How can a quantum dot be used as a qubit? Many people say that quantum dot is a potential physical representation of qubit. A qubit should have two distinguishable states which may carry quantum information. What are the two states of a quantum dot which could be regarded as the two states of a qubit?
 A: Quantum dots themselves are not the qubits. The reason to use them is because a quantum dot defines an "island" that can house one single electron which then stores the quantum information.


*

*There can be spin qubits realized within a quantum dot. Once we measure the charge stability diagram for a quantum dot, we can apply the corresponding bias voltages to make sure there is only one electron on that dot. Then the spin-up or spin-down state of that electron represents the logical $|0\rangle_L \equiv |\uparrow\rangle$ or $|1\rangle_L \equiv |\downarrow\rangle$ state. Alternatively, we can also make use of two adjacent quantum dots and drive the bias to the range where each dot has one single electron on it. If there is exchange interaction between the two electrons, they can be either in the spin-singlet or spin-triplet state $|0\rangle_L \equiv \frac{1}{\sqrt{2}}(|\uparrow \downarrow\rangle - |\downarrow \uparrow \rangle), |1\rangle_L \equiv \frac{1}{\sqrt{2}} (|\uparrow \downarrow \rangle + |\uparrow \downarrow \rangle)$ which serve as our two computational states. 

*Charge qubits are also possible. For example, we again use two adjacent quantum dots to define one qubit. This time $|0\rangle_L$ is defined as having one electron in the left dot and no electron in the right dot, and $|1\rangle_L$ is defined as having no electron in the left and one electron in the right. 


To see in more detail how these qubits can be initialized, read out and operated upon, here are two review articles from the past decade. Some even older ones can also be found from the references therein.


*

*Hanson et al, Rev. Mod. Phys. 79, 1217 (2007)

*Zwanenburg et al, Rev. Mod. Phys. 85, 961 (2013)

