the smallest qubit that had been experimentally created was a single electron trapped in a quantum dot. Quantum dots are nanoscale semiconductor structures that can confine electrons. In such experiments, the spin state of the trapped electron is used as a qubit, where the two spin states, typically referred to as "up" and "down," represent the computational basis states of the qubit.
It's important to note that the field of quantum computing is rapidly evolving, and new breakthroughs and advancements may have occurred since then. I recommend checking the latest scientific literature and research updates to find the most recent developments regarding the smallest qubits that have been created.