University of New Mexico and Google Quantum AI Develop Robust Quantum Computing Protocol

Researchers from the University of New Mexico and Google Quantum AI have developed a fault-tolerant quantum computation (FTQC) protocol using large spincat codes. The protocol corrects dominant error sources in quantum computation, surpassing the fault-tolerant threshold of standard qubit-based encodings. The team demonstrated the protocol’s practical application in neutral-atom quantum computing with qudits encoded in the nuclear spin of 87Sr. This approach, which exploits larger Hilbert spaces, could lead to improved thresholds and reduced resource overhead in quantum information processing, making quantum computation more robust.

What is Fault-Tolerant Quantum Computation Using Large SpinCat Codes?

Fault-tolerant quantum computation (FTQC) is a method that allows for reliable computation even in the presence of imperfect elementary components. This is achieved by ensuring that the error rate of individual components remains below a constant threshold, allowing for arbitrarily long quantum computation. A team of researchers from the Center for Quantum Information and Control and the Department of Physics and Astronomy at the University of New Mexico, along with a member from Google Quantum AI, have constructed a fault-tolerant quantum error-correcting protocol based on a qubit encoded in a large spin qudit using a spincat code.

The spincat code is analogous to the continuous-variable cat encoding. This protocol can correct dominant error sources, namely processes that can be expressed as error operators that are linear or quadratic in the components of angular momentum. Such codes tailored to dominant error sources can exhibit superior thresholds and lower resource overheads when compared to those designed for unstructured noise models.

How Does the Protocol Work?

The protocol works by categorizing the dominant errors as phase and amplitude errors. The team demonstrated how phase errors, analogous to phase-flip errors for qubits, can be effectively corrected. They also proposed a measurement-free error-correction scheme to address amplitude errors without relying on syndrome measurements.

A key component of the protocol is the CNOT gate that preserves the rank of spherical tensor operators. Through an in-depth analysis of logical CNOT gate errors, the team established that the fault-tolerant threshold for error correction in the spincat encoding surpasses that of standard qubit-based encodings.

What is the Practical Application of this Protocol?

The team considered a specific implementation based on neutral-atom quantum computing with qudits encoded in the nuclear spin of 87Sr. They showed how to generate the universal gate set, including the rank-preserving CNOT gate, using quantum control and the Rydberg blockade. These findings pave the way for encoding a qubit in a large spin with the potential to achieve fault tolerance, high threshold, and reduced resource overhead in quantum information processing.

What are the Advantages of this Approach?

The conventional approaches for FTQC are mostly devoted to structureless and uncorrelated noise. However, such decoherence models often entail stringent threshold requirements and result in significant overheads for FTQC. An alternative strategy involves seeking error-correcting codes tailored to the prevalent noise sources of the particular physical platform. When possible, these tailored approaches can lead to improved thresholds and reduced resource overhead.

Another advantage of this approach is the exploitation of the larger Hilbert spaces that can be controlled in individual subsystems for a given physical platform. While many platforms offer access to multiple levels, the focus is often on isolating two well-defined levels for qubit-based computations. However, a more advantageous approach emerges when we exploit these multiple levels to create qubits naturally resilient to dominant noise channels.

What is the Future of Fault-Tolerant Quantum Computation?

The research team’s work on fault-tolerant quantum computation using large spincat codes represents a significant step forward in the field of quantum computing. By harnessing the properties of qudits with multiple levels, they have established logical qubits that possess inherent resistance to the impact of dominant noise channels, paving the way for more robust quantum computation.

The concept of encoding a qubit in a large spin has been explored in previous works. However, this is the first time that a fault-tolerant quantum error-correcting protocol has been constructed for a qubit encoded in a large spin. This approach can be extended to a wide range of physical systems, bringing us closer to harnessing the full potential of quantum computing.