Using coherent spin shuttling to scale up quantum computing

This development, leveraging a technique known as coherent spin shuttling, is a significant stride towards realising large-scale quantum computing. This pioneering method addresses the long-standing challenge of linking quantum dots that are far apart, which is crucial in scaling quantum computers from hundreds to millions of qubits.

This advancement in connecting distant qubits is set to propel the development of larger and more intricate quantum computers, which rely on silicon quantum dots. RIKEN physicists have demonstrated an effective method to connect two qubits, the fundamental units of quantum information, separated by a physical distance.

Major IT corporations, including IBM, Google, and Microsoft, are fervently engaged in the race to develop quantum computers. Some of these entities have already showcased the superior capabilities of quantum computers over traditional ones in specific computational tasks. However, a significant challenge in the commercialisation of quantum computers is scaling them from a mere hundred qubits to millions.

Among the leading technologies for large-scale quantum computing are silicon quantum dots, with diameters in the tens of nanometres range. A notable advantage of silicon quantum dots is their compatibility with existing silicon fabrication technologies. Yet, a major obstacle has been the difficulty in connecting quantum dots that are not in close proximity. “In order to connect many qubits, we have to densely cram many of them into a very small area,” says Akito Noiri of the RIKEN Centre for Emergent Matter Science. “And it’s very hard to use wires to connect such very densely packed qubits.”

Noiri and his team have now successfully executed a two-qubit logic gate between silicon spin qubits that are physically apart.

“While there has been a lot of work in this area using various approaches, this is the first time that anyone has succeeded in demonstrating a reliable logic gate formed by two distant qubits,” says Noiri. “The demonstration opens up the possibility of scaling up quantum computing based on silicon quantum dots.”

Coherent spin shuttling

The technique employed by the team, spin shuttling, enables the movement of single spin qubits across a quantum dot array without disrupting their phase coherence. Phase coherence is a vital attribute for quantum computers as it transports information. The process involves propelling electrons through an array of qubits by applying a voltage.

Although the physical distance between the two qubits in this experiment was relatively modest, Noiri is optimistic about expanding this distance in future research. “We want to increase the separation to about a micrometer or so,” he says. “That will make the method more practical for future use.”

Understanding coherent spin shuttling

Quantum coherent spin shuttling is a concept, now proving a reality, in quantum computing and physics that involves the transfer of quantum information, specifically the spin state of a particle, between different locations without losing its quantum coherence. Quantum coherence is the property that allows particles to be in a combination of states at the same time, a fundamental aspect of quantum mechanics.

To understand this concept, let's use a practical analogy of a library book transfer system.

Imagine a library with an advanced book transfer system. Each book in this library represents a 'quantum bit' or qubit, and the content of the book (the specific arrangement of words) represents the quantum state of the qubit, analogous to the spin state in quantum coherent spin shuttling.

Under this analogy, the spin shuttling method would go as follows:

• Initial state (Encoding information): In a library, each book has a unique arrangement of words (its content). Similarly, in a quantum system, each qubit has a specific quantum state (like spin up or spin down).
• Transfer process (Shuttling): The library uses a special conveyor system to move books from one section to another. This conveyor is designed to keep the books open and upright, ensuring that no words are altered or pages turned during the transfer. In quantum coherent spin shuttling, we transfer the quantum state (spin) from one qubit to another, often using electromagnetic fields or other quantum mechanical interactions. This transfer must maintain the quantum coherence, meaning the transferred state must remain in its superposition (combination of spin up and down) and entanglement properties throughout the process.
• Preserving the state (Maintaining coherence): Just as the library's system ensures no words in the book change during the transfer, in quantum coherent spin shuttling, it's crucial to preserve the quantum state exactly as it is. This is challenging because interacting with a quantum system or exposing it to an environment can disrupt its coherence (akin to someone shaking the conveyor belt and messing up the book's pages).
• Arrival and further use (readout): Once the book reaches its destination, it can be read and used, with its content unaltered by the journey. Similarly, once the quantum state is shuttled to a new location, it retains its quantum information, which can be used for further quantum computations or measurements.

This analogy simplifies many complexities of quantum mechanics but helps illustrate the core idea of quantum coherent spin shuttling: transferring quantum information reliably without disturbing its delicate quantum state. This capability is vital for building effective quantum computers and quantum communication systems, where maintaining coherence over distances and interactions is essential.