Eighteen Qubits Function Simultaneously, Paving the Way for Practical Quantum Computers

J. J. Dijkema and colleagues at QuTech, Delft University of Technology, and imec have achieved a key advance in scaling up qubit registers. Their work details the successful operation of an 18-qubit array fabricated in germanium, utilising a modular architecture designed for future expansion. Simultaneous control and readout across all qubits with high fidelity, with average and median single-qubit gate fidelities of 99.8% and 99.9% respectively, represents a step towards realising utility-scale quantum computation with semiconductor spin qubits. Furthermore, the demonstration of high-quality controlled-Z gates and the generation of a three-qubit Greenberger-Horne-Zeilinger state validates this modular approach as a viable pathway for building planar semiconductor quantum processors.

Modular germanium qubit arrays use parallel processing for enhanced scalability

Parallel operation of modular unit cells underpinned the successful scaling of this qubit array. Rather than sequentially addressing each qubit, the processor was designed with repeating, identical blocks, akin to an assembly line where multiple products are worked on simultaneously, vastly increasing throughput. Each unit cell contains all the necessary components for qubit control and readout, allowing for parallel manipulation of multiple qubits, an important step in building more complex quantum circuits.

A modular approach simplifies the control architecture and enables future expansion, circumventing the limitations of custom layouts seen in earlier devices. A qubit is a basic unit of quantum information, like a bit in a regular computer but capable of representing more complex states. An 18-qubit array fabricated in germanium was successfully operated, utilising a 2xN architecture for scalability. Employing modular unit cells, this design enables parallel qubit control and readout, a method chosen to overcome limitations found in earlier quantum processor layouts. Detailed characterisation of each qubit’s properties determined single-qubit gate fidelities averaging 99.8%, with a median of 99.9% across the array, demonstrating high-performance operation despite the increased qubit count.

High-fidelity control and modularity in an 18-qubit germanium quantum processor

Single-qubit gate fidelity across the 18-qubit germanium array reached 99.9%, a substantial improvement over previous demonstrations limited to sequential operations and custom layouts. Achieving such precision across a register of this size was previously unattainable, surpassing the critical threshold for scalable quantum computation where maintaining high fidelity becomes increasingly challenging as qubit numbers grow. The new architecture, composed of repeating six-qubit unit cells, enables parallel control and readout, circumventing bottlenecks inherent in earlier designs.

Furthermore, successful generation of a three-qubit Greenberger-Horne-Zeilinger state validates the modular approach as a viable route towards building planar semiconductor quantum processors capable of complex calculations. An average single-qubit gate fidelity of 99.8% and a median of 99.9% across the 18-qubit germanium array was confirmed, and this consistency is vital as even slight variations in performance can rapidly degrade calculations in larger systems. The device utilizes a repeating six-qubit unit cell design, each incorporating a dedicated charge sensor, allowing for simultaneous readout of all qubits within a cell and sharply speeding up processing compared to sequential reading methods. The creation of a three-qubit Greenberger-Horne-Zeilinger (GHZ) state, a complex entangled state, validates the modular architecture’s ability to perform multi-qubit operations. While these results represent a major step forward, they do not yet address the substantial engineering challenges of scaling control electronics to manage thousands of qubits, or the implementation of strong error correction schemes necessary for fault-tolerant quantum computation.

Germanium qubits advance processor scale, but fault tolerance remains a key challenge

The creation of an 18-qubit array in germanium signifies progress towards building larger, more useful quantum processors. Semiconductor spin qubits offer a promising route to integrated quantum-classical systems. However, demonstrating complex algorithms or full error correction remains elusive, with only a three-qubit GHZ state generated as proof of principle. This highlights a critical tension; while scaling qubit numbers is achievable through modular designs, realising genuinely fault-tolerant computation demands significantly more sophisticated error mitigation strategies.

Achieving a functioning quantum computer demands more than simply increasing qubit counts. Generating a three-qubit GHZ state, a specific entangled configuration, is a valuable demonstration, but it is a far cry from the complex error correction needed for reliable computation. Researchers  have fabricated an 18-qubit processor in germanium, demonstrating high-fidelity control across the array. This modular architecture offers a pathway towards scaling quantum systems, and practical quantum computation will begin with strong error correction techniques already under development.

This demonstration of an 18-qubit array in germanium establishes a scalable architecture for building quantum processors. The team achieved simultaneous control and readout of each qubit using repeating, modular unit cells. Maintaining high-fidelity operation, with median single-qubit gate fidelity at 99.9%, across this many qubits represents a significant step beyond previous, smaller devices limited to sequential operations. Successfully generating a three-qubit Greenberger-Horne-Zeilinger state confirms the viability of this modular approach, and attention now turns to exploring how to connect these modules to create even larger, more complex quantum systems.

The successful operation of an 18-qubit array in germanium demonstrates that spin-qubit systems can be scaled while preserving high performance. Achieving average single-qubit gate fidelities of 99.8% across the array is particularly noteworthy, as it indicates a level of control previously unseen in larger qubit systems. Researchers also generated a three-qubit Greenberger-Horne-Zeilinger state, validating the modular architecture. The authors intend to explore connecting these modules to build even larger quantum systems, furthering the development of planar semiconductor quantum processors.