= Quantum circuits vs classical circuits
Just like a classic <software engineer>[programmer] does not need to understand the intricacies of how transistors are implemented and CMOS semiconductors, the quantum programmer does not understand physical intricacies of the underlying <quantum computer physical implementations>[physical implementation].
The main difference to keep in mind is that quantum computers cannot https://en.wikipedia.org/wiki/Observer_effect_(physics)[save and observe intermediate quantum state], so programming a quantum computer is basically like programming a combinatorial-like circuit with gates that operate on (qu)bits:
* https://quantumcomputing.stackexchange.com/questions/8441/does-a-quantum-computer-have-a-clock-signal-and-if-yes-how-big-is-it/9383#9383
* https://quantumcomputing.stackexchange.com/questions/8849/quantum-circuits-explain-algorithms-why-didnt-classical-circuits/8869#8869
For this reason programming a quantum computer is much like programming a classical combinatorial circuit as you would do with https://en.wikipedia.org/wiki/SPICE[SPICE], <register transfer level>[verilog-or-vhdl], in which you are basically describing a graph of gates that goes from the input to the output
For this reason, we can use the words "program" and "circuit" interchangeably to refer to a quantum program
Also remember that and there is no no clocks in combinatorial circuits because there are no registers to drive; and so there is no analogue of clock in the quantum system either,
Another consequence of this is that programming quantum computers does not look like programming the more "common" procedural programming languages such as C or Python, since those fundamentally rely on processor register / memory state all the time.
Quantum programmers can however use classic languages to help describe their quantum programs more easily, for example this is what happens in <Qiskit>, where you write a Python program that makes Qiskit library calls that describe the quantum program.
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