As far as I know, major quantum computing approaches include superconducting qubits, trapped ions, nuclear magnetic resonance qubits, and free space quantum optics. I guess two main technical difficulties in this field are creating entanglement and maintaining entanglement.
Are there any introductory review articles address the state-of-the-art status of various quantum computing approaches as well as the major challenges that each of them faces. Additionally, I am curious why the free-space quantum optics approach hasn't yet been "upgraded" to on-chip nanophotonic devices.
- I am a newbie to quantum computing research/industry. So, if there are any other promising approaches or other important technical difficulties, please add them.