Suggestion: use x and ~x instead of (x, 0) and (x, 1) for create/annihilate operators

[Strilanc]

Assuming modes must be non-negative integers, the bitwise complement will be unambiguous and significantly easier to read/write/code.

For example:

def get_fermion_operator(interaction_operator):
    n_qubits = count_qubits(interaction_operator)

    identity_term = FermionOperator((), interaction_operator.constant)

    one_body_terms = FermionOperator.sum(
        FermionOperator.ladder(~p, q,  # annihilate p, create q
                               coefficient=interaction_operator[p, q])
        for p, q in itertools.product(range(n_qubits), repeat=2)
    )

    two_body_terms = FermionOperator.sum(
        FermionOperator.ladder(~p, ~q, r, s,  # annihilate p, annihilate q, create r, create s
                               coefficient=interaction_operator[p, q, r, s])
        for p, q, r, s in itertools.product(range(n_qubits), repeat=4)
    )

    return identity_term + one_body_terms + two_body_terms

The downsides here are:

• ~ looks like -, so typos will be hard to spot and misreadings are somewhat likely.
• Mistakes go from fail-fast to fail-sneaky. If you accidentally create ((p, 0), 1) the code will probably fail, but accidentally creating ~~p will likely run to completion and create garbage. The -p instead of ~p typo is also at high risk to make something that almost works and eats an hour of your time to debug it.
• General weirdness / surprising to readers.

The upsides are:

• Performance. Ints are cheaper than tuples.
• Concise.

[Strilanc]

I've convinced myself this is too unpythonic to be a good idea.