Speeding-up quantum computing using giant atomic ions

Scientists from the University of Nottingham and University of Stockholm have sped-up trapped ion quantum computing using a new experimental approach -- trapped Rydberg ions; their results have just been published in Nature.

In conventional digital computers, logic gates consist of operational bits that are silicon based electronic devices. Information is encoded in two classical states ("0" and "1") of a bit. This means that capacities of a classical computer increase linearly with the number of bits. To deal with emerging scientific and industrial problems, large computing facilities or supercomputers are built.

Quantum entanglement enhancing capacity

A quantum computer is operated using quantum gates, i.e. basic circuit operations on quantum bits (qubits) that are made of microscopic quantum particles, such as atoms and molecules. A fundamentally new mechanism in a quantum computer is the utilisation of quantum entanglement, which can bind two or a group of qubits together such that their state can no longer be described by classical physics. The capacity of a quantum computer increases exponentially with the number of qubits. The efficient usage of quantum entanglement drastically enhances the capacity of a quantum computer to be able to deal with challenging problems in areas including cryptography, material, and medicine sciences.

Among the different physical systems that can be used to make a quantum computer, trapped ions have led the field for years. The main obstacle towards a large-scale trapped ion quantum computer is the slow-down of computing operations as the system is scaled-up. This new research may have found the answer to this problem.

The experimental work was conducted by the group of Markus Hennrich at SU using giant Rydberg ions, 100,000,000 times larger than normal atoms or ions. These huge ions are highly interactive, and exchange quantum information in less than a microsecond. The interaction between them creates quantum entanglement. Chi Zhang from the University of Stockholm and colleagues used the entangling interaction to carry out a quantum computing operation (an entangling gate) around 100 times faster than is typical in trapped ion systems.

Chi Zhang explains, "Usually quantum gates slow down in bigger systems. This isn't the case for our quantum gate and Rydberg ion gates in general! Our gate might allow quantum computers to be scaled up to sizes where they are truly useful!"

Theoretical calculations supporting the experiment and investigating error sources have been conducted by Weibin Li (University of Nottingham, UK) and Igor Lesanovsky (University of Nottingham, UK, and University of Tübingen, Germany). Their theoretical work confirmed that there is indeed no slowdown expected once the ion crystals become larger, highlighting the prospect of a scalable quantum computer.

Weibin Li, Assistant Professor, School of Physics and Astronomy at the University of Nottingham adds: "Our theoretical analysis shows that a trapped Rydberg ion quantum computer is not only fast, but also scalable, making large-scale quantum computation possible without worrying about environmental noise. The joint theoretical and experimental work demonstrate that quantum computation based on trapped Rydberg ions opens a new route to implement fast quantum gates and at the same time might overcome many obstacles found in other systems."

Currently the team is working to entangle larger numbers of ions and achieve even faster quantum computing operations.