Quantum computing is among the most exciting and rapidly changing branches of modern research. By harnessing quantum effects to make calculations, researchers around the world are continually developing cutting-edge computation techniques, whose capabilities would have astonished computer scientists just a few years ago.
Although phenomena such as particle superpositions and quantum entanglement are still far from being properly understood, they are now being used to send, receive, store, and process information at far greater speeds and efficiencies than conventional computers currently allow. Yet despite these exciting developments, the technology still faces many difficult challenges before quantum computers can be widely applied in everyday scenarios.
Among the most pressing of these issues is the need to seamlessly integrate the outcomes of real-life quantum processes – which play out within real physical systems on atomic scales – with those of the calculations initiated by users. In practical settings, operations must be carried out using large enough numbers of quantum bits, or ‘qubits’, to improve the capabilities of quantum computers over their conventional counterparts.
Inevitably, however, such deeply complex operations will lead to calculation errors, which must then be corrected: a costly and time-consuming process. To combat this issue, researchers need to develop systems that minimise errors in their calculations – but this is no simple task.
Monoliths and networks
In recent years, a diverse array of techniques has emerged to process quantum information with as few errors as possible. Each of them has its unique pros and cons, but according to Dr Xing-Yu Zhu at Suzhou University and his team, they can be divided into two broad categories, or ‘architectures’. In the first of these, qubits are arranged in large, orderly arrays, where each qubit will interact directly with its neighbours to process information. Dr Zhu names these systems ‘monolithic’ architectures.
Alternatively, systems can exploit the enigmatic phenomenon of quantum entanglement, where the observed quantum state of one particle can directly correspond with that of another ‘entangled’ particle, no matter how far apart they are separated. In what Dr Zhu’s team call ‘network’ architectures, the effect enables qubits in different parts of a system to become entangled by exchanging a photon, and then, to process information simultaneously.
As Dr Zhu explains, both of these architectures face challenges relating to calculation errors – but based on the results of previous studies, one appears to win out over the other. “The key issue of architecture design is to find a realistic path from the feasible state-of-art technology to quantum information processing in a fault-tolerant manner”, he says. “Compared to a monolithic architecture, a network architecture may be more achievable for many physical platforms.”
In their research, Dr Zhu and his colleagues propose new ways to achieve reliable quantum information processing using network architectures – based on a rapidly growing branch of semiconductor technology.
Introducing quantum dots
With electrical conductivities falling in between those of conductors (such as metals) and insulators (such as glass or plastic), semiconductors are a crucially important element in many modern technologies. Recently, it was discovered that when these materials are scaled down to just a few nanometres in size, creating structures named ‘quantum dots’, they can display remarkably similar properties to individual atoms.
These properties can be observed when a quantum dot is illuminated by ultraviolet light, causing it to transition from an insulator to a conductor. On a quantum scale, this involves a single electron in the dot transitioning from an insulating ‘valence band’ to a ‘conductance band’ as it absorbs the ultraviolet photon.
Just as orbiting electrons have discrete, or ‘quantised’ energy levels characteristic of their atoms, the difference in energy between both of these bands is engrained into the material composition of the quantum dot. As a result, when the electron transitions back to the valence band, it will release a photon with a highly specific energy, which perfectly matches this difference.
In Dr Zhu’s research, this effect is relevant since the quantum states of electrons contained in quantum dots can function as qubits, becoming entangled with other qubits to form robust network architectures. Compared with the qubits used in previous network architectures, such as trapped ions and single atoms, this approach allows for far more reliable information processing – significantly reducing calculation errors. For Dr Zhu and his team, these capabilities offer unprecedented opportunities for developing network architectures suitable for everyday use.
Developing a new module
The researchers propose that qubits can take the form of quantised ‘spins’ in quantum dot electrons. In this context, spin is a purely quantum property, comparable to the polarisation of photos – in which transverse light waves are aligned along one specific direction. Before these qubits can be integrated into practical network architectures, Dr Zhu’s team have identified three important challenges, and propose measures to overcome them.
The first of these relates to the ability of network architectures to read and write information onto their qubits, using only photons. “This is challenging because of the weak influence of a single electron’s spin on its surrounding environment, which limits the spin-photon coupling rates”, explains Dr Zhu. “We resolve this challenge by using a method called ‘spin-charge hybridization’.” This involves coupling the electric field of a single photon with the spin of a system comprising two entangled quantum dot electrons. As a result, system operators can read and write information onto the qubits far more easily.
The team’s second challenge emerges from the need to reliably generate and absorb photons within quantum dots, purely through manipulations of the photons coupled to electron spins. “This needs to be done in a way that can be extended to systems containing many quantum dots”, Dr Zhu continues. “We do this by applying a driving microwave pulse to the system, which mediates tuneable interactions between the electron spins and photons.” This enables operators to efficiently generate and absorb photons on demand, improving the functionality of the system.
Finally, within entangled pairs of qubits, it is critically important to ensure that the quantum information held within one qubit strongly correlates with that of the other. “The key challenge here is to establish high-quality remote entanglement in the presence of unavoidable errors”, Dr Zhu says. “Our approach exemplifies the feasibility of network architecture using currently available experimental techniques.” This is achieved through advanced mathematical procedures, which minimise the influence of calculation errors.
Building blocks for network architectures
With these measures in place, Dr Zhu’s team now present promising new routes towards feasible network architectures. The most significant result of these efforts has been a new design for a small, simple ‘spin-photon module’. Containing two quantum dots, whose electrons can be coupled to a photon through spin-charge hybridisation, these modules can process quantum information in many useful ways to produce clear, accurate output signals.
Perhaps the most important feature of the team’s design is its ability to be integrated with many other identical modules: acting as ‘building blocks’ for more complex systems. This enables researchers to construct and manipulate intricate networks of entanglement, suitable for advanced applications in quantum computing. “The spin-photon module we propose constitutes a kind of building block for the network architecture and all higher functions are built upon it. The results of our work indicate that the spin-photon network is consistent with present technology and might be achievable in the near future”, Dr Zhu concludes.
Without the need for further technological innovations, this approach could ensure that practical quantum information processing can be achieved through far simpler architectures than were previously thought possible. Through future work to realise their proposal experimentally, Dr Zhu’s team could soon bring the widespread use of everyday quantum computing a step closer to reality.
- Zhu, X.Y., Tu, T., Guo, A.L., Zhou, Z.Q., Guo, G.C. and Li, C.F. (2020). Spin-photon module for scalable network architecture in quantum dots. Scientific reports, 10(1), 1-9. Available at: https://doi.org/10.1038/s41598-020-61976-2
Based on a spin-photon module, Dr Zhu examines scalable quantum network architecture.
- National Natural Science Foundation of China (No. 11974336)
- National Key R&D Program of China (No. 2017YFA0304100)
- The Scientific Research Foundation of Suzhou University (No. 2020BS006)
- Tao Tu (USTC)
- Guang-Can Guo (USTC)
Dr Xing-Yu Zhu works at the School of Mechanical and Electronic Engineering, Suzhou University. He received his PhD in physics from the University of Science and Technology of China in 2020. Dr Zhu has been working on the theoretical research of solid-state spin quantum computing.
School of Mechanical and Electronic Engineering
Suzhou 234000, P. R. China
Key Laboratory of Quantum Information
University of Science and Technology of China
Hefei 230026, P. R. China