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Researchers at MIT and a non-profit corporation have achieved a significant breakthrough by demonstrating a scalable, modular hardware platform that integrates thousands of interconnected qubits onto a customised integrated circuit. This “quantum-system-on-chip” (QSoC) architecture allows precise tuning and controlling a dense array of qubits. Multiple chips can be connected via optical networking, forming a large-scale quantum communication network. This innovation marks a critical step toward practical quantum computing.
The QSoC architecture involves tuning qubits across 11 frequency channels, enabling a new protocol called “entanglement multiplexing” for large-scale quantum computing. This breakthrough required years to perfect a process for manufacturing two-dimensional arrays of atom-sized qubit microchiplets and transferring thousands onto a complementary metal-oxide semiconductor (CMOS) chip in a single step, ensuring the necessary scalability for practical applications.
“We need a large number of qubits and precise control to leverage the power of a quantum system. We propose a new architecture and fabrication technology that can support the scalability needed for a quantum computer,” said Linsen Li, an electrical engineering and computer science graduate student and lead author of the paper.
The researchers chose diamond colour centres as their qubit type due to their scalability advantages. Diamond colour centres are “artificial atoms” that carry quantum information and are compatible with modern semiconductor fabrication processes.
They are compact and exhibit relatively long coherence times, meaning the qubits’ states remain stable for extended periods due to the clean environment provided by the diamond material. Furthermore, diamond colour centres have photonic interfaces, enabling them to be remotely entangled with other qubits, enhancing their utility in large-scale quantum systems.
Linsen Li elaborated, “The inhomogeneity of the diamond colour centre is often seen as a drawback. However, we turn this into an advantage by embracing the spectral diversity of the artificial atoms. Each atom’s unique frequency allows us to communicate with individual atoms by tuning them into resonance with a laser, similar to dialling a radio.”
Achieving communication across qubits involves having multiple “quantum radios” tuned to the same channel, which becomes feasible at the scale of thousands of qubits. The researchers addressed this by integrating an extensive array of diamond colour centre qubits onto a CMOS chip. With built-in digital logic, this chip can rapidly and automatically reconfigure voltages, ensuring full connectivity.
“This compensates for the inhomogeneous nature of the system. With the CMOS platform, we can quickly and dynamically tune all the qubit frequencies,” Linsen Li explained.
To build the QSoC, researchers developed a fabrication process to transfer diamond colour centre “microchiplets” onto a CMOS backplane. They began by creating an array of diamond colour centre microchiplets from a solid diamond block and designed nanoscale optical antennas for efficient photon collection.
In the MIT.nano cleanroom, they post-processed a CMOS chip to add microscale sockets matching the diamond microchiplet array. Using an in-house transfer setup, they applied a lock-and-release process to integrate the diamond microchiplets into the CMOS chip sockets.
Since the diamond microchiplets are weakly bonded to the diamond surface, releasing the bulk diamond horizontally allows the microchiplets to stay in the sockets.
The researchers developed a method to characterise and measure the QSoC’s performance on a large scale. Using a custom cryo-optical metrology setup, they demonstrated a chip with over 4,000 qubits, all tuned to the same frequency while maintaining spin and optical properties. They also created a digital twin simulation to connect experiments with digitised modelling, aiding in efficient implementation.
Future improvements could come from refining qubit materials or developing more precise control processes. This breakthrough is a significant step towards realising quantum computing’s full potential, with wide-ranging implications for cryptography and complex system modelling.
