A quantum processing unit (QPU) is a type of computer processor that uses quantum mechanics to perform calculations. Unlike classical computers, which use binary digits or bits to store and manipulate data, a QPU (Fig. 1.) uses quantum bits or qubits. Qubits have the ability to exist in multiple states at once, which allows quantum computers to perform certain types of calculations much faster than classical computers. However, due to the inherent fragility of quantum states, QPUs require specialized cooling and shielding to operate effectively. Additionally, programming a QPU requires a different approach than programming a classical computer, as the algorithms used for quantum computation are fundamentally different from classical algorithms. Despite these challenges, QPUs have the potential to revolutionize fields such as cryptography, optimization, and drug discovery, and many tech companies and research organizations are actively working to develop practical quantum computers using QPUs.

Transmon qubit is a type of superconducting qubit that has gained popularity in the field of quantum computing due to its simplicity and robustness. It is a type of charge qubit that utilizes a Josephson junction to create a non-linear inductance, which allows it to be operated in a stable manner at higher frequencies than other superconducting qubit types. The transmon qubit (see Fig. 2.) has several desirable properties, such as a longer coherence time, a reduced sensitivity to charge noise, and an improved scalability compared to other superconducting qubit types. These features make it a suitable candidate for building large-scale quantum processors. However, one of the major challenges with transmon qubits is the difficulty in controlling individual qubits, which can limit the scalability of quantum systems. Nonetheless, research in the field of transmon qubits is ongoing, and the development of improved control methods and materials could lead to significant advances in quantum computing.