Revolutionizing Quantum Computing: New Two-Qubit Encoding Architecture in Ytterbium-171 Atoms

Researchers from the University of Illinois at Urbana-Champaign and the University of California, Berkeley, have developed a new architecture for encoding two qubits in neutral ytterbium-171 atoms. The architecture uses the optical clock transition and nuclear spin-1/2 degree of freedom, inspired by recent advancements in high-fidelity control of all pairs of states within a four-dimensional ququart space. The team demonstrated the benefits of ququart encoding for entanglement distillation and quantum error correction, showing superior hardware efficiency and performance. The research could revolutionize quantum computing by improving hardware efficiency and performance, and opens up new avenues for future research and applications.

What is the New Architecture for Two-Qubit Encoding in Neutral Ytterbium-171 Atoms?

A team of researchers from the Department of Physics at The University of Illinois at Urbana-Champaign and The University of California, Berkeley, have developed a new architecture for encoding two qubits within the optical clock transition and nuclear spin-1/2 degree of freedom of neutral ytterbium-171 atoms. This architecture is inspired by recent advancements in high-fidelity control of all pairs of states within this four-dimensional ququart space. The team has presented a toolbox for intra-ququart single atom one and two-qubit gates, inter-ququart two atom Rydberg-based two and four-qubit gates, and quantum non-demolition (QND) readout.

The researchers have demonstrated the advantages of the ququart encoding for entanglement distillation and quantum error correction, which exhibit superior hardware efficiency and better performance in some cases since fewer two-atom Rydberg-based operations are required. Leveraging single-state QND readout in their ququart encoding, they have presented a unique approach to studying interactive circuits and realizing a symmetry-protected topological phase of a spin-1 chain with a shallow constant-depth circuit. These applications are all within reach of recent experiments with neutral ytterbium-171 atom arrays or with several trapped ion species.

How Does the New Architecture Improve Quantum Computing?

Most quantum computing architectures focus on two-level quantum systems for encoding qubits. However, the need for auxiliary quantum systems or extra quantum states is ubiquitous. These extra degrees of freedom are used to mediate gates between qubits, perform measurements or cooling during a computation, or improve the qubit connectivity. For neutral atom and trapped ion quantum computers, these auxiliary degrees of freedom often take the form of a second atomic species and/or atoms that are moved during the circuit.

The potential of extra states within the atoms to address these needs has gained intense interest. The ability to use two or more portions of the atomic level structure to perform disparate functions simultaneously without crosstalk has recently enabled mid-circuit operations such as readout and cooling. It has also been shown how the ability to programmably repurpose an atom for these roles by moving it between the different sets of levels opens the door to improved hardware efficiency and more flexible circuit compiling.

What is the Significance of Higher-Dimensional Encodings?

When using auxiliary states within an atom to perform ancillary functions for qubits, a natural question arises: is it advantageous to include these states in the computational space and encode quantum information in a higher dimension? For instance, when using a pair of extra states to provide two distinct qubit encodings within one atom, we could alternatively think of this four-level system as a ququart composed of two qubits.

Higher-dimensional encodings with qudits have attracted much attention for neutral atoms, trapped ions, and superconducting circuits. Qudit systems offer improved hardware efficiency, including logical encoding, and potentially have better performance if the intra-atom gates have higher fidelity than the inter-atom gates. However, the required intra-atom operations quickly become numerous and challenging as the dimension grows, and thus relatively small internal spaces whose states are already widely used for myriad qubit operations offer an attractive starting point for architectures with higher-dimensional encoding.

How Does the Ququart Operations in Ytterbium-171 Atoms Work?

The researchers focused on the level structure and operations surrounding the ququart encoding. Specifically, they focused on operations that are easily translated to the more widely used language of qubits because they allow us to apply our ququart system to any qubit-based algorithm. The ququart is encoded in the F = 1/2 1S0 ground and 6s6p3P0 metastable manifolds, each with two nuclear Zeeman substates. Transitions between the ground and metastable states are driven by a direct optical clock coupling, and transitions between nuclear Zeeman states are driven by a stimulated Raman coupling.

What are the Future Implications of this Research?

This research builds upon the growing literature of higher-dimensional encoding schemes for neutral atoms, trapped ions, and superconducting circuits. It presents a comprehensive blueprint for qudit encodings that are specifically focused on a direct mapping with qubit circuits. The new architecture for encoding two qubits within the optical clock transition and nuclear spin-1/2 degree of freedom of neutral ytterbium-171 atoms could potentially revolutionize the field of quantum computing by improving hardware efficiency and performance. The researchers’ unique approach to studying interactive circuits and realizing a symmetry-protected topological phase of a spin-1 chain with a shallow constant-depth circuit also opens up new avenues for future research and applications in quantum computing.