Comparing technical limitations of major quantum computing approaches

Question

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.

Answer 1

For a general overview about hardware and the difficulties they present, I recommend section three of Quantum Computing: An Overview Across the System Stack. Another good introduction would be Quantum Computing Hardware Implementation Methods: A Survey over Categories.

Let me also give you some papers specific to certain implementations and their recent advancements.

Superconducting qubits:

• New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds.
• Superconducting Qubits: Current State of Play.

Trapped ion qubits:

• Realization of real-time fault-tolerant quantum error correction. This one may not be 100% about the hardware itself, but it still is a really important achievement using this technology.

NMR qubits:

• Quantum Computing and Nuclear Magnetic Resonance.

Topoligcal qubits:

• Understanding potential topological quantum bits. Not exactly a paper but an interesting article on the journey of this technology.