three types of quantum computer

1. Analog quantum computer (quantum annealer, adiabatic QC, direct quantum simulation) examples: D-wave
2. Noisy intermediate-scale quantum (NISQ) gate-based computer  
1. example: IBM quantum computer
3. Fully error-corrected gate-based quantum computers. 
1. not available yet

Two leading technologies for quantum computing—trapped ions and superconducting qubits—use very different strategies for embodying and operating on qubits. 

quantum computer difficuty

There are many technical challenges:

1) quits cannot intrinsically reject noise

2) error free quantum computer requires quantum error correction

3) large data inputs cannot be loaded into a quantum computer efficiently.

4) Quantum Computers Will Need a New Software Stack

5) The Intermediate State of a Quantum Computer Cannot Be Measured Directly

From "Quantum Computing: Progress and Prospects (2018)" by National Academies Press. The pdf file is http://nap.edu/25196