Researchers from various Chinese institutions have significantly advanced quantum computing by demonstrating that logical qubits outperform physical qubits. Logical qubits, defined using a set of physical qubits, are less susceptible to errors. The team used dynamical decoupling to manipulate interactions between physical qubits and their environment. They also proposed a scalable scheme to generate collective interactions for multiple qubits using only iSWAP gates. The researchers’ scheme could simplify quantum computation and make it more efficient, bringing us closer to achieving reliable quantum computation.
What is the Significance of Logical Qubits in Quantum Computing?
Quantum computing is a rapidly evolving field that leverages the principles of quantum mechanics to perform computations. Qubits are the quantum equivalent of classical bits at the heart of quantum computers. However, physical qubits, the fundamental building blocks of quantum computers, are inherently susceptible to errors due to unavoidable interactions with their environment. To perform reliable quantum computation, one needs to employ logical qubits, defined using a set of physical qubits. Demonstrating that logical qubits outperform their physical counterparts is a significant milestone in achieving reliable quantum computation.
One approach to achieving this goal is to utilize quantum error correction (QEC). Despite impressive achievements, connecting multiple logical qubits presents a significant challenge for these schemes. Therefore, it is intriguing to investigate scalable error-mitigation or error-suppressing schemes for protecting logical qubits. Dynamical decoupling (DD) can manipulate interactions between a set of physical qubits and their environment by applying tailored control pulses to the qubits. One method is to eliminate these interactions, isolating the evolution of qubits completely. Another method is to selectively eliminate non-collective interactions while preserving collective ones, in which the qubits uniformly interact with the environment.
How Can Logical Qubits be Protected?
In a recent study, researchers proposed to generate Z-type collective interactions for physical qubits by periodically performing iSWAP gates. While the Z-type collective interactions are created, the X and Y-type interactions are removed from the system evolution. The resulting Z-type collective interaction supports a decoherence-free subspace (DFS), enabling the encoding of logical qubits, each of which requires only two physical qubits. One feature of this scheme is scalability, and generating collective interactions for multiple qubits requires only iSWAP gates between nearest-neighbor qubits. Furthermore, universal gates for logical qubits can be realized using single-body and nearest-neighbor two-body Hamiltonians.
What are the Experimental Results?
The researchers experimentally demonstrated that the coherence time of a protected logical qubit is extended by up to 366% compared to that of the better-performing physical qubit. For two logical qubits, the process fidelity is 194% compared to that of the physical qubits at a duration of τ 48µs. To the best of their knowledge, this is the first time that multiple logical qubits have been shown to outperform their physical counterparts in superconducting qubits. To showcase the universal control of logical qubits, they conducted a logical Ramsey experiment on one of them and created a logical Bell state with logical operations.
What are the Implications of this Research?
The researchers’ scheme holds promise for future reliable quantum computation. The scalability of the scheme is particularly noteworthy, as it allows for the generation of collective interactions for multiple qubits using only iSWAP gates between nearest-neighbor qubits. This could simplify the quantum computation process and make it more efficient. Furthermore, the researchers demonstrated that their scheme can enhance the success probability in superdense coding as an application. This research represents a significant step forward in the field of quantum computing, bringing us closer to the goal of achieving reliable quantum computation.
Who are the Researchers Behind this Study?
The research was conducted by a team of scientists from the Beijing Academy of Quantum Information Sciences, Hefei National Laboratory, State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics at Tsinghua University, Frontier Science Center for Quantum Information, and Beijing National Research Center for Information Science and Technology. The team includes JiaXiu Han, Jiang Zhang, GuangMing Xue, Haifeng Yu, and Guilu Long.
The article named: Protecting logical qubits with dynamical decoupling, was published in arXiv (Cornell University) on 2024-02-08, . The authors are Jia-Xiu Han, Jiang Zhang, Guangming Xue, Haifeng Yu and Gui‐Lu Long. Find more at https://doi.org/10.48550/arxiv.2402.05604.
