues at Delft University of Technology. The design addresses key technological challenges in diamond fabrication and circuit complexity. It details a methodology to optimise both the chiplet containing the colour centre and its receptor, achieving an excitation-to-emission conversion exceeding 5.4%, crosstalk below -40 dB, and a bandwidth of 160 nm within a compact footprint of less than 2000 μm². The resulting structure is a vital step towards efficient and strong photonic quantum circuits utilising diamond colour centres.

High-efficiency diamond chiplet surpasses important threshold for scalable quantum readout

Excitation-to-emission conversion now exceeds 5.4% within this diamond chiplet, a substantial improvement over previous designs that struggled to surpass 3%. This level is vital for viable quantum systems, as signal loss below it overwhelms the faint light emitted by colour centres, rendering accurate quantum state readout impossible. The significance of exceeding this threshold lies in the inherent weakness of the signals generated by individual colour centres. These defects, acting as artificial atoms, emit photons carrying quantum information, but the emission rate is low, necessitating highly efficient collection and minimal loss. Previous designs suffered from significant optical losses due to imperfect waveguide coupling and scattering within the diamond material itself.

The newly designed cross waveguide efficiently accesses and collects emissions from these centres, enabling heterogeneous integration with other photonic circuits and paving the way for modular quantum technologies. Careful optimisation of both the chiplet and its silicon nitride receptor ensures strong operation, minimising crosstalk to below -40 dB and maintaining a 160 nm bandwidth. Crosstalk, a measure of unwanted signal leakage between adjacent waveguides, is critical for maintaining the integrity of quantum information; a value of -40 dB indicates exceptionally low interference. The 160 nm bandwidth allows for operation across a range of wavelengths, accommodating variations in colour centre emission and facilitating multiplexing of multiple quantum channels.

A compact structure, with a footprint under 2000 μm², demonstrates fabrication feasibility and mechanical stability for practical applications. This small size is crucial for increasing the density of quantum devices on a single chip, a key requirement for scaling up quantum processors. Larger footprints would limit the number of colour centres that can be integrated, hindering the development of complex quantum algorithms. Simulations utilising a 15 nm mesh within the Lumerical software suite confirm performance at a resonant wavelength of 620 nm, typical for silicon-vacancy (SnV) colour centres. The silicon-vacancy centre, a nitrogen atom substituting a carbon atom adjacent to a vacancy, is a particularly promising colour centre due to its relatively long coherence times and bright emission. The use of a 15 nm mesh in the Lumerical simulations provides a high level of spatial resolution, allowing for accurate modelling of light propagation within the nanoscale structures.

Alignment marks incorporated into both the diamond chiplet and its silicon nitride receptor aid in precise positioning during a “pick and place” integration process, relying on van der Waals forces to secure the chiplet. This pick-and-place approach is essential for assembling complex quantum circuits from individual chiplets, offering a potentially cost-effective and scalable manufacturing method.

Further investigation focused on the long-term stability of these forces and the potential for automated assembly. Achieving over 5.4% excitation-to-emission conversion, alongside minimal signal interference and a small footprint, presents a viable pathway towards scalable quantum technologies. Colour centres are atomic-scale defects within diamond used to store and manipulate quantum information. These defects possess unique spin properties that can be controlled and read out using light and microwave radiation.

The resulting chiplet, incorporating a cross waveguide to efficiently channel light, enables heterogeneous integration, combining different materials with silicon nitride photonic circuits. Silicon nitride is an excellent waveguide material due to its low optical loss and compatibility with standard microfabrication techniques. This approach allows for the creation of complex systems by connecting multiple chiplets, but the impact of numerous interfaces on signal quality requires further study. Each interface introduces potential sources of scattering and reflection, which can degrade the quantum signal and reduce the fidelity of quantum operations. Understanding and mitigating these interface effects is therefore paramount for building large-scale quantum networks.

Diamond colour centre integration confronts interface limitations for scalable quantum networks

Isolating diamond colour centres within a chiplet offers a pragmatic solution to the longstanding challenge of integrating these quantum emitters with scalable photonic circuits. Diamond, while possessing exceptional quantum properties, is notoriously difficult to fabricate into complex photonic structures. The material’s hardness and brittleness pose significant challenges for etching and patterning, limiting the complexity of on-chip photonics. Separating the diamond containing the colour centre from the supporting photonic circuitry allows each component to be fabricated using the most appropriate materials and techniques.

The work acknowledges a gap in demonstrating full system-level performance; the “receptor” component, while optimised in combination with the chiplet, remains the only other photonic element detailed. This raises a key tension: can this modular approach truly deliver the complex, interconnected networks needed for practical quantum computation, or will the accumulation of interfaces between numerous chiplets introduce unacceptable signal degradation and limit scalability? The performance of the receptor component, including its coupling efficiency to the diamond chiplet and its ability to route and manipulate photons, will ultimately determine the overall performance of the integrated system.

Separating diamond, which hosts colour centres—tiny defects emitting light used for quantum information—from the silicon circuits needed for control and communication has long been a hurdle. Efficient light transfer between a diamond chiplet and the receptor component provides a modular building block. The choice of silicon nitride as the receptor material is strategic, offering a good compromise between optical performance and fabrication compatibility. However, the refractive index mismatch between diamond and silicon nitride can lead to significant reflection losses at the interface. Careful design of the waveguide coupling structures is therefore essential to maximise light transmission.

This refined fabrication methodology successfully decouples diamond colour centre integration from the complexities of building entire photonic circuits, allowing focus on optimising the interfaces between these modules and developing strategies to mitigate signal loss across larger networks. Future research will likely focus on developing advanced coupling techniques, such as adiabatic mode conversion, to minimise reflection losses and improve the overall efficiency of the integrated system. Furthermore, exploring alternative receptor materials with better refractive index matching could further enhance performance.

The researchers successfully demonstrated a method for optimising the connection between diamond chiplets, containing light-emitting colour centres, and a silicon nitride receptor component. This modular approach addresses the technological challenges of integrating diamond into larger photonic circuits by separating the diamond material from the silicon circuitry. The optimised structure achieved an excitation-to-emission conversion of over 5.4% with minimal signal loss, indicated by a crosstalk of less than -40 dB, and operates across a bandwidth of 160 nm. The authors intend to explore advanced coupling techniques and alternative receptor materials to further improve efficiency and reduce signal loss.