Quantum computing (QC) has the potential to transcend classical limits of computing in applications of interest to the Intelligence Community (IC) and the DoD. Qubits—the indivisible units or “bits” of quantum information within a quantum computer—exhibit quantum coherence[1] and entanglement[2],[3] properties but are also subject to noise and couplings to the environment, all of which weaken coherence and lead to the loss of quantum information during computation and thus produce computational errors. Fortunately, qubits may be organized into computational units called logical qubits (LQs) that work to preserve quantum information and coherence by detecting errors within their boundaries, identifying corrections, and admitting repairs, all while maintaining fault tolerance.1,2 Theoretical and experimental advances in quantum error correction2 (QEC) have led to several demonstrations of fault-tolerant (FT) logical qubits in recent years across different hardware platforms. A further step towards universal, fault-tolerant quantum computing2 (UFTQC), however, comes by engaging separate LQs into entanglement, all while sustaining coherence and the protections of fault tolerance.

Entangled Logical Qubits (ELQ) is a four-year foundational research program directed at generating high-fidelity entanglement between two error-corrected LQs in a fully FT manner, and utilizing it to achieve logical state teleportation with high success. These accomplishments will lay the cornerstone for realizing the full potential of QC and make a profound advance on the path to UFTQC. The program is divided into four (4) phases, outlined in Table 1, Table 2, and Table 4, and described in detail in Section A.3. Proposals covering all four Phases are being solicited under this BAA; proposals covering fewer may not receive full consideration.

Broadly, ELQ seeks to develop and demonstrate schemes that preserve FT properties throughout an operational sequence that incorporates LQ entanglement. Importantly, the schemes must also exhibit modularity, where the entangled ensemble is built from, and separable into, decoupled, independently-operable LQs residing on the same physical platform. Modularity is also reinforced by the Program’s structure, with LQs being established separately during the first through third years of the Program before proceeding with entangling operations. We define modularity in Section A.2.1.2 and Box A in connection with architectural requirements.

While limited theoretical work exists, FT logical entanglement from the engagement of separate LQs is a frontier topic providing new ground for theory and practice to meet, cooperate, and evolve toward an objective critical to UFTQC. Achieving high-fidelity, maximally-entangled logical states, as evidenced, for example, by teleportation3 success rates of 95% or higher, is an ambitious yet attainable target in consideration of continuing advances in state-of-the-art performance of quantum hardware and related controls. The challenges—spanning developments of hardware, software, QEC protocols, benchmarking protocols—awaiting this deeper excursion into QC will be considerable. Successful teams will be interdisciplinary, adept at working at the interfaces between the disciplines involved, and capable of executing ground-breaking results.

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