  Qubits are the fundamental building blocks of quantum information science and
applications, whose concept is widely utilized in both quantum physics and
quantum computation. While the significance of qubits and their implementation
in physical devices have been extensively examined, now is the right time to
revisit this understanding. In this paper, we introduce an abstract qubit model
(AQM), offering a mathematical framework for higher-level algorithms and
applications, and setting forth criteria for lower-level physical devices to
enable quantum computation. We first provide a comprehensive definition of
"qubits", regarded as the foundational principle for quantum computing
algorithms (bottom-up support), and examine their requisites for devices
(top-down demand). We then investigate the feasibility of relaxing specific
requirements, thereby broadening device support while considering techniques
that tradeoff extra costs to counterbalance this relaxation. Lastly, we delve
into the quantum applications that only require partial support of "qubits",
and discuss the physical systems with limited support of the AQM but remain
valuable in quantum applications. AQM may serve as an intermediate interface
between quantum algorithms and devices, facilitating quantum algorithm-device
co-design.
