Definition may vary, but deep tech is commonly technology built on substantial scientific or engineering breakthroughs: solutions that represent a genuine paradigm shift in how a problem is solved, not an incremental improvement. I use the word "transformative" a lot in my day job, usually as marketing language.
Building a quantum computer that actually works is hard. Proving that a working machine does something economically worth doing is harder - and that proof requires spanning more disciplines at once than almost any single party currently holds. The challenges are usually presented as a list: qubits, error correction, talent, cost.
Overview Quantum computing is normally described as the deepest of the deep tech moonshots, but I think that is also precisely why it is so attractive – “We choose to go to the Moon... not because they are easy, but because they are hard” – John F. Kennedy. As a clear example